Shooting in Aperture Priority question...

[MinnieForMe]

Hi:
My assignment this week is to take a picture at a low f/stop and a high f/stop to compare the focus. I can get the f/stop to the high end (f/22) but can't get the f/stop lower than f/5.6. What am I doing wrong? I am set at AV on my pentax k-x. I'm thinking I'm using the wrong lens and need a macro lens to get down to f/1.4.

Please help this ignorant newbie!

 

[Snappy in OK]

It sounds like the max aperture on your lens may be 5.6, or maybe 5.6 at the focal length you are using. Check the lens to see if it is supposed to be any larger.

Good luck!

 

[disneyboy2003]

Hi:
My assignment this week is to take a picture at a low f/stop and a high f/stop to compare the focus. I can get the f/stop to the high end (f/22) but can't get the f/stop lower than f/5.6. What am I doing wrong? I am set at AV on my pentax k-x. I'm thinking I'm using the wrong lens and need a macro lens to get down to f/1.4.

Please help this ignorant newbie!

The lowest f-stop will depend on which lens you have. You'll have to look at the numbers on the lens itself to determine this.

I forget which lens you have, but let's say that you have the Pentax 18-55mm f/3.5-5.6 lens. You'll see that the maximum aperture you can achieve with this lens is anywhere between f/3.5 and f/5.6, depending on your focal length. In this case, at 18mm, you can get a maximum aperture of f/3.5. At 55mm, the maximum aperture is f/5.6. This is a variable aperture lens.

You can't a lower f-stop than what is stated on the lens. To get a lower f-stop, you'll need to buy a lens that has a larger maximum aperture, such as a 50mm f/1.4 lens, for example.

As a side note, there are zoom lenses that do have a constant aperture. For example, I shoot Canon, and I have the Canon 17-55mm f/2.8 lens. This means that my maximum aperture is always f/2.8, regardless of whether I'm at 17mm or at 55mm. Constant aperture lenses typically cost a lot more.

One final point, which is probably pretty obvious. Your minimum aperture will always be f/22, regardless of what lens you're using.

 

[MinnieForMe]

That is what I thought but you put it into words better. My 50-200 mm lens says 1:4-5.6 and my 18-55 says 3.5-5.6.

Am I processing this correctly? If I use f/22, more of my picture would be in focus and if I used f/1.4 one object would be in focus and the background would be blurry? I keep telling myself one step at a time but there is sooooo much to learn.

I'm off to play some more! I wish Understanding Exposure would arrive because I'm sure these are questions the book would answer when I read it.

 

[RBennett]

Am I processing this correctly? If I use f/22, more of my picture would be in focus and if I used f/1.4 one object would be in focus and the background would be blurry? I keep telling myself one step at a time but there is sooooo much to learn.

Essentially you are correct. When shooting with a larger aperture (f/1.4) you will get just a very small area in focus. And when shooting with a smaller aperture (f/22) almost all of your shot will be in focus, given that you have focused correctly. Sounds like you are getting it, even BEFORE the book arrived!

 

[DebºoºS]

Hi:
My assignment this week is to take a picture at a low f/stop and a high f/stop to compare the focus. I can get the f/stop to the high end (f/22) but can't get the f/stop lower than f/5.6. What am I doing wrong? I am set at AV on my pentax k-x. I'm thinking I'm using the wrong lens and need a macro lens to get down to f/1.4.

Please help this ignorant newbie!

Did they want you to use the same lens? If so then f22 and f5.6 are at the ends of that particular lens.

Here's a really good article that describes DOF not Peterson but as he would explain it
.http://www.11thstudio.com/blogs/all-posts/simple-guide-to-aperturef-stop-numbers-and-depth-of-field/
[FONT=Arial, verdana, helvetica, geneva]
[/FONT]

 

[seashoreCM]

If you set on depth of field.

 

[seashoreCM]

If you set up some test subjects quite close (you don't have to go so close as to need macro) you can still see the effect of aperture on depth of field using f/5.6. Use minimum zoom if it is a zoom lens.

For both the large aperture and small aperture test shots, include some distant background material. It would be better to use the same lens for both.

 

[zackiedawg]

Note a few things to consider...

When doing an aperture focus test like that, try to find something that has a nice repeating pattern from close to distant - fences and handrails are particularly handy for such tests...but you can even do it at home easily just by lining up some AA batteries standing on end spaced evenly apart. Shoot the subject from an angle of 30 degrees or so, that way there are parts of the subject line close to the lens, and parts growing farther and farther away, and put your focus point somewhere in the middle. This will really help visually show the depth of field difference between the smaller and larger apertures.

If you're unsure the max aperture of the lens, that part you posted above is exactly what you're looking for...the numbers that follow the "1:" are the aperture maximum. So 1:4-5.6 means that over the zoom range, the lens would have a maximum aperture of F4 at the wider end, and F5.6 at the longer end.

