Let's talk exposure

[MICKEY88]

You weren't slow at all! This has baffled me for the past 2 years but I was always too embarrassed to ask. I've done countless searches, read a dozen books and studied the manuals on several cameras and nowhere could I find anything that explained what "until your camera indicates the correct exposure" actually meant. No kidding, it's been driving me nuts. Thanks for having the guts to ask!

you should never be afraid to ask questions here, you are among friends,

if we don't have your answer we will be happy to make one up
..

 

[cpbjgc]

Nikon D80 -
The manual wasn't clear to me that the scale within the viewfinder indicated a correct exposure at 0. I thought perhaps that was it, and now I know for sure! Thanks, cpbjgc.
...guess I was a little slow to figure it out...

You are welcome. I have to say, I didn't have a firm grasp on this either until I read my operating manual a few times (after reading Understanding Exposure as well, and thinking, "What does he mean meter off the sky? How am I supposed to do that, exactly?").

Have fun playing with the settings, and remember there is no one "correct" exposure, just the creative exposure that captures what you want for your picture.

 

[oliviansmom17]

You weren't slow at all! This has baffled me for the past 2 years but I was always too embarrassed to ask. I've done countless searches, read a dozen books and studied the manuals on several cameras and nowhere could I find anything that explained what "until your camera indicates the correct exposure" actually meant. No kidding, it's been driving me nuts. Thanks for having the guts to ask!

I am SO glad to hear that...sometimes I feel like I might wear out my welcome...
And nice to know I'm not alone!

you should never be afraid to ask questions here, you are among friends,

if we don't have your answer we will be happy to make one up
..

 

[oliviansmom17]

You are welcome. I have to say, I didn't have a firm grasp on this either until I read my operating manual a few times (after reading Understanding Exposure as well, and thinking, "What does he mean meter off the sky? How am I supposed to do that, exactly?").

Have fun playing with the settings, and remember there is no one "correct" exposure, just the creative exposure that captures what you want for your picture.

Metering off the sky- I learned how to do that here, too. I'm so glad I found this board- help without intimidation- I love it!
I swear, I feel like I'm back in school again..
At least this time I enjoy the subject...
Thanks for the encouragement!

 

[pjacobi]

Unfortunately, the exposure is incorrect.

If you used an automatic exposure mode, the light meter reads all the light from the sky leaving the flowers backlit and under exposed.

To get the correct exposure, you need to meter only the flowers and set a manual exposure.

You would also need to use a polarizer or split nurtral density filter to keep the over exposed sky blue.

The composition is good, but there are some tricky exposure problems with the photo. Perhaps some of the exposure problems can be fixed with a little post processing.


-Paul

 

[MarkBarbieri]

I think it's important to know a little bit about how your camera's meter works. It helps you understand it's limitations.

The camera meter sees the world in black and white (shades of grey might be a better description). It tries to determine how much light is available by looking at how bright things are. When it sees something very bright, it assumes that there is a lot of light available and, conversely, when it sees something very dark, it assumes that there is very little light available.

For this to work, the camera has to guess at how much light things reflect. If you hold three different pieces of paper in front of the camera (one black, one white, and one grey), you'll get three different meter readings. The camera assumes that they are all grey because it doesn't have any way of knowing. When it sees the grey one, it sets the exposre properly. When it sees the white sheet, it thinks that it is a grey sheet with a lot of light shining on it. When it sees the black sheet, it thinks that it is seeing a grey sheet in the dark.

Think about how this applies in real life. In a typical scene, everything works roughly as expected. In a scene where there is an unusually light subject (perhaps a lot of snow, someone in a white dress, or a white pet), the meter may assume that there is more light available than is really the case. That will cause the camera to underexpose and what should have looked white in the picture will actually look grey. Conversely, in a scene with lots of dark colors (a person in a black suit, a black pet, or a vampire bat in a coal mine), the camera will assume that there is less light available than is really the case. That will cause the camera to overexpose and what should have looked black in the picture will look grey.

So what do you do? Normally, just trust your meter and don't worry about it. If you are taking a picture of something unusually light colored or dark colored, adjust your exposure. If you are setting a manual exposure picture of someone in the snow, try setting it so that the meter reads +1. Now the camera will let in twice as much light as it otherwise would. If you are setting a manual exposure of that vampire bat in the coal mine, set the meter to read -1. Now the camera will let in half as much light as it otherwise would.

