Olympus EVOLT - 8 MP Consumer DSLR (four thirds)

Discussion in 'Digital Photography' started by Richard Cockburn, Sep 27, 2004.

  1. It appears to have the standard optical viewer as well.

    Roger Halstead (K8RI & ARRL life member)
    (N833R, S# CD-2 Worlds oldest Debonair)
    www.rogerhalstead.com
     
    Roger Halstead, Oct 8, 2004
    #61
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  2. Richard Cockburn

    dj_nme Guest

    That lump on top of an SLR camera usually houses the pentaprism (some
    oddball SLRs have a fixed mirror box or porroprisms).
    The mirror that you can see from the front (with the lens off) reflects
    the image up into the prism.
    I don't think that it will make much difference to the brightness of the
    finder.
    Because it is an SLR, the mirror and shutter block light to the image
    sensor until the shutter is fired.
    No live LCD preview on this camera.
     
    dj_nme, Oct 8, 2004
    #62
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  3. And of course, Olympus has experience with SLR's with odd finders;
    remember the Pen F? This setup is somehow reminiscent of that. And I
    think the both use mirrors rather than a prism.
    But the fact that it uses mirrors rather than a prism will have some
    effect; Canon 300D has a rather dim finder because of this, as I
    recall.
     
    Stephen H. Westin, Oct 8, 2004
    #63
  4. Richard Cockburn

    dj_nme Guest

    Not true.
    The E-Volt uses a porroprism (according to the press release), like the
    Pen-F.
    I haven't used a Pen-F, so I don't know if it had a good viewfinder.
    It uses a porroprism, which is a series of triangular prisms put
    together to flip the image around like a pentaprism.
    I think it ends up being wider, but much less tall as a pentaprism for
    the same job.

    If memory serves correctly, an early full-frame 35mm SLR used a
    porroprism in the same way.
    I read a reference to this camera on camera history page, but it didn't
    have a great deal of detail (just a name that I've unfortunately
    forgotten) just stating that it used a porroprism in it's finder.

    There must be a good reason why no other full-frame SLR has them in
    their design.
    Perhaps bulk?
    Or the need to assemble it from pieces, rather than a solid hunk of glass?
    I dunno.
     
    dj_nme, Oct 8, 2004
    #64
  5. Richard Cockburn

    dj_nme Guest

    1947 Gamma Camera Co., (Hungary) introduces the Gamma Duflex - the first
    SLR with instant return mirror, metal focal plane shutter, bayonet lens
    mount (only a 50mm lens was ever produced) and internal semi-auto
    diaphragm. Its reflex focusing finder uses a porroprism rather than the
    now common roof Pentaprism. Way, way ahead of it’s time. Some 300 to 800
    are produced (estimates vary) and currently sell for over US$3,500 on
    the collectors market!

    Taken from this page in the Google archive:
    http://www.google.com.au/search?q=c...~dnr/a_brief_history.htm+porroprism+slr&hl=en
     
    dj_nme, Oct 8, 2004
    #65
  6. Right you are. The Canon 300D and Nikon D70 seem to use pure mirrors,
    and controversy has ensued,
    e.g. <http://www.photo.net/equipment/nikon/D70/>. Well, the d70 seems
    to have a prism:
    <http://www.nikonusa.com/template.php?cat=1&grp=2&productNr=25214>.

    So the only extra loss is the extra air-glass interfaces.
    Alignment, additional reflections and loss. Space and packaging,
    certainly.
     
    Stephen H. Westin, Oct 8, 2004
    #66
  7. Richard Cockburn

    Leonard Guest

    Can anyone quantify how much light is lost by a pentamirror as compared
    to a prism?

    And how that compares to the dimness resulting from using a ~f4 zoom
    rather than a ~f2 fixed lens?

    - Len
     
    Leonard, Oct 8, 2004
    #67
  8. Richard Cockburn

    Mike Henley Guest

    So, for the simple (simplicity-seeking) mind, what are you saying? The
    E-Volt is good or bad for this?
     
