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25 and 12-35 comparison, odd DOF difference

Discussion in 'Native Lenses' started by Kiwi Paul, Jul 24, 2012.

  1. Kiwi Paul

    Kiwi Paul Mu-43 Top Veteran

    729
    Aug 15, 2011
    Aberdeen Scotland
    Out of interest I compared the 12-35 and PL25 1.4 lenses at 25mm FL on Sunday.
    For most part the differences weren't too dramatic, the 25 wins overall, just better edge sharpness basically otherwise take your pick.

    However one anomaly that I noticed when viewing at 100% was the difference in DOF even though all settings were identical (focal length, aperture, shutter speed, ISO and same focus point).
    Here's an example, these are 100% crops of the original shots, both shot at 25mm, f5.0, 1/2500 sec, ISO160.

    PL25 is first, 12-35 second. Notice how the 25 has much wider DOF for the same aperture.
    Not sure why this is, I took 3 shots of the same scene with each lens and it was the same every time.

    Any ideas, just food for thought really.



     
  2. pxpaulx

    pxpaulx Mu-43 All-Pro

    Jan 19, 2010
    Midwest
    Paul
    I think the focus is just off (or rather, different). The 25 to me is properly focused, with the in-focus area starting just in front of the birds, while with the 12-35mm it is starting at the saturn wording on the boat (at least, perhaps closer). There may also be a small amount of shake in the 12-35mm image, to my eyes at least, which gives a slight appearance of being out of focus...which is kind of the same as blurred I guess :)

    The 25mm image overall just looks sharper and more crisp, whether that is blur or shake in the zoom I can't exactly tell, but either way the 25mm is the better shot.
     
  3. Kiwi Paul

    Kiwi Paul Mu-43 Top Veteran

    729
    Aug 15, 2011
    Aberdeen Scotland
    The focus points were the same, as I said I took 3 separate shots of the same scene with each lens and it's the same every time. There's is no shake the shutter speed was 1/2500 enough to eliminate any potential camera shake blur.

    Paul
     
  4. David A

    David A Mu-43 All-Pro

    Sep 30, 2011
    Brisbane, Australia
    If the 12-35 isn't as sharp as the 25, things that are equally in focus will look softer with it than they do with the 25. Depth of field will be the same, but the point of focus won't look as sharp, and things not at the point of focus but still within the depth of field won't look as sharp with the 12-35 as they do with the 25. The point of focus will still be the sharpest point for both lenses.

    Are you confusing a difference in sharpness with a difference in depth of field?
     
  5. Kiwi Paul

    Kiwi Paul Mu-43 Top Veteran

    729
    Aug 15, 2011
    Aberdeen Scotland
    No, if you look behind the boat, the ropes and mooring buoys and seaweed are all much softer (i.e. going out of focus) in the 12-35 shot caused by a shallower depth of field (DOF), it's like the 12-35 shot was taken with a larger aperture.
    Both lenses are very sharp in the focus zone.

    Paul
     
  6. MajorMagee

    MajorMagee Mu-43 All-Pro

    Feb 24, 2011
    Dayton, OH
    How far away were you from the focus point? The closer you were the more sensitive the depth of field is to having the focus point being slightly different and throwing off your test.

    The hyperfocal distance with those settings is 27.2 ft so with an out of focus background we can tell that it had to be something less than that.
     
  7. CPWarner

    CPWarner Mu-43 Veteran

    244
    Dec 24, 2010
    Cliff
    Great timing. I just was out on my deck comparing my 12-35mm to my 12mm, PL 25mm and both the Olympus 45mm and PL 45mm. Too many lenses, I know. I shot everything on a tripod with manual focus at the identical point. What I found was that the focus by wire can seem to give you the same point but in reality it may be further out or closer in than you think. I also found that the PL 25mm seems to focus further out than the 12-35 in my tests. I did not see any significant differences in depth of field, but subtle changes of where that range was in the photo. My guess is that my observations are due to focusing technique. I may not have approached focus from the same direction and therefore biased the range one way or another. With this focus by wire, it is a bit tricky.

    I did see differences in how smoothly the lenses moved in and out of focus. Interesting.
     
  8. ~tc~

    ~tc~ Mu-43 Hall of Famer

    Oct 22, 2010
    Houston, TX
    So, where are the results?

    Is this lens truly a "bag of primes"?
     
  9. CPWarner

    CPWarner Mu-43 Veteran

    244
    Dec 24, 2010
    Cliff
    Not ready to publish those yet. However, it is quite good. :)
     
  10. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    Looking at the full frame, is the FOV identical? Perhaps the zoom wasn't really at 25mm? Or the magnification at the marked focal length is a bit different?
     
  11. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    David may be on to something. DOF is a perceptual thing. There's no "absolute" DOF, regardless of what all the tables and web sites say.

    If the zoom is less sharp overall, it will be less sharp everywhere within the depth of the photo. So the point at which the level of sharpness deteriorates to a point where you no longer consider it in focus will occur closer to the point of focus.

