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| p.2 #2 · Canon 24-70mm f/2.8L II sample variation tested |
Roger, thanks for that information.
One question it suggests for further investigation is how much of the variation observed is attributable to each lens individually (as a permanent feature), and how much to the spread of results you might get from a single lens if it were re-zoomed and re-focussed between each of multiple tests.
If on multiple tries any particular lens always grouped tightly, we would know that the question of getting a good copy is more important than it would be if each lens produced a spread of results similar to the one on your graph. Maybe your experience allows you to just answer the general question from your present knowledge?
Just one other question. The subjective quality measurement you use is based on an 8x10 print, and of course some of us print much larger. Would smaller subjective quality variation numbers become more evident in say, a 30x20 print? I'm basically wondering if getting a great copy is more important to a landscape photographer who prints big than to a wedding photographer, for instance.
Thanks again for the terrific work.
I did that sort of thing a bit in this post: http://www.lensrentals.com/blog/2011/10/notes-on-lens-and-camera-variation
It's an older post and our technique with testing is a little better now, but it shows the principle (third graph down):
1) If I take 6 straight shots, the points are nearly touching but there's a bit of variation (5 points or so either way) which is simply variation in vibration or lighting.
2) If I manually focus 6 separate shots with live view, there's a bit more variation, maybe 20 points. That's the limit of Roger's ability to focus and why we focus bracket and take the best result.
3) If I autofocus (phase), though, there's quite a bit more variation shot-to-shot. AF variation on one lens can be around 30% of the variation seen for best shot on all lenses.
4) If I test with a different camera (next graph down) variation is again, about 30-40% for each lens.
Lets also add that I'm testing 70mm at 18 feet. Since the focus elements will change position at infinity or minimum focusing distance, there would be some variation between lenses if I was able to test at infinity.
Items 3, 4 and 5 are why I'm always saying (trying to explain it to a well known reviewer right now) you can't say "I want your best 24-70". I don't know which is best on your camera, at different focal lengths, or at different distances. There's a reason you don't shoot shotguns at a bullseye target.
For the other question, yes, if you're printing a huge print, than a difference in SQF of 5 would be noticeable, at least in a side-by-side comparison. But for all the reasons mentioned above I don't know if it's possible to predict more closely than that. I can tell you that even on lenses we're great at optically adjusting, that's about the best we can do, and that's about what the factory service center can do too.