touji wrote in post #14604641
A quick and dirty way is to line up batteries in a 45 degree angle, equally spaced apart (As best as you can). Throw your camera up on a tripod, focus on batteries, and BLAM! You should see if you are getting front or back focus depending on which batteries are in focus.http://cameralightlens.com/newsblog/?p=264
I am really glad I saw this and post 7150 (below).
I bought one of these off ebay after a couple of message exchanges with the owner assured me there were no focusing issues. It arrived as we were going out the door for a day long outing and I took it along. Knowing the focusing issues that the lens may have, I took a bunch of pictures and was sensitive to "front focus", but I only saw one where that may be a problem.
I decided to do some tests with focus charts (two of them) and UH-oh, looks like front focus. Started wringing my hands and cussing the previous owner for lying about the focusing issues.
That night as I was waiting in bed waiting to go to sleep, I started to thing about the tests I did and some of the things I had read about the lens and wides in general:
>>Unless a wide angle lens is perpendicular/square to lines, they will tend to converge at odd angles (I was at a 45 degree angle to both focus charts, hmmm)
>>Very thin dof at f/1.4 means that that not being perpendicular/square might affect the thin area of focus in front/behind the focus target. Hmmm
>> Use only the center focus sensor, not the full array (I have a Canon 60D). Hmmm
>> Hand held is much less preferable than tripod mounted, especially at "test" shutter speeds under 400 or so. Hmmm
Next day I used a tripod, 5 AA batteries lined up touching each other (front to back and offset a bit) and made sure I was perpendicular/square to the batteries (no 45 degree angle) and I used a flash. No front focus, no back focus issues. Whichever battery I focused on at f/1.4 was perfect and the batteries in front and behind were a bit blurred.
I haven't been at this very long (re-entry after 30+ year absence), but I have been pondering this question since the second re-test with the batteries - Did the lens begin to focus perfectly because 1) the test methodology changed to deal with all of the things noted above -OR- was it just because the lens was well-rested after having had an additional night's sleep?
Now that I've had some fun with this, I suspect that the lens may get a bit of a bad wrap because of the difficulty in working with a wide open f/1.4. Somewhere between actual lens focusing issues and operator error is likely where the truth lies. Based on all of the general usage on the first day (48 pics), the testing (30 pics in the focus chart test and 80+ pics in the batteries test) plus what I have read about the wide open wide angle dof, in my mind I am pretty comfortable thinking that this lens at f/1.4 will probably require some experience to fully maximize results in a photo and stability, center focusing and as square/perpendicular as I can muster will contribute to better results.
Just to keep the heat level down on this whole thing a bit, I am NOT an expert at this, just relating some experience based on what I have to work with. Also, I did the same tests with my Canon 15-85mm f/3.5-5.6 IS USM wide open at 15mm. On the focus charts it had a bit of back focus, on the batteries it was also perfect. Of course, it too had another night's sleep. Both lens are terrific in general shooting at various f-stops.
So, YMMV, but I hope some of this was helpful.
BTW - the pictures posted on this thread are a large part of the reason I wanted the Sigma 30, along with a fast lens to catch grandchildren in motion indoors w/o flash.