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Understandiing Depth of Field
Old 01-23-2005, 01:52 PM   #1 (permalink)
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I work with managing depth of field alot, but still have a question or two.

I have found Roland's depth of field calculator a great help. http://www.garageglamour.com/tips/depth.php

But one question has come up. Since converting to digital, I have to deal with a 1.5 magnification factor. So I am wondering if this creates an issue with the above depth of field calculator? If I use the calculator for a 105mm lens, set to F2.8, do I need to adjust for the fact that my camera sees this as a 157.5mm lens?

Any thoughts???

Thanks

ScottA
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DOF
Old 01-23-2005, 02:55 PM   #2 (permalink)
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First, you ALWAYS use the actual optical focal length of the lens when using formulas or DOF calculators.

But there is a bigger issue. There are two ways to look at DOF.

One is simple - using a calculator it tells you what is “in� focus (technically, what areas of the subject will have a circle of confusion equal to or smaller than a certain value). Everything that is not “in� focus is considered “out� of focus. This works if the amount of background blur is irrelevant to your image.

But focus is not really represented by a set value. If you are focused on a subject at 10 feet and the calculator says your back DOF is 2 feet, items 2.01 feet behind the plane of focus are not really perceptibly more out of focus than things 2.00 feet from the plane of focus. It is a continuum of change from sharply in focus to progressively less in focus (technically progressively larger circle of confusion). Similarly something that is at the plane of focus at 10 feet will be sharper than something that is within the DOF, but not at the plane of focus, such as at 10 feet 3 inches.

The second way to look at DOF, is to look at what the calculator says is “in� focus, and then redo the calculations using a circle of confusion 10 times greater, and consider everything beyond that area to be “blurred.� So instead of 2 zones (in-focus and out-of-focus) you have 3 zones from splitting the “out-of-focus area in two... creating an “out-of focus zone� and a “blurred� zone of things significantly out of focus.,

For the sake of an example, consider shooting a “head and shoulders� image with either a 105mm from10 feet away or a 300mm at 45 feet away..... compare the following using a standard CofC for 35mm film of 0.025mm:

105mm f6.9 focused at 10 ft. ... DOF is 0.96 ft
300mm f2.8 focused at 45 ft. ... DOF is 0.96 ft.

Exactly the same DOF. But if you look at the CofC 10 times larger (0.25mm) you get:

105mm f6.9 focused at 10 ft. ... DOF is 12.3 ft
300mm f2.8 focused at 45 ft. ... DOF is 9.7 ft.

So the background behind the model must be 12.3 feet behind the model to be blurred with the 105, but needs only to be 9.7 ft. behind the model with the 300.... even though you would get the exact same DOF of 0.96 ft. for both setups.

Now sure you can open up the 105 to blur the background, but then you get less DOF of your subject.

So using two different scenarios to capture the same shot (a head and shoulders portrait for example) and wanting a certain DOF, using the longer lens will isolate the subject by blurring the background more than a shorter lens.

The reason for this mathematically, is that the calculation of CofC uses the square of the focal length of the lens, so the longer the lens, the steeper the slope of the curve.
 
 
Re: DOF
Old 01-23-2005, 04:25 PM   #3 (permalink)
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at the risk of sounding like a smartass, which is not my intent, do many photographers really take the time to precisely and mathemantically figure this stuff out???
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Re: DOF
Old 01-23-2005, 07:15 PM   #4 (permalink)
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I hear you jimmyd - it loses artistic aspect when you break the calculators out.

I go about it two ways - blur the background or don't.
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Re: DOF
Old 01-23-2005, 11:54 PM   #5 (permalink)
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I agree... unfortunately the DOF issue... partucluarly comparing FF to cropped digital sensors is a qick flame war. If you don't do the math, no one believes you.

I have never used a calculator for any actual shooting.... but when I tell people that smaller than FF digital sensors make you use shorter focal length lenses (or move farter away) and thus it is harder to blur the background, a lot of people don't believe it. Hence the math.

 
 
zoom vs. prime
Old 01-24-2005, 05:50 AM   #6 (permalink)
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another area to address: achieving short DOF with zoom lenses vs. primes. my experience shows me its more difficult to shorten the DOF with a zoom than with a prime. i did some reading about it a while back, and as i remember, it has to do with how zooms focuse vs. how primes focus.

zooms focus with both the front and rear optical elements. but primes only focus with the front elements and the back remains fixed. i don't understand the physics, and i don't think i need to. i only need to know the results i can expect with zooms vs. primes.
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Re: DOF
Old 01-24-2005, 12:05 PM   #7 (permalink)
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Thanks Robert. Your insight will be a great help. Now I guess I just need to practice in my environment to get a more natural feel for acquiring the wanted results.

Your response got me thinking in the right direction. ie. I hadn't for some reason even considered the multiplication factor of the blurred background as you move farther back. Or increase the distance of the subject from the background.

Now what I am really trying to get a handle on, is the DOF of the foreground. Would like to do a series where the foreground is blurred, but subject is in focus. My assumption is the the same logic applies, only in reverse??

Thanks

Scott A.
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Re: DOF
Old 01-24-2005, 06:37 PM   #8 (permalink)
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The exact same logic applies. The slope of the distance to CofC curve is steeper for longer focal length lenses... that means the background AND foreground get blurrier faster than compared to a shorter focal length lens at a given DOF value.

If you want to get more blur, a wider apeture has a bigger effect in general, but the side effect is that you get less DOF of the subject itself. If you need both DOF of the subject, and a more rapid blurring of the foreground and background, go to a longer lens (and use a larger film format so you can be closer to the subject when using tghe longer lens. With a 300mm lens on an 8x10 view camera, you can get the center of a model's eye in focus, while the lashes at both the near and far corner are out of focus!)
 
 
Re: zoom vs. prime
Old 01-24-2005, 07:02 PM   #9 (permalink)
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Not true. Many primes and zooms have fixed front and rear elements and use internal focusing.

If you really want to throw a curveball, consider the variable de-focus lenses, like the "Defocus Control" (DC) nikors.
 
 
Re: zoom vs. prime
Old 01-25-2005, 03:39 PM   #10 (permalink)
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it doesn't ever get LESS complicated, does it?
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