Diffraction Limit Discussion Continuation

Started 7 months ago | Discussions thread
Mike Davis
Contributing MemberPosts: 690
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Re: Diffraction Limit Discussion Continuation
In reply to knickerhawk, 7 months ago

Answering Jim Pilcher's original question:

http://www.dpreview.com/forums/post/53151276

He did not communicate his desired print resolution and anticipated enlargement factor, so I'll answer for a fairly extreme combination: A desired print resolution of 5 lp/mm at the greatest enlargement factor at which his sensor's pixel count can support 5 lp/mm.

Please realize that there is no single f-Number at which diffraction becomes an issue for all combinations of desired print resolution and enlargement factor, with any given camera.

If you desire a print resolution of 5 lp/mm (equivalent to an un-resampled image resolution of 360 ppi from a CMOS sensor after AA filter and Bayer algorithm losses), using your 15.93 MP Olympus PEN E-PL5 (with sensor dimensions of 17.3 x 13.0 mm), the largest possible print dimensions would be 9.6 x 12.8 inches, for an enlargement factor of 14.68x.

The f-Number at which diffraction would begin to inhibit a desired print resolution of 5 lp/mm at an enlargement factor of 14.68x is...

f-Number "limit" = 1 / 5.0 / 14.68 / 0.00135383 = 7.9

f/7.9 is very close to the f/7.3 figure that Michael Meissner obtained using the Cambridge In Colour diffraction calculator.

http://www.dpreview.com/forums/post/53151553

Surprise?

Here's the formula, again:

f-Number "limit" = 1 / desired print resolution in lp/mm / anticipated enlargement factor / 0.00135383

Make a smaller print (decrease the enlargement factor) without decreasing the anticipated viewing distance and you can use a larger f-Number.

Make a larger print (increase the enlargement factor) without increasing the anticipated viewing distance and you must use a smaller f-Number - but look out for the ceiling imposed by pixel count.

Decrease your desired print resolution without increasing the anticipated viewing distance and you can use a larger f-Number.

Increase your desired print resolution without decreasing the anticipated viewing distance and you must use a smaller f-Number.

For any given camera, there is no single f-Number at which diffraction becomes a "problem" for all combinations of desired print resolution and enlargement factor/

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