Diffraction Limit

Started Aug 28, 2013 | Discussions thread
Dr_Jon
Senior MemberPosts: 1,490Gear list
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Re: Yep.
In reply to Great Bustard, Aug 28, 2013

It's a nightmare reading through all of this and I'm not sure I'm cool on people knocking Airy disks so much (well, if that's what they are doing, with my Astronomical hat on I tend to see them a lot).

Luckily I like the unquoted content of your last post on this, although it does tend to some pointing out of fine details over what's useful in the real world, IMHO, that can perhaps be less helpful to people. Can I add some stuff (feel free to disagree, I learned the other day rushing something off before going out can go spectacularly wrong on me... and I'm due out the door in 4 mins and counting...)

There is no such thing as a "diffraction limit" except when the resolution falls to zero.
- well, yes that's true, except it is actually useful to have an idea where diffraction is going to really start killing the sharpness so worth remembering this so-called non-existent limit, while understanding it isn't one you can't bypass (at a cost). On m43 I tend to remember f8 is somewhere not to go below without thinking carefully, for example (I try to stick to f5.6 and up a lot of the time, but I have lenses that are really good at f4-f5.6). There might be diffraction at f2.8 but at that level it's more something for scientific argument than for photographers to worry about.

There is a point where diffraction softening becomes the dominant source of blur, and this point will vary from lens to lens, as well as where in the frame we are looking (the corners typically, but not always, lag about a stop behind the center for DSLR lenses).
- not arguing

All systems suffer the same diffraction softening at the same DOF.
- Okay, except other factors will affect how much it troubles you, so again more readers here will find it not so useful choosing whether to use a FF or m43 camera to shoot something.

More pixels, all else equal, will *always* resolve more detail.
- True, although a chunk of the time it will be to so small a degree you don't care, the rest of the system needs to be in the ball-park. The number of times a friend's D800 out-resolves my 5DmkII are less than you'd think as the pixel effect gets lost in other factors. (He has consumery long lenses, for example)

All systems do not necessarily resolve equally at the same DOF, as diffraction is one of many sources of blur. However, as the DOF deepens, the resolution decreases, and the resolution differences between systems narrows, typically becoming trivial by f/16 on mFT (f/32 on FF and f/5.6 on an FZ200), regardless of how sharp the lens is or how many pixels the sensor has.
- okay

Anyway, I should ask a question - are you unhappy about applying Airy disk maths to either a CoC for viewing a picture of size X at distance Y or using a CoC for the presumed resolution limit of a camera in the 2-3 pixel pitch range (depending on AA filters, de-Bayering algorithms, etc.)?

Oh, I should say the point being working out when you are likely to really start caring about diffraction effects, but I'll avoid the "L" word. Okay, re-reading that maybe I should have said "Limit" word...

(I think it is something worth knowing, BTW.)

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