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Old Jul 25, 2010, 11:59 AM   #1
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Default DOF question

Hi, many questions today

but I was thinking about DOF, if I have understood correctly then higher aperture (lower number) makes the DOF smaller, using a higher aperture means letting more light in to the sensor, right? so to not get overexposure i must compensate my shutterspeed and/or iso-value to match the aperture ?

Here is my trickquestion, If I would use aperture 2.8 and shutterspeed 1/100 and iso 100 to get a small DOF, would aperture 4 and shutter speed 1/400 and iso 200 get me the same DOF?

(my values are taken out of nothing, just for the sake of argument)

Shooting with a:
Sony NEX F3
Sony 18-55 f/3.5-5.6 OSS
SMC Takumar 50mm/f1.4 (manual)
"unknown" 35mm/f1.8 (manual)
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Old Jul 25, 2010, 12:18 PM   #2
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Yes to the first part of your question - your understanding.

No to the second. The amount of DoF varies according to the aperture used and the distance to your subject - it is not influenced by the shutter speed or ISO.

Very close subjects - as in macro, means the DoF can be wafer thin, e.g it could be 1/4" at 2.8 and 6" away from your subject, but if you use 2.8 to shoot a distant landscape then of course the DoF is no longer 1/4" however the amount of your shot in focus will not be as much as if you used e.g. F8 or smaller (higher number).
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Old Jul 25, 2010, 12:24 PM   #3
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DOF is also affected by PHYSICAL focal length of lens and by sensor size. Now, your camera is not going to change sensor size, so for DOF concentrate on: Aperture, Physical focal length of lens, distance to subject. Here's a calculator - you can change some numbers to see how it affects DOF:
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Old Jul 25, 2010, 3:02 PM   #4
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While ISO and shutter speed has no affect on DOF, you are correct in your thinking of having to compensate with one or both to get the same exposure value. In other words, if your camera is set at a given ISO, shutter, and aperture for a correct exposure and you change the aperture to let less light in, then you will have to change one or the other to get the same exposure value but, the only time you would need to change both shutter and ISO would be if you couldn't get the shutter speed you wanted without doing so.

I hope I made that clear, sometimes my thoughts don't translate in writing like they should.

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Old Jul 25, 2010, 4:28 PM   #5
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Hi Michael,

This is a good subject to get you thoroughly confused, as I have done on many occasions. I'm sure you'll agree if you've read articles on it. It doesn't help that the rules change for different focal lengths and subject to camera distances. . .Truth be told, I'm very at advanced math, but I can only just barely get a working understanding of how all this works for me as a photographer.

You seem to have a reasonable understanding of the overview of exposure from your first question, but. . .

In your example, these are not equivalent exposures, and given the same subject distance, would not get you anything close to the same exposure or DOF. Try changing one value at a time, and you'll see this. f2.8-f4 is one stop slower 1/100-1/400 is two stops slower, and ISO 100-200 is one stop faster. In this case, "slower" means less light for exposure purposes, so your exposure would be (-1)+(-2)+1=-2 stops (underexposed by 2 stops) in direct comparison. The only change in DOF would be due to stopping down from f2.8 to f4, and 1 stop doesn't usually make that much of a difference.

When looking for exposure equivalents, it's usually easiest to just change one of the three variables (at a time) to compensate to start off, so I'd suggest doing this.

Av priority gives you the best direct control over DOF, and is my default mode for shooting, since I usually work with very thin DOF, shooting ultra teles (300+mm) at close distances (8-20 ft) for birds and at macro distances for critters.

With the song birds, I'm shooting subjects that are 4-12" long with DOF that is only a few inches max at close range and wide enough apertures to give me shutter speeds that are manageable for both camera shake and subject motion, and the noise at the highest ISO settings destroys fine feather detail, so I can only take this so far. . . but compromises sometimes have to be made, so I'll often find myself shooting wide open at higher ISO than I like just to get a usable shot.

With macros, I'm working with DOF that measures in a few mm, even at very small apertures (f11-16 at the lens, but effective apertures of f19-27 because of the TC I use), shooting critters that are usually bigger, but the use of P-TTL flash makes the shutter times and ISO a lot less important in getting the exposure close and freezing motion. I could get a little more DOF with even smaller apertures, but at the expense of sharpness -- very small apertures cause diffraction problems -- smaller holes bend light more -- much like a lens does -- and that works against what the lens is trying to do, so sharpness (resolution) suffers. . .Again, compromises have to be made, but since I can control the light to a significant degree, there are fewer that have to be made.

I personally don't use DOF charts and calculators much, except to get a feel for how much I can expect to get for a particular lens or TC combo. These and hyperfocal settings and distances are much more important with landscapes, architectural shots, and general wide-angle shooting than it is for what I shoot. I'm usually just trying to get the best mix of maximum DOF and the least amount of blur I can get away with for the lighting conditions presented to me, but I'd say that the way I normally shoot is about as far from the majority as you can get.

I tend more toward the technical end of photography than the artistic. Some of my shots have some artistic merit, but that's usually more due to circumstance and chance than from planning on my part, though I do take some time and trouble to try to get good backgrounds and angles. I think I've improved significantly in this aspect over the years.

DOF control and ultimate image quality is where DSLRs separate themselves from smaller sensored P&S cameras. Faster lenses and larger sensors give even shallower DOF, as well as a wider Field of View, so these are all reasons why I'm not particularly interested in the FF vs APS-C debate, as these would all work against me for the types of shooting I like to do.

I've ventured off topic pretty far here, but I thought that it might help to give some insight from a different perspective.

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