Is Canon EF-S 18-135 STM lens too soft in large apertures?

Please look at the following side-by-side comparison of exposures of my 18-135 STM lens in two different apertures (100x100 crop). As you can see, it is too soft in f/3.5 (the largest available aperture). I repeated this test with different focal lengths (50mm and 135mm) and images taken with the largest apertures are always too soft. I could not find any review/sample image on internet that confirms such a big difference in sharpness. Is this normal?

Left: 18mm f/3.5 1/100 Sec. Right: 18mm f/5.0 1/50 Sec.

Yes, such zoom lenses are always softer wide open. But unless you print really big, you will not see it in prints.

saisai wrote:

Yes, it is soft. The sony DT 18-135 SAM is much sharper than the canon's.

I'd be surprised if Sony's 18-135 is much better than the 18-135 STM. That would make it the first decent performing basis Sony lens.

Do not expect any lens in this class to be perfect wide open. From my short experience with the STM I found it to be quite capable though, definitely better than the non-STM 18-135. Fairly decent centre and not to bad corners either. Stopping down did help to improve sharpness, but mainly on the edges and in the corners.

If I were you I'd try to make some pictures with contrast detect focus, which means in live view mode. In that case you rule out focus errors (front or back focus). Compare the shots again after doing so.

Regards, Sandor.

Are those centre, edge or corner crops ?

massud wrote:

maarensv wrote:

Are those centre, edge or corner crops ?

center

OK, I expect those to be better than the samples you showed.

On the other hand, how many pictures do you take with aperture wide open ?

You showed only 18mm shots. How about the mid and tele FL ?

Regards, Sandor.

massud wrote:

maarensv wrote:

massud wrote:

maarensv wrote:

Are those centre, edge or corner crops ?

center

OK, I expect those to be better than the samples you showed.

On the other hand, how many pictures do you take with aperture wide open ?

You showed only 18mm shots. How about the mid and tele FL ?

Regards, Sandor.

I have taken more than 50 sample shots in different focal lengths. Always same results. Here are two more samples in 50mm and 135mm.

50mm f/5

135mm f/5.6

the 135mm is almost sharper than the 50mm, should be the other way round.

I presume you did live-view focusing with 10x.

It does look like your lens is softer than expect from e.g. photozone tests

http://www.photozone.de/canon-eos/776-canon_18135_3556stmis?start=1

i.e. at 18mm in the center, it should already be quite sharp.

photonius wrote:

the 135mm is almost sharper than the 50mm, should be the other way round.

I presume you did live-view focusing with 10x.

It does look like your lens is softer than expect from e.g. photozone tests

http://www.photozone.de/canon-eos/776-canon_18135_3556stmis?start=1

i.e. at 18mm in the center, it should already be quite sharp.

From my (short) experience I think both wide and tele should have been sharp in the centre wide open. Photozones test corresponds with that. It looks to me that your 18-135 STM is under par.

Regards, Sandor.

massud wrote:

Thanks. What a disappointment. Can this be a lens calibration issue?

Don't really know, but doubt it that calibration is the issue.

Maybe you can try another copy of the 18-135 somewhere ?

Regards, Sandor.

massud wrote:

maarensv wrote:

photonius wrote:

the 135mm is almost sharper than the 50mm, should be the other way round.

I presume you did live-view focusing with 10x.

It does look like your lens is softer than expect from e.g. photozone tests

http://www.photozone.de/canon-eos/776-canon_18135_3556stmis?start=1

i.e. at 18mm in the center, it should already be quite sharp.

From my (short) experience I think both wide and tele should have been sharp in the centre wide open. Photozones test corresponds with that. It looks to me that your 18-135 STM is under par.

Regards, Sandor.

Thanks. What a disappointment. Can this be a lens calibration issue?

in the first (18mm) image it looks like the focus could be behind the branch.

but you said you did life-view focusing at 10x (on tripod and 10 sec timer or remote ??) so front or back-focusing should not be an issue?

You might have a poor copy.  Take a look at my 85mm 1.8 prime shots (I don't own a single zoom lens except on MFT currently because I got tired of finding *good* quality zooms).

My wide open f1.8 shots do not degrade as much from f1.8 to f4.0 and that is saying a whole lot.  It only sharpens up significantly in extreme corners of FF area.

massud wrote:

photonius wrote:

massud wrote:

maarensv wrote:

photonius wrote:

the 135mm is almost sharper than the 50mm, should be the other way round.

I presume you did live-view focusing with 10x.

It does look like your lens is softer than expect from e.g. photozone tests

http://www.photozone.de/canon-eos/776-canon_18135_3556stmis?start=1

i.e. at 18mm in the center, it should already be quite sharp.

From my (short) experience I think both wide and tele should have been sharp in the centre wide open. Photozones test corresponds with that. It looks to me that your 18-135 STM is under par.

Regards, Sandor.

Thanks. What a disappointment. Can this be a lens calibration issue?

in the first (18mm) image it looks like the focus could be behind the branch.

but you said you did life-view focusing at 10x (on tripod and 10 sec timer or remote ??) so front or back-focusing should not be an issue?

As you said, the photo was taken in live view mode on a tripod. I am pretty sure it's not a focus issue. I tested the lens using a focus test pattern (45 degree) and there is no sharp area in the photo. Also, turning IS off did not help.

hmm, you should try another copy then.