Can anyone help with Camera Obscura project using a Projector len?

[stella_h]

<p>Hi everyone,</p>

<p>Apologies if I am posting this in the wrong place but it looked the most likely category for my query.<br>

To be as concise as I can, I'm wanting to build a camera obscura with a lens as opposed to a pinhole. I have acquired a projector lens that I am told came from a training simulator projector. As I was looking for something that would cover either 20x30 or 30x40 inches it seemed the lens could be a good idea. (It wasn't very expensive so worth the punt). The lens diameter is 14cm and the length 26cm. There is no shutter, of course, and no iris, and no other identifying information. If I can figure out a way to post a photo (sorry, newbie problems) I will. There are multiple lens groups inside and the lens comprises an inner cylinder that holds the lenses and an outer cylinder that has a grooved band that I thought would be a grip for focussing but it doesn't seems to move. (Although strangely enough, I thought it did when I first picked it up - I may have been mistaken or there may be some kind of locking mechanism?) I mocked up some giant bellows just to look at the focussing distance and was surprised that when I was behind the lens with the flange plate nearest me, focus using tissue paper as a ground glass was almost touching the back of the lens. As it's a projector lens I then turned it around (flange plate furthest away) and found focus on tissue paper several inches behind the lens. Either way of course not the large image circle I was looking for.<br>

<br>

Would anyone be able to tell me if this is likely to be just a focus issue and larger images will be attainable once I figure out what is going on with the focussing outer cylinder? Or am I missing something really obvious to everyone else and that it won't be possible to use a projector lens of this sort in the way I'm trying to?<br>

<br>

I very much appreciate any help or insight anyone can offer. I shall now go away and see if I can find out how to post a photo.<br>

<br>

Thanks a lot.<br>

<br>

Stella</p>

[stella_h]

<p>Ah-ha, I've just freed the focussing ring. I can go back to my walk-in bellows. I am still very interested in any advice you large format users have on using such a beast of a lens though. Somehow I'm guessing I'll probably need to fabricate a lens hood too? All advice will be greatly appreciated.</p>

<p>Thanks again.</p>

<p>Stella</p><div></div>

[dan_fromm2]

<p>First things first. Does the lens cover the format you want to shoot at the magnification you want to use? I ask because if it is from a projection TV it won't cover more than the CRT/LED panel/whatever whose image it projects.</p>

[stella_h]

<p>Hi Dan,</p>

<p>Thanks very much for your response. Just from experimenting with it just now it seems that the focusing ring does very little (at the distances I've been experimenting with). It looks to be an immensely heavy macro lens. So I'm thinking you might be right. Coverage seems to be somewhere between 5x7 and 10x8. Not exactly what I had in mind. I wonder though if the lens configurations could be altered to get the coverage I want?</p>

[john_shriver]

<p>See what size image it presents in both directions. If it's from a projection TV, it might be more suitable for your use backwards.</p>

[stella_h]

<p>Hi John,<br>

When I tried it earlier using tissue paper in the place of groundglass the focus point was virtually touching the lens when the lens was in 'projecting out' position. When I turned the lens around to use it backwards focus was a few inches away from the lens but (being so close) produced a small image circle. I'll try a different set-up tomorrow though and see if I can achieve better.<br>

I noticed something strange earlier just when looking through the lens. Having the lens pointed at a subject and me close to the back of the lens the image was sharp, as I backed away the image lost focus but as I kept backing away it came into focus again. I have to say I thought that was quite mysterious. I need to set up a longer bellows to follow the full projection I think.<br>

Thanks for responding to the post.<br>

Stella</p>

[dan_fromm2]

<p>Stella, lenses for large format cameras don't have focusing rings. They are focused by changing the lens-to-film distance by moving one of the camera's standards.</p>

<p>Your lens really smells like a lens for a projection TV of some sort. If so, probably not what your project wants.</p>

<p>There aren't that many lenses for LF cameras that will cover 20" x 30" (diagonal 36", ~ 900 mm) or 30" x 40" (diagonal 50", ~ 1270 mm) at infinity. Do you intend to shoot landscapes or near life size portraits. The latter can be done with somewhat more easily found lenses. None inexpensive. Your trophy's (no sarcasm intended) low price and obscurity are hints.</p>

[stella_h]

Thanks Dan,

 

I'm not worried, it's all learning. I actually really like the lens and was quite amazed that as close as I was to the back of

the lens the image still looked likely to be nearing the 10x8 inch coverage. It will still get used and I'll learn a lot in the

process. Thanks for your help.

