Difference between lenses for film and lenses for digital?

Discussion in 'Digital Photography' started by One4All, Jan 7, 2007.

  1. $http://photonotes.org/articles/eos-flash/ says:
    [...]
    $ Since
    $ digital cameras do not have film, digital EOS cameras do not
    $ support TTL.
    $
    $So given digital sensors are more reflective than film, why is the
    $same trick not used for flash metering?

    Some did; the first few EOS DSLRs (sold under different names as
    Canon and Kodak products) did their flash metering the same way as
    the EOS 1N film body upon which they were based: using a sensor
    looking at the light reflected off the "film" (OTF).

    Since Canon started making DSLRs on its own with the D30, this
    has been dropped; all Canon DSLRs starting with the D30 have used
    only E-TTL (or E-TTL II) flash metering, which uses the same
    evaluative metering sensor used for ambient metering, and don't have
    the OTF sensor.

    Why the change? I don't know. Canon has also been using E-TTL
    or E-TTL II for film bodies for over a decade now, though until
    relatively recently their film bodies also retained the OTF sensor
    and supported the older A-TTL and TTL flash metering systems which
    used the OTF sensor.

    I can make a couple of guesses. One is that the reflections
    off the digital sensor may be more directional than those off film,
    and perhaps this messed up flash metering. The other is that
    there are some pretty clear advantages to E-TTL/E-TTL II over
    A-TTL/TTL; the flash metering zones are much smaller, which offers
    the same advantages that smaller zones offer to ambient metering,
    and since the zones are the same for ambient and for flash, the
    amount of light added to a particular zone by the flash can easily
    be determined. So maybe Canon felt it would be wise to make a clean
    break here (and since many of Canon's film bodies since then have been
    relatively minor reworkings of older film bodies, perhaps they didn't
    feel that excising the OTF sensor from these was worth the bother).

    And just to be perverse, here's one reason why OTF metering
    could be *better* for digital than it was for film: it can be
    designed to know exactly how reflective the digital sensor is.
    With film, there's no guarantee that every film, past and future,
    from every manufacturer will reflect light the same way, so the
    flash metering system could come up with different answers for
    exactly the same scene depending on what film was used. But
    every body of the same model should have a sensor with exactly the
    same reflective characteristics; measure it during development
    and you're done calibrating the flash system for that model.[/QUOTE]
     
    Stephen M. Dunn, Jan 7, 2007
    #21
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  2. [A complimentary Cc of this posting was sent to
    Richard Kettlewell
    My conjecture is that the reason is the same as with "stealth"
    aviation: AFAIU, the principal feature of so called "stealth" aircraft
    is not that they reflect radiation much less (THIS holds too, but is
    not the MAIN reason for hard detection), but that they reflect is much
    less *in random direction*. A stealth aircraft is essentially a giant
    "flat mirror": it reflects the radar radiation via more or less in one
    direction; so the probability is much low that in any particular time
    it would reflect anything back in the radar direction.

    I would assume that the same holds for digital sensors: film reflects
    more or less diffusely, but sensors reflect "mirror-like". So what
    reaches the flash sensor is not very much related to what it *wants*
    to see.

    [All this is just a random musing, I have never seen any explanation.]

    Hope this helps,
    Ilya
     
    Ilya Zakharevich, Jan 8, 2007
    #22
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  3. Yes, very true. The Polachrome instant slide film was *much* less
    reflective than normal films, and really screwed up my flash
    auto-exposures. Luckily, the error was so big I noticed the flash
    running out of power when it shouldn't, and changed to A mode, and got
    the pictures anyway.

    I'm very annoyed about DSLR flash. Even my D200 with SB800 flash
    doesn't do anywhere near as well at flash exposure as my N90 and Sunpak
    555 in 1994.
     
    David Dyer-Bennet, Jan 14, 2007
    #23
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