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Digital Depth of Field
Old 05-18-2004, 12:28 PM   #1 (permalink)
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Rolando's DOF guide in the tips area of this forum is quite accurate, and I have used it many times. Now that I am moving to digital I am wondering about the differences.

With 35mm film, lens mm is accurate. When using digital I am working with a multiplier factor of 1.5. Now does this mean I need to select in the DOF guide, a lens size of 155mm in digital, even though it is truly a 105mm?

Thanks for your help
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Re: Digital Depth of Field
Old 05-18-2004, 01:23 PM   #2 (permalink)
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The short answer to your question: A 105mm lens is a 105mm lens--the "crop factor" of digital changes the angle of view, but it doesn't change the physical properties of the lens (including DOF at a given aperture and subject distance).

The slightly more complicated answer: Calculations of DOF take into account the typical magnification for a print, but the DOF calculator on GG doesn't allow you to adjust that parameter. Because a smaller-than-full-frame sensor requires more magnification to produce the same size print as a 24mm x 36mm piece of film, the results you get from the calculator will tend to overestimate the DOF.
 
 
Re: Digital Depth of Field
Old 05-18-2004, 05:02 PM   #3 (permalink)
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I use a Canon 10D, and in order to calculate DOF, I use an application called DOF Master ( http://dfleming.ameranet.com/ ). The trick to using this application is knowing an accurate value for the "Circle of Confusion" (for mine, I use a value of 0.008). This value is driven by the resolution of your camera's sensor. I found a suitable value for my camera by searching the forums on http://www.dpreview.com/. There is a forum for each major camera body, so search on the term "circle of confusion" or "coc". The DOFMaster site also has articles on DOF and CoC values for many digital cameras.

Once you have this information, the DOFMaster program allows you define focal lengths for various lenses. There is also a program that lets you produce charts.

This is a great tool, and I use it all the time to determine hyperfocal distances when doing wide angle shots outdoors.

I hope this helps.
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Re: Digital Depth of Field
Old 05-19-2004, 05:46 PM   #4 (permalink)
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Here's a link to an article which explains how DOF changes with the part frame sensors on a DSLR. In short, you get more DOF out of the same aperture with the same lens as 35mm providing you frame the same shot - because you have to stand further away due to the magnification factor.

DLRS DOF

In other words, when Minolta finally get their 1/2 frame DSLR sorted, to get the same tight DOF I get from my 85mm F1.4, I will have to use my 50mm F1.4, but to get the same limited DOF as the 50mm F1.4, I'll need a 26mm F1.4 which unfortunately does not exist.

Iain
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Re: Digital Depth of Field
Old 05-22-2004, 11:29 AM   #5 (permalink)
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No... you always use the actual focal length of the optics.

But the fact you have a multiplier (in reality, you are corpping down to the center of the 35mm frame) you will have to get farther away from your subject to get the same framing, and thus with the same lens on a digi body with a 1.5 factor compared to the same image shot with a full-frame film camera, assuming you position yourself to get the same framing, you will be 1) much farther from the subject with the digi body and thus 2) you will have greater DOF.

Thus you have the big problem with shooting digital glamor or any other genre where shallow DOF is desired... you need a full-frame sensor. The 1.5 multipliers suck. The only 2 cameras with full frame 35mm sensors suck. Some rumors are that the new Nikon is to be introduced with the summer olympics that has a full frame sensor, but I'll only believe it when I see it. Supposedly Nikonis coming out with something that is modular like a Fuji GSX but for 35mm (drool).
 
 
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