Quirky focus on the 300mm lens?


The 300mm lens I rented gave me a bit of a scare. I recently purchased a program, Reikan FoCal. This is a focus adjustment and performance evaluation tool that is actually pretty handy for its low price. I originally purchased it to fine tune the focus of my Canon EF 100-400mm f/4.5-5.6 L IS lens, which has been focusing soft on my 7D. FoCal is capable of automatically determining which AutoFocus Micro Adjustment (AFMA) setting will produce ideal results, which is extremely handy. I purchased the Pro version of FoCal in order to get some additional features, such as an aperture test series that will tell you which aperture produces the highest resolution. I decided to run a quick aperture test on the 300mm f/2.8 lens tonight, and its a good thing I did!

Every single shot was way off. The lens couldn’t focus correctly at any aperture, and every shot was very soft. I was barely getting more than 300lp/ph resolution at best, and considerably lower than that outside of the “best” aperture (which is probably not accurate). I ran through the aperture test about half a dozen times, and the results were all the same. At first, I thought I’d received a bad lens, however I have read LensRental.com’s blog for some time now, and I know how extensively they test each and every lens. I also know that they test each lens as they are returned, to ensure that they are in good condition before being sent out to the next renter.

I finally started to think that perhaps FoCal was incompatible with the lens…it is the Mark II version of the 300/2.8, which was only released last year. I started testing the focus myself, and to my dismay, real-world focusing had the same problem. Every single shot seemed to be backfocusing by a considerable amount. I decided to check the Autofocus Microfocus Adjustment settings on my 7D, and to my surprise…when the 300mm lens was attached the setting showed -92! That was rather curious, as the adjustment range only spans from -20 to +20, and to my knowledge there is no way to set AFMA to a setting higher than that. I reset the adjustment to zero, and tested again. The results appeared to be the same, so I set it to -20, then +20, no difference in any case. Powering my camera off and back on returned the setting to -92. I was rather worried at this point that I would have to return the lens for another copy, which would probably take more than a week, and my vacation would be over then.

In a last ditch effort to resolve the issue, I reset my camera settings. That seems to have saved the day…and my plans for my vacation. The lens seems to focus 100% perfectly with an AFMA adjustment of zero now. I ran it through the aperture test in FoCal again, and resolution is up to 2000lp/ph, a very good sign. I tested focus manually against some items that have very fine detail, and it seems to be focusing tack-sharp ever time, no front or back focus. Thank you, Lord! I’d become rather disheartened for a while, worried that I would be unable to use this wonderful lens I’d been waiting for for so long.

So, when in doubt…RESET!

P.S. I’m going to have to ask the LensRental guy about the -92 setting. I have no idea how that could have occurred, but they mess with lenses all the time, and might have a clue. I’ll update this blog if I get any more information.