FA 31mm f1.8 vs A 50mm f1.7 Help!

Discussion in 'Pentax' started by marius_mirea, Jan 17, 2010.

  1. Guys, I am pretty alarmed! I have done some testing in-house (with the built in flash) and I found out that my A 50mm is clearly sharper than the FA 31mm (1/100 sec, f6.3).
    Is this possible? please if someone has both lenses, do a comparative test.
    Either I have an ultrasharp A 50 or I need to pay a visit to the pentax guys where I have bought the FA 31.
    Thanks for replies!
     
  2. This is not a big deal unless the 31mm is really unsharp. Because of the design of wide angle lenses, it's much easier to make a sharp 50mm than a sharp 31mm. 50mm lenses are usually the sharpest lens in a manufacturer's catalog.
    I wouldn't worry unless you are seeing significant lack of sharpness from the 31mm that shows up in prints.
     
  3. Marius,
    We read these kind of "findings" all the time. How are you evaluating images? Are you looking at stuff onscreen? Post processed? Printed output? What camera are you using? File format, sharpening, all these things mean lots of variables.
    If you can provide more detailed info, some screen examples, etc. would help us validate (or not) your testing.
    ME
     
  4. At the short distances you are probably using, focusing errors will likely contribute significantly to any apparent lack of resolution. Mount your camera solidly on a tripod and if you have a camera with Live View activate it and zoom in as far as you can while focusing manually. If you don't have Live View you will need to focus bracket and examine your shots at full resolution to see which was focused most accurately, before comparing between the lenses.




    That said, I agree with the first comment, Pentax's 50/1.7 have traditionally been stellar in resolution, on par or even better than the 50/1.4s. Also, the 31/43/77 lens set were designed not necessarily for optimal resolution, but for optimal image quality (which includes a lot more than resolution).
     
  5. This is not a big deal unless the 31mm is really unsharp. Because of the design of wide angle lenses, it's much easier to make a sharp 50mm than a sharp 31mm. 50mm lenses are usually the sharpest lens in a manufacturer's catalog.
    I wouldn't worry unless you are seeing significant lack of sharpness from the 31mm that shows up in prints.​
    I agree with what Jim said. In order to test lenses, you need to compare apples with apples.
     
  6. Thanks for replies.
    I will do a full comparison of a test chart (well... homemade but I think it will be enough to test the sharpness) and I will put all my lenses to this test, using a tripod and same aperture, ISO and exposure time. For the money I have paid and after the reviews I have read (includingphotozone "Typical for most ultra-large aperture lenses the quality gets boosted quite a bit when stopping down to f/2.8 with the center reaching outstanding levels followed by very good+ borders. The center resolution peaks at f/4 whereas the borders reach their maximum quality at f/5.6")
    Well, I am looking for that outstandingness...:)
    I will post the results for your review also, and thanks for replies!
     
  7. >I will do a full comparison of a test chart (well... homemade but I think it will be enough to test the sharpness) and I will put all my lenses to this test, using a tripod and same aperture, ISO and exposure time.
    Be sure to test your camera/lens combination for front/back focus errors. There are charts available on the internet for doing this. A significant focus error with the 31mm can undo much of its sharpness.
     
  8. Make sure you shoot the same FoV for each lens so you can compare appropriately. If you're shooting a scene from the same distance, I'm not surprised your 50mm is sharper than the 31mm, because the magnification is higher.
     
  9. Be sure to test your camera/lens combination for front/back focus errors. There are charts available on the internet for doing this. A significant focus error with the 31mm can undo much of its sharpness.​
    Thanks Jim, I have thought of that, and could it be corrected in the repairshop? The k-x does not allow to correct lens focus errors. I will anyway try to focus it manually too.
    For now I will show some quickn'dirty shots
    00VWoo-211065584.jpg
     
  10. The kit 18-55mm
    00VWou-211065684.jpg
     
  11. DA 55-135mm
    00VWp0-211067684.jpg
     
  12. When testing lenses for sharpness, you should always use manual focus. Autofocus is simply too unreliable and introduces a second variable (the camera) into the equation.
    Klaus at photozone.de always focuses his lenses manually when testing them. Sometimes he will comment on the lens's autofocus performance, but he does not use autofocus when doing the resolution/sharpness tests.
     
