Stacking plus exposure blending -- seemed like a good idea

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pittendrigh
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Stacking plus exposure blending -- seemed like a good idea

Post by pittendrigh »

An experiment occurred to me this morning, which did not work out well.
But perhaps it might have If I had only known.........something.

I set up what would have been a simple manual focus stack in my office, on a small tripod. I shot 4 focus resets for an object up close, for two intermediate distance objects and one more for a book shelf on the far wall. The book shelf was more dimly lighted than the closer objects.

For each each exposure I not only changed focus I also twisted the "exposure compensation" dial one stop so more distant objects (which were dimmer) would be slightly more exposed. If the focus stacking chose each pixel by sharpness alone the furthest focus points (which were dimmer) would end up not only well-focused but also brighter as well. I used D-map with contrast set to include everything.

The result was a mess. The background (the far wall in this ensemble) seemed to have a chaotic mixture of lighter and darker pixels. This seemed like a good idea but it did not work out well. Have I over-looked something? Is there hope for this idea? For combining different focus and different exposure for successive images?

rjlittlefield
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Re: Stacking plus exposure blending -- seemed like a good idea

Post by rjlittlefield »

Usually when people ask about focus stacking plus exposure blending, they mean that they want to shoot the same multiple exposure levels at each focus plane, then combine the shots into a composite that is both HDR and focus-stacked. That can work well, because each focus plane is subjected to more or less the same transformation from HDR.

But it sounds like you have in mind to shoot just one exposure level at each focus plane, gradually changing that level from front to back.

This approach presents some difficulties for focus stacking. The issue is that there are always transitions from one focus plane to another, and along those transitions you'll have adjacent pixel positions that received different exposures. That will introduce a dark/light transition, which certainly will not match your original perception of the scene.

At best you might get a sort of contour banding, where each focus plane is rendered in its original exposure and the transitions are well behaved.

But very likely the focus stacking method will not choose exactly the right plane in all pixel positions, and then you'll get a blotchy appearance. This is most likely to happen in exactly the situation you describe, using DMap with contrast set to include everything. That's why the "How To Use DMap" tutorial cautions that
One common mistake is to leave the contrast threshold set too low. That causes the frame selection to be determined mostly by random noise, in areas that are out-of-focus or mostly featureless. The result can be an ugly mess of blotches
If you want to pursue this technique farther, I suggest to raise the contrast selection threshold so that all the blotchy areas go "black in preview". That will give DMap a better chance to select all the same source frame over larger areas. If you're lucky the result will suffer from no more than brightness halos on the transitions between focus planes. PMax is worth a try, also.

But in general, I am not optimistic about changing exposure levels from one focus plane to another. I expect you would do better to go to full HDR, and hope that the HDR method can do a better job of evening out the brightness differences without introducing annoying transitions.

--Rik

pittendrigh
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Joined: Tue Oct 16, 2012 11:02 am
Location: Bozeman, MT
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Re: Stacking plus exposure blending -- seemed like a good idea

Post by pittendrigh »

Good answer. I'll work with all of that. Bracketed at each step might be a better direction for me.

With big magnification on insects I might use 20 exposures, all the same lighting and exposure.

This morning I was thinking about dabbling in commercial real estate photography--where indoor lighting is often difficult and where focus from closeup (perhaps flowers 10" inches from lens) all the way over to the opposing wall, perhaps in a large room, would be both helpful and distinguishing. Real Estate is a big deal in my home town. The trendy local real estate photographers I notice are all using video made from drones, which is cool. But their indoor still shots (that end up on brochures as well as websites) tend to have narrow depth of field, despite wide angle lenses.

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