detail (100% magnification), Canon 24-105mm L f/4 lens, at 105mm at f/9 copyright Douglas Stockdale
Last Friday I had found myself in the local Canon Service shop in Costa Mesa regarding some issues with the bottom left corner of my images and was informed that I need the flange back repaired for my 5D Mark 3. sigh. I picked up the “adjusted” body yesterday (Note: next to me was a guy with a sad face as he was just informed that his 5D Mark ? needed the same repair. He was also holding a big 300mm prime lens. Canon crappy camera design and construction strikes again) so this morning I decided to perform a quick camera check on my adjusted body, just to make sure everything is okay.
I mounted my 5DMk3 on a tripod and then set the aperture to f/9 to make sure that the lens was stopped down enough so that a narrow focus would not be an issue. My first test was my Canon 50mm f/1.4 prime lens; results exhibit nice sharpness edge to edge, image below. Appears my flange back adjustment is fine. Then being a curious cat, I then mounted what I suspect was the bad-boy lens that created the flange back to go out of alignment, my “heavy” 23 oz Canon 24-105mm L f/4 lens. Similar to the 50mm f/1.4, I set the f/9 for exposures for the 24mm, 35mm, 50mm, 70mm and 105mm focal length tests.
At the 24mm, 35mm and the 50mm focal lengths exposures, excellent edge to edge image sharpness (bottom, below). My surprise was the amount of corner softness that occurred at 70mm (below) at 100% magnification, now more like “average/poor” and even more corner softness at the 105mm focal length (above) bordering on “poor/unacceptable”. I am guessing on first looking at the image below that these might all look acceptable, but when making 16×20″ prints or larger, the corner softness starts to become more noticeable.
I have had the Canon 24-105mm lens (purchased new) since I acquired my used Canon 5D in 2010. I had never tested this lens out since the initial images on my monitor appeared fine. I usually am composing with something in the 24 to 50mm focal length, so probably unknowingly I was in the sweet zone for this particular lens. On a couple of occasions I composed using a focal length between the 70 to 105mm range for some informal portraits, but now remember being a bit disappointed that the images appeared a bit soft. Since portraits are not my usual thing, I just kept moving on, also I did not think I would use a 100mm prime lens that much to make an investment.
Bummer about the Canon 24-105mm lens results. So this lens is now sitting on the storage shelf as I contemplate selling it, while the 50mm f/1.4 is on the 5DMk3 (camera on it’s back with the lens straight up, no off-center weight on the flange back). As potential replacements for the 24-105mm I am thinking of a lighter Canon 35mm f/2 (prime) lens and a Canon EF 100mm macro f/2.8 (prime) lens. I all ready have a Canon 17-40mm L lens (which I have not test yet, but think I will sometime soon) so I have the 24-35mm focal lengths covered. For the 100mm macro, I need to consider if I want the heavier L lens which now has the IS (Image Stabilization) feature, although only 1 oz difference in weight it’s not really much difference. The 100mm macro lens is close to the weight of the Canon 24-105mm lens, so I would need to be careful how I carry it when it’s mounted on the camera body, or just mount it when I need it.
No more trips back to Canon to repair the flange back again! I hope you enjoyed my morning in the back yard.
Cheers!
full frame (above), 24-105mm lens at 105 mm focal length and f/9
full frame (above), 24-105mm lens at 70mm focal length and f/9
full frame (above), 24-105mm lens at 35mm focal length and f/9
full frame (above), 50mm f/1.4 lens at f/9
Singular Images 
Doug, when I first read about that flange back issue, I could not imagine that this created such corner specific softness, and your discoveries of those lens performance specifics prove my deliberations right. However it’s bitter to read that such a highly esteemed lens has such flaws.
Markus, The 24-105mm lens is still a pretty good lens, even at the 105mm focal length, as much of the interior of the image was sharp, just the soft/blurry issues at the four corners (maybe what the reviewers call barrel distortion). I have no issues with this lens at the 24 thru 50mm focal lengths, just the corners beyond the 70mm focal length. I might be hyper-sensitive to the corner softness after the flange back issues (actually created even larger amounts of corner softness, but specific to the lower left, and I think I now have a better understanding why).
For a zoom, none of these lens are perfect as compared to a prime lens of the individual focal lengths; thus to use a zoom is to accept some compromises.