Diffraction Limit Discussion Continuation

Started Feb 21, 2014 | Discussions thread
Mike Davis Contributing Member • Posts: 829
Enlargement factor has been ignored

Bob's got it right, but I can reconcile the two camps - those who believe pixel density has no impact on diffraction and those that are convinced that it does.

Yes, you are missing something, which is that using a sensor with a finer pixel pitch does not change the f-number at which the resolution starts to drop due to diffraction. It changes the resolution that you get, but the drop starts at exactly the same f-number. Here for example is the MTF50 of a Nikon lens on two different Nikon cameras with different pixel pitch:

View: gallery page

Notice that in both the resolution falls away after f/5.6. The 24MP camera extracts more resolution from the lens than the 12MP one.

Notice also there is no defined 'limit' where the resolution suddenly falls due to diffraction, it is a smooth and even drop-off. The 'limit' is just a bogus idea. McHugh has taken a well defined optical term - a 'diffraction limited' system is one so good that diffraction is the only limit on its performance - turned it inside out and made it into something senseless.

If you examine a flat view of the prior thread and search all the text on all eight pages, you will not find any of these strings:

"enlarge" "enlargement" "magnification" "print size" "viewing distance"

And that's what's been missing from this discussion - a consideration of the impact of enlargement factor on diffraction's ability to inhibit a desired print resolution.

Bob's statement is absolutely true as long as long as the enlargement factor imposed by the ratio of the final image size to sensor size remains constant. If someone decides to exploit a higher pixel count on a like-sized sensor to make a larger print (without simultaneously enforcing a proportionately greater viewing distance), otherwise using the same f-Number, the Airy disk diameters at the sensor will suffer a greater enlargement factor in the final image and thus, could inhibit your desired print resolution.

It's the larger enlargement factors that one might use with higher pixel densities on a like-sized sensor (without somehow enforcing proportionately greater viewing distances), that can require a diffraction-savvy shooter to use a wider aperture, to maintain a given print resolution. If you're making the same-sized print, from the same-sized sensor, to be viewed at the same viewing distance, pixel density has no impact on diffraction. At best, pixel density's impact on diffraction is only incidental with the higher enlargement factors that are encouraged.


It's funny that no one seems to notice that Sean McHugh's advanced-mode Diffraction Limit Calculator includes, as its first variable, a user-specifiable input for "Max Print Dimension." Ask yourself why that variable is there. Are we all making 10-inch prints (the default value)?

His "Eyesight" variable is an attempt at determining your desired print resolution.

"Camera Type" is going after your sensor size.

Max Print Dimension / Max Sensor Dimension = Enlargement factor.

Yes - enlargement factor must be considered when determining the f-Number at which diffraction will begin to inhibit a desired print resolution.

And yet, almost no one includes enlargement factor or viewing distance in discussions of either DoF or diffraction. Why?

With equivalent Pixel Count, DoF, Diffraction, Print Size, and Viewing Distance...

Small Sensors can give us the same DoF and diffraction as larger sensors, but with faster shutter speeds at smaller f-Numbers, and thus, fewer "diffraction-free" f-Numbers from which to choose.

Large Sensors can give us the same DoF and diffraction as smaller sensors, but with slower shutter speeds at larger f-Numbers, and thus, more "diffraction-free" f-Numbers from which to choose.

It's the higher enlargement factor required by small sensors having the same pixel count as larger sensors that forces the use of smaller apertures (smaller Airy disks at the sensor) before magnification, to produce like-sized, like-resolution prints.

To answer the OP (of the original thread), Jim Pilcher, see this post and those that follow:


By the way, I absolutely love the way Jim included this text in his OP:

"I'm unaware of the calculations and perceptual considerations."

You are not alone, Jim, but at least you intuitively understand that perception is something that must be taken into consideration!  (Hint: Enlargement factor, viewing distance, and desired print resolution.)

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