# Diffraction Limit Discussion Continuation

Started 10 months ago | Discussions thread
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 Re: Diffraction Limit Discussion Continuation In reply to Mike Davis, 10 months ago

Mike Davis wrote:

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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Mike:  Thanks for your post(s).

Several people posting here (in very unhelpful ways) have serious misunderstandings regarding the term ‘diffraction limited’ and specifically regarding just how it PRACTICALLY relates to a lens’ ‘sweet spot’ (or the consequences of using aperture stop settings of smaller diameter than that associated with such sweet spot). Worse still, some of these individuals have made it abundantly clear that there is absolutely nothing else that they could be taught – because they have mistakenly convinced themselves that they already know everything!

The following quote has been attributed to multiple famous individuals, so I cannot be sure just who to credit here, but their saying is absolutely true: “It’s what you learn AFTER you know it all that counts.”

When all the dust has finally settled, a particular individual (and several of his kiss-up cronies, repeatedly trading ‘likes’ with him and each other) will have to suffer the perpetual awareness that these recent threads will long-proclaim his (their) repeatedly proven ignorance regarding such matters. The real question is whether any of them will ever have the humility to admit their errors and recant their various posts and related website entries that propagate their persistent misunderstandings. At least one of these individuals (guess who) is so irretrievably committed to bashing the entire (engineering proven) concept of diffraction limiting that he will likely never relent – no matter what the facts!

For the rest of you readers, who really do want to get something useful out of all the machinations of these recent “Diffraction Limit” debates, please stay tuned. There are, in fact, a few genuinely intelligent and persistent “learning teachers” contributing to this discussion – despite the utterly childish responses they keep receiving from these know-it-all supposed ‘luminaries’. (And I, for one, value their efforts.)

Although “bobn2” clearly has evidenced considerable knowledge regarding photography on his website, his errors and behaviors relating to this diffraction subject (and his constant bashing of the CoC site) should be setting off alarms for those reading his various writings. Having seen this side of him, I now have no plausible alternative but to view all of his other statements with a heavy dose of skepticism (and I believe others should do the same).

As soon as practicable, I and others will be furthering the real discussion. Till then, everyone might want to carefully reconsider their position, before further entrenching themselves.

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