Diffraction Limit

Started 8 months ago | Discussions thread
Great Bustard
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Re: Yep.
In reply to Dr_Jon, 8 months ago

Dr_Jon wrote:

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...)

Let's begin, then!

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.

Well, the point where "diffraction is going to be really start killing the sharpness" depends heavily on the display size of the photo, the viewing distance, the visual acuity of the viewer, the other sources of blur in the photo, and the QT (quality threshold) of the viewer.

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

Not arguing because you agree, or not arguing for other reasons?

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.

I can make it more simple -- if you are going to use the larger format to shoot the same DOF and shutter speed as a smaller format, then you will be better served by the smaller format in almost every instance.

The advantage of the larger format comes from using a more shallow DOF and/or longer shutter speed for a given DOF.

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)

Goes along with what I said above -- allow me to paraphrase for the new paragraph:

Well, the point where "it will be so small a degree you don't care" depends heavily on the display size of the photo, the viewing distance, the visual acuity of the viewer, the other sources of blur in the photo, and the QT (quality threshold) of the viewer.

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

Here, I make a value judgment and claim that by f/16 on mFT (f/32 on FF and f/5.6 on the FZ200) the resolution differences are trivial between systems. That's not to say that there aren't differences in resolution still, as well as other differences, but just that, in my opinion, they are trivial by that point.

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.)?

The CoC does not depend on the pixel size, although one can make a strong case that the smallest meaningful *choice* for the CoC does depend on pixel size.

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.)

When people start "caring about diffraction" depends, as I said, on a host of other factors, namely the display size of the photo, the viewing distance, the visual acuity of the viewer, the other sources of blur in the photo, and the QT (quality threshold) of the viewer.

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