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Sigma 100-300 f/4 Front Focus (1 Viewer)

Rapala

Well-known member
It seems that my copy of a Sigma 100-300 f/4 has some serious focusing issues. After suspecting the lens was front focusing, I printed off a test chart and conducted the test. The test itself was pretty simple, requiring the test chart propped up at about a 45 degree angle and shooting straight at it from a tripod. I aligned the center focus point with the center line on the chart and shot with a 10sec timer to reduce shake. The results I got are much worse than I had expected. The first photo attached shows the chart from about 7ft @ 300mm, 1/2000 f/4 ISO 100. Right about near the number 1 seems to be the perfect focus, showing prominent front focus. The second photo below is from about 12ft away @ 300mm, 1/1600 f/4 ISO 100. This one looks even worse, with focus around the number 2. BTW, both photos have been cropped to the center of each frame.

To show what this equates to in the field, the third photo would have been a nice photo of a young chickadee. The photo is uncropped, so the center focus point was directly on the bird's head. Instead I came away with a detailed photo of the Chickadee's alula and primary feathers. (Okay, maybe I looked up alula ;)) This is very frustrating since it would have otherwise been a great photo. In this thread I am looking for guidance since I have never had this issue before. I assume that my lens requires micro adjusting to fix this issue, but I am not sure. I also don't know how or where to get it micro adjusted if that is the case. Any help here is much appreciated.
 

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I am using a Canon T4i. As far as I know, micro adjusting is only available on high-end Canon DSLRs.
 
Also, how can I tell if it is my lens or camera that needs adjustment? Is it usually one of the two?
 
Also, how can I tell if it is my lens or camera that needs adjustment? Is it usually one of the two?

This is a complex problem as in an ideal world both camera and lens have a certain range that is considered within tolerance. So your lens could be fine on a different body and your body could be fine with another lens, depending on how the tolerances of both combine. This article explains the whole can of worms (and some of the comments show how tricky it can get).
https://www.lensrentals.com/blog/2008/12/this-lens-is-soft-and-other-myths/https://www.lensrentals.com/blog/2008/12/this-lens-is-soft-and-other-myths/

Only way for you would be to get body and lens tuned to each other by a service station. Not sure if Canon will do this for a non-brand lens or Sigma is able to do this on a Canon body, nor what the costs for this would be. Them the question is how other lenses one owns will behave after the tuning.

One day when we all use mirror-less cameras and AF is done not by a AF-module and straight from the sensor this could be a thing of the past. For now it is just sometimes a major pain "you know where".

Love Sigma (and Tamron) for giving some of their newer lenses AF fine tuning ability in the lens itself. It took me over an hour to set-up my 120-300 for a camera body (values for several different focal length at several different distances had to be determined), but the result could have never been reached with in camera one value/lens AF micro adjustment. Unfortunately your 100-300 f/4 isn't one of those lenses.
 
Thank you seaspirit.
I will be off to Colorado for the next couple of weeks so I will try to find a camera shop there to calibrate my gear (no such shops exist in the U.P.). Hopefully all goes well.
 
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