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Advanced relative DOF comparison: Cropping/enlargement/format reduction as single indep var?

ZoneV

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ZoneV Silver Member Nikonian since 08th Jan 2005
Sat 17-Jul-10 08:47 PM | edited Sat 17-Jul-10 08:48 PM by ZoneV

Let's say you keep everything constant except for:

a.) your lens focal length

and

b.) the amount of cropping/enlargement (effectively a reduction in capture format size)

Now, say you take two pictures from the exact same location, using the same f/number (eg. f/8), and use a 50mm lens and then a 100mm lens. You then crop the image taken with the 50mm lens so that the angle of view equals the image taken using the 100mm lens.

Both images are captured using the same film or sensor. Both images are printed at 8x12 inches (so the 50mm image has undergone more drastic enlargement).

According to wikipedia: "Cropping an image and enlarging to the same size final image as an uncropped image taken under the same conditions is equivalent to using a smaller format; the cropped image has less DOF (Stroebel 1976, 134, 136–37)."

The logical question then, is: Does the above statement simply mean that the cropped image will have less DOF than the original version of that same image (uncropped)?

And how can I apply this effect to predict what will happen in my above scenario?

I found an excellent calculator online: http://www.cambridgeincolour.com/tutorials/DOF-calculator.htm

But what I'm struggling with, is figuring out how to translate the crop of the one image into a larger print size. I don't know how much I would have to enlarge a given image in order to equal the coverage of a similar image taken at a given longer focal length. And I don't know what is a bigger variable in the DOF calculation: the focal length of a lens or the amount of cropping/enlargement.

Optics experts, please weigh in!

Nikon user since 2000

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