Let's talk exposure

[caniswolfie]

Oh and BTW,,,, "North-South-Carolina-bbq-seafood-eating-picture-taking-sketching-extravaganza" Sounds like something I'd love to take part in. Throw in some good beer and I'm all over that. BBQ and Seafood!! Two of my most favoritests!!!! Especially if the seafood is BBQ'd!!

Mmm.. BBQ.. I want some good pictures of it. That way my cholesterol won't skyrocket. Since if I had my way I'd eat way too much BBQ.

 

[Snurk71]

I'm trying to get better at setting the controls properly on my new XT, so I check the histogram to see how things look and make adjustments based on what I see in the histogram. But I thought I should make sure I'm reading the histogram correctly.

I understand the histogram goes from underexposed (dark) to overexposed (light) as you read across the graph from left to right. I've been trying for a bell curve on the histogram for general pictures. Is this even correct? Would you shoot for a different looking histogram based on the conditions (sunlight, night, sunset, etc)?

Here is a sample of a histogram from a picture I took. Is this bad, fair, good, perfect? The picture was taken indoors this morning with overhead ceiling fan lights as the light source. If it makes any different, here are the settings I had:

1/25 shutter
3.5 aperature
800 ISO
18mm
Tungsten WB

Thanks

 

[Anewman]

I've been trying for a bell curve on the histogram for general pictures. Is this even correct? Would you shoot for a different looking histogram based on the conditions (sunlight, night, sunset, etc)?

Pretty much yes a "bell curve" would be a decent starting point, for a balanced scene.

And yes different scenes would yeild much different histograms and should be taken into consideration when looking a the histogram. You would not expect the following image to produce the same type of "bell curve" since about 70% of it would fall on the far right of the histogram. Using the same logic a night scene could easily have 80% on the far left of the histogram while the stars/lights would produce little spikes on the far right.

IMO the biggest thing to look for is not having any data FALLING off either end of the histogram, especially if that data is important to you. So if you are shooting a wedding couple any thing falling off to the right means you are losing the detail on the wedding dress, data falling off the left means you are losing detail on the tux(to me not as important).

The histogram is only so big and you can not always capture the entire range you would like too. Using the histogram you would need to know what is the most important for that paticular scene.
For the following shot I was pretty much shooting directly at the cloud covered sun, if I exposed any more for the planes the background would have been way over the right of the histogram. So instead I allowed a little fall off on the left.

 

[MarkBarbieri]

I agree that the left and right edges of the histogram are the most important. In the sample histogram, there is a little line on the right side of the histogram that appears to represent blown out highlights. This is a section of the histogram that is irrecoverably too bright. You could lower the exposure a bit, but that would probably result in some sections being too dark. It's up to you to decide which is preferable.

The key is understanding what you are losing. Many cameras give some indiciation on the picture of where you have overexposed. You might see a bit of sky flashing or perhaps the glare off of someone's cheek. As the photographer, you've got to make a decision as to what is acceptable and what's not.

I would focus much more on trying to get an exposure that doesn't result in blow highlights or lost shadow detail rather than the shape of the histogram. To change the shape itself requires that you change the lighting on the scene, which is considerably more complicated than just adjusting your exposure level.

One exception to this that is easy to correct is when you have a backlit subject. This often happens when your subject is in the shade but part of the scene is in full sun. In that case, you can illuminate the subject with your flash to brighten it up. Now the difference in brightness between the subject and the background should be less and you are more likely to get the entire scene without hitting either edge of the histogram.

If you happen to be taking a shot where you aren't in danger of going off either side of the histogram, some people suggest that you purposefully overexpose and then correct in post processing. To be honest, I don't completely recall the logic in this approach. I think it has to do with brighter exposure levels having less noise and something else to do with the ability of sensors to get more information on the right side of the histogram rather than the left. Honestly, I can't say that I've ever bothered with this, but I thought it might be worth mentioning.

If you've got time and a tripod and you have a scene that you really want to capture but you can't do it without something important being over or under exposed, you still have an option. You can take two identical shots with one overexposed and one underexposed. You can then mate the two together in Photoshop using the dark bits from the overexposed shot and the light bits from the underexposed shot. The result is that you get everything reasonably exposed. I've done this a few times with sunrise and sunset photos and it works quite well. The drawback is that you have to have time, a tripod, and a subject that doesn't change dramatically between the shots.

 

[rtphokie]

Consider the following

Canon Rebel XT with the kit lens
ISO 400
F5.6
1/60 sec exposure
Focal Length of 77mm (equiv)

What could I have done to prevent this from being so overexposed? Did the higher ISO make that big of a difference? Or is a lost cause due to the really cloudy day and bright background?

 

[MarkBarbieri]

I'm not really sure that I understand the question. The answer seems straightforward enough - lower the exposure level. Perhaps the question is how one should go about lowering the exposure level.

