Will the Real 20mm Please Stand Up

Discussion in 'Olympus Cameras' started by CWRailman, Jun 29, 2015.

  1. CWRailman

    CWRailman Mu-43 Top Veteran

    564
    Jun 2, 2015
    Scottsdale, Arizona
    Denny
    My new/previously owned Panasonic 20mm lens arrived today so I decided to play with it a bit. I set up my E-M10 and first took a few shots with the Panasonic 20mm lens then, without moving the camera, switched to the Olympus 14-42mm kit lens and set it to 20mm. This was supposed to be a comparison of which lens was sharper which is important for my use but what I found about the difference in perspectives of the two lenses became more of an interest to me. Note the crop factor difference between the two shots. In order to get a perspective/cropping similar to the Panasonic lens I had to set the Olympus zoom lens down to 17mm. (That is not demonstrated here).

    This was shot with the Panasonic 20mm lens
    Olympus%2020mm%20test%20003a.
    This was shot with the Olympus 14-42mm kit lens set to 20mm.
    Olympus%2020mm%20test%20004a.

    These are JPG images, both have been downsized the same amount for display but if you want to see the full size images.
    http://cwrailman.com/Images/Olympus 20mm test 003.jpg
    http://cwrailman.com/Images/Olympus 20mm test 004.jpg
     
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  2. nuclearboy

    nuclearboy Mu-43 Top Veteran

    850
    Jan 28, 2011
    USA
    I saw a similar thing when I was shooting lens charts with the 20mm a few years ago. I always figured the P20 was a P18.5 or something like that (at least for jpg output).

    The PL 25 and the Oly 25 also have a slightly different field of view.

    The P12-35 @12mm is also wider than the Olympus 12mm prime.
     
  3. colbycheese

    colbycheese Mu-43 Veteran

    378
    May 1, 2012
    Way up there.
    Wow i didn't realize there is such a drastic difference. I have recently noticed this with my 20mm as well. It seems a bit wider than it should be. Maybe this can justify me keeping both the 20mm and the 25mm.
     
  4. eteless

    eteless Mu-43 All-Pro

    Jun 20, 2014
    It probably has a massive case of focus breathing, the focal length is measured at infinity thus as you focus closer the focal length changes.
     
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  5. pellicle

    pellicle Mu-43 All-Pro

    Feb 10, 2010
    Southport, OzTrailEYa
    pellicle
    my first question is:

    how do you know that the focal length you were taking with on the zoom was exactly 20mm? Just by the markings printed on the plastic?
     
  6. nuclearboy

    nuclearboy Mu-43 Top Veteran

    850
    Jan 28, 2011
    USA
    I don't know about the op but I assumed the readout in the evf (and the exif data) would be correct for the zooms.
     
  7. CWRailman

    CWRailman Mu-43 Top Veteran

    564
    Jun 2, 2015
    Scottsdale, Arizona
    Denny
    While I did use the scale on the barrel to get close to 20mm or what I thought it would be, and it did seem that I was around 20mm on the zoom lens based on what I was seeing in the view finder, that was not how I verified I was at 20mm. If you download the files and check the image properties, or EXIF file depending on what you use, you will note that they both recorded 20mm.
     
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  8. agentlossing

    agentlossing Mu-43 Hall of Famer

    Jun 26, 2013
    Andrew Lossing
    Is the camera body in the same position relative to the subject in both shots? Oh right, you said it was. Does distance from sensor plane make all the difference with this, or does distance from front element play a part, possibly?
     
  9. CWRailman

    CWRailman Mu-43 Top Veteran

    564
    Jun 2, 2015
    Scottsdale, Arizona
    Denny
    It would be interesting if someone who had the Olympus 17mm lens as well as the 14-42 Zoom set to 17mm could do the same comparison. That way both lenses would come from the same company. (No I am not going to purchase an Olympus 17mm just to do this test but if someone wants to donate one to me……..) Last year I did this test using the Pentax Q series 01 fixed focal length lens which is 8.5mm and compared it to the Pentax Q series 02 Zoom set to 8.5mm I came up with almost exactly the same composition. That is why this difference surprised me a bit.
     
  10. pellicle

    pellicle Mu-43 All-Pro

    Feb 10, 2010
    Southport, OzTrailEYa
    pellicle
    Hi

    well all of that is just going to actually reproduce markings from a different place (such as where the machine readable markings are to report the position to the microprocessor) and I've found myself with my 14-45mm that there is some difference in FoV to be had with the EXIF showing the same number. You don't see 21.5, 22, 22.5 ... and so on. While you had good incidence with the Pentax as these are plastic low cost optics I wouldn't put much faith in them being accurate in that way.

    When I get home I'm going to use the FOV calculator data (which shows horizontal angle to be 46.8deg) to determine what that angle translates to in horizontal distance (at a specific distance). I'll report back what I see.

    :)
     
  11. CWRailman

    CWRailman Mu-43 Top Veteran

    564
    Jun 2, 2015
    Scottsdale, Arizona
    Denny
    You are correct, the EXIF will not report focal lengths of .5, however in our Pentax Q7 cameras, it does report the .5mm focal lengths when reviewing the image in the back LCD. However when imported into the puter the focal lengths all show up as round numbers such as 8.0mm verses 8.5 when viewed on the back LCD screen.
     