    Mike Henley, Oct 9, 2004
    #68
  9. Richard Cockburn

    dj_nme Guest

    Perhaps.
    I have two accessory viewfinders for my RF cameras, a Leica Vidom
    multifinder and a Steinheil multifinder.
    Both use a porroprism to flip the image around.
    Considering that the objective lens is so tiny on the Steinheil version
    (2mm, 4mm or 6mm diameter depending on focal length), the image seen
    through it is suprisingly bright.
    I could only assume, given that a typical APS or 135 sized lens usually
    has an objective element that is at least 10 (or 20 or 30) times the
    diameter as this, then I would expect the viewfinder on such an SLR
    camera to be more than sufficiently bright and clear.

    Alignment of the porroprism elements would seem like a pretty good
    reason against using it a mass-produced item.
    Perhaps Olympus has found a short-cut to manufacture them?
     
    dj_nme, Oct 9, 2004
    #69
  10. Richard Cockburn

    Alan Browne Guest


    If there are two similar cameras from the same maker (one prism, the other
    mirror), look at the AF or metering performance at the low end. Difference will
    be close to the loss difference between the mirror and the prism versions.
     
    Alan Browne, Oct 9, 2004
    #70
  11. Richard Cockburn

    Leonard Guest

    ? Does not the AF system operate on the other side of the focus screen?

    Anyway, I looked it up. You lose about half a stop of finder brightness,
    assuming that the mirrors are ordinary cheap ones (and they surely are,
    otherwise why not have a prism).

    - Len
     
    Leonard, Oct 10, 2004
    #71
  12. Richard Cockburn

    Steve Hix Guest

    Because a normal pentaprism wouldn't permit reducing the size of the
    body as much as the porroprism solution?
     
    Steve Hix, Oct 10, 2004
    #72
  13. Richard Cockburn

    Alan Browne Guest

    Yep.
    What I meant was : look at the specification for the metering, the difference
    will indicate the losso of performance from the difference in transmission
    between pentaprism and pentamirror (a good chunk of the loss difference).
    (Better to look at the metering than AF difference as the AE usually has the
    longer path).
     
    Alan Browne, Oct 10, 2004
    #73
  14. Well, it's a different situation from a 35mm film SLR. What is awkward
    and bulky for the one might be the perfect packaging solution for the
    other. And presumably there's a tradeoff between simpler prisms with a
    greater challenge in installation (porroprisms) and a prism that's
    harder to make, but contains most of the critical alignment in a
    stable manner (pentaprism). It's interesting that Olympus made this
    choice; I wonder if the company heritage in the Pen F had any
    influence. Of course, the designers in charge on that camera have
    presumably retires long since.

    Anyway, I'm not about to predict relative performance based on a vague
    idea of overall layout; not only am I not a qualified optical
    designer, but I bet an inferior layout with superior detail design and
    manufacturing will beat a fundamentally better one that has been
    sloppily done.
     
    Stephen H. Westin, Oct 11, 2004
    #74
  15. In a back-of-envelope calculation, a good aluminized front-surface
    mirror could have reflectance about 95% over most of the visible
    spectrum. So after the two reflections needed, you wins up with about
    90% of the light getting through. Since a prism uses total internal
    reflection, the reflectance approaches 100%, but there are two
    transparent faces where reflectance is about 0.5% each, assuming good
    antireflection coating. So that would come out to 99% of the light
    getting through. Probably not a difference you would notics. Assuming,
    of course, that everything is clean and properly aligned; dust on the
    mirrors would make things worse.
    Much less, if my calculations above are correct.
     
    Stephen H. Westin, Oct 11, 2004
    #75
  16. Richard Cockburn

    Leonard Guest

    Sorry, I had digressed into mirror-vs-prism for traditional designs.
    If the evolt has the same light path as the pen-f then you could use
    whatever combination of mirrors and prisms you wish, with the similar
    pros and cons as you get with the more usual style.

    - Len
     
    Leonard, Oct 11, 2004
    #76
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