If you are shooting in full manual mode, that's as simple as stopping down the aperture, shortening the shutter speed, or reducing the ISO.

With the camera doing the metering, adjusting the ISO, shutter speed, or aperture won't help because the camera will just adjust the other values to get the exposure that it thinks is correct. You need to either overrule the camera's notion of correct exposure or change the way the camera meters the scene.

To override the camera's sense for correct exposure, you can dial in exposure compensation, perhaps to -1/2 stop. Just be sure that you turn off the exposure compensation when you are done or you really will be underexposing shots later.

Adjusting exposure compensation is most useful when shooting a scene that will fool the camera's meter. A common example is shooting scenes with lots of snow. The camera's meter assumes that everything in the world is a lightish shade of gray. When it sees a predominately white scene like someone in the snow, it thinks "wow, that gray is brightly lit" and reduces the exposure to make the snow look gray. Conversely, if you shot someone in front of a lot of black stuff, the camera thinks "wow, that gray is poorly lit" and increases the exposure to make the black look gray.

In your case, it looks like the camera was off because it metered off of the center of the subject. It decided that you wanted the flower to look really good and weren't concerned that the surrounding sky would get blown out. When shooting a backlit subject, it is often difficult for the camera to determine what it should do. If it meters for the subject, the background gets blown out. If it meters for the background, the subject ends up too dark. You can force it to meter off of whatever you want by aiming the center of the lens at your desired subject and pressing the spot-meter button.

Another option would be to use a flash for the shot. That would illuminate your subject so that it is closer in brightness to the background. The camera can only capture so many different light levels (called the dynamic range), so reducing the dynamic range by lightening dark areas with a flash is sometimes the only way to capture everything in a scene.

 

[rtphokie]

I'm not really sure that I understand the question. The answer seems straightforward enough - lower the exposure level. Perhaps the question is how one should go about lowering the exposure level.

If you are shooting in full manual mode, that's as simple as stopping down the aperture, shortening the shutter speed, or reducing the ISO.

I didn't ask my question very completely.

If I crank down the ISO, aperture and shutter speed, wont I loose the detail of the center of the flower?

 

[mabas9395]

I think an important point to remember that not every picture will be able to have every element properly exposed. Try taking a picture of a black cat in the snow, you can set your exposure to get detail out of the cat or the snow, but not both.

Which leads to point number two, which is that there is not ONE correct exposure for a given scene. To a certain degree, whether a photo is too over exposed or not is a matter of opinion. It depends on the creative look you are going for.

I think you have captured a technically acceptable and creatively pleasant exposure, even if it wasn't the exposure you were going for.

 

[MarkBarbieri]

If you reduce your exposure by reducing any of the ISO, shutter speed, or aperture, you will risk a loss of detail with the center of flower. In your particular picture, I think you've still got about a 1/2 stop of lattitude before that happens.

This is where a histogram comes into play. Here is the histogram for the photo you posted:

The x-axis shows the range of brightness with the far left being totally black and the far right being totally white. The y-axis represents the proportion of pixels with that brightness value. The tall line on the far right side represents the pixels that are irretrievably overexposed. The area on the far left shows the darkest parts of the picture. Because there are very few pixels shown their (short bars or none at all), that means that you will not lose much if you lower your exposure.

After you take a shot, do a quick check of the histogram. If there is room on the left, you can safely lower your exposure. If there is room on the right, you can safely raise it. If there is no room on either end, you either have to change the lighting or be prepared to have some parts of the picture underexposed and others overexposed.

One more thing to worry about is that your camera is taking pictures with three colors but most have histograms that show only total luminosity. It is possible to overexpose or underexpose one of the three color channels and not notice it with a histogram that doesn't show separate colors. This is most often true when shooting extremely saturated colors.

Here is a good article on understanding histograms.

 

[Anewman]

I will try to keep it simple.

Always EXPOSE for the Highlights(in this case the bright sky background), which yes may leave the flower a bit on the darker side.

Options.
1. fill flash, may not be ideal for this shot.
2. Reflect some light back onto the flower with like some white poster board postitioned out of the frame.
3. You lighten the flower back up in post.
4. live with the silhouette type look.
5. Live with some clipping in the background
6. any combination of the other options.

 

[MarkBarbieri]

Always EXPOSE for the Highlights(in this case the bright sky background), which yes may leave the flower a bit on the darker side.

It's a question of style. I'm usually willing to blow out the sky if I have to in order to get my subject. My philosophy is do what you can within reason and the demands of the situation to get everything to an achievable dynamic range. If you can't, worry about your subject and let the rest of the picture over or under expose as necessary. If the shape of your subject works for a silhouette, that's great. If it doesn't, have a nicely lit subject in front of a white sky than a dark blob in front of a blue sky.

