Ok, help with RF's. R2a, R3a, speed, etc...

Discussion in 'Rangefinder Forum' started by g0tr00t, Oct 25, 2004.

  1. g0tr00t

    g0tr00t Member

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    Ok, now that I have been playing with this Agfa for a couple of weeks....I am hooked. I leave my EOS3 at home now and walk around with the Agfa and my light meter.

    I was trying to understand the difference between the R2a and R3a of the Bessa's. "Bessa R2-A has a x 0.7 magnification ratio and the Bessa R3-A has x 1.0 ratio" -- Why would I want the .7 mag over teh 1.0 mag?

    Actually, I am trying to understand RF's....How slow can you hand hold a RF? I mean with SLR's the rule of thumb is not set the shutter slower than your focal length.

    Although, with RF's it seems you can shoot much slower due to lack of a mirror and also since the lens is closer to the film plane, my 2.8 is much brighter than a 50mm 2.8 on an SLR..

    Correct? Sorry for the noob question, but I gotta start somewhere....
     
  2. clogz

    clogz Subscriber

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    Generally you can sefely go one shutter speed slower. It's of course best to test your handshaking yourself.
    Regards
    Hans
     
  3. garryl

    garryl Member

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    Then Political candidates should only use the higher speeds- cause they handshake all the time :D
     
  4. Helen B

    Helen B Member

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    'Why would I want the .7 mag over teh 1.0 mag?'

    David,

    With everything else being equal, 'in general' and off the top of my head, the viewfinder magnification affects:

    Rangefinder precision - the higher the magnification, the higher the precision.

    Minimum equivalent focal length of viewfinder field of view - the lower the magnification, the wider the field of view. A 1x might be able to show framelines for a 35 mm lens while a 0.7x may be able to show them for a 28 mm lens.

    Brightness - the lower the magnification the brighter the image, but this is usually barely noticeable and it is affected by other factors.

    Ability to use both eyes - with a 1x magnification you can keep both eyes open, and you see the brightline frame 'floating' in space. With other magnifications it is still possible to keep both eyes open (a lot of us do), but it takes a little getting used to, and it doesn't look so perfect.

    'my 2.8 is much brighter than a 50mm 2.8 on an SLR..'

    That's a new one on me. How are you measuring or judging 'brightness'? There may be a tiny difference because of the slightly higher light transmission of a simple lens compared to a lens with more glass and more air-glass surfaces, but one is unlikely to be 'much brighter' than the other.

    Best,
    Helen
     
  5. g0tr00t

    g0tr00t Member

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    Thanks for the responses. I am taking a photo class now and the teacher said that since the lens elements are closer to the film plane in a rangefinder than they are in an SLR, you get "more light" using a RF at 2.8 than you would using an SLR.

    Confused me too and am still confused...
     
  6. garryl

    garryl Member

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    So am I! If true, then the "same" lens used on an RF and then an SLR (at the same F/stop)would require differing exposures.
     
  7. John Koehrer

    John Koehrer Subscriber

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    Sounds like a very creative logic. Remember those that can't do, teach.
     
  8. g0tr00t

    g0tr00t Member

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    See, can't be true. I think it may only relate to zoom vs prime. That makes more sense to me. Otherwise, my light meter would have to be different for a RF compared to an SLR. I'll bug him again later this week about that....
     
  9. Ed Sukach

    Ed Sukach Member

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    Confuses me too!!

    I would say the instructor was asked a question, and not knowing the answer, tried to create something.

    A wide angle lens on a single-lens reflex camera tends to be more complex ... requiring more elements (read: retrofocus). That usually means more air-to glass surfaces... and less efficiency. Very little less efficiency. DAMNED little less efficiency.
    It may be of concern in certain photometric (read wild scientific) applications. but for anyone with a modicum of sanity, it means *nothing* in any type of photography in which the most fastidious photographers - as we know them - might be involved.
     
  10. bjorke

    bjorke Member

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    For RF lenses, more of the light that strikes the lens ends up in the correct place on the film. There is less diffusion. The OVERALL amount of light at f/2.8 is theoretically the same. But correct placement contributes to better sharpness and higher contrast, hence the high regard for RF glass.

    Actually few lenses hit their stated apreture f/stops accurately. Cine lenses, which need high tolerances, use "T/stops" which are set lens-by-lens by actually measuring transmission for individual lenses, rather than trusting the theoretical numbers printed on the barrel by the mfgr
     
  11. Robert Kennedy

    Robert Kennedy Member

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    In the case of the Bessas, the R3-a has a 1:1. This means that you can't get a 35mm frameline. You get the 40mm instead. This could mean a lot if you own a 35mm lens(es) or it could mean nothing.

    As to aperture, ummm....f2.8 is f2.8.

    Period.

    F stop numbers are ratios, so they are the same on all lenses. F8 is f8 no matter what.
     
  12. Ed Sukach

    Ed Sukach Member

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    Uh ... I don't want to start anything, but ... Do you mean to suggest that the light from an "ordinary" (non-retrofocus) lens DOESN'T "end up in the correct place on the film"?
    When you speak of "diffusion", do you mean "dispersion"? - With more glass, there should be more, not less...
     
  13. bjorke

    bjorke Member

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    Yes, that is indeed what I mean, though SLR lenses are generally retrofocus lenses, which is just why they have problems versus RF lenses (or LF lenses).

    Some of the light ends up in the correct place, but some photons are scattered inappropriately, due to more glass, more surfaces, etc. SLRs == Retrofocus designs == more surface, more glass, and more diffusion of the irradiance ("dispersion" is the same thing). Non-retrofocal designs, such as SLR tele lenses, are on a more even footing with their RF cousins. Of course, for those of us 35mm shooters who treat anything longer than 45mm as a rarely-used exotic lens, I guess that particular SLR benefit goes largely unrealized :smile:

    (None of this means you should change exposure when shooting with a rangefinder or LF camera as opposed to an SLR, though)