  13. R.T. Dowling , Jan 17, 2010; 05:09 p.m.
    When testing lenses for sharpness, you should always use manual focus. Autofocus is simply too unreliable and introduces a second variable (the camera) into the equation.
    Klaus at photozone.de always focuses his lenses manually when testing them. Sometimes he will comment on the lens's autofocus performance, but he does not use autofocus when doing the resolution/sharpness tests​
    Good point. I have just uploaded some photos taken at f4, ISO200, 1/20 sec from a tripod using autofocus and it seems to me that the winner are the 18-55 kit or the DA 50-135!
    I will do some further testing but what I have seen until now makes me doubt a little the qualities of my copy of FA 31mm.
    We'll see....
     
  14. Unfortunately Klaus never tested the 50/1.7 in its various incarnations. He did, however, test the 35/2 and the 50/1.4, and it is interesting to compare the resolution charts of those two lenses to the 31/1.8.
    To summarize:
    At f/2, the 35/2 is the sharpest of the bunch.
    At f/2.8, the 31 is just slightly sharper than the 35/2, but not by much. It is much sharper than the 50/1.4 at this aperture.
    At f/4, all three lenses have nearly identical results. The 31/1.8 wins, but only by a hair.
    I would expect a 50/1.7 to perform very similarly, and based on the results from the 35/2 and the 50/1.4, I am not at all surprised that your 50/1.7 is producing such sharp images. It is a very good lens.
    Still, doing another test would not be a bad idea, and focusing both lenses manually will remove the camera variable from the equation.
     
  15. It looks like your 40 and your 31 may be exhibiting a bit of front focus or back focus. Definitely try again with manual focus; the results should be better.
    It's too bad that the K-x doesn't allow focus adjustment.
     
  16. For me it is still puzzling how the 50-135 which is a zoom lens could be clearly sharper than the FA31? I will go for sure to the Pentax shop to have this lens checked.
     
  17. Miserere Mei [​IMG], Jan 17, 2010; 05:01 p.m.
    Make sure you shoot the same FoV for each lens so you can compare appropriately. If you're shooting a scene from the same distance, I'm not surprised your 50mm is sharper than the 31mm, because the magnification is higher.​
    I took care of that, too, thanks for input
     
  18. R.T. Dowling , Jan 17, 2010; 05:24 p.m.
    It looks like your 40 and your 31 may be exhibiting a bit of front focus or back focus. Definitely try again with manual focus; the results should be better.​
    Thanks for the input, I will try some "focus bracketing". Anyway, for the 1400 USD I have paid for the FA, I expect to see "prime-like" performance
     
  19. I have redone the quick tests with a better lightning and a shorter exposure time (1/30), hand held and the results are as follows. I think it was a false alarm after all, although the results from the tripod shooting were a bit ... unresting.
    00VWs2-211103584.jpg
     
  20. Thanks all for the replies, I will try anyway to do a proper home-test with my lenses...and post it on the forum.
     
  21. There is another factor, and very hard to compensate for in a home test. That is distance. It is possible, being that the 31mm is a wide angle lens in its original 35mm film body concept, that it may not be optimized for its very best performance at close distance. The 50mm lens is not wide angle at all, and may perform better at close distance. Of course, when performing a home test, in order to approximate the same image size, it is necessary to move in closer with the 31mm LTD than with the other lenses.
    The photozone procedure is for lines of resolution, which I understand is done at long distance focus only. The older Pop Photo testing was done using film, at long, medium ( field test) and also close focus distance. Certainly a more thorough procedure than testing as done now.
    One good example of this aspect is the old Pop Photo test of the 43mm LTD. This lens is justly held in very high regard, but occasionally with some reservation about being somewhat soft wide open. Well, one poster on this forum a while back custom adjusted his camera's focus for this lens, and it produced very sharp images wide open. But the interesting thing is, that when the old Pop Photo test protocol did their close focus test of this lens, the results about knocked the socks off the lab tester with outstanding lines of resolution, even very good wide open.
    Using a tripod, why not do your test shoot using something much larger? Like perhaps a building with plenty of detail. Or maybe a billboard. You'd still have to move in more with the 31mm LTD compared to the 50mm, but at least both would still be at some distance.
     
  22. "shorter exposure time (1/30), hand held"​
    I don't think tests of this kind should be done hand-held. Not only should it be on a tripod but also with mirror-lock-up engaged (2-second self timer) which should also disable SR. Also these shots are all underexposed--shouldn't the white backgrounds of the cards be white?
    Also, I assume these are 1:1 pixel crops? From the center of the image or borders?
     
