*D 300 F/11 diffraction too limited?*

Started May 16, 2008 | Discussions thread
monte12345 Senior Member • Posts: 2,672
Diffraction and print size

Diffraction effects are independent of the camera resolution. They have always been, what is happening now is that cameras have gained enough resolution so that the effects are now quite visible in a 100% peep. What has to be remembered is that it's the final magnification of the image that determines if the effect of diffraction will have an effect on the print. As camera resolutions increase, the effect of diffraction will become visible in 100% peeps at wider and wider apertures. So, at some point we'll all have to start to choose our shooting aperture based on the intent of the image that we are taking a picture of and quit worrying about what we see in a 100% peep.

So, what's all that mean? Simple, if you make a 4 x 6 inch print you won't be able to see the effect of the diffraction of shooting at f22 because the magnification is too low to make the softness due to diffraction visible in the print. Make a 24 x 36 inch print from that very same shot and it will look terribly soft because the higher magnification will make the effect of diffraction quite visible.

Now for some basic math and science. First, the human eye is only capable of resolving about 6 to 8 lines per millimeter on a print at 12 to 18 inches without assistance. To give us a cushion, and because I like round numbers, I'll use 10 ll/mm. Also, for those who don't how to determine the total magnification of a print, it's really quite simple, divide the vertical dimension of the print in millimeters by the vertical dimension of the image sensor. (I use the vertical because some print sizes have the ends of the image cropped off, such as an 8 x 10 inch print). Also, there are 25.4 millimeters to an inch. so a 4 x 6 inch print measures 101.6 x 152.4mm.

Now for some math. BTW, I am basing the theoretical lens resolution on Nikon's e-line table for resolution versus aperture. Which is as follows.

f1.0 = 1560 ll/mm . . . 24 x 36 inch
f1.4 = 1072 ll/mm . . . 24 x 36 inch
f2.0 = 750 ll/mm . . . . 24 x 36 inch
f2.8 = 536 ll/mm . . . . 24 x 36 inch
f4.0 = 375 ll/mm . . . . 24 x 36 inch
f5.6 = 268 ll/mm . . . . 16 x 24 inch
f8.0 = 188 ll/mm . . . . 11 x 17 inch
f11 = 136 ll/mm . . . . . 8 x 12 inch
f16 = 94 ll/mm . . . . . . 6 x 9 inch
f22 = 68 ll/mm . . . . . . 4 x 6 inch
f32 = 47 ll/mm . . . . . . 3 x 4 inch
f45 = 32 ll/mm . . . . . . 2 x 3 inch
f64 = 24 ll/mm . . . . . 1.5 x 2 inch

So, keeping in mind that we want to achieve a resolution of 10 ll/mm on the final print it's now easy to do the math. At f8, because there are 188 ll/mm available in theory, we have to limit the total magnification to 18.8 X. For the DX format that means that we multiply 16mm by 18.8 to determine the vertical dimension of our final print. (16 x 18.8) 25.4 = 11.84 inches so call it good for an 11 x 17 inch print. Which is how the above table was constructed. Note, the table stops at 24 x 36 inches for the maximum print size for one simple reason, even the very best Nikkors can only approach theory at an aperture of f4.

Finally, we are making prints of pictures and not patterns of lines. So we can cheat on those prints sizes in the above chart by quite a bit. In general, I would consider a 50% increase in size quite reasonable with proper post processing so shooting at f8 for a 16 x 24 inch print shouldn't be any problem. However, there is a limit to how much we can "cheat" with Unsharp mask and that limit is probably about at a 80 to 100% gain in size and going that far will be subject dependent. So, in most cases trying to take a shot taken at f11 to 16 x 24 inches will be pushing the limit and may result in a print that looks just a bit soft if it's examined closely. Which means that you either hang your print so people can't look too close at it or you just don't print that image that large. The simple fact is that what we can see drops off with the distance. So if you have a long viewing distance, you can make really big prints without anyone complaining.

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