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My Sigma 70-300 APO DG lens is known to be "soft" and I've been getting quite varying quality with it. I've put most of this down to my technique but today I decided to test it to try and find out how much the lens contributes to this variation.
I took a total of 48 shots of a brick wall at every aperture and 3 focal lengths, each end of the range and roughly mid-way - the camera doesn't display the FL on the screen so I had to guess using the scale on the lens. 185mm is the mid-point but they actually came out at 168mm
I took the pics from ~7m using a tripod, an infra-red remote, mirror lock-up with a 2 sec delay. The camera was my EOS 400D.
Obviously I expected some variation but was surprised about how poor the sharpness was at the wide end. OK, this is a £150 lens, not Canon L-glass, but even so I didn't expect it to be as poor as this. Is this reasonable performance for this lens?
One other interesting thing is that in some of the sequences the increase/decrease in sharpness doesn't appear to be constant - it's getting better as the aperture closes down, then it goes worse for one setting, then improves again - 168mm f/5-f/5.6-f/13 and 300mm f/8-f/9-f/10. Maybe I wasn't allowing enough time for the camera to stop moving after changing the aperture? I did leave it about 5 secs. and then there was another 2 sec. with the mirror lock-up.
One thing I noticed was that the focusing ring moved (only very slightly) between some of the shots. Why was this? Was it the A/F trying to correct the fact that the lens couldn't focus? I thought that the focussing is done before the iris closes down, in which case the conditions would be constant (for each focal length) - what do people think?
All the pics are 400x400 crops at 100% from the centre of the image.
70mm f/4 1/400
70mm f/11 1/50
70mm f/22 1/13
168mm f/5 1/400
168mm f/5.6 1/250
168mm f/13 1/125
168mm f/29 1/30
I took a total of 48 shots of a brick wall at every aperture and 3 focal lengths, each end of the range and roughly mid-way - the camera doesn't display the FL on the screen so I had to guess using the scale on the lens. 185mm is the mid-point but they actually came out at 168mm
I took the pics from ~7m using a tripod, an infra-red remote, mirror lock-up with a 2 sec delay. The camera was my EOS 400D.
Obviously I expected some variation but was surprised about how poor the sharpness was at the wide end. OK, this is a £150 lens, not Canon L-glass, but even so I didn't expect it to be as poor as this. Is this reasonable performance for this lens?
One other interesting thing is that in some of the sequences the increase/decrease in sharpness doesn't appear to be constant - it's getting better as the aperture closes down, then it goes worse for one setting, then improves again - 168mm f/5-f/5.6-f/13 and 300mm f/8-f/9-f/10. Maybe I wasn't allowing enough time for the camera to stop moving after changing the aperture? I did leave it about 5 secs. and then there was another 2 sec. with the mirror lock-up.
One thing I noticed was that the focusing ring moved (only very slightly) between some of the shots. Why was this? Was it the A/F trying to correct the fact that the lens couldn't focus? I thought that the focussing is done before the iris closes down, in which case the conditions would be constant (for each focal length) - what do people think?
All the pics are 400x400 crops at 100% from the centre of the image.
70mm f/4 1/400
70mm f/11 1/50
70mm f/22 1/13
168mm f/5 1/400
168mm f/5.6 1/250
168mm f/13 1/125
168mm f/29 1/30