You can also adjust your exposure without being in manual exposure mode. Most DSLRs (and many P&S) allow you to set "exposure compensation." That is a setting where you tell the camera that you want it to still handle all the metering and exposure settings but you want it to set the exposure higher or lower than what it thinks it should be. When I go shoot in the snow, I often set my exposure compensation at +1 and still use an auto-exposure mode.

This type of metering (looking at things and guessing at how brightly lit they are) is called reflective metering. That's the way cameras work. There is another type of metering called incident metering. They work the opposite. With an incident meter, you carry the meter over to your subject and it directly reads the amount of light coming in. Because it isn't looking at reflections, it isn't influenced by how light or dark colored your subject is. This type of metering is impractical for casual shooting, but it's great for studio shots where you want to carefully balance different light sources.

All of this probably sounds terribly complicated if you are new to it. Fortunately, today's cameras are amazing at guessing the right exposure level, so you can almost always just trust them and they'll do a good enough job. The one time that you have to be very careful is when you use a "spot" metering mode. In that mode, you are telling your camera to meter off of the little bit of the picture covered by the spot meter (usually about 2% of the picture right in the middle). If that spot happens to be unusually dark or light, the exposure for the whole picture will be thrown off. I recommend that you either don't use spot metering until you understand metering fairly well or that you be careful to always meter off of something not too dark and not too light. Blue sky and people's skin usually work OK.

If you aren't sure whether you metered correctly, there is a wonderful tool available to digital shooters to check. Your camera almost certainly can display something called a "histogram." Reading histograms is another great subject to learn about, but I'll leave that for another time. I'll just say that if you want to master exposure, you will do yourself a great favor by learning to read a histogram and checking your histogram whenever you are in doubt about an exposure.

 

[MarkBarbieri]

Whether the exposure is correct or not is, I think, an artistic judgment - an opinion if you will. I agree, however, that I would like to see more balance between the brightness of the flowers and the sky. It's a lovely composition, though.

If you metered off of the flowers, that would increase the exposure and make the flowers look right, but it might overexpose the sky. A polarizer could help by darkening sky more than it darkens the flowers. I don't think that a split neutral density filter would work well with this scene because the dark and light areas are so intermingled. They tend to work best when there is a relatively clear dividing line between the darkly and brightly lit parts of the picture.

In this situation, I would first look to see if there was an angle that would show the flowers light by the sun that still showed the sky in the background. If there wasn't, I might try using a flash or a reflector to light up the flowers. However you slice it, the trick is to find a way to either make the sky darker or the flowers lighter so that they can all appear well exposed. If none of that works, I'd take it with the brightest possible exposure that didn't blow out the sky and then I'd drop the luminance level of the blue color chanel in post production (which is just a fancy way of saying I'd darken up the blue stuff in the picture on my computer).

 

[barrie]

Thanks Mark - I was thinking "he should put this on his blog", then I saw that you did!

So what do you do? Normally, just trust your meter and don't worry about it. If you are taking a picture of something unusually light colored or dark colored, adjust your exposure. If you are setting a manual exposure picture of someone in the snow, try setting it so that the meter reads +1. Now the camera will let in twice as much light as it otherwise would. If you are setting a manual exposure of that vampire bat in the coal mine, set the meter to read -1. Now the camera will let in half as much light as it otherwise would.

You can also adjust your exposure without being in manual exposure mode. Most DSLRs (and many P&S) allow you to set "exposure compensation." That is a setting where you tell the camera that you want it to still handle all the metering and exposure settings but you want it to set the exposure higher or lower than what it thinks it should be. When I go shoot in the snow, I often set my exposure compensation at +1 and still use an auto-exposure mode.

OK, now this is another thing that has always confused me. If I'm reading this right, in order for my photo to come out a little darker I should set the meter or exposure compensation to "+_" and if I want it to be brighter, I should set it at "-_". Is that right? That just seems counter-intuitive to me. I always want to do the opposite.

 

[MICKEY88]

Thanks Mark - I was thinking "he should put this on his blog", then I saw that you did!



OK, now this is another thing that has always confused me. If I'm reading this right, in order for my photo to come out a little darker I should set the meter or exposure compensation to "+_" and if I want it to be brighter, I should set it at "-_". Is that right? That just seems counter-intuitive to me. I always want to do the opposite.

