Recently I needed to get some screen shots for a presentation. Normally I would just use the built-in screen grabber, but some of the images I needed were of startup screens during the boot process before the UI becomes available. So I figured I would just grab my camera and get on with it.

That was about the time I realized that it wasn’t quite as easy as I thought.

As far as absolute sharpness goes, my Tamron 90/2.8 Macro lens is as good as I have, so I put it on my D700 and set up for the shots. Here is what I got first at f/5.6, its sharpest aperture:

D700, 90mm, ISO 200, F5.6, 1/50

Even after fiddling around with other lenses and different distances, I could not get around the interference pattern between the pixels on the screen and the sensor. So I started trying different things to reduce the sharpness.  First I opened the lens up:

D700, 90mm, ISO 200, F2.8, 1/160

Plan B. Since the pattern appeared to be chromatic, I thought maybe getting rid of the color would solve the problem:

D700, 90mm, ISO 200, F5.6, 1/50

I was about to give up when I remembered two articles (here and here) I had read recently about the detrimental effects of using too small an aperture, and how that minimum aperture depended on the size of the photo sites on the sensor. It said that on a full frame sensor like the D700, the effect began to appear around f/16, but I tried the smallest aperture the lens offered, f/36.

D700, 90mm, ISO 200, F36, 0.8s

Interference gone! But the image was very soft, and since I needed as much detail as possible for my presentation, I tried opening up to f/22 to regain some detail, but hopefully stay below the threshold.

D700, 90mm, ISO 200, F22, 1/3

Better detail, but with noticeable moire. Next I tried f/32:

D700, 90mm, ISO 200, F32, 0.6s

That gave the least detail loss without any visible interference. Finally!

So the solution was to leverage the reduction of sharpness due to diffraction. Not exactly what I expected when I began the project, but an adequate solution, nonetheless.