Can this strong directional blur at wide apertures still be explained by the usual arguments?

Question

I recently bought a used Canon EF 24-105mm f/4 IS II USM from MPB. I'm pretty happy with it, except when using it wide open where it's very blurry.

Now, I understand that lenses (especially zoom lenses) don't have their best sharpness at the widest aperture. That's why I initially dismissed my concerns and assumed "that's just the way it is". Add to that that I'm an amateur with not exactly the most advanced camera (it's a 14 year old EOS 600D). I assumed that if this was out of the ordinary, surely MPB's tests would've shown that?

But today I decided to compare the lens to two other (much cheaper, and both 10+ years old) lenses that I have; a Canon EF 50mm prime and a Sigma 50-200 / 4-5.6.

And the difference is so huge that I'm starting to think this maybe can't be explained away as "that's just the way it is":

All three are center crops of shots made at 50mm, f/4, on a tripod. (These are using the autofocus, but even manually it doesn't get any better.) Happy to provide uncropped or even RAW files if that's useful.

I also noticed that there seems to be some directionality to the blur -- it almost feels like motion blur. So I checked if changing the camera orientation changes things, and it does:

Clearly the blur is more vertical in the left image and more horizontal in the right image -- so I guess there's something in the lens that's happening top-to-bottom?

Once I stop down the lens, the sharpness seems fine; at f/8 I don't have any complaints.

I also thought there might be something wrong with the image stabilizer, but turning it off doesn't make a difference. Different shutter speeds also doesn't seem to have an effect.

To be clear, I'm not exepecting razor-sharp focus with the equipment that I have, just something reasonable.

So, here are my questions:

• Is this blur still in the realm of what I should expect in terms of variability? Just a tradeoff I have to make?
• If so, do you have any tips for shooting and/or editing to deal with this as best as possible when I need the faster aperature?
• If not, any idea of what the problem might be and if there's something I could do myself, before I call in the warranty?

Answer 1

That appears to be a pretty bad case of a decentered element.

Here's a simple test using a siemens star; you don't even need to print it, it can be displayed on your monitor.

Answer 2

Is this blur still in the realm of what I should expect in terms of variability? Just a tradeoff I have to make?

No. Not even remotely.

If so, do you have any tips for shooting and/or editing to deal with this as best as possible when I need the faster aperture?

No. The lens should be returned or repaired. That is nowhere near expected performance for that lens.

If not, any idea of what the problem might be and if there's something I could do myself, before I call in the warranty?

Warranty? It it were me I'd be exercising the return policy. I'd want nothing more to do with that lens. Something bad has happened to it. It's either been damaged and misaligned by a very hard bump or someone who had no idea what they were doing opened it up for whatever reason and didn't make adjustment marks so it could be reassembled properly. (I'm guessing the latter, and probably to clean dust or something worse - such as fungus - off an interior lens element.)