so if this test is no good, then how are you to accurately adjust the lens?
I just photograph a line of cars, at an angle, at full aperture. No need for anything more. Here's my thinking:
I see two things wrong with many popular forms of lens testing: a) In order to reveal small errors, extreme tests are devised, b) When these tests take the lens too far from its intended method of use, you need to be very careful when interpreting the results, or you'll get led up the garden path.
Such is the case with the A4 paper, shot at very close distance, at 45degs. If this is the kind of thing you photograph regularly, then it's fine. It's a good test for macro lens focusing accuracy. But not for much else. This particular test is notorious for revealing slight errors that do not exist in 'normal distance' use, or that are in practise so slight that they are within accecptable tolerance and can be ignored.
No lens is perfect, or camera, and we must accept that there are tolerances. It is also useful to remember that photographers are not perfect and when it comes to the examination of the problems shown in real pictures taken by real people, well, you know where the problem usually lies
But it's nice to have our kit working as close to 100% as we can get it, and I'm as keen on testing as anybody. I just try to keep it relevant and in context. To that end, I'm generally not keen on test targets whose only real virtue is, often, in repeatability, ie you can do exactly the same test, day after week after year and still have some hope of making valid old vs new comparisons. Buit if you just want to do a one-off check on a lens for focus accuracy, why not shoot a real life, relevant subject, at the sort of distance you would normally use?
For this focus test, I would choose something like a line of cars in the supermarket car park. For focus accuracy, I actually prefer an angled target as it gives you an instant visual if the focus is out, and also by how far, and in which direction. A flat target won't do that.
Stand at an angle to the line, set centre point AF and max lens aperture (lowest f/number). Use IS or tripod is there is any danger whatsoever of camera shake. Focus on a number plate, or one digit of the number plate if it's a long lens. If your camera has a good LCD screen and you zoom in, you can usually see any major errors there and then. If not, then take a peek on the PC.
If you do this, you will quite often find that it's very hard to see
exactlywhere the camera has actually focused, as it all looks quite sharp. This is what you'd expect using a normal lens for normal photography, especially if it's f/4 or something modest. So no worries then. And we shouldn't use this as an excuse to come up with an extremely severe test which artifically reduces depth of field to a miniscule level so we
can see what the camera is up to.
The problem is that in doing so we have taken the lens so far from its intended method of use that the results are, unsurprisingly, often prone to error. We see problems that actually don't exist in normal photography. But it can be worse than that: if we then make lens/camera adjustments based on the erroneous test, we are automatically screwing up the focus accuracy for normal picture taking.
My experience is that with the ten Canon lenses I've bought and tested, only three made me think that they weren't spot on, but even then it was not sufficient cause for correction. None of my cameras/lenses have been adjusted for focus, and I'm quite happy that. I suspect that this problem overall is more imagined than real
For sure, if you speak to technicians at Canon/Nikon service centres, this is often what they find.
Regards,
Richard.