  12. tkbslc

    tkbslc Mu-43 Hall of Famer

    Two thoughts:

    1. Focal length is measured at infinity. Zooms often have a bigger variance between near and far FOV than primes.
    2. This appears to have been a very close scene. Since the zoom is about 3x longer than the prime, the 14-42 was actually closer to the scene if the camera was held at the same position. If you were close, this 2 inches or so could be 5-10% of the distance. This would also explain the apparent perspective shift.
     
  13. agentlossing

    agentlossing Mu-43 Hall of Famer

    Jun 26, 2013
    Andrew Lossing
    That's what I was alluding to (poorly it seems). At close distance small space becomes more significant, and if the focal length is determined by something other than absolute distance from the sensor plane, then that may explain what's going on.
     
  14. pellicle

    pellicle Mu-43 All-Pro

    Feb 10, 2010
    Southport, OzTrailEYa
    pellicle
    Hi
    I seem to recall it being even more obtuse with my 14-45
     
  15. tkbslc

    tkbslc Mu-43 Hall of Famer

    Interesting article:
    http://photo.stackexchange.com/ques...the-focal-length-of-a-lens-is-calculated-from

    The optical center can be almost anywhere. This means we cannot assume that because the camera is in the same spot that two lenses of the same focal length are actually being shot the same distance from the subject. And usually the only time it would be exactly at the sensor plane would be for very, very simple lens designs.
     
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  16. pellicle

    pellicle Mu-43 All-Pro

    Feb 10, 2010
    Southport, OzTrailEYa
    pellicle
    indeed interesting .... I guess that the best way to "test" the lenses then would be at a scene pointed to infinity.

    I identified back years ago that my 300mm lenses (I had an OM and an FD 300 f4) showed quite different magnification on close focus because the OM focused by extension and the FD by inner focus.

    So when focused at 4 meters from the camera the lenses each have different magnification. See below:

    The FD:
    sunflowerFD.

    The OM
    sunflowerOM.

    The Panasonic 20mm f1.7 focuses by extension (all groups moving in unison) ... so while that's a small extension it may be enough to contribute further to this.

    Many artifacts are generated by testing assumptions (which may turn out to be wrong) so my advice to the OP is to test photo more or less what you intend to use the lenses on ... or if that's harder make sure that most of the parameters are the same.

    otherwise your outcomes will not be what you may experience in real use.
     
  17. pellicle

    pellicle Mu-43 All-Pro

    Feb 10, 2010
    Southport, OzTrailEYa
    pellicle
    when trying to test my Sigma 30mm f2.8 for its usefulness as a shallow normal I decided to take along my OM with a 50mm f1.8 So as to compare it at "normal" sorts of distances I tried to use a static subject in the local mall

    So here I am hanging with my mate at the mall. I'm just a step away from him and I take this shot. Which seems a common range for using a normal...

    film739.

    I focus on his ears and use f1.8 to render the background soft and out of focus so its clear its a shot of him, not the bicycles or the lady walking towards us ... just my mate on the bench.

    Now with the Sigma the background is substantially clearer (the bicycles are now more clear as is the other bench over on the right).

    p1030739.thumb.

    But looking a bit more carefully the focus really is on his face. My bag is clearer as well as the background.
    Lets have a clearer look

    normalsDetails1.

    Yep, same angle of view, but just so much tighter in. Focus is on the face and on the ears and background diffuse on the larger aperture diameter obtained by the 50mm f1.8.
     
  18. CWRailman

    CWRailman Mu-43 Top Veteran

    564
    Jun 2, 2015
    Scottsdale, Arizona
    Denny
    That sounds plausible however if that was the rule then why does the Pentax 01 fixed focal length lens give the same perspective as their 02 Zoom when set to the same focal length? In a week when my Olympus 45 mm arrives I will test it against the Olympus 40-150 zoom and see if I get similar results. I more suspect that it’s dependant on the manufacture and if you took a 20 mm lens from another manufacture you would find the perspective again different. That is why I was wondering how the Oly 17mm would compare to the 14-42mm zoom when set to the same focal length.

    Note that I am complaining about this. The field of coverage that I am seeing from the Panasonic 20 mm suites my shooting style and with it’s apparent wider, than the Oly 14-42 zoom, perspective makes me glad that I did not opt for the Panasonic 14mm lens which I had also considered. I just found the difference in perspectives to be interesting, not critical.
     
  19. pellicle

    pellicle Mu-43 All-Pro

    Feb 10, 2010
    Southport, OzTrailEYa
    pellicle
    just shooting from the hip here, but:
    - both inner focus mechanism design
    - better marking on their zooms
     
  20. agentlossing

    agentlossing Mu-43 Hall of Famer

    Jun 26, 2013
    Andrew Lossing
    I think herein lies the answer. It's unpredictable, at least from a lay perspective, and the slight effect is magnified when close focusing.