Any standard sample underexposed digital photos on web?

Discussion in 'Digital Photography' started by Walter Dnes (delete the 'z' to get my real address, Oct 10, 2005.

  1. Some time ago, I asked here about binning ("Trading pixels for ISO")
    as a method for brightening underexposed photos. Since then, my
    computer died, I got a new one, experimented with the install, did some
    Google searching, installed and learned how to use ImageMagick and
    FreeBasic, and did some programming.

    I now have clumsy, but working, "proof-of-concept" software that will
    convert a 2560x1920 digital photo to a brighter 1280x960 digital photo...
    in a bit over 2 minutes... on an AMD64-K8 3000+... on a new PC with
    2gigs of RAM... running optimized-to-the-reasonable-max Gentoo linux. I
    did say it was proof-of-concept, not a polished production system.

    I took a few test shots Sunday at -2EV, which came out dim. I ran
    them through my software. The default (4X) brightening resulted in an
    overexposed final picture. Fortunately, I had built in a program
    parameter to control brightening from the command line. About half the
    brightening, i.e. 2X, seems to work OK. Is there a formula correlating
    shutter/aperture/ISO with EV for the Panasonic FZ5?

    I'm happy with the results so far. Are there any standard underexposed
    test photos on the web? I'd like to test if my software works as well as
    standard programs for brightening digital photos. If so, I'll publish
    my code on the web.
    Walter Dnes (delete the 'z' to get my real address, Oct 10, 2005
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  2. Is there a formula correlating
    OK, I'm probably barking up the wrong tree here because I'm sure you know
    this and/or want something else, but here we go anyway, in what will no
    doubt seem a horribly patronising tone due to my probable misreading of your

    Modifying these settings (shutter/aperture/ISO) in their normal increments
    has the effect of halving or doubling the amount of exposure (Which is known
    as moving up or down a 'stop'). These are settings are standardised across
    all cameras and always have the same effect.

    This allows you to easily calculate equivalent exposures:

    [1/500sec, f5.6, ISO400] == [1/250sec, f5.6, ISO800] == [1/500sec, f4.0,

    The EV is just the value assigned to each equivalent set of shutter/aperture
    values, regardless of ISO, going from [1sec, f1.0] (EV 0).
    So EV1 would be [1/2sec, f1.0] == [1sec, f1.4]
    and EV2 would be [1sec, f2.0] == [1/2sec, f1.4] == [1/4sec, f1.0]
    and so on.

    This page has a nice table of equivalent EVs:

    So all that setting -1EV on a digicam will do is either put you 1 stop down
    on the aperture or halve the exposure time. Maybe a combination of shutter
    speed and aperture that add up to one stop will be used, but it's the same
    idea. -1EV = -1 stop of exposure. I'm not sure if all/some cameras will also
    adjust the ISO rating when you modify EV, but in that case -1EV may just
    mean going from ISO200 to ISO100 without changing the other two settings.

    Well, that was probably no help at all, but my fingers needed a walk and I
    needed to reinforce some memories!

    More random EV/LV info here:

    Captain Blammo, Oct 10, 2005
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  3. Actually, your explanation is exactly what I was looking for. I'm a
    relative newbie to photography equations, and I got a camera just this
    summer. I did some reading and picked up the tradeoffs between aperture
    and exposure time and ISO. EV ties it up into one neat equation.
    Another mystery (to me at least) explained. Thanks.
    I see that an aperture multiplication/division of 1.4 (approx square
    root of two) is one full stop. Given that the area of a circle is
    proportional to radius (or diameter) squared, this makes sense. To sum
    things up,
    (ISO )
    EV = log_base_2(--- * exposure_time * f_ratio^2)
    (100 )

    The sign is tricky, because we normally deal with 1/f_ratio and ditto
    for exposure time.
    Thanks again. I normally run -2/3rd EV, and yesterday was sunny. So
    -2 EV was approximately 1 stop below my regular setting. That explains
    why the photos only needed 2X brightening.

    The FZ5 is a great little daylight camera, but has problems in
    low-light situations due to running at ISO 80. It does have 100, 200,
    and 400, but at the cost of noise. Being able to push it a couple of
    extra stops, without the noise of regular ISO boosting, will make it
    that much more useful. I might even try 3x3 binning for 9X brightness
    multiplication in dark situations. That would cut 2560x1920 down to

    Since that level of deliberate underexposure is beyond the limits of
    the camera's EV offset, I'd have to do it in manual mode.
    Walter Dnes (delete the 'z' to get my real address, Oct 10, 2005
  4. Actually, your explanation is exactly what I was looking for. I'm a
    Phew! Glad to hear I could be of some help!
    The annoying thing about that equation is that it never gives integers.
    Floor or ceiling don't work, either. I'm not sure, but I *think* rounding to
    the nearest 0.5 does the trick.

    Captain Blammo, Oct 15, 2005
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