Is my 24-105mm f/4 L unreasonably soft?

[RS2021]

One of the unsung underrated aspects of this zoom is the beautiful contrast it produces straight out of the camera. With some lenses the images come out "fey".  As a package, most owners come to love this zoom if not at once, with time.

[iKenndac]

Thanks for your advice, everyone.

Well, I bit the bullet and emailed Canon's Swedish service partner, and they told me they'd want 700kr (£70, $110) to even look at it, at which point they'd tell me how much to would be to fix it, if it needed fixing at all.

Since I'm not 100% there's a problem, instead I bought FoCal, an excellent lens tuning and measuring application. First, I tuned the AFMA (which was a little bit out), then ran some sharpness tests vs. my new 40mm f/2.8 STM.

Attached are the results. As you can see, at 65mm or so my 24-105L is sharper than the 40mm, but at 24 and 105mm it's considerably worse, with 65mm being over twice as sharp as at both ends.

Now I know this, I'm less sure there's a problem. Now I know where the softest parts of the lens are I can avoid them. Still, I'm curious if such a large difference is normal.

[Sporgon]

@iKenndac, apologies if I have misunderstood your data, but there is no way the 24-105 is anything like as sharp as the 40mm at f4, so there is something very wrong with your focal result.

[zim]

That green curve does look strange to me, resolving better at f4 than f8 (I'm assuming the sweet spot would be around f8) and again better at f20, maybe I'm miss reading?

[iKenndac]

No, you're not misreading. Just to make sure, I re-did all the tests multiple times today when the light was better (I needed to add non-natural light yesterday) — this time it's all natural light.

Interestingly, the 105mm sharpness rose to the same-ish levels as 65mm, although the shape of the curve stayed the same. Everything else was entirely within a reasonable margin of error from yesterday.

Anyway, although the results for my 24-105L are kinda confusing between f10 and f22, they basically confirmed that my lens is behaving like one should expect — weakest at 24mm. Time to stop obsessing over numbers and go take some photos!

[Hannes]

Your curves do look a bit odd. I would have expected them to be more like the 40mm in shape.

My lens on the other hand is sharper at 24mm than 105mm, at 24 it hardly improves on stopping down whereas on the long end it becomes sharp only after stopping down to f8. 50 and 70 are nearly as sharp as 24 but do benefit from stopping down to f5.6 when looking at 100% crops, for normal viewing it doesn't really matter. Probably just down to the good old sample variation.

[Mt Spokane Photography]

Something is wrong with the curves, a lens always loses sharpness above f/8.
 
However, lenses with decentered elements do exhibit strange curves.  That's one quick way to spot a lens that needs adjustment.
 
This are some typical  curves.  They doesn't go above f/11, but resolution continues to drop off slowly as aperture increases due to diffraction.