Negative accessory lenses

We are used to using positive lenses as accessory lenses, usually as close-up adapters. These are typically placed in front of the main lens and allow one to focus on closer objects that would otherwise be the case. They work by increasing the strength of the main lens, i.e. decreasing the focal length. They are usually quoted in dioptres: +1, +2, +3, +4, +10, etc.

Telek lenses

In the good old days there were plenty of other lens adapters. One particularly interesting sort is the Kodak Telek lens (sadly no longer made) which were negative adapter lenses. For example, this one fits "series VI" fittings and is -2 dioptre.

I suppose the reason these are not popular now is that most cameras have fixed fittings and to use a Telek lens on a camera on would have to move the lens further away from the camera. With the right camera, that could be done (just). You could add a Telek lens and an extension tube. Then the result would be a longer focal length, so would magnify more, a bit like a telephoto lens. Definitely worth a try if you have the necessary parts.

I had a Telek sitting in my box of spare parts for some time. I never quite found a particularly good use for it. Until, that is, an interesting vintage lens came in the post which I wanted to try on my 10x8 camera.

This lens dates to about 1880 give or take a few years, and is by M.P. Tench. Tench had worked with Dallmeyer until he set up on his own. This lens is a simple doublet with an aperture set back well behind, and might have been used as a landscape lens for 1/2 plate, or possibly 4x5.

The problem was that this lens was just a bit too small for my 10x8 camera and (to see it working as I wanted to, the bad parts of its image as well as the good) it was a bit too large for 4x5. The image it made on 10x8 was a circle, but I wanted it to fill the whole frame. I measured its focal length at about 150mm, and realised I wanted something just like it that was about 200mm.

Since my Telek lens increases focal length (at some loss in quality, which would not be significant when one is contact printing 10x8 negatives) it would be just the thing I need. There was a small matter of selecting whether to put the Telek on the front or the back. It turned out that it fitted beautifully at the back, which was probably the best place for it anyway. So that's what I did.

I mounted my Tench lens on a lens-board and fitted the Telek lens to its rear. Since it was a very good fit, and the Telek is very light, I just used blu-tack. Maybe I shall revisit this arrangement one day, especially if I decide this lens is a "keeper", but works surprisingly well for the time being.

That F number thing

One doesn't get something for nothing. By making the lens longer, but with the same amount of light going through it, the light intensity on the film is less. It is this light intensity that is measured by the "F number". (Or at least that's true without so-called "bellows factors", which we will ignore here.)

The rule that was chosen for F numbers way back is the F number is the focal length of the lens divided by the diameter of the aperture. So a f=50mm lens with an aperture 25mm in diameter is F50/25 or F2. This information is important for any new lens. I measured my Tench/Telek combination as f=200mm and aperture of 12.5mm so decided it was F16, which seems slow by modern day standard but is fairly typical of vintage wide angle lenses and perfectly adequate for what I wanted.

Testing it

Armed with the information about F-numbers I took my camera off to try it out. I loaded some film holders with photographic paper, which I find is much quicker to develop and scan for testing purposes.

For the first test I wanted to see how the lens behaved when focused at infinity.

This was a difficult contrasty subject, and I reduced the contrast here: any lack of contrast is not the fault of the lens. You can see in the corners where the lens is not quite covering the frame, but it gets close and the effect is quite pleasing. The lens has a very nice "antique" look to it, especially in the corners.

For the second test I want to to focus up close and study the out of focus background.

When focused close, there are no longer any coverage problems. Again, it is very pleasing look, with a noticable but subtle swirl in the background.

The Ross epidiascope

It's been a long time since I posted here - sorry folks. The day job got in the way, and then somehow I got out of the habit. I'm going to try and post about once a week for the next few weeks. (Maybe once a fortnight might be more realistic.) Let's see. Anyway the plan is to try something regular, if not frequent.

A Ross Epidiascope

I like old gear, and I like fiddling with old stuff to try and get it to work. And I like my photographs big. So when the possibility to buy a beast like this came up I went for it, even though I hardly need another project, and have only just enough space to keep it.

OK. What is it?

An Epidiascope is like an old-style slide projector or magic lantern and overhead projector built into one. It would have been used mainly for displaying images during lectures, talks and so on. So education was the main market, though no doubt they were used in cinemas, military etc. The gadget has a dual function, and I guess this is made economical because of the powerful light source.

Dia function

The more familiar function is that of a slide projector. That's the "dia" part of the name, from the Greek for "through", and in most languages but not English the word for "slide" is based on "dia". ("Das Dia", in German.) This slide projector is no ordinary 35mm projector like I remember being terrorised with by neighbours just back from their holiday. ("You must come round and see our slides.") Instead of projecting a 24x36mm image (or even a 56x56mm image if you are lucky enough to work with MF slides on a Hasselblad) the slides are three and a quarter inches square. So images of 80x80mm or so can be displayed, with correspondingly better resolution and punch. And instead of a perhaps 250W bulb, the epidiascope has a 1000W bulb for four times the brightness, or four times the area. That at least is the theory. I want to try it out.

The lens used is a 10.5 inch projector lens of about f/5. That was obviously intended for a fairly long throw, as well as coverage of the larger slide. I will have to see how it performs for me, but a wider lens might work better in my limited space. This is for experimentation as well. In any case the lens seems a good one and may also be useful as a taking lens for 4x5. Watch this space.

Epi function

The other function is the "epi" one, and this refers to projecting the surface of an opaque object or drawing. So in this respect the epidiascope behaves more like a modern day visualiser. The object to be projected is placed on a movable tray under the projector and can be brought flat against a glass underneath it. The light from the huge bulb is split into two paths and reflected off two mirrors in the side of the gadget. It illuminates the object, and the light reflected off it is reflected by a mirror and projected by a huge 17inch lens. The combination of the mirror an the lens restores the object to normal way round. (As opposed to upside down or mirror image.)

The area projected is about six inches (150mm) square. So quite large images should be projected. Once again, I need to try it to see how well it works in practice. Watch this space!

History

I have no idea how old this object is. Ross marketed models very like this as early as 1932 and up to the 1960s. Mine appears to be a later model as it has a fan added on the front. Mine is similar but not quite the same as this one http://www.curzon.org.uk/content/public/curzoncollection/RossEpidiascope.aspx and a contemporary advert for a very similar one is available on-line here: http://www.everystockphoto.com/photo.php?imageId=11740710. Mine is Type B.E model 2, which suggests it's not the oldest. More information would be gratefully received!

My plans are firstly to get it going and see how good it actually is. That means cleaning it carefully and re-wiring it inside. If the bulb inside is no good or blows, and it looks like I will continue to tinker with it I will probably replace the original bulb with a modern and more easily replaceable halogen one. I would also like to try making old-style three and a quarter inch slides. If all else fails these are very interesting lenses to try out. We will see...