do diffraction effects vary by focal length?

Started Nov 23, 2011 | Discussions thread
Mike Davis
Contributing MemberPosts: 686
Re: do diffraction effects vary by focal length?
In reply to Brev00, Dec 18, 2011

Hi Brev00,

Brev00 wrote:

That is certainly more than I can digest without brain fatigue. What do you make of the part of the Cambridge Colours article that states that without a sturdy tripod and top quality lens, all talk of diffraction has no meaning? That is, you need excellent resolution in the first place to even notice the effects of diffraction; the damage caused an image by an average lens without stabilization far outweighs (or obscures) the effects of diffraction regardless of viewing distance, etc. At least, that is how I read it. This, of course, it theoretical.

I'll answer by quoting my third post, above:

"Lastly, please realize that there are many factors other than available pixel count, defocus and diffraction that can prevent you from achieving a "desired" final image resolution. What I've written here only describes an approach to controlling defocus and diffraction, with no attention given to lens resolving power at various apertures, the smearing caused by subject or camera motion at inadequate shutter speeds, film resolution and in-camera flatness (for those who are still shooting film), etc. Defocus and diffraction are, however, among the most controllable of factors affecting final image resolution, if you're willing to exercise that control instead of just rolling the dice every time you make an exposure."

In other words, if you aren't willing to take control of as many variables as can be controlled to secure a desired print resolution (assuming you actually have a desired print resolution in mind), then there's no point in worrying about diffraction - especially at reasonable enlargement factors in combination with small f-Numbers. It's when people use tiny 12-, 14- or higher MP sensors to produce really large prints (as encouraged by the pixel count) that they had better know enough to shoot wide open or they will SEE the impact diffraction when viewing those prints at close range.

Look again at the formula I provided for calculating the f-Number at which diffraction will begin to inhibit a desired print resolution - specifically, look for the variables used in the formula:

f-Number = 1 / anticipated enlargement factor / desired print resolution / 0.00135383

Thus, for ANY particular desired print resolution, the ONLY variable that comes into play is the anticipated enlargement factor. If you try to make a 24x30-inch print from a one-inch tall sensor, the enlargement factor will be 24x, but if you try to make the same size print from a sensor that's only 1/4-inch tall, your enlargement factor will be four times greater, or 4 * 24 = 96x. This means that even if the smaller, 1/4-inch tall sensor offers 100 or even 1000 Megapixels, the enlargement factor required to produce a 24x30-inch print would be FOUR TIMES greater than the enlargement factor required by the one-inch tall sensor. A diffraction-savvy photographer would have to open up two stops wider when shooting with 1/4-inch tall sensor to shrink diffraction's Airy Disks at the sensor by a factor of four.

What I like about what you wrote (at least as far as I could understand it) is the importance of viewing distance--a real life variable. But, since so many only view images on their computer screen, how does viewing on an lcd monitor effect the equations you offered (at a comfortable distance and without pixel peeping)?

Whether viewing with a computer, looking at backlit color slides in a slide viewer, projecting film onto a theater screen, or looking at prints, the same formulas apply to every situation.

In the case of a computer monitor, if that's your final media (as with photos for web pages), the limit of final image resolution will be dictated primarily by the display resolution. A 1920 x 1080 monitor having a pitch of 100 dpi, can be more than satisfied with a 4 MP camera, but the impact of diffraction can still be calculated by using the equation above. Measure the diagonal of the image on-screen, then divide by the diagonal of the sensor to get the enlargement factor. Note, however, that your "desired resolution" shouldn't be any greater than 2 lp/mm when your monitor has a dot pitch of only 100 dpi.


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