Besides the heft, is the 50mm 1.4 worth the extra cost over the 1.8

Started Apr 13, 2013 | Discussions thread
Keith Z Leonard Veteran Member • Posts: 6,038
Re: Besides the heft, is the 50mm 1.4 worth the extra cost over the 1.8

Just setup some targets and use a tripod to test the focus accuracy.  Sometimes people will use some batteries to do this kind of testing, setting them up in a row at different distances to the camera, as in this article.

You then focus (single AF point) on one of the batteries using phase detection (not live view contrast AF, which should always get it right) and see if the battery you focused upon is in focus.  If it is not see if one closer or farther away is, that would indicate the lens is front or back focusing.

Now that's ok, a lens can do that and still be usable with micro focus adjustment, what you want to look for is if it consistently does it or not.  If the lens is always slightly back focused, you can correct it with MFA (micro focus adjustment), but if the lens front focuses for targets at 10 feet and back focuses for targets at 30 feet, then you have a problem.  You cannot correct for this type of bad lens behavior and would need to send it to Sigma to have them correct the lens for you at their optical bench setup.

The crux of the issue stems from the fact that Sigma has reverse engineered the behavior of the EF mount, this information is not public as Canon wants to keep it secret and thus be the only company able to make lenses for their cameras.  Sigma has done will here overall, but some lens/body combinations can have issues that need to be corrected by Sigma.  There have been some bad events in the past (not recent) where Sigma has dropped support for old lenses and they won't update them to for newer Canon cameras.  The 4 year EX warranty means that they will do this for you free of charge (though you do pay shipping 1 way, the pay for the return shipping).

I will say that my 5Dmk3 does AF my Sigma 50 much better than my 7D does, though I had it corrected for the 7D.  I suspect that the 5Dmk3 AF system is just more accurate so I have less misses in the f1.4-f2.0 range.  When doing testing you should test wide open as that will be most likely to exhibit any problems.  If you are shooting at f4-f5.6 it's a lot more forgiving, but also at this point most zooms are good in that range, so why bother with a prime??

Also keep in mind that focus shift happens to 50mm prime lenses quite often, that is when a lens focuses wide open and then stops down for the exposure and it moves the focus a bit in the process.  So it's best to test at the larger apertures that you intend to use.

 Keith Z Leonard's gear list:Keith Z Leonard's gear list
Canon EF 70-200mm f/4.0L USM Canon EF 100-400mm f/4.5-5.6L IS USM Sigma 50mm F1.4 EX DG HSM Canon EF 24-105mm f/4L IS USM Canon EOS 400D +16 more
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