,

if you want your pic darker you set minus exposure comp..

for lighter you set plus exposure comp..

 

[oliviansmom17]

I think it's important to know a little bit about how your camera's meter works. It helps you understand it's limitations.

The camera meter sees the world in black and white (shades of grey might be a better description). It tries to determine how much light is available by looking at how bright things are. When it sees something very bright, it assumes that there is a lot of light available and, conversely, when it sees something very dark, it assumes that there is very little light available.

For this to work, the camera has to guess at how much light things reflect. If you hold three different pieces of paper in front of the camera (one black, one white, and one grey), you'll get three different meter readings. The camera assumes that they are all grey because it doesn't have any way of knowing. When it sees the grey one, it sets the exposre properly. When it sees the white sheet, it thinks that it is a grey sheet with a lot of light shining on it. When it sees the black sheet, it thinks that it is seeing a grey sheet in the dark.

Think about how this applies in real life. In a typical scene, everything works roughly as expected. In a scene where there is an unusually light subject (perhaps a lot of snow, someone in a white dress, or a white pet), the meter may assume that there is more light available than is really the case. That will cause the camera to underexpose and what should have looked white in the picture will actually look grey. Conversely, in a scene with lots of dark colors (a person in a black suit, a black pet, or a vampire bat in a coal mine), the camera will assume that there is less light available than is really the case. That will cause the camera to overexpose and what should have looked black in the picture will look grey.

So what do you do? Normally, just trust your meter and don't worry about it. If you are taking a picture of something unusually light colored or dark colored, adjust your exposure. If you are setting a manual exposure picture of someone in the snow, try setting it so that the meter reads +1. Now the camera will let in twice as much light as it otherwise would. If you are setting a manual exposure of that vampire bat in the coal mine, set the meter to read -1. Now the camera will let in half as much light as it otherwise would.

You can also adjust your exposure without being in manual exposure mode. Most DSLRs (and many P&S) allow you to set "exposure compensation." That is a setting where you tell the camera that you want it to still handle all the metering and exposure settings but you want it to set the exposure higher or lower than what it thinks it should be. When I go shoot in the snow, I often set my exposure compensation at +1 and still use an auto-exposure mode.

This type of metering (looking at things and guessing at how brightly lit they are) is called reflective metering. That's the way cameras work. There is another type of metering called incident metering. They work the opposite. With an incident meter, you carry the meter over to your subject and it directly reads the amount of light coming in. Because it isn't looking at reflections, it isn't influenced by how light or dark colored your subject is. This type of metering is impractical for casual shooting, but it's great for studio shots where you want to carefully balance different light sources.

All of this probably sounds terribly complicated if you are new to it. Fortunately, today's cameras are amazing at guessing the right exposure level, so you can almost always just trust them and they'll do a good enough job. The one time that you have to be very careful is when you use a "spot" metering mode. In that mode, you are telling your camera to meter off of the little bit of the picture covered by the spot meter (usually about 2% of the picture right in the middle). If that spot happens to be unusually dark or light, the exposure for the whole picture will be thrown off. I recommend that you either don't use spot metering until you understand metering fairly well or that you be careful to always meter off of something not too dark and not too light. Blue sky and people's skin usually work OK.

If you aren't sure whether you metered correctly, there is a wonderful tool available to digital shooters to check. Your camera almost certainly can display something called a "histogram." Reading histograms is another great subject to learn about, but I'll leave that for another time. I'll just say that if you want to master exposure, you will do yourself a great favor by learning to read a histogram and checking your histogram whenever you are in doubt about an exposure.

Thanks, Mark! I'm working on the whole histogram thing...that'll be my next thread, I'm sure .
Gotta give Momma some time to let this gel...

Thanks Mark - I was thinking "he should put this on his blog", then I saw that you did!



OK, now this is another thing that has always confused me. If I'm reading this right, in order for my photo to come out a little darker I should set the meter or exposure compensation to "+_" and if I want it to be brighter, I should set it at "-_". Is that right? That just seems counter-intuitive to me. I always want to do the opposite.

I always think of it as:
- = take away light...+ = add light. Can you tell I live with a 3rd grader??

