How to easily reduce the flange distance

[alex ph]

Some of projection and enlarger lenses have a very large flange focal distance, from 15cm to 25cm. This is an example, an interesting lens which makes part of the "problematic" list (the photo is the seller's courtesy)



Among other inconveniences, such as the need for a long and quirky adapter tube (which completely disbalances the whole setup), the downside of it is a huge loss of the light transmission. Being afapted, the lenses whose original largest apertures vary from f2.2 to f8 lose their light value in times.

I wonder if there is a simple way to shorten the flange distance, adding a correction lens or two. If yes, which lens(es) should be used? If you have an experience of DIY lens construction or adaptation of the kind, could you please share your findings.

[eggplant]

In the telescope world, there is such a thing known as a 'folded refractor', where a series of flat mirrors are used with a long focus achromat (e.g. 1000mm focal length - 1000mm flange distance!) to reduce it's overall physical length.

Whilst the examples online are quite long, there is nothing stopping you getting creative like this:





This is the only way to my knowledge to shorten the flange distance of the lens without significant and fundamental changes to the optical prescription. You will lose some transmission from the mirrors. You could rearrange it to be more convenient. I don't know what sensor/film size you intend to cover but the mirrors could be quite small for anything full frame or smaller.

This is a much, much easier idea and doesn't require too much expertise.

I'm not sure why you say a long tube causes a "huge loss of the light transmission", and I'm not too sure its really worth it for a 150mm and 250mm lens- maybe the need for this will go away with adjusting expectations, but I just wanted to float the idea anyway.

[RokkorDoctor]

[alex ph]

What an elegant solution, thank you for the idea eggplant! I also see a version with two mirrors, so that the lens alignment is straight.

I wonder if there are some ready solutions in the astro market, something of the kind of chinese helicoids.

As for general ideas you comment, I have to say I am not expert in optics. But what I know from practice, is that when you put a lens on extension rings long enough, you observe the loss of light transmission. In my experience, a 2x extender ring raises the shutter speed twofold with the same aperture, a longer bellow even more.

RokkorDoctor, what kind of doublet could work in this case? I'd be curious to try and to to see how it affects the IQ, waiting for the folded refractor to come.

Aside the aforementioned Minolta, I have for example an Elite 4.2/250 projection lens. It came with an adapter for slide projector which gives an idea of the long flange distance the lens has. Too long, in fact.




It is for sure an interesting lens, judging by a quick freelens experience. But operating with it is a true PITA.

#1


#2


So, an elegant mirror solution or a quicker lens reducer seem both interesting to me.

[DConvert]

[alex ph]

[eggplant]

[eggplant]

P.S. That lens looks like a rebadged ISCO Projar- sorry I can't find a better pic:



They are put in long lens tubes just like yours (this one for 350mm):



This seems to be an earlier/earliest body type of these lenses.

Funnily enough, the one I was talking about is the later-body version, and is also 250mm f4.3, and looks like this:



Not to derail the thread either, but- it splits into three, all the optics are contained in the front section, I used a 51mm step ring in the middle part which isnt the exact pitch but fits snugly enough.

Even in this later version, I can tell you- it has really bad issues with veiling glare even on the dullest of days and not next to any direct sunlight, maybe because the tube on mine is quite shiny.

The LoCA is pretty lairy, but again these are designed to cover much larger film than the APS-C camera I was testing it on. No glow, decent bokeh.

This model and similar are in an awkward spot, because I've got 7.5 inch (190mm) slide projection lenses at a similar aperture which are a bit nicer to handle, and 250mm or longer there are some fast aperture (f3.5-f2.8 ) slide projection lenses that are a bit more interesting proposition.

[-rageQuit-]

(Not intended as the last word, because I might be wrong)

1. Because binoculars invert the image so that it appears the right way up, if those binocular components were used with a camera then that image would appear upside-down, wouldn't it?

2. The alternative binocular design is the Schmidt-Pechan aka the 'Roof' prism - which is the straight binoculars as opposed to the familiar zig-zag (Porro) mentioned before.

The advantages of hacking with such a component ought to include:
a) my understanding is that, because of the difficulty in aligning the two prisms, they come as a sealed unit.
b) hopefully easier to mount securely to the camera - just stick 'em inside a dumb extention tube?
c) consequently, a bit neater and more compact

I suspect there is a limit to how big the lenses, and consequently the prisms, can be in binoculars, because of the need for the tubes to align with the eyes. The ones in monoculars might be larger. Then again, half-busted binoculars are probably easier to acquire.

[eggplant]

[alex ph]

Wow, so many insightful details, thank you!

A special thanks, eggplant, for the ISCO reference. The Elite does look like ISCO. I did not find any consistent information in the net regarding both, and I think, the reason is the too long flange distance.

After the first iteration of our exchange I thought about a Chinese ready-made monocular solution, something like that which is pretty cheap in general, or even a small cheap telescope. But then, reading your comment on the small exit pupil, I realize that this migh not be the best solution.

[paulhofseth]

Last year I tried to use a Zeiss Planitar 150mm (intended for flat field reproduction of A4 size originals). I mounted it in a suitably long plastic tube. But my blackening attempts were insufficient, so its wide light cone resulted in abundant haze-

p.

[DConvert]

[DConvert]