 

[Groucho]

One thing I read recently about histograms (possibly from a link on here) pointed out that the average camera has about a 5-stop range - meaning you'll get exposure on things two stops above and two below what you've chosen. Anything above or below that will be lost (blown out in highlights or just black).

Real life has more dynamic range (obviously), and there are certains shots that just cannot be gotten properly. What you'd probably have to do to get a good sky behind that photo is use a tripod, take the photo once exposure for the flower and another time for the sky. Use Photoshop (or similar) to combine the photos together. Unfortunately, there's no simple magic that can be done to make the camera capture more range than it's capable of.

That being said, I did recently change my camera to overlay the histogram on its post-picture preview, so I can quickly see just how the exposure is immediately.

 

[fuse04]

I am in need of some help or maybe reassurance. My first SLR came about 10 days ago (400d). I have been getting some idea about shutter speeds etc from this board and have also been reading Understanding Exposure. However, on some test shots in the garden I seem to be getting underexposure...well I think I am

For example this morning at 8am, a relatively light morning, no clouds, I took some snaps in Av...8.0 and the metering identifed shutter of 1/125 but the picture was as if I had taken the picture in cloud cover looking very dim. I took similar shots at the weekend and had similar results although it was a little overcast. I was concerned how I would get decent pictures on a overcast day using a slow zoom.

This evening, again reasonable light but not direct sunshine, ISO 200, Av 5.6 and shutter speed set itself to 1/60, again the image was noticably less bright than how my eye saw it.

To my eye, when I delibrately 'overexpose' according the the display bar I get a truer image. Is this to be expected that a 400d underexposes, is it that the camera is not a good copy or do I have much to learn!!!

thanks
Rob

 

[MICKEY88]

the problem is either the bright sky or the light wall. or a combination of the 2......the camera is reading that and then underexposing trying to get a good exposure of the sky....

if you were to spot meter a darker or more neutral area of the scene lock exposure and then recompose before shooting you woud get a better exposure, the same happens if you are metering a really dark spot in the scene..the camera will over expose to lighten that area...

 

[ukcatfan]

From what I have heard and from personal experience, it has more to do with the lens. You did not mention it, so I assume you used the kit lens. If so, that is probably the reason. Try playing with exposure compensation to account for it. My Pentax kit sometimes underexposes around 2/3 - 1 stop. My 50mm and 80-210mm both seem to overexpose by about 1/3 - 2/3 stop. You can always just use manual controls to set it where you want it. Another thing that might help is to lock the exposure. If you are wanting the garden exposed properly, try filling the frame with it, locking the exposure, and then re-framing to what you want to be in the shot.

Kevin

 

[boBQuincy]

I'm with Mickey88 on this, the bright sky is making the meter think it is much brighter than it really is, leading to underexposure in the foreground. This scene has too much range to be fully captured by the sensor and some decisions have to be made (that the camera can't make).

Metering off a patch of green grass is close to the 18% gray the meter is (supposed to be) calibrated to. Try metering so the grass fills the viewfinder, set the exposure for that, then step back and take the photo. It should look like a bright day but the sky will completely wash out.

If you are really serious, set the camera on some support and take a series with the exposure adjusted by plus and minus 1.5 stops. One of these should be suitable and you can always combine them to capture the sky and the foreground.

 

[fuse04]

Thanks for the advice. I did wonder about the sky so took a picture of just the wall (AE mode 8.0, ISO200 and shutter defaulted to 1/100) Sigma 17-70 at 70mm. I used the exp comp to +1

This is starting to get closer to actual colour and feel that maybe +1.5 or maybe +2 might have done the trick. Perhaps using the wall is not a good subject matter, however from the test shots I have taken in learning about exposure, I already have a feel that the camera is returning darker images and have been disappointed.

Here is a picture with my PnS which is closer to the mark.

I have yet to do battle with my historgram but as a rule most of the shape for my images are in the first 2 fifths of the histogram, with the peaks (is this brightness) in this zone as well.

Another one with 400d AE 8.0, 200 ISO, camera set 1/60 for shutter, focal 33mm....another rather dull result

 

[Master Mason]

One other thing to check is the WB. Try setting it to cloudy and/or sunny rather than just AWB. My colors have been much better on my 350d since I stoped using the AWB.

 

[MICKEY88]

with just the wall it is still fairly light so the camera will underexpose...

with the pic taken with the p&s you eliminated the sky, so the overall exposure of the scene with the light wall and darker vegetation averages out more closely to 18% grey so the exposure is better....

to do an accurrate comparison of the 2 camera you have to take the exact same shot, or as close to it as possible...

another way to test the camera is to set it for bracketing if you have that option, that way the camera will take 3 quick shots...-1, normal, and +1...

 

[MICKEY88]

another thing that might be throwing you off is the amount of processing done in the camera, your p&s might be programmed to increase saturation, whereas the dslr, might be programmed for little or no saturation increase