    More technically, "sharp" for most people is defined as a Circle of Confusion (CoC) of about 0.2mm on the viewed image, whether it's on a print or a screen.* At the exact plane of focus, the CoC is (theoretically) zero. As subjects in the photo get further from the actual plane of focus, the CoC gradually increases. At some point is exceed the threshold of sharpness and we judge it as out of focus.

    Now compare two lenses. One is much sharper than the other. The sharp lens may have a CoC at the plane of focus very close to zero. Say, .002mm. The bad lens might have a CoC of .01 (extreme example) due to various lens aberrations.

    The actual sharpness of both lenses will fall off by the same amount as your subjects get further from the plane of focus. Just as an example, say that at 5 ft behind that plane, the "good" lens produces a CoC of .007, the poor lens a CoC of .015. Both are still below the threshold of .02mm, and appear to be "sharp," or within the depth of field. Move another 5 feet back, and say the good lens makes a CoC of .012, the poor lens .02. Another 5 feet, the good lens produces .017, the bad lens .025.

    Bingo: at 15 feet behind your plane of focus the sharp lens still appear to be sharp. The "poor" lens appear to be out of focus. Same FL, same aperture, different subjective DOF.

    I don't know if this is the answer, but it's possible. I'm sure the PL 25 is sharper at the plane of focus than the zoom, so it's going to be sharper at any distance in front of and behind the zoom, too. (Assuming the same FOV, aperture, and camera to subject distance.)

    *This means that the great the magnification of the print or screen image, the shallower the apparent DOF. Standard DOF tables from the 35mm era typically assumed an 8x10 print. Larger prints, or images shown on large monitors (or at 100%), will also show less DOF than the standard tables show. DOF tables are useful for comparing lenses, but not so useful in determining actual DOF unless they take total magnification into account.
     
  12. CPWarner

    CPWarner Mu-43 Veteran

    244
    Dec 24, 2010
    Cliff
    I am not seeing differences in sharpness in my tests of the same lenses. DOF seems similar as well. What I do see is a difference in how the two lenses react to manual focusing, and what ends up in focus.
     
  13. D@ne

    D@ne Mu-43 Top Veteran

    593
    Feb 23, 2012
    Toronto
    I compared these two as well...they looked consistent, which is all I cared to know. I do see what the OP is seeing in the photos posted...I have no idea why this is, but I'm happy enough knowing that a zoom(!) is comparable to a superb prime.
     
    • Like Like x 1
  14. jloden

    jloden Mu-43 Hall of Famer

    May 15, 2012
    Hunterdon County, NJ
    Jay
    I haven't compared my 12-35 f/2.8 and 25mm f/1.4 directly like this for DoF, but I'm with D@ne. Just the fact that this zoom compares with the primes I own for sharpness and IQ makes me a pretty happy camper :thumbup:
     
    • Like Like x 1
  15. fredlong

    fredlong Just this guy...

    Apr 18, 2011
    Massachusetts USA
    Fred
    I can't see any difference as I'm looking at this on my iPod. But I have a thought anyway.

    Some lenses transition from sharp to unsharp very smoothly, others have a sharper transition. This sharper transition can look like greater depth of field. Modern, highly corrected lenses are more likely to exhibit this characteristic.

    Just a thought.

    Fred
     
  16. Psalty

    Psalty New to Mu-43

    6
    Jun 27, 2012
    Can someone that has both the O12 and 12-35 comment on the difference in video autofocus between the two? The O12 is such a great performer in this regard on my OM-D. I'm curious how the 12-35 stacks up.
     
  17. Kiwi Paul

    Kiwi Paul Mu-43 Top Veteran

    729
    Aug 15, 2011
    Aberdeen Scotland
    Thanks for the replies, the camera reports the same focal length for both lenses. I used the same focal point for each shot so in theory both lenses should have focussed on the same spot.
    I think I need to repeat this with the camera on a tripod and manually focus to ensure accuracy.
    I was only curious if anyone else had experienced anything like this, I was wondering if maybe a zoom lens with more lens elements had different characteristics to a prime of the same focal length or something like that. I did take multiple shots of the same scene with both lenses and each time the results are consistent so any focusing errors etc should have been accounted for.
    I'm not really bothered but thought it was interesting food for thought.

    Here's the 2 pics with no cropping, hard to tell the difference now.

    Paul

    25mm....



    12-35

    P1040005.
     
  18. Kiwi Paul

    Kiwi Paul Mu-43 Top Veteran

    729
    Aug 15, 2011
    Aberdeen Scotland
    I'd just like to add that I well impressed with the 12-35, it's an excellent lens, sharp, detailed, good resolution and contrast and very well built with a good "feel" to it.
    I was merely curious how the PL25 and 12-35 compared, one thing comparing these 2 lenses has revealed is that while the 12-35 is truly great, the 25 is still the better lens.....(well at 25mm anyway lol) but in my view really only because it has better edge quality.

    Paul