Lens Test

JSPhotography

How should I go about testing a lens? I have a Sigma 120-400 F4/5.6 that is currently being used on a 7D(previous to that a 40D and an Xti before that) that I have struggled with from the beginning. Even the best shots seem soft. At this point I feel like I am trying way too hard and still not getting the sharpness I would like. By trying too hard I mean I am keeping my aperture above F7, ISO below 1500, focal length between 150 and 350. All in an attempt to hit that sweet spot everybody talks about. I have done some simple pics of yard sticks etc, some of which I thought might show some issue. In regards to that, if I shoot my yard stick at say 20' away and say it shows the DOF that is in focus seems to be slightly shifted short of my target mark will that amount be multiplied as the distance to my target is increased? What is this micro adjustment for focus that I read about to adjust for a certain lens's error?

I need help here, I am really struggling with confidence in this lens. I keep blaming it on my abilities but I have been taking a lot of shots over the last few years and I have to be getting it right some of the time. The other day I was hired to do some work for a small christian college by the AD, shot a soccer game in the afternoon with the 120 - 400 and then a volleyball game in the evenning. My Sigma 70 - 200 2.8 in a poorly lit gym just kicked the crap out of the 120-400 in nice level overcast light.

ziggy53

When I test a new lens I often choose a few different subjects for comparison:

1) A brick wall, shot straight on and squared with, using a tripod. This is a pretty good test for front-focus, back-focus, field curvature and vignetting issues. If a camera/lens combination does not focus accurately in this test, something needs to be serviced. Make sure to test at least twice MFD from the wall.

2) A fence line or similar, shot at an angle to the subject. Put a singular strong-contrast target on the top of the middle post and use a single focus point in the camera to focus against the target. This shows focus accuracy and/or how easily distracting for/aft objects influence AF accuracy (compared to the above wall shot). It also shows bokeh tendencies at different aperture settings.

3) A focus target/chart like in the following:

http://regex.info/blog/photo-tech/focus-chart

I do recommend testing these charts at twice minimum focus distance or greater. Most lenses, especially most zoom lenses, do not do their best at MFD. True macro lenses are the major exception.

4) A US stop sign, or similar very high contrast signage, both centered and off center and to the side and/or corner. This can show chromatic errors and off-axis errors.

5) A spectral highlight at night. (Point source ideally, but a street light at some distance will do.) Try to avoid "blooming" of the source but allow some white clipping. This shows internal reflections and dispersion problems. Again, test both in the center and with the subject at the sides or corners.

6) Now just shoot "typical" subjects for your use of the lens(es). It's important that the lens perform correctly for your intended uses.

Feel free to use JPGs and just give image links or directory links, but any significant issues and comparisons you may want to add inline to a post describing particular issues. These may be crops and display 100 percent if you wish. 100 percent crops aren't really fair unless you intend to print "very" large, but it can be easier to display comparisons for examination and to demonstrate problems.

JSPhotography

OK, so I downloaded all the test pages you recomended. My autofocus would pick up even the lightest grey scale. I am very confident though that my single point was spot on the zero line. I did learn quickly that I needed the center line of the lens to be the same height as the zero line on the chart. I started out shooting down at the chart which is tilted back making the plane of the chart perpendicular to the lens. Made my DOF really big!! Anyway, wow, to me these results make me wonder how I ever get shots that are any good. The Sigma 120-400 4/5.6 is showing back focus. The Sigma 70-200 2.8 is showing front focus. The Tamron 17-50 2.8 is pretty good. It really becomes apparent when you look at the at 100%.

120-400


70-200


17-50

ziggy53

Please do the brick wall test first.

JSPhotography

Well ziggy, not sure exactly what your after. Thanks for the help

70-200 @ 70


70-200 @ 200


120-400 @120


120-400 @ 400


120-400 @ 400 Crop

ziggy53

JSPhotography wrote: »

Well ziggy, not sure exactly what your after. Thanks for the help

...

I'll look at these tomorrow, but thanks for trusting me for the need to have them.

JSPhotography

OK, thanks. Let me know if you want me to do anything else.

ziggy53

Sorry for the delay.

