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aligning lens optical axis with rail movement

 
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Ultima_Gaina



Joined: 28 Jan 2017
Posts: 74

PostPosted: Fri May 12, 2017 5:06 pm    Post subject: aligning lens optical axis with rail movement Reply with quote

I'm trying to figure out the best way to align the optical axis of my lens with my automatic rail movement.

Is there an accurate method, hopefully already discussed on this forum?
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 20185
Location: Richland, Washington State, USA

PostPosted: Fri May 12, 2017 7:36 pm    Post subject: Reply with quote

I don't recall any clever tricks discussed in the forum.

However, this alignment is not as critical as might be imagined. Lining things up "by eye" works fine for me at all magnifications.

If you are fabricating a mount and want to compensate for subtle issues like sag, then you might make a careful measurement by mounting a long macro lens or even an ordinary telephoto lens on the camera, and using it to look at an alignment grid that you have carefully positioned some distance away from the rail and camera.

--Rik
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elf



Joined: 18 Nov 2007
Posts: 1384

PostPosted: Mon May 15, 2017 6:22 pm    Post subject: Reply with quote

If the lens is not aligned with the rail, you should see the image drifting in the direction of (or opposite depending on your point of view Laughing) the misalignment. If the leadscrew isn't precise or has a bend in it, then the misalignment effects could easily be masked.

The effect is more visible when using bellows draw to change focus planes as the distance traveled is usually quite a bit more than moving the subject or the camera/lens combo.
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Searust



Joined: 11 Mar 2016
Posts: 22
Location: Austin TX

PostPosted: Tue May 16, 2017 10:38 am    Post subject: Reply with quote

http://www.photomacrography.net/forum/viewtopic.php?t=32159&highlight=

there were a few good answers in this post on the subject
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Ultima_Gaina



Joined: 28 Jan 2017
Posts: 74

PostPosted: Tue May 16, 2017 6:57 pm    Post subject: Reply with quote

Searust wrote:
http://www.photomacrography.net/forum/viewtopic.php?t=32159&highlight=

there were a few good answers in this post on the subject


Thanks for the link.

In general, I'm not concerned with the my rail linearity and I'm not using bellows.

I realise that I deal with quite a few fuzzy details from behind foreground elements, details that should normal be hidden.

As Rik mentioned, it is not a critical issue, but I believe a proper alignment could lead to better quality and maybe less retouching.

I could parse Zerene's logs and plot them on a graph (after an alignment session) to figure-out how much off axis I "eyeballed" my lens. Then I could realign and check if I improved anything.
This effort can be worthwhile for I have a fixed setup, indeed.

But since I have to remove the camera/lens quite often, aligning and realigning by decoding Zerene logs is very time consuming.

I also tried to remove the microscope objective, then zoom 10x with live view on a distant detail. Unfortunately moving 10cm on my rail, has no impact on the distant live image I see.
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elf



Joined: 18 Nov 2007
Posts: 1384

PostPosted: Tue May 16, 2017 10:22 pm    Post subject: Reply with quote

Here's a model of an adaptor plate for my Olympus EM5-II and Manfrotto quick release:


The two posts register against the front of the camera, so the camera always stays aligned.
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ChrisR
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Joined: 14 Mar 2009
Posts: 8407
Location: Near London, UK

PostPosted: Wed May 17, 2017 4:39 am    Post subject: Reply with quote

I keep thinking it ought to be possible to shine a spot laser through a viewfinder and have it coincide with its reflection in a mirror, back behind the viewfinder using split prisms. I'm positive that it's doable, but I haven't spent time on it. I have an epi- lamp-aligning objective which I think would work. It directs the source 90ยบ onto a target in the side of the "objective". If your laser source stays on target as you "focus", job done?
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Ultima_Gaina



Joined: 28 Jan 2017
Posts: 74

PostPosted: Wed May 17, 2017 8:52 am    Post subject: Reply with quote

I'm thinking at a perfectly centered rod attached to the camera, instead of the lens.
Assuming the whole rig is precisely vertical, a suspended weight could be used to make sure the rod is exactly above the mid-section of the rail, through-out the whole traveling range of the rail

Something like in this "mouse-held" drawing.

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elf



Joined: 18 Nov 2007
Posts: 1384

PostPosted: Wed May 17, 2017 2:29 pm    Post subject: Reply with quote

Zerene Stacker is the only affordable sub-micron measurement tool that I know of Shocked

I expect building your centered rod to sub micron tolerances will be very difficult to do in a home shop. You could mount a lens with a long smooth barrel and measure the deviation with a test indicator.
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Ultima_Gaina



Joined: 28 Jan 2017
Posts: 74

PostPosted: Wed May 17, 2017 3:44 pm    Post subject: Reply with quote

elf wrote:
Zerene Stacker is the only affordable sub-micron measurement tool that I know of Shocked

I expect building your centered rod to sub micron tolerances will be very difficult to do in a home shop. You could mount a lens with a long smooth barrel and measure the deviation with a test indicator.


This is probably the best way. Thanks!
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