 

[SrisonS]

Wow!!! That's quite a bit of analysis on that pic. I was mainly just going for composition, without the sky being too overexposed. And I was just taking quick pictures, so i didn't want to sit there and mess with all the settings. And as far as a different angle goes; this was the only one I could get without a building probably being in the background (these flowers were located at the bottom of the pillar of the "winged lion"). And I think the building behind me was blocking some of the sunlight.

I almost always forget about using the flash to light up my dark subjects....because I RARELY do it. And usually when i think about it, I'm already home, disappointed on how something looks (not just with my nicer pics either...but "touristy" ones too). But I'll probably be going back to Epcot soon, so maybe I'll try a few things to get the picture "right". Thanks for the breakdowns though.

 

[vettechick99]

Funny, I took this shot with that same picture of his as inspiration...

(small town in Mexico on a HOT summer day)

I think I metered off the cacti, but it was last year and I can't remember. I can't remember half the stuff I do, therefore I never seem to get any better!

 

[MarkBarbieri]

The term "exposure" keeps coming up in different threads. It's critical that you understand it if you are going to learn to take good pictures in a variety of conditions. Even if you leave your point and shoot camera in full manual, you'll be better off knowing how it handles exposure.

First, it helps to understand that your camera builds a picture from lots and lots of little dots called pixels. When the picture is taken, each pixel is either red, blue, or green (they get blended into more colors later in the process). Each of these pixels ends up somewhere between totally dark (black) to totally light (white). Each pixel start out as black when you take a picture and the more light that hits it, the farther it moves towards being white.

The first step in understanding exposure is understanding ISO. That's how sensitive your camera is to light. A high ISO number (like 1,600) means that your camera is very sensitive. A low ISO number (like 100) means that it is not very sensitive. When your camera is very sensitive, that means that it doesn't take much light to get a pixel to go from black to white. If your camera is not very sensitive, it takes a lot of light to go from black to white.

With just about all digital cameras, you can adjust the sensitivity (ISO) from a low number to a high number. On a bright, sunny day, you probably want ISO 100 (or the lowest number your camera can be set to). On an overcast day, you want it higher - perhaps ISO 400. When it gets even darker, you want to go higher still.

After ISO, the next thing to understand is "shutter speed." That's the term for the amount of time your camera is gathering light to make the picture. When someone says that they are using a shutter speed of 1/250s, that means that the camera is gathering light for one two hundred and fiftieths of a second. The longer the shutter speed, the more time your camera has to gather light. It’s as simple as that – it’s just the amount of time those pixels get to collect light. The more light they collect, the brighter they are in the picture.

The last piece of the puzzle is the “aperture.” It’s just a fancy way of saying the “size of the opening in the lens that the light goes through.” If your lens has a big opening, it lets a lot of light through. That means that your pixels brighten up more quickly. If it has a little opening, it doesn’t let much light through.

We refer to apertures with “f-stop” numbers. We can’t just talk about the size of the opening itself because the world is a little more complicated than that. What matters isn’t the size of the opening but how big the opening is compared to how long the lens is. That’s too confusing to keep up with, so we use f-stops. You don’t need to know anything about the size of the opening or how long the lens is if you just know the f-stop number.

F-stop numbers are really fractions, but most people don’t like fractions, so we pretend that they are just normal numbers. If you want to know the truth, an f-stop of f5.6 (sometimes written as f/5.6) really means that the size of the lens opening is 1/5.6 of the length of the lens. You don’t really need to know or care about that. The one thing that you do need to know is bigger f-stop numbers mean smaller openings. So a big lens opening is f/4 and a really small lens opening is f/22.

Some lenses open bigger than others. Those are called “fast” lenses because the big opening can let a lot of light in. With all that extra light coming in, the camera doesn’t need to take as long making the picture. So they take pictures faster.

Now let’s bring our three terms together – ISO, Shutter Speed, and Aperture. All of them affect how pixels gather light (which makes them go from dark to light). The ISO is how much light it takes to go from dark to light. The shutter speed is how long the camera spends gathering light. The aperture is how wide the opening is that lets light in.