Also note there is one other factor that needs to be considered when you start to make the aperture really small - when you're up in the F22 range...diffraction. The general concept is...the smaller the aperture, the bigger the depth of field, therefore the more you can get in focus. However, if the aperture gets too small, the expanding depth of field from the really tiny aperture begins to be negated by diffraction - that is, the aperture hole is so small that the lens is unable to obtain maximum sharpness on subjects due to the small amount of light peeping through the tiny hole in the middle - so you start to get softness and lack of details. The key to any lens is figuring out what is commonly referred to as the 'sweet spot' - the higher end of the sweet spot is usually the smallest aperture you can go for maximum sharpness and detail without getting any effects of diffraction.

 

[mom2rtk]

Also note there is one other factor that needs to be considered when you start to make the aperture really small - when you're up in the F22 range...diffraction. The general concept is...the smaller the aperture, the bigger the depth of field, therefore the more you can get in focus. However, if the aperture gets too small, the expanding depth of field from the really tiny aperture begins to be negated by diffraction - that is, the aperture hole is so small that the lens is unable to obtain maximum sharpness on subjects due to the small amount of light peeping through the tiny hole in the middle - so you start to get softness and lack of details. The key to any lens is figuring out what is commonly referred to as the 'sweet spot' - the higher end of the sweet spot is usually the smallest aperture you can go for maximum sharpness and detail without getting any effects of diffraction.

I found this out shooting a few things at f22 on our trip. They were scenes I where I wanted to get as much in focus as possible. I didn't have the information in my brain yet about the affects of diffraction. My lenses are fairly new yet, so I haven't worked with them enough to know their "sweet spots". Does this vary more my copy of lens? Or is it fairly consistent between copies? So I guess the question is, can I ask others what is typical, or is it going to be different for each one?

 

[VVFF]

I found this out shooting a few things at f22 on our trip. They were scenes I where I wanted to get as much in focus as possible. I didn't have the information in my brain yet about the affects of diffraction. My lenses are fairly new yet, so I haven't worked with them enough to know their "sweet spots". Does this vary more my copy of lens? Or is it fairly consistent between copies? So I guess the question is, can I ask others what is typical, or is it going to be different for each one?

IN GENERAL, for most wide angle shots you shouldn't need to go beyond F7.1 or F9 or so to get everything in focus. It's generally better in these types of shots to focus fairly close than to focus on the distant mountain.

Try out some DOF calculators like those you can find at www.dofmaster.com. They should shed some light on what apertures you actually need.

IN GENERAL, again I wouldn't exceed F9 unless you are in macro range. There really isn't much that you'll need beyond this aperture to capture. On pretty much any resolution camera you will be in the diffraction range at this point.

The larger the MP in the same size sensor, the earlier the diffraction limit. This does not mean that a lower resolution sensor will capture more detail at higher apertures. It simply means you are not achieving the highest possible resolution of the sensor anymore.

In general for normal shooting I stay between F2.8 and F7.1. F2.8 would be for low light or portraits and F7.1 would be my landscapes. The shorter the focal length the lower the aperture you need to get everything in focus. So while F4 may be enough to get everything in focus at 17mm(35mm equiv) this may not be the case at 100mm.

The only times you really need higher apertures are:

Long exposures without a ND filter(sometimes diffraction is worth it for the ability to get a longer exposure...fireworks, water, etc)
Starburst effect(with rounded apertures this only occurs at F11 or higher typically)
Macro - Sometimes F11 or higher is necessary to get enough in focus. With the MP-E lens from Canon it can be used til F96 or so!

Those are about the only situations I can think of at the moment...maybe somebody will have some others.

 

[zackiedawg]

I'll actually shoot down to F14 or so on some of my lenses...though most of the time for normal daylight shooting I'm probably between F5.6 and F11 or so. It depends on the lens, and its minimum aperture. With an F1.4 lens, the sweet spot can start even as low as F2.8 or so, and go to F7.1 or F8...no need to explore smaller apertures. But with a big zoom, like my 200-500mm birding lens, the telephoto end has a maximum aperture of F6.3 - so stopping that lens down to F11-14 in very good light will still be far from the effects of diffraction.

 

[mom2rtk]

Thanks VVFF and Zackiedawg! That was all very helpful. There really is no substitute for more experience with my lenses. It's just so much more fun to play with them at Disney than to settle for mundane suburbia...... I'm learning though, and for this I thank you!

 

[VVFF]

I'll actually shoot down to F14 or so on some of my lenses...though most of the time for normal daylight shooting I'm probably between F5.6 and F11 or so. It depends on the lens, and its minimum aperture. With an F1.4 lens, the sweet spot can start even as low as F2.8 or so, and go to F7.1 or F8...no need to explore smaller apertures. But with a big zoom, like my 200-500mm birding lens, the telephoto end has a maximum aperture of F6.3 - so stopping that lens down to F11-14 in very good light will still be far from the effects of diffraction.

Diffraction has nothing to do with the maximum aperture of the lens. It's a constant based on the sensor so it will occur at the same aperture no matter what lens. Whether this is offset by a increase in lens resolution at smaller apertures is the only question.

