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.

Yesterday's photo

Here's the photo I took yesterday.

There are lots of things wrong, most notably a light leak that comes as a big white splodge on the right. But I was happy with this: because there's no similar splodge on the left. That means the film holder I made works on one side. When I worked out which side is good, which bad (remembering that the image on film is upside down) I could see that one side is slightly better than the other. So all I have to do is fix up the other side too, and that does not seem so difficult now I know that all I have to do is make it like the other one.

Experimental design

There's another tip here. Every experiment should if possible test only one thing. Then you know if it didn't work what was wrong. I'd already tested the film and the developer I was using in another camera and knew that was fine. So that eliminates one possible source of problems. And so on. It's sometimes tempting to rush ahead and try lots of new things all at once, but it doesn't always pay in the long run.

There's something else strange about this photo, but not really so strange because I'd seen it before somewhere else and was half-expecting it. That's the big sideways-on moon-shaped curve at the top. This is because the lens I was using didn't "cover" the huge negative. This is a common problem with large cameras and easily identified. In fact some photographers made it almost a trade-mark, and used such vignetting due to lack of lens-coverage as a feature of their pictures. Eugène Atget was one such photographer.

In my case, I'd rather have this vignette when I choose to have it, and not leave it to chance. That means I will have to modify something to get rid of it, possibly introducing it again when I really want it. But I was surprised it was so noticeable - I was expecting the lens to very nearly cover the negative. When I had put this lens together I had calculated what to expect, and it wasn't supposed to be this bad. I checked my calculations, they were OK. Eventually I decided to check the +1 close-up lens I was using.

The focal length of a lens

A good quick way to estimate the focal length of a lens is to project an image of the sun (which is effectively "at infinity") onto board or paper and measure the lens-to-image distance. (I will have to say more about this later too.) In my case it was about 650mm, and the lens was supposed to be +1 dioptre or 1000mm. That's appalling - very inaccurate. Soon I will be measuring the focal length of some of my other lenses to see if they measure up to what the manufacturer says.

When I redid the calculations with 650mm they came out in line with what I had observed. Again this is bad news in a way, but good in that one more variable has been eliminated. And so it will go on.

Scratches

To finish today, here is a detail from that 20x24 image.

Close-up you see a nice feature - lots of little scratches on the film. This really is an interesting film-hack to know about. There are some wonderful effects with scratched film that would take a photoshopper a month of Sundays to make. (Or do you buy things in photoshop?;) But again, I don't want them all the time so I will have to investigate. There's a clue - all the scratches are parallel. That means I think that they were made when the whole film slid as one piece either cutting or mounting the film in the holder. Another thing to investigate.