When you take a picture in full auto-mode, your camera chooses all three of these things for you. Often, you choose two of these and the camera chooses the third. Let’s say that you pick an ISO of 200 and an aperture of f8 and your camera picks the shutter speed of 1/250s. You camera’s meter determined that with a sensitivity of ISO 200 and an opening of f8, it will take a one two hundred and fiftieth of a second to gather enough light so that some pixels get very bright while others stay fairly dark. If it gathers light for longer than that, too many pixels will get too bright and your picture will look too bright (overexposed). If it gathers light for less time than that, pixels won’t gather enough light and your picture will be too dark (underexposed).

In the same situation, you could adjust the ISO or the aperture and the camera would change the shutter speed. If you changed the ISO from 200 to 400 (doubling it), you made the camera twice as sensitive to light. Now it will change the shutter speed from 1/250s to 1/500s. Because it is twice as sensitive, it only takes half as long to gather the light.

Every time you double the ISO (from 100 to 200, 200 to 400, 400 to 800, 800 to 1600, etc), you make the camera twice as sensitive, so it needs half as much time to take the picture. The word used for making the camera twice as sensitive, or making your shutter speed twice as fast, or opening your lens opening twice as wide is a “stop.” The reason for the term is unimportant (some historical thing no one cares about anymore). Just know that when someone says that they changed their ISO, shutter speed, or aperture by one stop, they either made it twice as much or half as much.

Here’s where the stupid aperture fraction stuff comes back to bite us. With ISO and shutter speed, stops are easy – they are just double or half the other number. With f-stop numbers, it’s just a mess. If you want to understand it, I’ll try to explain it, but I’m warning you now that it involves the square root of two and probably pi as well. The simple fact is, you’ll just have to take my word for it that these are the f-stop numbers that matter: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f11, f16, f22, and f32. Every time you move up one of the numbers on this list, you are letting in half as much light (remember, bigger numbers mean a smaller opening – sorry, not my fault). Every time you move down one of the numbers on the list, you are letting in twice as much light (smaller number = bigger opening).

You might see aperture numbers that fall in between these. A common lens is an f/1.8 lens. That means that it has a bigger opening than an f/2 lens but smaller than an f/1.4 lens. Don’t get hung up on the exact difference because you don’t really need to know.

OK, so back to our ISO 200, f8 (f-stop), 1/250s (shutter speed) example. If you told the camera that you wanted to keep the shutter speed at 1/250s and you doubled the ISO to 400, now what would happen to the aperture? We made the camera twice as sensitive, so it would want to make the opening half as big. That means moving up on our f-stop list to f11 (one last time – bigger numbers mean smaller openings).

The camera wants to keep everything in balance. When you adjust one of the three – ISO (sensitivity), shutter speed (how long it gathers light), or aperture (how big the opening is), the camera wants to adjust one of the others to keep things in balance. This is where those exposure modes – P, Tv, Av, and M come in.

First, all of these modes assume that either you or the camera has picked an ISO, so we’re going to forget about it right now and just focus on aperture and shutter speed. When you shoot in P (program) mode, the camera picks both a shutter speed an aperture.

When you shoot in Tv (shutter speed – no, I don’t know why Tv stands for shutter speed, it just does) mode, you get to pick the shutter speed and the camera picks the aperture. If you make the shutter speed twice as fast, the camera makes the lens opening twice as big. If you make the shutter speed twice as slow, the camera makes the lens opening half as big.

When you shoot in Av (aperture) mode, you get to pick the aperture and the camera picks the shutter speed. If you pick a bigger f-stop number (smaller opening), the camera picks a slower shutter speed. If you pick a smaller f-stop number (bigger opening), the camera picks a faster shutter speed.

In Manual mode, you’re on your own. You can pick the shutter speed and the aperture. If you pick a faster shutter speed and forget to make your lens opening bigger, your picture will be darker because you gave it less time to gather light. If you pick a slower shutter speed and leave everything else along, your picture will be brighter because you gave it more time to gather light. While you are making these adjustments, your camera will show you on a little meter whether it thinks your picture will be too bright, too dark, or just right.

So why not just shoot in P all the time and let the camera worry about it? The answer is that you may want a particular shutter speed or lens opening or ISO. There are side effects to each of these settings.

For ISO, the side effect is noise. Noise is what we call those little spots on pictures that are weird colors or too dark or bright. The higher you make your ISO, the more of these dots you get and the weirder looking they get. You want to use the lowest ISO you can and still get the shot so that you have the least noise possible. The amount of noise you get at any particular ISO depends on your camera. Some can take noise free pictures at very high ISO numbers and others are noisy even with low numbers.

