Homemade lens

I’ve often thought about making my own lenses, but unfortunately I’m so bad at making things I’ve always avoided it (apart from making a pinhole “lens”). But this week I was fortunate enough to be left with a broken 35mm slide projector, a Braun Novamat 515 AF-I, to be specific. It had a Braun Color-Paxon multicoated 85mm f/2.8 lens, with the usual screw mount for projectors. What’s the worst that can happen to a broken projector?

Braun Novamat 515 AF-I
Canon L lenses(that’s the pro series) have a red ring around them to distinguish them. Perhaps if you don’t look too closely you might think I’ve got an expensive lens here. Red ring? 85mm? Yep, must be expensive.

Braun Color-Paxon MC 85mm f/2.8
I dismantled the projector and removed the lens mount from inside it. “Removed” sounds so tidy… I had to use a hacksaw. Then I drilled out a Canon body cap, and glued the projector’s lens mount onto the body cap.

See, I told you I wasn’t good at making things. I believe that anything that can’t be made with a saw and a glue gun can’t be made at all 😀

Canon EF mount for projector lenses
The lens simply screws into the barrel, and can easily be focused like any other manual-focus lens, although the steep thread means it’s hard to focus accurately.

I seem to have cocked up the focal-flange distance and the lens can’t quite focus to infinity. However, a quick zap with a file will allow the lens to slide further back into the barrel and will fix this. I’m not too bothered though – 85mm is traditionally used for headshots.

As the lens screws so far out of the barrel it has very good macro capability too.

Braun Color-Paxon MC 85mm f/2.8
Canon 600D with 85mm f/2.8
Crucially – how well does it work? Surprisingly well, actually. Unlike camera lenses, projector lenses usually don’t have a variable aperture. This lens is stuck at f/2.8 all the time. This gives narrow depth of fieldand can make it hard to get a sharp picture.

Perhaps more importantly, as this is a projector lens and not a camera lens it suffers from worse aberrations. Stopping down would probably help reduce them. Perhaps in the future I will make a set of interchangeable apertures out of black cardboard. I reckon this lens would look nice at f/5.6 or f/8.

The contrast of the lens is a little low, despite the multicoating. I think it would perform badly outdoors in sunlight, since it just isn’t designed for that. However, the front element is recessed in a sort of hood, which will help.

These photos have had the contrast boosted a little and the white balance fixed (I shot with auto white balance under ugly CFL lighting, combined with golden evening sunlight from the window).

In this photo of the mains plug, you can see how shallow the depth of field is, by how blurry the skirting board goes. But the lens is capable of sharp images – just look at the dots on the plug for my toothbrush.

Mains plug
In this picture of Mittens, the whiskers, eyebrows and hairs on her left ear are quite sharp. I could probably have focused more accurately given some time, but kittens never sit still for long. The black shapes in the background are cables hanging off my computer desk.

And finally, this picture is of my jeans. Obviously when doing macro work, you need to stop down a lot to achieve decent depth of field, and that just isn’t possible with this lens. But it’s not too bad.


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