mFT - will diffraction be a problem with smaller lens?

Started Aug 10, 2008 | Discussions thread
John Sheehy
Forum ProPosts: 16,054
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Re: You're both wrong....
In reply to Nuno Souto, Aug 19, 2008

Nuno Souto wrote:

Presumably, the Airy disk (d) needs to be smaller than the pixel size
otherwise one gets diffraction.

No; you always get the diffraction. You get to see the effect of it when the pixel density is higher, because it isn't being hidden by the relatively humongous blur of the big pixels.

At least, that is what this:
http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm
says.

No. Re-read the paragraph at the bottom with the title, "Are smaller pixels somehow worse?".

Play around with the interactive table there, it's very educational.
In particular, recall this:
"the airy disk can have a diameter approaching about 2 pixels before
diffraction begins to have a visual impact (assuming an otherwise
perfect lens, when viewed at 100% onscreen)"

So if you reduce pixel size, you have to reduce Airy disk or you get
diffraction. Period.

No "period". What you say is only true at the pixel level.

And to make "d" smaller according to above equation and assuming
λ stays the same, we need to reduce N.

What is N again? The fnumber! Doesn't that define as well the
amount of light we can collect?

We then have two contradictory requirements: if we reduce Airy disk
size so it won't be affected by smaller pixels,

Airy disk size is NEVER, EVER affected by pixel size.

we need to reduce the
amount of light reaching those same pixels!

But let's make N smaller anyways. How do we do that? If N is still
= focal length/diameter, we decrease focal length with diameter
constant, or we increase lens diameter and keep focal length constant.

Now, with smaller sensors we know we can reduce the focal length to
get the same angular coverage. But we also have to reduce the
diameter because we want to end up with a smaller lens! So we have to
reduce focal length more than we reduce diameter. A bit of a problem
if we want to keep things small.

It appears to me that the above indicates and demonstrates the well
known phenomena that the smaller the pixel size, the smaller the
fnumber at which one starts to get diffraction and the smaller the
range between max lens aperture and smallest lens aperture before
diffraction.

Catch 22? You bet!

Only in your impractical model.

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John

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