For shutter speed, the side effect is how motion is displayed. Usually, you want a pretty fast shutter speed so that your picture isn’t blurry. If it takes too long for you to take a picture, you might not hold the camera still enough and everything will look blurry. Even if you have the camera on a tripod, a long shutter speed will give time for the things you are photographing to move. That might mean that your running child is a blurry blob instead of a kid.

Sometimes a little blur is a good thing. Waterfalls look better with longer shutter speeds because the water blurs and looks softer. Action sometimes looks better with a little blur because our eyes see that blur as movement. The main thing to understand is that the longer your shutter speed is, the more things will blur with movement.

For apertures, the main side effect is “depth-of-field.” That’s a photography term that refers to how much of the picture is in focus. When you have a lot of depth of field, things that are very near and very far are all in focus. When you don’t have much depth of field, things that are closer than your subject or further away than your subject are blurry. That sounds bad, but sometimes having very little depth-of-field (DOF) is good because it makes your subject stand out against a blurry background. Big lens openings (small f-stop numbers) mean less depth-of-field. Small lens openings mean more depth-of-field.

So let’s say that you want to take a picture of your friend, it’s a cloudy day, and you want the background behind them to be blurry. Because we want that background blur, we want to use Av mode so that we can control the DOF. Because it’s cloudy, let’s start with an ISO of 400. Because we want the background to be blurry, let’s use a big opening and pick an aperture of f/2.8 (or whatever the smallest f-stop number your lens will allow). The camera will pick the shutter speed that, given the amount of light it sees, will work best with that ISO and that aperture. Let’s say that it picks 1/250s. Great. We take our picture and all is good.

Now let’s say that we want one with the background in focus. We increase out aperture from f/2.8 to f/4 to f/5.6 to f/8 to f/11. We just moved it up four steps so the camera will double the shutter speed in half four times 1/250 to 1/125 to 1/60 (I know that’s not really double, but camera round things off in their own weird way) to 1/30. Now, 1/30 of a second doesn’t seem like a long time, but that might be long enough to make the picture a little blurry. To be safe, I may want a higher shutter speed…let’s say 1/60s. In that case, I can either go back to f/8 and get a blurrier background or increase my ISO from 400 to 800 and have more noise. Because I am giving the camera half as much time to gather light, I either have to let twice as much light in (f/11 to f/8 on our scale) or I have to make it twice as sensitive ISO 400 to 800. It’s all a tradeoff. If I didn’t keep them all balanced, my picture would get darker.

I do have one other option in addition to the big three (ISO, shutter speed, and aperture). I can add more light. The exposure numbers that the camera keeps trying to balance are based on how much light is available. On a brighter day, I might have been able to use a much lower ISO and a much smaller aperture and still gotten a fast shutter speed. All that extra light would mean that I didn’t need to let as much in or be as sensitive and I could still gather all the light I needed in a hurry. I can move things in my favor adding a flash or other lights.

Well, that’s all the time I have at lunch today, so that wraps up my attempt at explaining exposure. I hope it helps someone.

 

[Geoff_M]

Mark, when I teach the concept of "exposure" to people, I like to use a plumbing analogy and I find people can really gasp it when I do so.

When talking about ISO, it's like needing a certain amount of "water" (substituted for "light") to make a proper picture. You can pick an arbirtrary amount and say that ISO 400 = 10 gallons of water. Doubling the ISO requires 1/2 as much water (ISO 800 = 5 gallons) and vice versa (ISO 200 = 20 gallons). What you're taking a picture of doesn't (for the most part) determine the amount of light you need, as that's determined by the ISO you use.

There's lots of ways to get X gallons of water into a bucket, but it boils down to three variables: the time you leave the facet open, the diameter of the pipes leading up to the facet, and the water pressure.

The time you leave the facet open represents the shutter speed... the longer you leave it open, the more water flows into the bucket and the reverse is true.

The diameter of the pipes is the analog of the lens aperture. At the same water pressure, a fire hose will deliver a lot more water per minute than the amount that a pipe the size of a straw can deliver. The same is true for wider and narrower lens apertures.

