Can an iris be used with a scanner lens to get greater DoF?

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Beatsy
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Can an iris be used with a scanner lens to get greater DoF?

Post by Beatsy »

I tried one with my new DImage F-2400 lens and it just vignettes. With the adapters I have it's around 20mm from the back of the lens but that works OK with Mitties. Thinking it might just be too far away, I hand held the lens against the iris and it still vignetted. Am I trying something that doesn't work anyway or do other factors come into play (e.g. distance from sensor etc).

Ta

rjlittlefield
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Post by rjlittlefield »

Vignetting is always due to two (or more) apertures fighting with each other. In your case it's the aperture inside the lens versus the one added by the iris.

When the iris is behind the lens, the thing to think about is the relationship between the diameter and location of the exit pupil of the lens, the diameter and location of the iris, and the size and location of the sensor. Vignetting happens when the geometry is such that some or all light rays that would otherwise reach the corners of the sensor, passing through the iris, would have to come from outside the exit pupil of the lens. The lower the ratio of distances, sensor-to-iris versus iris-to-exit-pupil, the more likely you'll get vignetting.

One thing to try is placing the iris in front of the lens instead of behind it. That changes the concern from exit pupil to entrance pupil, so if the lens is asymmetric, switching sides may help.

--Rik

Beatsy
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Post by Beatsy »

Thanks Rik. I did try the lens both ways round (the rear element is recessed more than the front ) with the same result. I didn't try an iris in front though so I'll try that next. Note: I don't see a separate aperture in the lens. I assume the retaining ring in the back end counts as one though.

rjlittlefield
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Post by rjlittlefield »

Beatsy wrote:I assume the retaining ring in the back end counts as one though.
Yep, wherever a limiting restriction is, that's an aperture. Something retains the lenses in the front also, so that's another possibility.

If you don't need rapid changes of aperture, and the lens elements are recessed very far into the housing, then another thing to try is punching a hole in a piece of black paper that can nestle right up against the lens elements.

--Rik

Beatsy
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Post by Beatsy »

Yes, I could use a temp paper-aperture for taking hand-held macro snaps but only on the front (if it works) when stacking. There I just want it for first and last shots of partial stacks to improve transition between in and out-of-focus regions. I wouldn't want to risk unscrewing things then.

Thanks again.

rjlittlefield
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Post by rjlittlefield »

Beatsy wrote:I just want it for first and last shots of partial stacks to improve transition between in and out-of-focus regions.
For this application, one concern is that changing the aperture location also changes the image perspective. Placing an added aperture in front will cause the image to have the perspective of a closer viewpoint. This may (or may not) introduce problems with alignment. For reference, see http://www.janrik.net/PanoPostings/NoPa ... xPoint.pdf, Figure 3 and the animated gif that goes with it.

If that's a problem, then you can attack it by using two added apertures, one small to get the increased DOF, and a second one much bigger, just enough smaller than full aperture to establish the paper as the limiting aperture even though the lens is almost wide open.

--Rik

enricosavazzi
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Post by enricosavazzi »

In general, I found that, when adding an aperture immediately at the rear of a lens causes a visible vignetting at a given magnification, the vignetting disappears or reduces substantially if the magnification is increased by a sufficient amount (to be found empirically).

This is only a rule-of-thumb, and there may be lenses where this does not work. This method is also not applicable when increasing the magnification pushes the lens out of its optimal magnification range.
--ES

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