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Image Overlay--explain Gain?

Discussion in 'General Technical Discussion' started by jczinn, Jan 6, 2006.

  1. Can someone explain to us non-engineering types what "gain" means as it relates to the image overlay and multiple exposure functions in the D200, and how it relates to exposure? For instance, what does "gain =1" or "gain=.5" really mean? If I want to overlay two images, how do I figure what the gain should be?

    Please, explain for engineering/mathematical dummies like me :biggrin:
  2. MontyDog


    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
  3. Thank you Paul! That does help, a bit, but the thing I'm still confused about is how this relates to exposure. Let's say I have two images I want to overlay, and I want to keep them at their value as exposed--does that mean I'd set the gain at 1, or at .5 (.5 +.5=1)? Or does setting the gain at .5 mean I'm halving the exposure of each? I'm assuming the latter, according to your explanation--

    A "practical" typical example--let's say I have a landscape with an empty sky in one image, and a full moon in another, and I want to overlay. Both properly exposed. I'd want to keep both exposures as they are (I assume) so as to have the moon and the sky correctly exposed in the final image. Do I set the gain at 1, or at .5? (Let's forget about the possibilities of unequal weighting, for the moment!)

    Thanks for your patience ;-)
  4. MontyDog


    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
  5. nfoto

    nfoto Guest

    Cannot see much need for the overlay feature, unless you want to improve high-ISO noise by overlaying a number of shots of the same, static subject taken with a tripod-mounted camera. However, given the pitiful quality of tripod mounts of many lenses these days, you'll likely get blurriness from imperfect frame registration in exchange for enhanced S/N ratio.

    On the other hand, I find the Multiple Exposure very neat and use it a lot, most often with auto gain switched off, though. I do the corrections of exposure on a per-frame basis like in the "old" silver-halide days. Only gripe is the limited number of multiple exposures that can be made, just 10 does not cut it for many occasions (with the F5, my personal high "record" was 4096 exposures onto a single frame, entire shooting duration slightly less than 2.5 hours).
  6. Chris101


    Feb 2, 2005
    THis feature might find some use in astrophotographic imaging, where some image averaging, also known as 'stacking' can be done in camera.

    Janet: You can think of gain as a sort of 'exposure compensation' that happens without affecting the actual exposure parameters. By reducing the gain, you will reduce the amount of exposure the image receives.

    For example, suppose you want to take a picture of a flying bird against a blue sky, and show the bird in two different positions superimposed on one another. If you shot a multiple exposure of the bird without gain, you would get the bird properly exposed where it did not overlap with the other exposure, but both the bird where it overlapped and the sky would be overexposed. Using overlay plus a gain of one half, the parts of the image that overlapped (bird and sky) would recieve half the exposure for each shot, so they would be properly exposed.
  7. Okay, I think I got it now--thanks Chris, Paul, and Bjorn. Honestly I don't really know how I'd use this feature--as was said, the same effect I suppose can be done in post with greater control-- but it seemed like a cool thing to try, and I just wanted to get a better handle on what the terms were. Anyway as always, the best thing to do is just try it, and I will, as soon as I get a chance. Chris' example seems like a fun possibility.
  8. nfoto

    nfoto Guest

    Multiple exposures with some subjects can be very tricky or near impossible to achieve in post processing. As to the overlays, this is so-so, as Chris explained you can get useful combinations, but being forced to decide which approach to take (or NEFs to combine) during the actual shooting can be an unwanted stress factor, at least when your CF card(s) fill up and it's time to delete the non-keepers :frown:

    However when the camera has a feature and it doesn't sit prominently in front of you to prevent normal use of the camera and you actively have to select a feature to access it, I guess it's OK. Always someone out there who sees the enhanced possibility of that gizmo. The rest of us can just ignore it.
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