I agree that the 70-200mm wall image tests at both 70mm and at 200mm both look soft, indicative of focus error. I suggest trying the micro-focus adjust with that lens with the brick wall again as the target. If the wall is in shade and with little contrast, you might use a flash at an oblique angle just to show the wall texture a bit better for visual assessment. (Keep the camera square to the wall however.)

I do not suggest using the other targets for micro-focus adjust (MFA) because there is too much chance for autofocus distraction. The brick wall gives a much better target for the AF sensor to work against.

The 120-400mm zoom looks much better and that may be all that zoom has to offer when the aperture is wide open. You can try the MFA with that lens too, but I rather doubt that it will improve much. Still, it's easy to try so I recommend trying MFA.

Once you do the MFA calibration on the brick wall to the limits of the adjustment improvements, then move on to the fence line and focus charts, knowing what the lenses are capable of with the brick wall test. In other words, use the best you can achieve from the brick wall test as your benchmark for all other tests.

ziggy53

I should add that Canon provides a couple of links to methodologies in testing and setting MFA:

http://www.learn.usa.canon.com/app/pdfs/quickguides/CDLC_Accurate_EOS_AF_QuickGuide.pdf

http://www.learn.usa.canon.com/resources/articles/2011/af_microadjustment_article.shtml

Use these methods against the brick wall for best results.

Jeffro

Not nearly as scientific, but when I noticed my 300L f4 was missing the shot, and appeared to be back focusing I used the "mailbox method" to check the bf. I shot a mailbox down the street from my house at micro adjustment of -5, 0, +5 (three shots at each) then viewed them at 100%. The -5 ended up being spot on.

Today I noticed my 17-40L was a bit soft.....actually I thought there was something a bit off a few days ago, but today it really bothered me. After doing some -5, 0, +5 shots I found -5 was close, but not quite. Then I did -5,-7,-10. Turns out -7 was dead on, well it appears to be anyway. The testing was done in doors tonight, tomorrow I'll head out and take some more shots and see if my testing was right.

JSPhotography

Wow, I had a chance to play with the 10x live view focus vs auto focus. Even at max MFA I could not make these lenses auto focus match the manual live view. They are much closer but not matching. I screwed up the test documentation, I thought the Exif would give me manual vs auto focus and MFA so I did not stay consistant or note settings. I'll play with them again when I get a chance and repost.

JSPhotography

Boy is this frustrating. I have thousands of dollars tied up in these lenses and according to all the tests and restests and playing around I have done they are crap. Now I'm into my busy indoor season with the 70-200 2.8. I can't afford to be without the lens but I am questioning every shot that they are not as good as they should be. If they are not as good as they could be what the h... am I doing charging people? ARGHHHH. I go for a walk the other day, squirel sitting in a tree at about 10 yards, take a shot, zoom in, looks like crap, flip on live view, 10X, manual focus, snap, zoom in at 100% and still looks pretty good. Try that at a show jumping event, vollyball game, or basketball game!! I am going to a sports shooting workshop with a Sigma photog next weekend. He probably won't have much help for me but I'm gonna bend his ear.

ziggy53

I highly recommend renting a similar Nikon lens so that you can do a proper comparison.

JSPhotography

So I contacted my Sigma dealer the 120-400 came from. He is 60 miles away so he got a call instead of another visit. His take was that it was not going to be a lens issue that it was going to be my 7D. So tonight I slapped the lenses on my 40D(remembered that it had live vieew) and it appears they matched up pretty good. I had to do my tests indoors as it is now dark when I get out of work. This weekend I'll do the live view brick wall tests with both the 40D and 7D. I don't understand why the 7D would be back focussing on one and front focussing on the other. Seems like it should be consistant.

JSPhotography

Thanks Ziggy and Jeffro for helping me out. Just got my 7D back from Canon. My winter photo work that kept me from sending my 7D to Canon finally ended. They replaced the complete auto focus mechanism and software. Ran tests this morning. Looks really good. IT WASN"T ME!!!

ziggy53

JSPhotography wrote: »

... Looks really good. ...

Thanks for getting back to us and good luck with the 7D.