Problem (or not?) taking frames for stacking with CombineZM

[Bruce Williams]

Hi folks,

I've just tried out my Nikon D80 on a couple of quick stacks using CombineZM.

ISO=400. Exposure was Ap. priority f/11 giving 1/10s. I used a 5sec self timer and shutter was delayed 0.4secs after the mirror.

Notes: ISO400 was a mistake - meant to set ISO to 100. Also had intended to use f/7.1 but forgot to reset.

I used a fairly easy test subject (a rather poorly prepared, 6cm long muskrat skull). Because of the subject's relatively large size, the camera had to be positioned some distance from the subject (approx 33cm from front of lens to subject).

I found that at that distance the skull only required 3 frames to complete the stack. Result was ok but not as good as I'd hoped.

Ok here's the reason for the posting:

Previously when using my Minolta A2 to take frames for a stack, I set the camera up on a tripod and used manual focus to step focus through the depth of field (typically 10 to 20 frames). The thing is, the size and registration did not change as I moved the point of focus. This meant that CombineZM always reported an X and Y offset of zero and a scale factor of 1. With the Nikon lens (105mm VR f/2.8 macro) the image size changes noticably and so CombineZM has to resize and adjust registration.

My questions are: Is this normal with dedicated DSLR macro lenses or does differing lens construction mean that some do and some don't? Does the fact that CombineZM has to make size and registration adjustments mean loss of quality?

Bruce

[rjlittlefield]

Previously when using my Minolta A2 to take frames for a stack, I set the camera up on a tripod and used manual focus to step focus through the depth of field (typically 10 to 20 frames). The thing is, the size and registration did not change as I moved the point of focus. This meant that CombineZM always reported an X and Y offset of zero and a scale factor of 1. With the Nikon lens (105mm VR f/2.8 macro) the image size changes noticably and so CombineZM has to resize and adjust registration.

My questions are: Is this normal with dedicated DSLR macro lenses or does differing lens construction mean that some do and some don't? Does the fact that CombineZM has to make size and registration adjustments mean loss of quality?

Short answers: 1) Yes, 2) to some extent, and 3) most likely not.

"Yes", it's normal for size and registration to change. Registration normally doesn't change very much if you refocus by turning the lens focus dial, but it can change quite a bit if you refocus by moving the camera or subject, and the movement happens to be not quite along the optical axis.

"To some extent", differing lens construction does affect the resizing. Resizing depends in part on exactly how the focusing is done -- for example some macro lenses actually change focal length while refocusing. It is also determined by the location of the lens's entrance pupil, which can vary between designs that have the same focal length.

"Most likely not", because with almost all lens setups, resizing is required to get the best results. See http://www.photomacrography.net/forum/v ... php?t=1418 for some discussion & illustration of why this is true. Note particularly cases B and C in the second panel of the first post. Basically, resizing makes subject features line up between neighboring images, so that perfect decisions are not required to get good composites.

What is really surprising to me is your report that the A2 did *not* require resizing. Consider for a moment a very simple camera with an ideal thin lens. Then image formation is governed by two basic equations:
1) focus: 1/f = 1/o + 1/i, where f = focal length of lens, o = distance from lens to object (subject), and i = distance from lens to image (sensor);
2) magnification: m = i/o.
Draw some pictures, play with the math, you reach the same conclusion: to focus closer, you increase i, which reduces o, and m increases at the same time. To get images that stay perfectly aligned without any resizing at all, requires a lens that changes focal length while it is refocusing, and does that in a very special way so that m remains constant. I'm surprised the A2 acts that way. I have never seen a camera that did.

Your image looks great. Don't worry about the resizing.

If you must worry about something, it should be about keeping the entrance pupil from moving while you shoot your stacks. Moving the entrance pupil means that the perspective changes from frame to frame. In extreme cases, it's possible to get stacks that have really ugly areas around edges because detail in farther back planes is never actually seen in focus!

Hope this helps,
--Rik

[Bruce Williams]

Rik - Thanks for your reply and helpful links. When I get the time I'll do a comparison stack with the 2 cameras - something a little more challenging.

