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Old October 11th, 2006, 06:01 PM   #16
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Again, though, Barry, how does this reconcile with years of DOF calculators that clearly say the opposite?
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Old October 11th, 2006, 06:26 PM   #17
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Is this perceived DOF difference due to magnification when using the tele end?
I believe this is what your stating Barry, it makes sense.
My eyes like Barry and Nate point out show us something different....

I believe we all agree we need to have distance from our subject and zoomed in to get the narrow DOF look.

The calculators seems to agree that DOF stays close to the same when the focus distance and focal length are changed to keep the subject at the same magnification.

http://www.cambridgeincolour.com/tut...h-of-field.htm
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Old October 11th, 2006, 08:16 PM   #18
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I answered my own question.

I was talking about if all other thing are the same, including the focus distance, then increasing the focal length would shorten the DOF. But if you adjust focus to create the same image size, then all things are equal.

So sorry Stephan that I doubted you, but the referenced page wasn't giving me enough info, and you weren't filling in the blanks either. As it turns out, this phenomenon is not at odds with my DOF calculator. It's just a nifty sidebar. All it took was a much better explanation on the second page referenced by Steven (along with numbers to back it up). See? I'm not ignorant after all.
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Old October 11th, 2006, 09:05 PM   #19
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That Luminous Landscapes page is a little misleading. The point about DOF being the same if the subject size is the same is only approximately true and only when the focus distance for the shortest lens is less than about 1/4 of the hyperfocal distance for that lens. Basically that means it only works when the subject is relatively close to you. If you took the extreme case of a 400mm lens versus a 28mm lens, it would only be true within the first couple of feet. Beyond that the DOF curves diverge rapidly. See this page and its graphs:

http://www.dofmaster.com/dof_imagesize.html

Anyhow, I still think the rule of thumb that a longer focal length will produce a more pleasing out of focus background is true. Reason being, the compressed perspective isolates more of the background so the out of focus background elements look bigger and appear closer to the subject. This is way more pleasing than the jumbo mess of multiple out of focus background elements in a wide angle lens positioned to have the same subject size. Even that Luminuous Landscapes page clearly shows this.
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Old October 11th, 2006, 10:46 PM   #20
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Nate: Precisely, a longer lens has inherently smaller DOF, but moving the focal point away increases the DOF and they tend to (mostly) cancel each other out. It's not exact but it's a good rule of thumb. BTW, I probably got a little too nasty earlier and I sincerely apologize for that.

My basic point is that you should worry more about the composition of your scene than the DOF because it will mostly just take care of itself. I don't really know any shooters who worry all that much about DOF or making their images "filmlike." They just concentrate on making good-looking pictures.
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Old October 11th, 2006, 11:15 PM   #21
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It's all in the math

Click Here

DOF does get shallow at the long end of the ENG lens or when you're using macro. Of course at the wide end you can infiniti focus and get everything in the scene in focus and DOF becomes moot.
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Old October 12th, 2006, 06:29 AM   #22
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If DOF is defined by what is PERCEIVED to be in focus, then using a longer lens does affect the DOF because it emphasises the blurriness by magnifying it. Therefore the depth of what is “perceived to be in focus” does reduce as lens length increases. In my book, DOF is a practical term, not a mathematical one.

You have to be careful with experiments trying to keep the aperture constant because F and T stops are misleading as they state aperture in relation to lens length. I can’t remember whether the focus is solely dependant on aperture – I had a physicist explain it to me once but most of it went over my head! Researching this subject leads to more untrue flak than proper scientific explanation.
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Old October 12th, 2006, 09:27 PM   #23
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Now this is a good discussion!
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