Optimal aperture position?

Started 2 months ago | Discussions thread
OP E Dinkla Senior Member • Posts: 1,843
Re: Optimal in what sense?

aidaho wrote:

E Dinkla wrote:

ProfHankD wrote:

E Dinkla wrote:

Optimal position for the aperture in the optical path?

Obviously, in front of the lens is optimal in the sense that it is most accessible.

Basically, there are multiple positions within, behind, and in front of most lenses that can be suitable places for an aperture stop. However, there are various differences between the different locations because rays don't get clipped the same way -- so, for example, some locations might result in better control of aberrations than others. It really takes either modeling the lens or physically testing it to determine the most viable aperture positions. A valid aperture stop position is one in which the opening does not cause vignetting (i.e., isn't a field stop); ironically, too large a hole is generally how you get vignetting.

If at all possible, I prefer the front mounting of a Waterhouse stop. It's easy to mount/unmount them on unmodified lenses and they are easy to make very precisely the size and shape desired, whereas an iris is really a pretty challenging precision build. My Waterhouse stops are generally cut on a programmable paper cutter, laser, or CNC router/engraver; larger aperture ones also can be 3D printed.

The projection lens that I have is the Leitz Elmaron 150mm 2.8 for 6x6 slides. Basically a Triplet but 4 elements 3 groups with a two element negative group in the center. Trioplan related, more resemblance to old Hektor + Thambar designs though. Lens diagrams for these types show that an aperture can be in front of the rear element and close to that element. I was thinking of a Waterhouse stop too with two openings in the slider, one keeping 2.8, the other closing to best performance. Should be based on trials. However it will be easier to add the waterhouse stop behind the rear element and in front of the focal reducer that can also be fitted within the projection lens barrel. Milling two slits in the barrel of the Elmaron at the rear of its optics is possible. 3D printed parts to secure a lightight fit for the two apertures Waterhouse slide. The Antares 2" x0.5 is a focal reducer option, a 4 element would be better. Main difficulty is finding them secondhand and with a wide aperture.
http://allphotolenses.com/lenses/item/c_4586.html#prettyPhoto[pp_gal][lens]/1/
https://www.largeformatphotography.ihttp://allphotolenses.com/public/files/optical/2caec131cb96a2dfe33c4f5574346281.png nfo/forum/showthread.php?113984-Rathenower-Optische-Werke-Cinerectim-62-5-180-(Projection-lens)/page2

An interesting discussion / project on a DIY focal reducer for M4/3 before the Speed Booster even appeared, I wonder whether Brian in that thread got the idea to market it, could be another Brian/Brain of course:
http://www.personal-view.com/talks/discusshttp://allphotolenses.com/public/files/optical/2caec131cb96a2dfe33c4f5574346281.png ion/577/using-focal-reducers-with-gh12-and-old-lenses

Met vriendelijke groet, Ernst
No photographer's gear list is complete without the printer mentioned !

Is this the lens you are speaking of?

I see no good place for the aperture at all. The least bad one is likely as close as possible to the middle element, not sure which side of it. Second worst is the front element, as close as possible. Doesn't matter which side. The worst one is behind the lens.

Speedbooster trace might give us more options, but I can tell you right away: the gap between the lens and booster is the worst option here. Aperture behind the back element should work significantly worse than the one behind the first: it will cut into corners before it will cut the center.

This placement will induce strong vignetting.

Well for most triplet and triplet related designs they decided to go for the second air gap and the aperture can be quite far to the rear in some:

left item is the plane glass with silver center spot of the soft focus thambar

The point is whether the addition of the focal reducer can alter the optimal position of the aperture for the total. And I doubt it will bring it forward in the triplet designs.

Met vriendelijke groet, Ernst
No photographer's gear list is complete without the printer mentioned !

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