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Is that Pentax Auto 110 lens REALLY f/2.8?

Discussion in 'Adapted Lenses' started by n3eg, May 17, 2016.

  1. n3eg

    n3eg Mu-43 Regular

    Another adapted lens expert on another forum pointed out that the aperture in the original camera may be the thing limiting these lenses to a fixed f/2.8 and not the lens itself. That got me thinking, and I did a few tests on mine comparing them to existing lenses.

    The results were surprising: The 24mm was exactly f/2.8, the 50mm was 1/3 stop faster, and get this - the 20-40mm is one whole stop faster at 25mm! The zoom lens varies +/- 1/3 stop from 40 to 20mm, and at 20mm it is about f/1.8! This could explain why there are so many reports of it not being as sharp as the others.

    In any case, I bought the 20-40 for $21, so I'm happy.
     
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  2. pellicle

    pellicle Mu-43 All-Pro

    Feb 10, 2010
    Southport, OzTrailEYa
    pellicle
    yes

    and remember that there is transmission to account for, not just the f number. Recalling that the f-number is just a ratio of the diameter of the pupil over the focal length and thus does not take into account the actual amount of light that reaches the sensor ... we have T Stops for that ;-)

    but when you say:
    are you discussing this from measurement of the shutter (determined by light getting to the sensor) or measuring the pupil (which determines the f number) .

    For instance after I bought a 45-200 I realised that with both at f5.6 my earlier purchase (FD200f4) resulted in higher shutter speeds
    in my view ...: Panasonic 45-200mm
     
    Last edited: May 18, 2016
  3. Klorenzo

    Klorenzo Mu-43 All-Pro

    Mar 10, 2014
    Lorenzo
    Yes, up to half stop is common even with modern lenses (this is based on DxO transmission measurements), up to 1/3 is almost the norm.

    Then I'm wondering how did you test this because it is not simple as it involves the camera metering. If you change the subject (zooming in/out for example) the camera will compute a new exposure depending on the reflectance ("brightness") of the new elements that are in the frame.

    The only reliable way to test this at home (that I can think of) is to use spot metering on a flat subject with uniform color, uniformly lit (on tripod so you do not change the angle, constant light). And double checking that you get an near identical histograms for the element you metered on only (with a crop, for example). Then the difference in the shutter speed should give you the transmission (with fixed ISO and Aperture priority mode).

    Trying to match the histograms/tones later in post processing is not as reliable IMO: I would not trust much the EV values from the exposure correction tool.
     
    Last edited: Jun 8, 2016
  4. n3eg

    n3eg Mu-43 Regular

    That's what I did. Spot metering pointed at a white surface with the camera stationary, and the shutter speed set to .3EV steps.
     
  5. n3eg

    n3eg Mu-43 Regular

    That's a different situation entirely. In your case, both lenses have an aperture built in. In the Pentax 110 lens, there is no aperture and the lens relies on the combination "shutter aperture" in the camera for stopping down. Mounted on an adapter with m4/3, these lenses are always wide open. The maximum aperture is only limited by the lens itself instead of the Auto 110 camera's internal shutter/aperture mechanism.
     
  6. pellicle

    pellicle Mu-43 All-Pro

    Feb 10, 2010
    Southport, OzTrailEYa
    pellicle
    I think you've missed my point ... while there is no "apeture control" on the 110 lenses but they do indeed have an aperture, it is simply a function of the pupil diameter ... its just that you can't change it. Read my questions again and you may see my meaning.

    I feel you are confused with the fact that the word aperture is now synonymous with the pupil itself (or so called wide open) and an iris system to effectively reduce the diameter of that pupil. It wasn't always so, and older lenses didn't have any system to change this ... the 110 lenses are simply the same idea.

    When my 200f4 is wide open you can not see the aperture iris either ... so its just the pupil diameter which detemines that its f4. Yes I know that the 110 cameras had an aperture slit in the camera but you'll find that when its wide open it does not obstruct light that reaches the sensor: thus the aperture is (as I said above) determined by the lens pupil diameter.
     
    Last edited: May 19, 2016
  7. barry13

    barry13 Super Moderator; Photon Wrangler

    Mar 7, 2014
    Southern California
    Barry
    I checked my 70mm today, and it meters the same as my 40-150 Pro.

    @n3eg@n3eg, I'd be interested in seeing samples from the 110 zoom.

    Barry
     
  8. dwig

    dwig Mu-43 Top Veteran

    622
    Jun 26, 2010
    Key West FL
    I've done this a lot of times, though for checking meters rather than comparing lenses. The best low-tech method I've found is to use a good quality (read: very uniformly lit) lightbox like those used long ago for sorting slides. You first need to manually focus the lens to infinity and lock it there and then place the lightbox very very close to the lens so that its surface is massively out of focus. With modern "zoom" lenses you would need to refocus after zooming as most are not actually true zooms but merely varifocals which distinct focus shift as they are "zoomed". The designers rely on the AF systems to trim the focus allowing them to concentrate on other optical corrections.
     
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  9. barry13

    barry13 Super Moderator; Photon Wrangler

    Mar 7, 2014
    Southern California
    Barry
    Anyone have samples or more info on results with the zoom wide open?