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DOF brain buster..


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#1 James Leonzio

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Posted 27 November 2007 - 01:09 AM

well maybe not a brain buster, but something I was thinking about the other day...

So here's the situation:
1) we are photographing something relatively small (like a phone number on a post it pad). This small subject will fill the frame.
2) we are shooting fairly wide open, say f/2.0. Why? cause we have to, at least for the sake of this exercise.
3) We need to maximize our depth of field (the right side of the post it pad is closer to the film plane than the left side, and we need the whole thing to be in focus) without changing our exposure, and the subject must be full frame (ext. CU).

In order to maximize depth of field, would you...

A) get the camera nice and close to the subject and use a wider angle lens? or
B) move the camera farther away from the subject and use a longer lens? or

or will the two options give you similar results in terms of DOF?

I'm guessing that this might depend at least somewhat on the specific lens design, but maybe not. Maybe there's a really simple answer to this, and I'm just going overboard.

Anyway, let me know what you're thinking...

thanks,

James
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#2 David Mullen ASC

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Posted 27 November 2007 - 01:31 AM

or will the two options give you similar results in terms of DOF?


Yes.

Looking at the charts in the ASC Manual...
A 25mm at 2' has the same view as a 50mm at 4'.
The depth of field for the 25mm at f/2 at 2' = 1" 11" to 2' 1" (2" depth of field).
The depth of field for the 50mm at f/2 at 4' = 3' 11" to 4' 1" (2" depth of field).

I mean, I know it's just a what-if because there's no reason why an insert of a post-it note pad can't be lit to f/22 if necessary -- with a 1K!

The only visible difference will be in the perspective of the closer but wider-angle lens versus the farther but longer lens.

You could use a tilt-focus lens to hold the angled pad in focus.
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#3 Adam Frisch FSF

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Posted 27 November 2007 - 10:44 AM

Interestingly enough, DOF is just a function of the magnification. There was a very useful link put up by someone on this board explaining it with images, which was rather educational. Does anyone remember and can put that link up again?

Ah, found it. This is really interesting - take a look at the photo examples of the 28mm compared to the 100mm further down this side. The DOF is constant for the same magnification for all lenses. In fact, it's a bit like pixel resolution on a jpeg picture. If a fuzzy image is reduced in size enough, it will look sharp. That's what a wide angle lens does - it has very little magnification.

http://www.vanwalree...optics/dof.html
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#4 James Leonzio

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Posted 27 November 2007 - 03:25 PM

Thanks for your answers guys. This really clears it up for me. That DOF article is very interesting as well.
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#5 James Leonzio

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Posted 28 November 2007 - 05:46 PM

just thought of two things. Does it matter if we are using a zoom lens vs. two primes? What about if we are using a macro lens vs. a standard prime? does this throw off the relationship that Mr. Mullen has explained? I guess what I'm really asking is does the lens design throw off this relationship at all, or is this a strict optics relationship?

eager to hear responses. there is a large bet between me and my friend weighing in the balance
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#6 Chris Keth

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Posted 28 November 2007 - 09:39 PM

just thought of two things. Does it matter if we are using a zoom lens vs. two primes? What about if we are using a macro lens vs. a standard prime? does this throw off the relationship that Mr. Mullen has explained? I guess what I'm really asking is does the lens design throw off this relationship at all, or is this a strict optics relationship?

eager to hear responses. there is a large bet between me and my friend weighing in the balance


It's a strict optics thing. A prime, a zoom, a single meniscus lens will all follow the rules. What may (probably will, actually) change between those options, though, is the speed of the lens and the close focus distance.
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#7 Tony Brown

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Posted 02 December 2007 - 09:23 AM

Undercrank.
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#8 Walter Graff

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Posted 02 December 2007 - 10:05 AM

And don't forget my article:

http://www.bluesky-web.com/dofmyth.htm

And pay attention to the bottom two pictures in my article. I do exactly what you plan on doing, shooting her both wide lens, closer camera, and farther camera, longer lens. While the depth of field is technically the same in both shots, what does change is the apparent depth of field. The longer lens compresses the foreground to background so in the long lens example the background looks softer but really isn't. Think about it all and how it might help you create the best illusion of depth of field.
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