Auto-step-focus-stacking

[feppe]

In real life, I have over a meter of monorail and bellows, and a support rail...

¿Any problems?

This is all new to me, but I just had to do the math on this for the insane hilarity of the discussion: assuming you have 1 meter of steel rail, and there's a 1 degree (Centigrade) increase in ambient temperature during your shooting, that rail will expand 0.013mm. To re-iterate, this is 13 times more than the 0.001 accuracy you "need," rendering that level of accuracy entirely useless. With just 1 degree change in temperature.

So if you manage to get that stepping motor accurate to 0.001mm, be sure not to touch it, breath at it, or even get close to it during the shoot, as I'm sure the infrared radiation from your body will change the rail's temperature enough to make that level of accuracy irrelevant. Even if you wear an aluminum apron and operate the contraption from the adjacent room with a remote, heat generated by the camera electronics and friction heat from moving up and down the rail are another challenge.

Of course you could measure the temperature (passively) and take thermal expansion into account when moving the rail. But this assumes you have a perfectly rigid, perfectly straight rail bolted to a 1-ton granite slab in a cooled, temperature-controlled clean room built on top of bedrock way out in the sticks so passing traffic doesn't disturb it. Hell, I bet a a stray butterfly wing flap, a burrowing earthworm or moon phase would wreak havoc on the results

Source for thermal expansion calc. I used 13 as CLTE from here.

[Dick Roadnight]

This is all new to me, but I just had to do the math on this for the insane hilarity of the discussion: assuming you have 1 meter of steel rail, and there's a 1 degree (Centigrade) increase in ambient temperature during your shooting, that rail will expand 0.013mm.

An instant 5 or 10 degree rise in ambient temperature would have no significant effect on the length of the rail in the second or two between exposures - one more good reason for an automatic system.

If the whole system was instantly engulfed in molten lava... you would no longer be worried about the length of the rail.

[feppe]

An instant 5 or 10 degree rise in ambient temperature would have no significant effect on the length of the rail in the second or two between exposures - one more good reason for an automatic system.

If the whole system was instantly engulfed in molten lava... you would no longer be worried about the length of the rail.

Even then, and in case you are actually serious, the challenges posed by extreme needs for rigidity, straightness, stability and isolation from imperceptible vibrations can't be overcome outside of a lab. I would think the weight of the camera moving up and down the rail would cause it to deform much more than 0.001mm, let alone the other factors - but I'm sure you have thought about a solution to that already.

[pchong]

I do too...and I tend to agree with you. I use a H3D-39, mostly with the 80mm plus rings on a Manfrotto macro rail. I gently tweak the camera position with the micrometer screws on the rail. I find this technique better than moving the focus ring, as on macro, the macro ring hardly moves to change image focus, and its is easier to move the micrometer screw to move the camera by a fraction of a milimeter....and perhaps its psycological, I find blending the images to be easier by moving the camera than changing focus on the lens.

But having the automated rail is a neat solution, and can guarantee repeatability. For me, this is important, because I shoot macros of product (and product parts), and would like to retain some consistency between shots.

I use focus stacking alot in the field with my H3D-50 (now H4D-50) and 50-110mm. There really is no need for automation (though it would be nice). THe beauty of the H camera is the mirror stays locked up. All I have to do is gently tweak the focus ring. Before I start I have noted where the near and far points are. I can work quickly and have done a 10 step stack. The biggest issue is part of the frame moving. I found the camera doesn't move at all as long as it is on a stable tripod. The other big plus with the H system is the leaf shutter doesn't create any vibrations.

[Dick Roadnight]

But having the automated rail is a neat solution, and can guarantee repeatability. For me, this is important, because I shoot macros of product (and product parts), and would like to retain some consistency between shots.

My thought is that for regular professional work the auto-rail would be hassle-fee, quick and easy... and studio macro is something I could still be doing a a decade or two, even though I am now 61.

[Dick Roadnight]

Even then, and in case you are actually serious, the challenges posed by extreme needs for rigidity, straightness, stability and isolation from imperceptible vibrations can't be overcome outside of a lab.

I appreciate that 25:1 is a challenge, inside or out.

The Sinar system is rigid and stable.

Vibrations are not a problem if the subject is mounted on some thing attached to the monorail... I would not try photographing a bee's eye (@ 25:1) on a windy with the bee on a waving flower and the . camera on a tripod.

I would think the weight of the camera moving up and down the rail would cause it to deform much more than 0.001mm, let alone the other factors - but I'm sure you have thought about a solution to that already.

I would use an additional support rail, and, as long as there was no significant movement between exposures, it would not make any difference.

Zeiss are a serious company, and I do not think they were joking when they made the 16mm f2.5, which is recommended for 10* to 40*, see macrolenses.

[Bart_van_der_Wolf]

I appreciate that 25:1 is a challenge, inside or out.

