Understanding vignetting with infin Objective + zoom camera

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Croskell
Posts: 2
Joined: Mon Mar 25, 2013 6:39 am

Understanding vignetting with infin Objective + zoom camera

Post by Croskell »

I am a new member and don't quite know where to post this question - 'Equipment' seems appropriate. I can't see a thread approprite to tack onto.

I am planning to build myself a macro table for the following kit - Nikon D800E + Nikon 70-200 VRII zoom + Mitutoyo 10x Plan APO LWD (not yet purchesed) + StackShot. My question concerns the un-vignetted FOV I will get at differing zoom focal-lengths, and hence overall mag.

Am I correct in thinking that at 200mm, I should get a full image on the FF sensor of the D800? If so, as I zoom out (towards 70mm) at what point does vignetting start to occur at the corners and at what point do I end up with a 24mm diameter circle? Note the Nikon lens is internal focus, internal zoom, so the external size does not change. It is constant F2.8 throughout. Is it a matter of figuring out at what point the lens iris becomes less than the FF diagonal 43mm in diameter (focal lenght/2.8)? Or isn't it as simple as that? I'm trying to figure out if I can expect a x5 mag at 100mm zoom, or even x3.5 at 70mm.

A second question - can someone please point me to the formulae to calculate DoF (for Stacking purposes) for the above setup?

Thanks.

rjlittlefield
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Post by rjlittlefield »

Welcome aboard!

There's another issue that's not on your list but should be: limited FOV of the objective.

My copy of the Mitutoyo Plan Apo 10X NA 0.28 has a high quality image circle that is notably less than 5 mm in diameter on subject. At 5X (on a 100 mm tube lens), it gets soft and astigmatic even in the corners of an APS-C sensor. See the discussion and images linked HERE. I think you would not like its appearance at 5X on full-frame.

As for the vignetting, I can definitely confirm that it's not as simple as just looking at the diameter of the lens pupil. Pupil diameter provides a sort of worst-case estimate that applies when the pupil is also located far back toward the camera. When the pupil is farther forward, the vignetting is not so severe. In the extreme case, if the pupil is located near the front of the tube lens and there's no extension to the objective, then a pupil as small as 15 mm will cover full frame with no vignetting.

For DOF, a safe value for NA 0.28 is 0.007 mm. The formula is 0.00055/NA^2 and corresponds to 1/4-lambda wavefront error for green light. It's pretty common to use step size around 0.005 mm (5 microns) in this case. Pushing it much beyond 0.010 will produce focus banding when you get down to pixel-peeping.

I don't know your interests and priorities, but it may be worth considering the Nikon CFI Plan Achromat 10x NA 0.25 WD 10.5mm (part number MRL00102) instead of the Mitutoyo. As shown at the above link, the Mitutoyo has far less longitudinal color aberration, but the Nikon delivers better detail over a larger field. Given the rest of your gear, you might consider getting them both -- the MRL00102 is only about $250 new from Nikon, and it can often be picked up for about half that used.

--Rik

Croskell
Posts: 2
Joined: Mon Mar 25, 2013 6:39 am

StackShot at x10

Post by Croskell »

Thanks for your prompt and helpful reply. Realise your quoting DoF as c.6 microns for an x10/NA 0.28 setup, leads to another Q. I have understood StackShot is good (consistent) down to 10micron increments. Does this imply x10 work is already beyond its capabilities, in the opinion of those that have used it?

Thanks again,
John

rjlittlefield
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Post by rjlittlefield »

I regularly use my StackShot with 50X objectives. My unit has demonstrated good performance down to below 2 microns when High Precision mode is turned on. See the detailed test report What does StackShot "High Precision" mean?.

In my personal opinion, the StackShot has suffered a lot of unjustified bad press due to its disarmingly simple mechanism combined with outdated information.

Early units indeed were not usable beyond 10X, because of the big-step problem documented in the article linked above. The problem was made worse by a user interface that made it almost impossible to set a natural focus increment of 1 full motor step. This obscured the regularity of the big-step problem. For a long time the irregular steps were incorrectly attributed to mechanical issues like stiction. After I characterized the problem better and convinced Cognisys that it was an electrical drive issue, they were able to retrofit High Precision mode as a firmware fix. Initially that mode was available only through the Zerene Stacker interface, but then they made it available also through the buttons and menus on their own controller.

At this time I have no reservations about recommending StackShot for 10X. Beyond that it may get fiddly. 50X works fine with my unit and my procedures. I have not seen test reports from other people.

I confess, I find this issue to be kind of frustrating. For a long time, the StackShot web site continued to say 10 microns despite the improved firmware that made it good well below that. In a quick look today, I see that the website finally says 2 microns, which is probably a good spec given the tests that I've done. (I'm not aware of other tests at this level of precision.) But still there are a lot of web pages that quote the older 10 micron spec, and that number and the corresponding general impression seems burned into people's heads. It's like the old saying: "You can't un-ring a bell."

--Rik

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