Lastly, there's the water pressure. This is the equivalent of the relative illumination of the subject. The more well light a subject the more "pressure" (OK, ignore for a second that the speed of light is constant) it has. If you've stayed in a hotel with 800 high schoolers that are all trying to get ready for a 8 AM meeting (I have), then you know that the lack of water pressure will make for a longer shower (though the lack of hot water may offset the desire for more time).

So let's say you need 10 gallons of water to make something the right way. You basically have to figure out the combination of water pressure, pipe diameter, and the time you're going to leave the tap open to get the 10 gallons. Often times, one variable is fixed for you. Unless you're going to add a booster pump, the water pressure that the city delivers to you is out of your hands, you're left with changing the diameter of the pipe or the time you leave the facet open... or both to some degree. The same is true for photography... The only way to add "pressure" is to turn on some room lights, or use a flash, but otherwise you're a victim of the natural light of your scene.

Of course you'll have to also work around certain limitations. You may only be able to use pipes up to a certain diameter (i.e. the maximum aperture of the lens) or you may only want to leave the facet open for so long (if you use a shutter speed that's too long, you'll get motion blur). If you hit the limits of one variable, you'll have to adjust another to compensate. If you're using the widest pipes you've got and are leaving the facet open for as long as you can and you still can't get 10 gallons, then you may want to double the ISO so you'll only need 5 gallons. If you have the ISO jacked up as high as you want to take it, then you'll need to get some bigger pipes (i.e. a "faster" lens with a larger maximum aperture.) to deliver more water faster.

 

[Master Mason]

]First, it helps to understand that your camera builds a picture from lots and lots of little dots called pixels.

But Pixels are rectangular not dots

 

[ksloane]

Thanks for these lessons. I was actually reading online and starting some beginning notes so that I could begin to understand.

You guys are much simpler and easier to understand than the other websites I've been reading today.

 

[mabas9395]

When you shoot in Tv (shutter speed – no, I don’t know why Tv stands for shutter speed, it just does)

They say that Tv stands for "Time Value". And we all know that "they" are never wrong.

 

[seashoreCM]

But Pixels are rectangular not dots

Pixels can be square (or rectangular) or round or (rarely) triangular and so long as you are not sitting too close to the TV screen or holding the photograph too close to your eyes, you won't notice the difference. In almost all cases pixels are arranged in rows and columns so as to imply a square or rectangular space for each pixel. Early color TV sets and even some modern CRT monitors had round colored dots although usually "one pixel" in terms of the video format spanned more than one dot.

When the format has square (or circular) pixels, images can be rotated 90 degrees and maintain the same proportions without the need to reprocess them.

The spots made by ink jet printers are theoretically circular although the bleeding of the ink on the paper results in the spot being some irregular shape. Also, one pixel in terms of the image file format usually spans more than one spot made by the printer.

Digital camera hints: http://members.aol.com/ajaynejr/digicam.htm

 

[Geoff_M]

They say that Tv stands for "Time Value". And we all know that "they" are never wrong.

Also, for my fellow Nikonians... Tv = "S" (Shutter Priority) and Av = "A" (Aperture Priority) exposure modes.

I'll also delve a bit into the "P" (Program) Mode. With a given illumination of a subject (think a fixed "water pressure"), if you create a line graph with aperture on one axis and shutter speed on the other you can draw a line that would represent each aperture (pipe diameter) value and the corresponding shutter speed (time that the tap is open) needed for enough light (gallons of water) to reach the camera sensor. All the "P" Mode does is select where on the graph to set the shutter speed AND aperture for the photo. The settings it selects takes into account such things as a needing a shutter speed fast enough to minimize blur, the focal length of the lens, etc.

Certain "scenic modes" are varations on the standard "P" mode. For example, the "Sports" mode simply will bias the settings selected towards faster shutter speeds. Other modes will skew things toward using smaller apertures for increased depth of field.

 

[boBQuincy]

The subject was handled in typical Mark fashion...that is, accurate and thorough!

If I might add something to "exposure", it is that all this deals with setting the camera to what the internal meter sees as the correct exposure. For various reasons the internal meter is usually quite accurate but it cannot make artistic interpretations.

Setting the ISO, aperture, and shutter speed to what the meter sees as a proper exposure is a very important step but it is only the first step toward getting the exposure we want. The next step is how to interpret the meter reading, the scene, and our own artistic wishes to come up with our "best" exposure.