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Tack Sharp Portraits

Until few months back, I used to wonder: why is it that my portraits aren’t tack sharp? I would feel that I could have eeked out more sharpness. However I wasn’t sure what was it that I was doing wrong. I would make sure that I:

  • Use spot focusing and choose a focus point so that I could aim for the eyes,
  • Choose a shutter speed that is fast enough to freeze motion when I want it to,
  • Choose an aperture so that I could blur the background,
  • Choose the lowest possible ISO,
  • Try to adjust my exposure settings to expose for highlights and
  • Choose a focal length greater than 50mm.

However, irrespective of the output, I would always feel that the portrait could have been sharper. I had seen stunning portraits (with the same lens) and I knew it could be done. Yes, the glass matters, but my Tamron 17-50mm f/2.8 wasn’t too shabby.

After much research, I found that not all apertures of the lens capture images with the same sharpness. (What? How can this be true? This called for a serious investigation.)

I googled for a lens-test chart. To test the lens, I stuck the chart to a wall. After arranging sufficient ambient lighting, I setup the camera on a tripod and switched to aperture priority mode. After setting the ISO at 100, manually focusing on the chart with live view and using a release cable to trigger the camera, I took photographs for all aperture settings at different focal lengths.

The results were astonishing. See the pictures below at 17mm. With all other parameters fixed (except for shutter speeds, which is understandable) the photograph at f/2.8 is not as sharp as the one at f/4.0. Similarly, the one at f/4.0 is not as sharp as the one at f/5.6. However I started to lose sharpness at the other extreme too. I observed the same behavior at all focal lengths, though with slight variations. Nevertheless, the trend was the same. Sharpness was poor at extreme apertures, which in my case was f/2.8 and f/32.

f/2.8, 1/400 sec, 17mm, ISO100 f/4.0, 1/200 sec, 17mm, ISO100
f/5.6, 1/60 sec, 17mm, ISO100 f/22, 1/4 sec, 17mm, ISO100

The lesson was that by stopping down once (f/4.0) I would get a pretty good image. Stopping down twice (f/5.6) would result in a much more sharp photograph. From there on I would observe diminishing returns, until the sharpness started to deteriorate.

Therefore, your f/1.4 may not produce pictures as sharp as you would have expected it to produce at f/1.4. While it is important to consider other aspects — e.g., avoid camera shake, making sure that you’ve focused where you intended to and choosing other exposure settings — avoiding extreme apertures is important to obtain tack-sharp photographs.

I have not tested the same with L series lenses. I would expect similar trends though.

There is another aspect to this discussion; I also realized that we should not confuse the lack of sharpness due to extreme aperture settings with the blurry feel of photographs that have a narrow depth of field. When we choose a small aperture (say f/16), we are increasing the depth of field; when we choose a wider aperture (say f/2.8), we are narrowing the depth of field. Depending on your focal plane, a photograph with a large depth of field may have a “feel” of being sharper than a one with a narrow depth of field. However, the discussion above is not with reference to the overall feel of the photograph, but instead about the sharpness of the subject(s) that fall within the region that is in focus.

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