After critically examining some landscape images I took a couple of years back I was disappointed with their critical sharpness, specifically on the Tokina 12-24 lens , which I use mainly for landscapes on my D80 & D300.
Initially I thought the lens was less than great, but I did some research into the effects of diffraction and after a bit of experimentation, realised that it was diffraction having its wicked way with the resolution I was getting...not a faulty lens.
What is diffraction? Well my understanding is that its a physical limitation that cannot be stopped and means you have to use your lens in ways that avoid its worst effects. Basically, its the effect of light rays passing through the pupil created by the aperture diaphragm and as the ray contacts the edges of the pupil, it scatters slightly. When this scatter exceeds the dimensions of a pixel its getting destructive, as the light ray is spilling onto adjacent pixels instead of falling on only one. Therefore cameras with very high pixel densities are worse affected. When the aperture diaphragm is wide open the proportion of light rays that contact the edge of the pupil is small compared to those passing undisturbed through the large pupil. When the aperture is stopped down, the proportion of rays contacting the pupil becomes much higher compared to those passing undisturbed...hence a higher proportion are scattered and diffraction gets worse.
When doing landscape shots on a tripod and including some foreground interest, I was routinely stopping down to F14 and even F16 and F20 on my DX (APS) sensor cameras. This was mostly out of "making sure everything would be in focus" by extending the depth of field...playing safe.
If you look at any lens tests on APS sensor cameras its quite obvious from the MTF plots that diffraction starts to creep in and rob images of sharpness as low as F8. Beyond F11 it really starts to get destructive and F16-22 is simply a no-go zone. If you have a 12MP SLR, let alone a 16, 18 or 24MP SLR your lens will nowhere near out-resolve the sensor and will be the limiting factor in sharpness and resolution.
I did some tests with my own 12-24 lens and found that sharpness held up well to F11, then declined. I was keen to see if this limited my depth of field enough to prevent me getting sharp images with close foreground interest, while still retaining sharpness at infinity.
I was surprised to learn that with careful focusing I could easily get objects in focus from around a metre away to infinity at F11. That was more than enough in all but the most close-up of situations. To make sure I nail the focus I often shoot in manual focus and set the distance scale so I'm not wasting DOF by focusing beyond infinity. I often shift the focus point and use the focus indicator on both the close and distant subject to confirm focus, before shooting.
In this image the Lion was under a metre away and even at F9 both it and the house are in focus at 12mm focal length. The image is sharp and has good contrast.
On full frame cameras diffraction effects tend to kick in perhaps a stop or two smaller than on APS cameras and many better lenses hold out to F16, but of course there is less depth of field anyway than on an APS camera.
As a result of this understanding I never stop down unnecessarily now, try to optimise my DOF and get better image quality as a result. Give it a try.
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