 

[Groucho]

It's already been said, but I'll restate it slightly. You can't go lower than F5.6 because you are zoomed in. Zoom out as far as possible, then you will be able to select the fastest speed (lowest number) - F3.5 on the 18-55mm, F4 on the 50-200. Once you're there, watch the Info screen then slowly zoom in. You'll see the aperture change a couple times as the highest available changes. Usually you can only get the fastest speed in this type of lens at the very widest; even a slight zooming in will bump it up. If you're in a low-light situation, you are probably better off to shoot wide then crop it down later on your computer.

I don't remember for sure if the K-x has it (I think it does), but the K-7 allows you to change the "Program mode", which is what it tries to get when you are in Program mode. One of the settings is called "MTF", which is a measure of sharpness. In this mode, the camera will attempt to choose the aperture that it knows is the sharpest for that lens - no need to test it yourself. Of course, this only works for lenses that have been profiled - so no third-party lenses, and no older manual lenses (since they don't tell the camera what lens they are.) It's a pretty cool setting if you are in a situation where you want maximum sharpness from a given lens.

 

[seashoreCM]

Diffraction has to do with the absolute size of the aperture, say in fractions of a millimeter. The smaller the sensor, the smaller the aperture for a given f/stop and a given field of view (or 35mm film camera equivalent focal length) and the sooner you depart from the sweet spot when making the aperture smaller.

F/11 is probably the smallest you can go with a small sensor camera (compact point and shoot) without getting some softness from diffraction. A typical SLR should be good to F/22.

If your camera stops down far enough that the effects of diffraction can be seen, then a third shot with the aperture that small can be included with your assignment. That would be for the effects of sharpness versus aperture although not relevant for depth of field observations. Include the absolute focal length and the sensor size in the photo caption to help others who did the same thing make meaningful comparisons.

The effects of diffraction are always there. With larger apertures, there is much more "undiffracted" light compared with diffracted light so you don't see the blurriness.

 

[boBQuincy]

As usual with cameras, there is more to depth of field (and diffraction) than it seems. See Bob Atkins web site for a good review of how crop sensors affect DOF.

As for diffraction, diffraction problems vary with the lens and the sensor, and there is no hard rule as to what is too much diffraction. The image degradation starts at maximum aperture and continually gets worse as we stop down, until the loss of sharpness passes an arbitrary threshold and /or becomes noticeable. Smaller sensors (smaller pixels) have more trouble with diffraction and these cameras (P&S) are sometimes limited to f/8 to avoid a lot of diffraction. dSLR sensors have larger pixels and can use smaller apertures before diffraction becomes an issue. Most dSLRs are ok to f/16 or even f/22.

Lens sweet spots are somewhere between where abberations are minimized and where diffraction becomes a problem. Most lenses are at their best about 2 stops closed from maximum, up through about f/11. This is not much of a range but for best sharpness it is where we should be.

I use f22 if I need maximum depth of field and don't worry much about it. Chances are if we are not using a camera support we will lose more sharpness from camera motion than from diffraction anyway.

 

[VVFF]

Diffraction has to do with the absolute size of the aperture, say in fractions of a millimeter. The smaller the sensor, the smaller the aperture for a given f/stop and a given field of view (or 35mm film camera equivalent focal length) and the sooner you depart from the sweet spot when making the aperture smaller.

F/11 is probably the smallest you can go with a small sensor camera (compact point and shoot) without getting some softness from diffraction. A typical SLR should be good to F/22.

If your camera stops down far enough that the effects of diffraction can be seen, then a third shot with the aperture that small can be included with your assignment. That would be for the effects of sharpness versus aperture although not relevant for depth of field observations.

The effects of diffraction are always there. With larger apertures, there is much more "undiffracted" light compared with diffracted light so you don't see the blurriness.

Where diffraction limiting starts depends entirely on the size of the pixel on the sensor. The smaller the pixels, the earlier you lose the ability to resolve to the maximum resolution of the sensor. This starts MUCH earlier than F22 on Crop SLR cameras:

This is one of Canon's sharpest lenses on a 50D and it starts softening due to diffraction at F5.6-F8

Virtually the same lens on a 12MP camera starts diffraction limiting at F8:

I chose these graphs because they illustrate the diffraction limitation point fairly clearly. However, you will see that all lenses on the same camera diffraction limit at the same point.

For maximum sharpness with a crop camera you'll want to stay in the F4-F7.1 range. That's not to say you can't go higher...per the graphs above it doesn't get REAL bad til F16-F22. Even then, for most prints the resolution is probably more than enough.

Here is the same lens again on a Full format Camera(5D Mark II). Here, softness due to diffraction doesn't kick in til F11.

Again, this doesn't mean that the images are spoiled after this point by any means. It simply means that in order to get the maximum resolution out of your sensor you have to stay above a hard "diffraction limit" AND balance that with how the lens performs at different apertures. If you had a lens that was horrible til F8(rare) and used it at F5.6 to avoid diffraction then it really wouldn't matter because the lens is the limiting factor still. With higher performing lenses released nowadays, the diffraction limit is often becoming the limitation of resolution.

All credit to www.photozonde.de for their resolution graphs. Excellent website for anyone interested.