[pakinjapan]

What in focus at infinity on my DA*55 seems to move closer and closer with larger and larger aperture setting.

Please see the sample image below. You can right click on the images and save as to see the full resolution.

How did I take the sample images:
I turn to manual focus mode and manually turn the focus ring to the infinity mark
I didn't refocus when taking each sample.
I use Av mode and change f-stop number start from f1.4 to f8. Let camera adjust shutter speed and keep ISO fixed at 100.
I didn't adjust/compensate the autofocus. (I think it has nothing to do with the manual focus on this test.)

Questions:
1. Is it common?

2. If it is common, how can I manual focus at infinity distance on live view? (non of my older lenses acting this way.)
My usual approach is digital zoom in to 100% and manual focus on a distant object then shoot. Like the white building in the sample image. I set the camera at f8. The building is out of focus while I try to manual focus because the lens is remained wide open (f1.4). (The f8 setting doesn't stop down the lens before I hit the shutter button and I think this is common.)

3. If it is uncommon, do you think what wrong with the lens? or I miss something in the camera setting? It is still under a 1-year warranty. In fact, I just get the lens yesterday.

Thank you in advance.

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Last edited by pakinjapan; 08-26-2018 at 07:51 AM.

[stevebrot]

It appears that the infinity mark/stop on your lens is not properly set. Since you just received the lens yesterday, you might want to return it for credit or replacement since it does not attain infinity focus.

Note: Other comments may mention something called "focus shift" on stop-down. This is not uncommon on some older lenses, but is not the issue here. Infinity should be attainable wide open.


Steve

[pentageek]

This is definitely not common - last picture shows 'infinity' is somewhere between 10 and 15 meters from the camera. Even if the focus ring goes past infinity mark (what is common), something is wrong with the lens - this is one of Star lenses, and should focus properly no matter how wide aperture is set.

[othar]

It appears that the infinity mark/stop on your lens is not properly set.

+1
I don't know if you can adjust the stop easily with this lens (for Samyang lenses this is typical and not hard to fix yourself). If you don't want the hassle just return the lens.

[stevebrot]

+1
I don't know if you can adjust the stop easily with this lens (for Samyang lenses this is typical and not hard to fix yourself). If you don't want the hassle just return the lens.

Given the price point and premium designation (*) of this lens, a return/exchange with no additional postage would be the expected seller response. This is a serious and unacceptable issue.


Steve

[pakinjapan]

Thank you for all the suggestion.

I found this Spherical aberration article on Wiki, and I just want to confirm if this is the problem you guys talking about on my *55?


I will go back to the store this Saturday for a replacement.

[stevebrot]

Thank you for all the suggestion.

I found this Spherical aberration article on Wiki, and I just want to confirm if this is the problem you guys talking about on my *55?


I will go back to the store this Saturday for a replacement.

No, probably not spherical aberration. Your lens appears to fail focus at infinity. Normally one would expect near focus to fade to the distance as the aperture is opened and far focus to stay pretty much the same. Instead, what your examples show is near focus staying about the same and far focus decreasing. In human terms, your lens is myopic, "short sighted". There are a number of possible causes, but since your lens is quite sharp over all, the most likely is that the infinity stop of the focus mechanism was set a few notches short of the appropriate distance for "infinity" when it was made.

Note that I added the word "appears" with emphasis, though in this case I believe the infinity focus issue is real. If your examples had been cropped, I would have had my doubts. What we are seeing in your last example is the far limits of depth of field for your lens at its infinity setting. Normally, resolution is poor enough at maximum aperture that the far limits at "infinity" are no more fuzzy than at "infinity".

I would expect some wrinkling of the nose at this point. After all, infinity is supposed to be amazingly far away and a far limit for DOF would not be defined. That would be true for an ideal lens, but you lens was made in a factory and infinity was set on an optical bench against a target at a virtual distance that approximates infinity as the point of apparent convergence of two parallel lines. By convention a distance of somewhere between 100x and 200x the focal length is used.* Assuming that that 200x is being used, infinity would be set to 110 meters from the camera. At f/1.4 on a K-3 one would expect everything from 55m to forever to appear sharp, assuming no pixel peeping and normal viewing distance. If 100x were used, everything from 34m to ~95m should be acceptably sharp. Given the expected performance of your lens, I would expect 200x.


Steve

* I am going on memory here, so the actual distances might be something else and might be further depending on system resolution, visual acuity of the final observer, size of final image, and so on.

[torashi]

That's not common, nor normal for a new lens. Send it back and demand a new one. This one might have fallen during shipping and handling. Externally, it can be ok, but maybe the internal elements misaligned from a hard blow.

[stevebrot]

maybe the internal elements misaligned from a hard blow.

Also a likely possibility.


Steve

[pakinjapan]

I just want to say thank you all again. I went to get a replacement and do infinity test with the seller before I leave the store yesterday.

One thing I notice after left store and do a few more test on my own, the last lens need no autofocus adjusting on my K3 but the new one need +8. I still have 2 more weeks if I am not happy with this copy. Anyway, it is so far so good and happy ending now. Thank you, everyone.