Orquid "Phoenix"

Orquid "Phoenix"

by ceinmart

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Shot some shoots with Hasselblad H4D-50 and in (at least) one of them the following artifact(s) appeared:

cropped area from a Hassy H4D-50 shoot

Of course some questions arise:

  • What is this? (moire?)
  • Why it happened? (aside of lack of AA filter in front of the Bayer sensor. I'm interested, if there are other factors involved in this)
  • How can I avoid it?
  • How can I fix it? (using eg. Photoshop)
  • The "fix" will induce secondary effects?

    ps: The image crop can be found at http://i.stack.imgur.com/O1vk5.jpg

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How was this taken?? it appears to be a composite of 4+ shots (note the 4x4 driving along the pier... –  Darkcat Studios Jan 15 '13 at 9:38
    
@DarkcatStudios: One of my colleagues shot it (cannot ask him now) - I don't know if he used the Hassy's in-camera multishoot mode. But in any case, it isn't a post-production composite. And even if there is an in-camera multishoot, the things shouldn't be like this, isn't it? Anyway, thanks for asking. –  John Thomas Jan 15 '13 at 10:21
    
The multiple ghost images of the driving car and the walking person on the dock should be strong clues. The apparent motion blur of the rocks at lower left shows the camera was not held still very well between the multi-shots either. –  Olin Lathrop Jan 15 '13 at 15:03
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1 Answer

up vote 13 down vote accepted

That is the result of using the H4D-50MS in multishot mode with an uncontrolled subject. Notice that everything that is absolutely still comes out just fine, but anything that moves, even slightly, shows registration problems. The vehicle is most obvious since it's moving at a decent clip, but the motion is also visible in the water, grass, and so forth.

In multishot mode, the camera takes four pictures (at about a second or so apart), moving the sensor by one pixel left/right/up/down between shots. That's a wonderful thing for studio still life—it means that every pixel of the resulting image was imaged by each of the four pixels in a Bayer quad, so each of the pixels has its own complete colour coverage (and doesn't have to rely on inference from surrounding pixels for colour info). If the subject moves at all—and keep in mind that the sensitivity is a single pixel in any direction—at least the colour information will be wrong, and you may get extreme ghosting (as with the SUV).

The solution is to ensure that mulishot mode is only used for controlled still life set-ups. (Or, perhaps, for extremely stable landscapes, like rock formations or deserts in still air and clear skies.) If there's any possibility of something moving, there will be artifacts in the multishot image, often more than can easily be fixed in post.

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