That's correct.

Not only are you going to be faced with very narrow DOF (e.g. ~6.4 micon @ f/8.0 when you use a sensel pitch of 6.4 micron and want to avoid diffraction as much as possible), but you'll need to find a way to get a lot of light (676x more than a regular exposure at infinity) on the object, and you'll wind up with a very small working distance which makes it difficult to get frontal lighting. I'm not saying it's impossible, but agree it's a challenge.

Moving the object/subject stage will require a lot of precision (given the narrow object side DOF), and it will change perspective during the incremental focus plane steps. Moving the sensor plane would have the benefit of unchanged perspective (and lighting!), and a larger depth of focus (~4 millimetres) thus requiring less positioning precision (can even be done by hand!). So using a view camera system has it's benefits.

Cheers,
Bart

[Dick Roadnight]

That's correct.

Not only are you going to be faced with very narrow DOF (e.g. ~6.4 micon @ f/8.0 when you use a sensel pitch of 6.4 micron and want to avoid diffraction as much as possible), but you'll need to find a way to get a lot of light (676x more than a regular exposure at infinity) on the object, and you'll wind up with a very small working distance which makes it difficult to get frontal lighting. I'm not saying it's impossible, but agree it's a challenge.

Moving the object/subject stage will require a lot of precision (given the narrow object side DOF), and it will change perspective during the incremental focus plane steps. Moving the sensor plane would have the benefit of unchanged perspective (and lighting!), and a larger depth of focus (~4 millimetres) thus requiring less positioning precision (can even be done by hand!). So using a view camera system has it's benefits.

Cheers,
Bart

Frontal lighting would be a major problem with a P2 lensboard, unless I paint it white and use it as a reflector.

So using a P3 is the obvious solution... except that P3 bellows are short and expensive... the solution I have for this problem is to have two P3/P2 conversion bellows, so I can use P2 standards and bellows to extend a P3!

If you focus on the back of a subject (away from camera) then increase extension by moving the sensor standard, the magnification increases as you move focus towards the front of the subject, give the effect of enhanced natural-looking perspective, as the front of the subject will be reproduced bigger. It should be possible to mount the rear standard on an auto-focusing rail, or connect a stepper motor to the focusing knob.

Moving the subject would result in ¿no? perspective as the reproduction ratio would be the same for each focus slice?

[Bart_van_der_Wolf]

Frontal lighting would be a major problem with a P2 lensboard, unless I paint it white and use it as a reflector.

Which is exactly what I would do, use the surroundings of the lens as reflector, it even allows to use more light from one side to maintain the impresssion of depth by sculpting 3D surfaces with light.

It should be possible to mount the rear standard on an auto-focusing rail, or connect a stepper motor to the focusing knob.

Yes, that would be the best solution IMHO. The stepper motor doesn't even have to have such a fine stepdistance, because the depth of focus (so at the image/sensor side) becomes bigger than the depth of field at the object side of the setup at larger than 1:1 magnification. You'll have plenty of leeway (several millimetres) for focusing with the rear standard, but only microns at the object end.
 

Moving the subject would result in ¿no? perspective as the reproduction ratio would be the same for each focus slice?

Moving the subject will change it's distance to the entrance pupil, and with that the perspective will change. It may or may not be an issue for the kind of object you're shooting, but it's best to avoid it if possible because it might result in halos with focus stitching.

Cheers,
Bart

[pchong]

Hi Dick...I am now very curious...can please you show your setup and the pics you have taken to illustrate the gear and technique?

[Dick Roadnight]

Hi Dick...I am now very curious...can please you show your setup and the pics you have taken to illustrate the gear and technique?

I am glad that I have given you the impression that "I know what I am doing because I have done it" but that is not the case... I have been acquiring kit and info for macro for some considerable time, and and auto-step-focus-stacker might be about the last piece of the jigsaw to make 25:1 practicable. Until I told him about this concept Doug thought that > 10:1 was not practicable.

[Dick Roadnight]

Dick,
One thing I may have missed and one I didn't see:  When the lens is moved for focus, the object image tends to not only go in and out of focus, but also change size slightly.

If you move the lens (or the sensor/rear standard {relative to the lens}) you change the focus distance and the magnification,, and, as above, this can give the effect of perspective... If you focus by moving the subject (or moving the whole camera relative to the subject) the magnification does not change, but you still get a perspective difference in size, but anything in the plane of sharpest focus will theoretically have the magnification ratio.

Second: the circle of confusion for digital for really sharp images would be at least .007 I believe.
Is this true in the case of your experience or am i way off?

For DSLRS with Anti-Aliasing filters, I would initially try a CoC of three times the pixel pitch, for single-shot medium Format backs without Anti-Aliasing filters about double the pixel pitch, For multi-shot Medium format backs 1.5 times the pixel pitch... but suck it and see what works for your setup.