 

Stella

[mike_earussi1]

<p>There's a lot of old Xerox copy lenses floating about. They will have very short focal lengths and cover about 8x10. This may be what you have.</p>

<p>And Dan is right, there aren't many (if any) LF lenses that cover 30x40 at infinity, and if they do exist they won't be cheap. So if you do want to cover that size at infinity an actual pinhole is your only real option.</p>

<p>Of course, if time and money are no object then you could try this:<br>

http://petapixel.com/2012/04/03/wet-plate-photography-with-a-giant-format-van-camera/</p>

[stella_h]

Thanks Mike,

 

I believe the lens came from a training simulator projector. And thanks for the link. I did stumble across Ian Ruhter's van

camera a while ago when I started researching camera obscura. Crazy guy, huh? I'm puzzled as to why each image costs

upwards of $500, I'm guessing it's his road crew and travel expenses factored in. But making images beyond 14x11, for

me, it is pretty much essential to be pouring your own emulsions, cost wise. And in ULF size wet plate and pinhole aren't

an easy combo because of the length of time of the exposures. Still, it's all part of the territory. Photography is never

without trade offs. Thanks for posting.

 

Stella

[mike_earussi1]

<p>Stella, in order to both simplify your life and reduce your costs you can also expose photographic paper directly, which will make a negative that you can then contact print for a positive. This was actually the original way photos were made: http://en.wikipedia.org/wiki/Calotype</p>

<p>and though they are a bit soft compared to glass negs, they do have their own unique look. </p>

[allan_f.]

If you don't have much invested, why not try taking the lens apart? One or more of the elements or groups might be much closer to your

requirements. The lens looks like it has at least a few larger glass lens elements that could be reused. For example I have a loupe made

from the front element and barrel of on obsolete TV camera lens.

[stella_h]

Mike, That's a good point. That would be an easy and less costly way to get to A3. For some images that softness would

be perfect. I am a sucker for the crispness of ambro and tintype though. Thanks.

 

 

Allan, that is a great idea. As the element configuration at the base is obviously intended to project the small sized image

outwards onto a wide screen, I'm wondering if even removing that element might be all that is needed? It's at times like

this I wish I could get my head around Physics and Maths better. I have to learn by physically trying things out what most

people can with paper and pencil.

 

Something struck me as odd when I was looking at it though. I had placed an object in front of the lens and was looking in

through the back of the lens, quite close to the lens and the focussed object was sharp. As I moved backwards the object

appeared out of focus (as one would expect) but as I backed away even further the object came back into focus. I'm not

sure if this is what would normally happen with a multi element lens and was just more noticeable due to the large lens

size, whether this is particular to projection lenses or whether I'm just being daft. I've been struggling to get a large

enough area in total darkness to experiment with it. I'll have await the arrival of my camera obscura tent, I think.

 

I don't want to destroy the lens but I'm buoyed from having yesterday managed to strip a process lens, clean it and put it's

disassembled iris back together successfully, so feel it's worth a try. I need to get a bigger lens wrench, that's for sure. It's

definitely worth attempting. Thanks for the idea.

 

Stella

[rodeo_joe1]

<p>Stella, the physical size of a lens is no guide to its coverage. You need to know the focal length [ http://en.wikipedia.org/wiki/Focal_length ] of the lens, and/or what it was designed for. A projector lens of any sort is unlikely to be useful as a taking lens to cover A3 or a larger area. For the simple reason that they're designed to project from a small-sized image to a large screen. Replace the screen with the subject, and you can see that the image projected by the lens will be a lot smaller. Maybe only 2 x 2 inches or less.</p>

<p>To cover A3, which is approximately 16" x 12" (nearest common photographic paper size), you'll need a lens with a focal length of about 20" or 500mm. This won't be too easy to find. Ignore the physical size; such a lens could be as slim as 1 or 2 inches thick and maybe 2" to 3" diameter to give a reasonably bright image, and therefore useably short exposure times. As already posted above, such lenses can sometimes be found in commercial photocopiers capable of producing A3 or larger copies. These are usually symmetrical design lenses and can often be split in half to produce two lenses capable of covering an even larger format - with reduced image quality of course.</p>