I'm stuck indoors w/ a cold, so decided to test my 16-80 a bit further & learn something new. I read up on decentering, and tried to test it out.
1. is the test valid
2. what do the results mean? Am I seeing field curvature, decentering, or I think it's decentered, but would appreciate some discussion as lens testing is not my forte, obviously.
I setup the camera on a tripod w/ remote, pointing straight down as verified with level... Camera is sitting over a grid (heavy cutting mat).
Any insight is appreciated. Please note, I shot this lens in the field and thought it didn't look healthy, so the testing is just further exploration of that. I am not trying to nitpick it to death without having taken it out and used it. Reviews generally say it should have good corner performace...
I have never seen anyone testing for decentering on such a short distance, and I sincerely doubt you can be sure anything you observe is not because of the sensor and the test target not being perfectly parallel. Doesn't one usually test such things at infinity, for example using a horizon going from the top left to the bottom right or from the bottom left to the top right corner?
As for the lens: I have had a couple of copies and sent them all back. Some were clearly decentered. The two copies that were not clearly decentered exhibited what looked like extreme field curvature at large distances and with short FL. This meant subjects at infinity placed at the short borders could never be in focus at all for 16mm and f8. It's far from horrible, but also far from sharp. I am still not sure whether these copies performed as they should, as in Kurt Mungers review I see better performance. So it may be a problem with build quality and quality control. I really wanted reasonable WA performance at large distances, which is why I gave up on the 16-80, planning to try the 16-50 (which was just rumored at that time).
Most reviews stating the 16-80 has really good corner performance use imatest, I think, and at short distances the lens can be quite outstanding. FC disappears, and the corners should become really good.
OT: BTW, I have a similar problem with the 16-50 now, of which I have had two copies with truly outstanding corner performance on one side of the image. Unfortunately, the other side was rather horrible. I had decentered copies of that lens as well, btw. The construction may feel much more solid than that of the 16-80, but I have yet to find a single copy that is actually assembled properly.
Ok, so what I saw in the field out taking desert photos is that the corners were various degrees of mushy - I took about 40 photos in the set that day, all at f/8, mostly focused 1/3 of the way into the scene. I don't know alot of technical lens performance stuff, but I have quite a bit of good glass and have been shooting long enough to know when something looks off - and to me something seems off about the lens.
I had looked at Kurt Munger's site today reading up on something or other and noticed how he did some of his lens tests, with the main subject in various parts of the frame (center, mid, corner, etc.). So I decided to try it myself - moved the tripod between shots, staying parallel to my subject, which is easy with the greenhouse tile to follow. I refocused with the aperture open after I moved the tripod and used fully magnified liveview to get peak sharpness. However one side of the images comes out looking out of sharp focus. Sample below, shot at f/5.6. Lens is at ~50mm and not anywhere near MFD. I took 6 sets of photos to compare, so not just basing it on single image.
Results are generally like this (this was with my container of tea bags, the container is midway to the lower corners - see 2nd image for full view):
And it's noticably off from about the center over to the side. There is one side of the lens that I can focus very sharply.
I suspect that is why my images all had a mushy looking side. I will try a similar set tomorrow of a mountain ridge at infinity, that is easy enough to do here.
ETA: larger view of scene for reference - I like the color and contrast the lens produces:
Ok, I know interest in this lens is very low, but I'm going to post my last set of images to followup and then send the lens back.
I shot a ridge in good daylight to look at inf. focus. I shot at 35, 50, and 80mm zoom length w/ tripod, remote, etc.
35mm was OK across the frame. 50 was good in center, poor at edges. 80 was good in center, very poor at edges. It pretty much matches what I thought I was seeing out in the field in less controlled conditions.
I think it's decentered, but would appreciate some discussion as lens testing is not my forte, obviously.
I am not trying to nitpick it to death without having taken it out and used it. Reviews generally say it should have good corner performace...