  23. Actually, there is a way to get the autofocus adjustment mode on the K-x. Instructions are here:

    http://www.scribd.com/doc/23723765/Pentax-Kx-Pentax-K-x-AF-adjustment-through-undocumented-debug-mode

    I would clarify the instructions a little bit:

    1) Make sure you hit return after the line of text inside the file.
    2) Make sure you save it as "MODSET.492", not "MODSET.492.txt" (but upper or lower case doesn't seem to matter)
     
  24. Marius,
    There are a lot of methodology holes in your approach here. Andrew G. is spot on. Hand-held has way too much margin of error. Also, at least in your original post you noted that you are using the built-in flash. I'm assuming that this will trigger a P-TTL protocol and the lighting will not be as uniform for each lens. Thus you will be examining images lighted differently, especially if you are maintaining standard aperture and shutter settings for lenses of differing focal lengths. That can affect perceived focus. Since the area seems quite dark to me, how can you be sure that you focused each lens by hand correctly to begin with? If you have to do this kind of testing indoors, use stable, consistent modeling lights, the camera on a tripod, MLU, and a remote control. The MLU will disable anti-shake. And as Michael K. says, try to equalize the target for both focal lengths. This difference alone, is sufficient enough to make me question any judgments about sharpness here.
    This may help you determine if you have a fine lens here (and to me the odds of that are very very high): read Mike Johnston's review of the DA 35mm that was posted on PNet in August 2008. I think Mike is one of the most coherent photography writers anywhere. Ignore the specific findings about that lens (though I agree with Mike), for your purposes, I would examine how he evaluates the lens. He functionally tests for factors besides simple sharpness. Then read his article about How to Stress a Camera Lens. May I suggest that you then take the 31mm outside and follow Mike's steps, taking notes along the way to help your evaluation? My guess is that you will better appreciate what a fine tool you have purchased.
    ME
     
  25. I engaged in a similar exercise in the delirium of cabin fever last winter. I came away with some vague, probably half-baked, impressions regarding relative sharpness but I learned a lot about shooting technique and lens qualities. My key finding was that, despite my attempts to control variables, there were significant differences between shots using the same lens with the same settings at the same distance: in essence, that my relationship with the gear was fairly dynamic, even in controlled circumstances. I came away with a deepened appreciation of the need for multiple takes of the same shot whenever possible and a sense that lenses need to be evaluated in terms of their suitability and performance in real world shooting situations.
     
  26. Marius: to me, none of the shots you posted seem very sharp to me. They also seem shot in low light and underexposed quite a bit. The information that Andrew asked for is also important - are these 100% crops or shots that were resized? For proper comparison, you should focus on the same area and then post 100% crops of that area.
    Frankly, while the K-x is a great camera, its viewfinder is not that nice to use for manual focusing in low light. I was thinking of recommending focusing via Live View, but due to the smaller resolution of the LCD screen, I didn't find that to be as precise as it could be either.
    I think the simplest test is to use as target some piece of cloth. Shoot it at an angle and you'll certainly get some part of it in focus - look for that area and then you can test how much detail you captured about its threads. To eliminate camera shake, put it on a tripod, use a remote shutter, and set the shutter delay so the mirror is raised some time before the image is exposed.
    Hope this helps.
     
  27. Thanks all, you have been very helpful. In the meantime I have used the lens more extensively and I think it is a good copy :)
    I admit that my testing methods were ...let's say... approximate and there is more to a good photo than sheer sharpness.
    Anyway I will try sometime whet I'll have the time to set up a more proper test but not to evaluate the FA 31mm (which now I am now convinced has no fault) but to try to compare all my lenses, because I think it would be very interesting to test a very different bunch of lenses and see how they compare.
    Overall I am very impressed by the speed and the usefulness of the responses I have got and I think that this forum is among (if not the most) friendly and usefull for a dslr beginner like me
    Cheers to you all!
     
  28. You can easily check the photozone website, which runs testing under very well-controlled lab conditions. They will provide specific lines of resolution figures for lenses.
    Actually, Hin Man reports that in shooting high-quality JPEG images with his K-x, the default results are slightly soft. He has increased sharpness setting by a notch or so, which I believe will mean +2, as the default bright mode would likely already be at +1. The camera is deliberately engineered this way to allow more leeway for adjustment.
     

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