Funny 50mm 1.8 behavior (long)

Discussion in 'Lens Lust' started by Removed User 2, Jul 4, 2005.

  1. That 50mm lens is a tricky one. I have a love/hate relationship with it.

    Okay, here's the deal:


    I shot a same scene with the 45P, 50 1.8 and 28-70 2.8 set at 45 and at 50mm. All from the same spot. All on tripod and on manual setting, all shot at f2.8.

    Results (and surprise at the end):

    -the 50 1.8 clearly overexposes a bit. Colors ok. Somewhat washed-out. Sharpest.

    -45P Excellent colors and it seems it can contain information in the high lights much better then the 50. Sharp.

    -the 28-70 has same colors as the 45P, which are excellent. Also holds information in the highlights very, very well. The lest sharp but very acceptable.

    Now the surprise:


    The 50 1.8 set at f2.8 (like both other lenses) clearly shows less DOF. A whole stop less I would say. Also, the bokeh is much creamier.

    This is technically impossible. From a same spot, all f2.8 lenses set at 50mm shall have the same dof. What gives?

    I even went much further away with the 50mm in order to match the 45P and I even went further back and still it had less DOF.

    There's something going on and I can't explain it. I looked at all the possibilities and still, that 50mm lens produces pictures with less dof.

    Conclusion:

    The 50 is VERY sharp. Colors not as good as 45P and 28-70. I DO recommend it for portraits, however. It's bokeh seems better then both other lenses. The lens itself cannot handle highlights as well and it overexposes (shot a same scene on same manual exposure as other 2). Blown highlights where there shouldn't be. It's not a contrast thing either. No sticky blades by the way.

    The 45P has much more "crunch" and is very sharp. It has that comething I can't explain but it's not in the bokeh. It's its overall rendering of an image: Powerful colors and crunch. There's that "something" unexplainable but very present.

    The 28-70: Very simple: Same as the 45P (same colors, same "crunch") but less sharp wide open.

    The 45 and 28-70 Show the SAME depth of field (while the 50 1.8 shows substantially less at same aperture! and no, it's not a sticky blades problem).

    So I think I finally UNDERSTAND why I always had this love/hate relationship with that 50 1.8. Never could really point it but now it's done. Still highly recommend it. Bokeh is excellent IF you don't have pin light sources directly in the frame.

    ---------------------------------------------------


    Can someone also test their 50 1.8 set at f2.8 Versus their 28-70 or 17.55 set at 50mm?

    Very curious...
     
  2. Chris101

    Chris101

    Feb 2, 2005
    Arizona
    Without seeing the photos and doing the shooting, please allow me to speculate.

    What you are seeing as DOF is made up of two parts, focus and contrast. We know that even at a small aperture, only a single distance - one plane in the 3D space you are photographing, will actually be in focus. There are cones of ever decreasing focus emminating in front and in back of this plane.

    Generally, the smaller the aperture, the narrower these cones are, so that it appears the depth of the plane that is in focus expands to take in more of the scene.

    The contrast (and to an extent, abberations of the lens) however can modify the focus in the cones. Very high contrast will illustrate the oof nature of parts of the subject within the DOF, but outside the plane-of-exact-focus. Lower contrast will not be as exact in showing that points near (but not on) the plane of exact focus are actually out of focus.

    This explains how a lower contrast lens (such as a zoom) could show more apparent DOF than a contrasty lens like the 50. I think your 45 is probably just magic, and thus outside the explanations of science. But that's just what I think (and it has been rumored that I am "crazy as a loon.").
     
  3. MontyDog

    MontyDog

    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
     
  4. Since I'm working now, all I could do on the spot was a quick test. Still the differences show.

    All shot manual from the same spot and focus on the same spot also.

    Here is the 45P lens@2.8.
    Good bokeh but what I really notice is the "crunch" (sorry, no other way to explain and no technical terms. I'm not Bjorn nor a measurebator usually)
    [​IMG]

    Here is the 28-70 set at 45mm@2.8
    Looks like the 45P heh?
    [​IMG]

    Here is the 28-70 set at 50mm@2.8
    [​IMG]

    Here is the 50mm@2.8
    Please notice the whites aren't well controled and they almost get hot. Much more then with the other 2 lenses. Also, the dof is less.
    [​IMG]

    Here is the 50mm backed up to match the 45mm view.
    This is the most interesting. I went further back and still, the depth of field seems to be less then with the 45P lens. This fov is less then 45mm fov too.
    [​IMG]

    My conclusion is: the 50mm has creamier bokeh (!?). What else? The focus was spot-on on all the shots.
    I can understand the zoom lens is bigger (so shorter focus distance) but that would only translate in shorter depth of field. Wouldn't it?

    I'm not a measurebator and I really don't actually care that much but having a conversation and understanding lenses in general is always great IMHO.

    At last, I understand this test can absolutely be seen as invalid but in all the situations/tests I have done there is a constant: The 50mm can't contain highlights as good and it seems to have either less DOF or it has a creamier bokeh (when there are not pin light sources in the pictures).

    Please share your opinions :D
     
  5. If anything, do you at least see the almost total loss of informations in the pics shot with the 50mm?

    I've noticed the 50mm lens is set at f2.7 (versus 2.8) but quite honestly I can't believe this is the reason.
    I'm not ready to call it overexposure. Contrast? Maybe but the 45P lens has much better contrast and it's still able to keep informations in the highlights much better.

    Maybe the coating... Might it be the lens coatings?

    Bjorn, where are you? :D (Ok Bjorn, just don't hurt me too much with your criticism, play it cool will'ya? :wink: )
     
  6. nfoto

    nfoto Guest

    First of all, the only valid comparison is that with all lenses set to the exact same distance. Otherwise the perspective will be different and hence DOF also would differ. Now, this would seem an easy task to accomplish, but for the short focused distance used here, matters become a little more complex than so. It is the distance from the *front* nodal point to the subject that needs to be equal, and unless this location is known, the distances involved will not be equal and neither will DOF. For ordinary picture taking, the difference is negligible, but not for close distances.

    Secondly, while the DOF equation states the DOF zone will match if the front nodal point to subject distance is equal and apertures are the same, it doesn't say anything about *how* the out-of-focus zone will be rendered. As we all know, there is this elusive concept of "bokeh" and this might confuse the interpretation of what really is in focus. Remember there is no such thing as an abrupt change from in-focus to out-of-focus, the transition is very gradual and where the border is drawn is subjective at such relatively small apertures (For really fast lenses f/1.0 or better, the subject is either in or out of focus, the transition is very abrupt).
     
  7. MontyDog

    MontyDog

    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
     
  8. nfoto

    nfoto Guest

    Just a quick correction: Nikon does publish all the optical data, including nodal points, for their large-format lenses. I have used this data to good advantage for calcuating exposure compensation for my close-up work with some of these lenses. When you shoot with 4x5" sheet film, you simply cannot afford to fumble around to get the perfect exposure.
     
  9. MontyDog

    MontyDog

    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
     
  10. I'm kind of impressed that the 28-70 held up so well against those fine prime lenses... particularly at f/2.8, since its best range doesn't start until f/4.
     
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