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Quantifying the Differences in "25mm" Lens Fields of View

Discussion in 'Native Lenses' started by Turbofrog, Dec 8, 2015.

  1. Turbofrog

    Turbofrog Mu-43 Hall of Famer

    Mar 21, 2014
    It's been noted already that the different 25mm lenses in the system have slightly different fields of view, with the Olympus 25mm/1.8 having a wider field of view than the others. The question is, what is the "true" focal length that all the 25mm lenses have?

    I noticed that in theory we already have all the information that we need to calculate the "true" focal length, at least at one focus distance, and that's the minimum focus distance, thanks to our handy Optics 101 formula: Focal Length = Working Distance / (1 + (1 / Magnification))

    Here's where it gets weird, though. Using the published data, the results are a little bit out there:

    Olympus 25mm/1.8
    MFD: 238.8mm
    Magnification: 0.12x
    FL: 25.6mm

    Panasonic 25mm/1.7
    MFD: 250mm
    Magnification: 0.14x
    FL: 30.7mm

    Panasonic 25mm/1.4
    MFD: 300mm
    Magnification: 0.11x
    FL: 29.7mm

    Voigtlander 25mm/0.95
    MFD: 170mm
    Magnification: 0.26
    FL: 34.7mm (!)

    I imagine that there must be some form of focus breathing happening with the Voigtlander especially due to some floating elements or something, but the results are kind of puzzling to me. Especially since they all claim the exact same 47 degree FoV (which is clearly not true).

    As a bonus, I checked the Sigma and Panasonic 30mm lenses, since the "true" FLs seemed to be clustered around there.

    Sigma 30mm/2.8
    MFD: 300mm
    Magnification: 0.125x
    FL: 33.3mm

    Panasonic 30mm Macro
    MFD: 105mm
    Magnification: 1
    FL: 52.5mm

    So obviously the macro is doing special stuff internally to get to 1:1 (which also explains why the lens is 64mm long perhaps?), but it's curious to see the Sigma 30mm so close to the calculated FLs of the Panasonics and the Voigtlander.

    Anyone got a Sigma 30mm and one of the other lenses to test out the hypothesis a bit more?
     
  2. Brian Beezley

    Brian Beezley Mu-43 All-Pro

    When using data accurate to just two significant digits, results may vary enough to make comparison difficult. For example, presumably the magnification of the 25/1.4 is between 0.105 and 0.115. The focal lengths calculated for these two values are 28.51mm and 30.94mm. If minimum focal distance has 10mm precision, which the figures other than for the 25/1.8 suggest, combining that with the magnification precision yields a range of 28.03mm to 31.46mm. This assumes all figures are rounded to the nearest value at the intended precision. But I can imagine that someone responsible for publishing a minimum focal distance figure, for example, might truncate a measured value of 307mm to 300mm rather than round it to 310mm.

    Brian
     
    Last edited: Dec 9, 2015
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  3. wjiang

    wjiang Mu-43 Hall of Famer

    MFD is the probably worst end to measure this, since many lenses rely on automatic extension (i.e. focal length changing) to achieve close focus.
     
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  4. Brian Beezley

    Brian Beezley Mu-43 All-Pro

    While true focal length values are interesting, I find a visual comparison of a scene as captured by several lenses to be more compelling. You can immediately grasp the significance of any differences in field of view. Lately though, the figure that has captured my attention most when comparing 25mm lenses is $99.

    Brian
     
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  5. b_rubenstein

    b_rubenstein Mu-43 All-Pro

    Mar 20, 2012
    Melbourne, FL
    Measured focal length is done at infinity focus distance. The different designs for how a lens can be focused renders focal length at MFD meaningless, except for very close focus distances.
     
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  6. Turbofrog

    Turbofrog Mu-43 Hall of Famer

    Mar 21, 2014
    Yes, I figured with the floating elements that the focal length at MFD would vary a bit from that at infinity. But I wonder if the relative relationships between them is similar, as at least in the case of the AF 25mm lenses it seems to bear itself out even at infinity. I totally agree with all the caveats that you guys have pointed out.

    Without the actual lenses in front of me, I just don't have a better way of trying to quantify the real field of view.
     
  7. kingduct

    kingduct Mu-43 Veteran

    298
    Oct 12, 2013
    I appreciate the effort, even if the results aren't too helpful.

    It would be really nice to have a reliable index for lenses, because this could matter for some people. For example, I own the 20mm and am happy that it turns out that the new Panasonic 25mm I just bought is relatively narrow compared to the Olympus. It means the lenses won't be quite as close to each other in focal length as otherwise.
     
  8. Turbofrog

    Turbofrog Mu-43 Hall of Famer

    Mar 21, 2014
    By the same useless metrics, the Panasonic 20mm/1.7 is 22.2mm. So you may be right. :flowers_2:
     
  9. piggsy

    piggsy Mu-43 All-Pro

    I think you / owners of the lenses are a photo of a tape measure at MFD away from determining a pretty accurate magnification to make it work :D
     
  10. SojiOkita

    SojiOkita Mu-43 Top Veteran

    617
    Feb 23, 2014
    France
    I don't understand your calculation.
    Your formula only works when you use the lens at minimum focus distance.
    And when using the lens at this distance, you don't really care for the field of view.

    At a "normal" focus distance, the field of view will be approximately the same.
    The field of view is supposed to be the same at infinity, after distorsion correction & crop (that can be pretty severe for some lenses)

    I also guess all values are rounded so some 25 mm may be 24.something or 25.something.
     
    Last edited: Dec 10, 2015
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