Bruce

[DaveW]

I thought most macro lenses changed their focal length up close now since autofocus lenses came in? In the old days with the helical focusing mount the lens simply extended to focus closer, but as this would take a powerful autofocus motor and be a big battery drain, moving elements to focus was "borrowed" from zoom lens technology because a smaller motor and less battery power is needed to move these. But this does mean the focal length of the lens usually changes slightly as it focuses closer?

DaveW

[rjlittlefield]

I thought most macro lenses changed their focal length up close now since autofocus lenses came in? In the old days with the helical focusing mount the lens simply extended to focus closer, but as this would take a powerful autofocus motor and be a big battery drain, moving elements to focus was "borrowed" from zoom lens technology because a smaller motor and less battery power is needed to move these. But this does mean the focal length of the lens usually changes slightly as it focuses closer?

DaveW

I don't know about the "most", but otherwise I think that's a pretty good description. My Sigma 105 1:1 acts like it's just extending, at least to the limited precision that I've ever measured it. I believe that Canon's MP-E 65 just extends also, while their newer 1:1 macros change focal length.

--Rik

[Mike B in OKlahoma]

My Sigma 105 1:1 acts like it's just extending, at least to the limited precision that I've ever measured it. I believe that Canon's MP-E 65 just extends also, while their newer 1:1 macros change focal length.

The MP-E-65 definitely just extends, inviting lewd remarks from photographers who watch me do it! My old Vivitar 100mm macro lens looked like it was just extending.

As for Bruce's original question, I definitely notice a change in sizing in CombineZ as I focus my 180mm macro through different distances. Haven't tried CombineZ with any other lenses. The MP-E-65 seems like a "natural" if I have a static subject, but the lens mounting ring won't work with my 1Ds, the bulge of the battery compartment below the lens mount interferes. So I don't use the MP-E-65 on tripod much. I've tried it using the arca swiss mount screwed into the base of the camera, and that's tolerable for short, flash-only exposures, but for any lengthy exposure, it is too wobbly, and using Combine Z would make this worse, I suspect.

I'm hoping Canon will upgrade/replace the EOS 5D, in which case I'll shell out for it and hopefully be able to use the lens and lens mount on a tripod.

[Epidic]

I thought most macro lenses changed their focal length up close now since autofocus lenses came in? In the old days with the helical focusing mount the lens simply extended to focus closer, but as this would take a powerful autofocus motor and be a big battery drain, moving elements to focus was "borrowed" from zoom lens technology because a smaller motor and less battery power is needed to move these. But this does mean the focal length of the lens usually changes slightly as it focuses closer?

DaveW

There are two way to focus, the usual one is to move the lens further away from the image plane which means the effective focal length of the lens increases when focusing and the focus range does not change that. (Although, moving the image plane from the lens would be a much better solution for macro photography, but that does not change the fact effective focal length is increasing.)

The other way is usually described as IF or Internal Focus where the focal length of the lens changes to maintain an image at the image plane without moving the lens further away. This is done by decreasing effective focal length.

In both cases, magnification is changing. So as you focus, there is a shift in image scale. It is just much easier to calculate if you are moving the lens.

There are a lot of AF lenses that move the entire optics to focus and it is not a large drain in the batteries as you would imagine. Since the AF system is geared, it is quiet easy to build a motor to do this.

[Bruce Williams]

Thanks for the input guys.

Just for info. the AF-S Micro Nikkor 105mm 1:2.8 G ED VR lens does not extend. I think maybe the very noticable change in size (in the viewfinder) was perhaps greater as I was photographing quite a large subject (6cm skull)? I've since done a couple more stackes with smaller subjects and the change in size was not so dramatic.

Sometime soon I'll do a 1 for 1 comparison stack with the Minolta A2 and the Nikon D80 (just for the fun of it) and post the results.

Bruce

[rjlittlefield]

There are two way to focus, the usual one is to move the lens further away from the image plane which means the effective focal length of the lens increases when focusing

But be aware that "effective focal length" means different things to different people.

I think Will is using it to mean distance from lens to image -- the same number I called "i" in 1/f = 1/o+1/i .

But Wikipedia, Melles-Griot, and photonics.com all define "effective focal length" as being a property of the lens alone, not affected by moving the lens for focusing.

This is, of course, a different situation from the phrase"effective aperture", which as commonly used definitely does change with focusing. Sigh...

--Rik