Lens Tests

[duranash]

I recently bought a Canon 100-400IS lens and, given all the hype on the internet about poor quality control, I'm trying to make sure I "got a good copy" .  So I made one of the 24"x36" pegboards that you described (I shoot a Canon 5DMII) and printed several of your resolution targets; then set up a test area in my basement shop where I have good shop lighting and enought space to cover the entire board at 400mm.  I first tested my Tamrom 90mm Macro, assuming it was probably the sharpest lens I own.  Looking at the Tamron images on screen at 100%, I can see down to 3.2 on your target, with a suggestiion of 3.6.  Based on that I assume my setup is OK.  The 100-400 lens at various focal lengths seems to give me at least 2.5 and often 2.8 clearly with 2.8 and occasionally 3.2 in the category of "barely' able to see the resolutiion lines on screen.  Would that seem to indicate that my lens is decent in terms of sharpness???

A couple of questions for Bob......given that these tests are done at pretty short distances, has it been your experience that the test results are pretty much consistent out at longer focus distances?  In other words, can I expect these results to be representative at all or most distances?

An area where I'm not having much luck in my tests is Auto Focus.  I also printed off your AF Chart and mounted it on a nice flat piece of MDF.  I first tried just clamping it to the miter jig on my table saw (@ 45%) and then set up the camera on tripod shooting toward the table saw.  The results seemed to indicate front focus by a pretty good amount.........but I think my test was flawed because I couldn't figure out how to really get the camera set up at exactly 45%, or straight on toward the saw base.  But when I did my resolution tests, I used Live View at 10X and it seemed to me that the resolution chart was in pretty darned good focus.  I would usually AF first, then turn AF off and tweak focus manaually, but I'm not sure I really did much better anyway.  At 400mm in Live View, just laying my hand on the lens would really bounce things around.  Just this morning, I tried laying the focus chart on the floor (actually on the stair landing) and set up the tripod at 45% and shot down the stairs (to get a bit more distance).  That result still seem to show some front focus.  What should I believe....the live view at 10x results or the 45% angle chart?

Thanks for providing all the useful information 

[duranash]

Let me amend this post re: AF.  I just spent some time trying to make sure the camera and chart were lined up as well as I could (hopefully a little better than before) and took 5 images.  I cranked the lens out of focus between each exposure so the lens would have to AF each time.  I must say, these 5 images look much better.  So I suspect my previous issues were simple "operator error"!
Thanks

[duranash]

Bob - if you have time, any thoughts?

[Bob Atkins]

It's hard to comment on tests I didn't do myself because there are so many variables. You have to be sure focus is good. You need a sturdy tripod (especially for long lenses), you may need to use mirror lock up for telephoto lenses too (depends on your shutter speed, but it never hurts at any speed).

Sounds to me like your lens is probably fine. As you've found, you have to do things right to get the best results, but that's part of the learning process!

[duranash]

I realize you can't comment on the specific test results, but, given that these tests are done at pretty short distances, has it been your experience that the test results are pretty much consistent out at longer focus distances?  In other words, can I expect these results to be representative at all or most distances?
Thanks

[Bob Atkins]

Yes. Though they seem like they are taken at close distances, as long as you're more than about 25-30x the focal length the difference from infinity is small.

At very close distances, say 1-5x the focal length, the image quality can change a lot, so results in the "macro" and infinity range may be quite different.

This is because as you move in from infinity, focus only changes by a small amount at first, but it changes by a increasingly large amounts when you move to the macro range. So for a 400mm lens the focus for infinity is quite similar to the position of focus at 10 meters (25 focal lengths).

For a 400mm lens focus shifts by 16mm at 10m, by 62mm at 3m and by 267mm at 1m distance.