Including high aperture frames in stacks

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TimmyG
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Including high aperture frames in stacks

Post by TimmyG »

I've encountered a number of photographers who state the number of images they've used for a stack, the aperture settings and sometimes the addition of one or two images at a higher aperture than the rest of the stack.

I've done some (admittedly brief) searches and cannot find the reason for including the higher aperture images in the stack. I assume it must be for some kind of alignment reason, or maybe to help smooth the transition between frames but I thought I would ask if anyone knew the benefits of doing this here before bothering the individual photographers.

The examples I have seen tend to be field stacks (some hand held, some not) of live subjects. I'm not sure if this is relevant to the question.

Thanks

johan
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Post by johan »

This is typically done to make a smoother more gradual transition between the in focus elements and the oof background, to make it less binary.
My extreme-macro.co.uk site, a learning site. Your comments and input there would be gratefully appreciated.

TimmyG
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Post by TimmyG »

Thanks Johan. So I take it it helps make the "edge" around the subject look more natural, rather than anything to do with the transition between in-focus areas.. interesting. I don't suppose you are aware of any examples comparing stacks including and excluding these frames to help get my head around it?

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Post by rjlittlefield »

I don't suppose you are aware of any examples comparing stacks including and excluding these frames to help get my head around it?
See http://www.photomacrography.net/forum/v ... 7644#57644 and the surrounding thread.

In my mind, the primary effect is not to soften the transition between in-focus elements and OOF background, but rather to make the OOF background less blurred than it would be without the extra stopped-down frame(s).

In the example linked above, one extra stopped-down frame makes the difference between a background that is blurred so badly as to be unrecognizable (upper left), and a background that still conveys some hint of shape (upper right).

Additional stopped down frames can extend the amount of background that is in focus but is also diffraction blurred so that it's not as sharp as the main stack.

In that extended region, the main value of stopping down is not to get more blur, but rather to reduce the number of additional frames by permitting a larger step size. Essentially the same result could be achieved by shooting the entire extended stack at wide aperture, followed by one stopped down frame to make the far background less blurred. But that approach would require shooting a lot more frames to avoid focus banding in the extended region. It's unfortunate that these additional aspects did not get discussed in that original thread, but that's what happens as our understanding evolves about new techniques.

--Rik

MaxRockbin
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Post by MaxRockbin »

I was thinking about using this technique of closing down an aperture behind a microscope objective (with the most common setup of putting the m42 iris sold by ebay vendor "jinfinance" between an infinite microscope objective and the tube lens), but I'm curious about how people handle the exposure difference.

With flash lighting, it's not that big a deal to up the flash a couple of stops and iris down a couple of stops, but ... 2 questions:

How do you adjust continuous lighting like the ikea Jansos?

How do you know how many stops to adjust?
If you put the iris behind the objective, do you close it down till you start to see darkening in the viewfinder or liveview and then start counting how many stops you close the iris from there? (I'm assuming that when the iris is wide open, it's not impinging on the light path yet).

Thanks!
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Post by rjlittlefield »

MaxRockbin wrote:How do you adjust continuous lighting like the ikea Jansos?

How do you know how many stops to adjust?
The simplest method would be set the camera on frame-average metering, note how the camera meters the scene at wide open and any appropriate shutter speed, then after stopping down readjust the shutter speed to give the same meter reading. You might be able to get a more accurate adjustment by metering some small area, if you can find a suitable area that's in focus. Or work the whole problem beforehand using a gray card instead of a real subject.

I'm not so optimistic about counting stops. That's because of the difficulty of finding the exact start point, which may be between click points, if the iris even has those and if they're accurately placed. (My iris has no stops at all.)

It's tempting to say "match the histograms", but that doesn't actually work very well because typically the shapes of the histograms change quite a bit as the aperture gets smaller, the OOF blur circles shrink, and the overall contrast goes up.

--Rik

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Post by ChrisR »

I've put 1/3rd stop marks on my iris. Yes you have to close it to find the point where it starts to have an effect.
How do you adjust continuous lighting like the ikea Jansos?
Shutter duration? :P

One thing to watch for in your stopped down frame, is the alignment done by the stacker. Because there's more to work from, it may not be the same as previous frames. The one with more dof will have detail already "covered" so you can get a double image in the output.

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Post by rjlittlefield »

ChrisR wrote:One thing to watch for in your stopped down frame, is the alignment done by the stacker. Because there's more to work from, it may not be the same as previous frames. The one with more dof will have detail already "covered" so you can get a double image in the output.
Good point.

In addition, there's another problem that has similar appearance but a completely different cause. That's the one in which switching from a lens-limited aperture to an iris-limited aperture causes the perspective to change, so that foreground/background relationships are altered and the subject appears to change shape. When that occurs, a good workaround is to never set the iris full open. Instead, always keep the iris closed far enough that it is the limiting aperture for all images, even when the aperture is almost wide open. This aspect is discussed in the thread that I linked earlier.

--Rik

TimmyG
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Post by TimmyG »

rjlittlefield wrote:
I don't suppose you are aware of any examples comparing stacks including and excluding these frames to help get my head around it?
See http://www.photomacrography.net/forum/v ... 7644#57644 and the surrounding thread.

In my mind, the primary effect is not to soften the transition between in-focus elements and OOF background, but rather to make the OOF background less blurred than it would be without the extra stopped-down frame(s).

In the example linked above, one extra stopped-down frame makes the difference between a background that is blurred so badly as to be unrecognizable (upper left), and a background that still conveys some hint of shape (upper right).

Additional stopped down frames can extend the amount of background that is in focus but is also diffraction blurred so that it's not as sharp as the main stack.

In that extended region, the main value of stopping down is not to get more blur, but rather to reduce the number of additional frames by permitting a larger step size. Essentially the same result could be achieved by shooting the entire extended stack at wide aperture, followed by one stopped down frame to make the far background less blurred. But that approach would require shooting a lot more frames to avoid focus banding in the extended region. It's unfortunate that these additional aspects did not get discussed in that original thread, but that's what happens as our understanding evolves about new techniques.

--Rik
Thanks for that! The penny's dropped now and I have some experimenting to do :)

Interesting stuff...

MaxRockbin
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Post by MaxRockbin »

I'm mostly shooting with flash - and gauging exposure by eye with the laptop connected. So this process seems a little tricky exposure-wise.

Marking stops on the iris and figuring out where the iris starts to have an effect on exposure - as Chris R suggested - and compensating with the flash might be workable with some twiddling.

If the double image and alignment issues are a big hassle, maybe it would make the most sense to "align all" in Zerene before processing the stack and then run stack selected leaving out the image with the iris closed down. Then that image could just be used for retouching fuzzy areas where it makes sense and alignment isn't critical. Maybe easier than retouching artifacts.
If your pictures aren't good enough, you're not close enough. - Robert Capa

rjlittlefield
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Post by rjlittlefield »

MaxRockbin wrote:If the double image and alignment issues are a big hassle, maybe it would make the most sense to "align all" in Zerene before processing the stack and then run stack selected leaving out the image with the iris closed down. Then that image could just be used for retouching fuzzy areas where it makes sense and alignment isn't critical. Maybe easier than retouching artifacts.
Good idea. I haven't heard that one before.

--Rik

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Post by ChrisR »

I'd forgotten about the perspective effect of moving the aperture. I think that can be more of an issue than alignment itself.
Yes, "Align all" is the only way I've dealt with it, then retouch with everything you want from the stopped down frame(s). It's going to be mostly edge and background, so it's fairly easy to select.

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