Elmo Zoom projection Lens F:1.2 (or 1,4) f=25-50mm

[Arnau Pell]

Good morning companions! I have a doubt with this lens and after using the search engine I have not been able to find the information I want
Is it worth it for a projection distance between 3.7 to 4 meters? I have an f1.1 12.5-25mm and I don't know if with the 25-50mm I could get a larger image with the same distance. Thanks in advance!

[Maurice Leakey]

Arnau
At 3 metres distance: 25mm lens gives 63cm x 47cm. 50mm lens gives 31cm x 23cm.
At 5 metres distance: 25mm lens gives 106cm x 79cm. 50mm lens gives 53cm x 39cm.
These details given from the instruction book of my Elmo ST-600.

[Arnau Pell]

thank you so much Maurice

[Joe Taffis]

The F1.2 and F1.4 lenses are both “long throw” lenses, used for places with a large distance between projector and screen.

[Stuart Budd]

It's the smaller number that gives the larger image. 12.5 to 25. The 12.5 is the wide angle and thus a larger image at any distance comparable to a narrow angel focal length (25-50).

The wider the image, the less brightness on the screen for any given lens as the light is spread out.

But a 25-50mm lens will give a smaller image overall. An 11-30mm lens will be wider and give a larger image using the 11mm part of the zoom than your 12.5mm zoom. It'll also give a smaller image using the 30mm part of the zoom than the 25mm part of your 12.5 to 25mm.

Focus is the next issue. A F1.1 lens has a shallow focus field. Meaning the thickness of focus at any given distance. Like reading glasses. That thickness of focus is linked to distance. The greater the distance the thicker that focus field is. Project an image a few feet using a fast lens like a F1.1 may be harder to focus the whole image because the thickness of focus may not extend to the imperfections in the screen/wall projected onto. That is, the top of the screen may be slightly further away than the bottom of the screen. Once the top is in focus, the bottom is out of focus. Making the image smaller will help at close quarters as the imperfections are over a smaller area. The idea, as mentioned, to throw the image further and get a greater thickness of focus to account for those kinds of errors.

So, in general a slow F1.4, F1.6 lens at close distances can focus sharply easier than quicker F1.1 lenses. They're not as bright but as the image is closer it looks bright.

Eventually, they're realise space isn't expanding. The redshift is a weakening of the information. I really should get back to number crunching, it's like vacuuming, it won't get done on it's own. Oh! Astro C3! I might get a bunch!

[Steve Klare]

Yes, the fundamental idea is to actually get a smaller image than the 12.5-25mm lens. When I do normal projection I use the 12.5-25mm, but when I put an anamorphic in front of this the image spills off the screen on both sides even zoomed small as possible.

At first, the solution was to move the projector forward, which put it right in the middle of the dining room table. "It's a centerpiece" didn't play at all well with my wife! (Some people don't like their centerpieces to have cables trailing across the table...)

Since I got the 25-50mm auditorium lens, everything can stay exactly where it started and projecting 'scope is much less traumatic! I can let it stay that way as long as I want.

[Stuart Budd]

Yes, the fundamental idea is to actually get a smaller image than the 12.5-25mm lens. When I do normal projection I use the 12.5-25mm, but when I put an anamorphic in front of this the image spills off the screen on both sides even zoomed small as possible.
...

Didn't think of that!

The old forum info:
https://8mmforum.film-tech.com/cgi-b...04951;go=older

It's quite an all round and interesting hobby.

A 25 mm lens will give a picture 1.2 m wide at a throw of 6m with Super 8 film.
Double the throw, you double the width. Double the focal length, you halve the width.
All quoted figures are a close approximation as different machines may have slightly different apertures, but they are close enough for all practical purposes.
Martin

Martin practically copied me word for word!!! Hash Tag Stamp Out Plagiarism

[Steve Klare]

Yes, I had the opposite problem with 16mm: the standard 50mm lens put the machine exactly where my wife's recliner belongs, so I had to find something shorter and move it forward.

(Optics and marriage just don't get along!)

[Arnau Pell]

It's the smaller number that gives the larger image. 12.5 to 25. The 12.5 is the wide angle and thus a larger image at any distance comparable to a narrow angel focal length (25-50).

The wider the image, the less brightness on the screen for any given lens as the light is spread out.

But a 25-50mm lens will give a smaller image overall. An 11-30mm lens will be wider and give a larger image using the 11mm part of the zoom than your 12.5mm zoom. It'll also give a smaller image using the 30mm part of the zoom than the 25mm part of your 12.5 to 25mm....

Master class! Thanks Stuart!