Setting a lens at infinity

[PeterW]

Hi all,

Several people have asked me how I set a front-cell focus lens to infinity after cleaning it or overhauling the shutter. Quite a few people have success with using a piece of ground glass taped in the film plane and using a 10x loupe to scrutinise the image of a far-away object, but though I’ve used this method I’ve never been entirely happy about it. Probably my old eyes and focusing with a loupe. So, for a long time now I’ve been using the two-camera method. There’s nothing new about it, it’s been around for years, but it seems a lot of people haven’t heard of it so here goes:

What you’re doing is basically using two cameras as a very much simplified infinity collimator, but it’s none the less accurate for being simple. As well as the camera on which you want to set the lens (we’ll call that camera A) you’ll need an SLR. One with a standard 50mm lens will do, but I prefer to use a 135mm or 200mm because the image jumps in and out of focus much more quickly. You’ll also need a piece of ground glass or a piece of flashed opal plastic sheet or a piece of plain glass and a length of ‘invisible’ Scotch Magic Tape. There are other brands, and it’s similar to Sellotape but is thinner and has a slightly frosted look to it. Plus a couple of rubber bands and a desk lamp, or a window if it’s daylight.

OK, here we go. First, camera A that you need to adjust. I’m assuming for the moment that it’s the usual type of front cell focus lens set-up where you’ve got an outer ring on the front cell, usually with the distance scale engraved on it. This will be fixed to the front cell with three (usually) very small grub screws round the outside. Loosen these off a turn or so, but don’t take them out unless you enjoy hunting around the floor for lost very tiny screws. The outer ring should now lift off the front cell.

The piece of ground glass/flashed opal sheet/plain glass and tape should be cut to fit in the film plane. Make sure it fits on the film rails and not on the edges of the body or any end rollers or runners, and is big enough to hold in place with a couple of rubber bands or a couple of pieces of drafting tape. On the ground side of the glass, or on the tape side of the plain glass/tape, scribe a cross with a scalpel or craft knife blade in the centre – or near enough in the centre, and fill the scratch with black marker pen to make it more visible. Fit the glass to the back of the camera with the cross facing the lens so it is where the emulsion would be with a film. Set the lens aperture to its widest setting and open the shutter on T, or B with a locking cable release, so it stays open. If you don’t have a T setting and don’t have a locking cable release use an ordinary cable release and hold the plunger in with a piece of sticky tape. (If you haven’t got a cable release, adapt, improvise and overcome!)

Put the camera on a tripod, or a table, and arrange a light source, either a desk lamp or a window, behind it so that it illuminates the open back of the camera.

Now take your SLR fitted with a lens that you KNOW is accurate when set on the infinity mark. Set it on infinity, set it on its largest aperture and open the shutter on T, or B with your finger held on the release button. Aim the SLR right into the front of the lens of camera A. The distance away isn’t critical, about a foot to 18 inches is about right. Look in the viewfinder and you’ll see an image of the scribed cross. If the lens on camera A is set at infinity, the cross will be dead sharp. If it isn’t, DON’T refocus the SLR. Turn the front lens cell of camera A (or get a helper to do it for you) until the image of the scribed cross does come into sharp focus. This is the setting at which both cameras are ‘seeing each other’ at infinity. In other words, BOTH lenses are set on infinity.

Now put the focus scale ring back on the lens of camera A without moving the position of the lens, and turn it till the infinity mark is opposite the fixed mark, or against its stop or whatever method your camera uses. Tighten the grub screws and the job’s done. It takes longer to describe than it does to do.

You can hand hold the SLR, or you can put it on a tripod, whichever is more convenient. If you’ve got two tripods, mount a camera on each and set them up facing each other. If you’ve got a copying stand and a light box you can do what I do to make life really easy. I put the light box on the copying stand table and put camera A back downwards on top of the box, supported by a book each end if necessary. Then I mount the SLR on the column looking down at the other camera.

OK, that covers front-cell focus lenses (or I hope it does!). What about lenses that focus by moving the whole body of the lens in and out on a big helicoid? If these are out of focus when set at infinity the chances are that the lens flange is missing some shimming where someone has had the flange off and lost some paper shims, so the lens in too far back. I can’t recall ever coming across one with the lens set too far forward. I suppose it could happen if someone goofed and cut some shims that were too thick, but I haven’t found this. Check infinity in exactly the same way as above, but this time unscrew the lens very slightly and adjust it till the image of the scribed cross is sharp. Now use feeler gauges, or even a couple of pieces of paper, behind the lens mount to measure how far forward you need the lens to be set. Take the lens flange off, make up paper shims to this thickness, and test again.

If you’ve got a coupled rangefinder, don’t rely on the rangefinder setting. Use the above method to set the lens at infinity then, if necessary adjust the rangefinder coupling. How you do this depends on what make and model of camera you’ve got. Sorry, but you’ll have to refer to a repair manual for this.

Sorry, anyone with a screw-thread Leica, or a Russian bottom loading camera. This method obviously won't work if you can't open the back. The only way I know to adjust them is by using a depth micrometer or depth caliper gauge on the lens flange and referring to the manual for the correct setting.

Hope all this has helped. If you get problems, post them and I’ll do my best to sort them out.

Good focusing!

Peter

[Just Plain Curt]

Thanks Peter, that info will come in darn handy. I've printed off a copy to add to my files. Keep up the great work.

[herron]

Ditto, Peter.

Thanks.

[doubs43]

Peter, that's a great write-up. I'll be saving it for future reference! Thanks.

Walker

[John Parry]

Hi Peter,

You are a fountain of knowledge - all useful, practical and above all do-able!

Just a thought. I have achieved disappointing results when using a big lens with an extender. The lens is 300mm F4, (a real CZJ monster), which should give me loads of light to play with - even with the extender. Do you think this method would be valid for checking the infinity focus of the combination?

If it would, it would be useful to mark the infinity focus position on the lens for use with the extender. Of course if it turns out that the combination can't achieve focus at infinity then I'll be snookered, but at least I won't have to struggle trying to achieve the impossible any more with my rather deteriorating eyesight!

Regards - John

[PeterW]

Thanks for your kind words, John, and thanks Walker, Ron and Curt.

John, have a go if you like, but you might get misleading results. The so-called but often misnamed teleconverters (they should, as you did, be called focus extenders) are not often at their best with really long lenses, nor wide angle lenses. Most popular ones are better with 50mm to 135mm.

All except the hideously expensive ones, like Canon's 9-element extender (wish I had one!), don't just reduce the resolution, they tend to bring in a phenomenon known as focus shift, the same thing that you get on low and some medium price zoom lenses when you alter the focal length. In some cases this can mean that the combination never quite reaches infinity. They should be used only on an SLR where the viewfinder is WYSIWYG, not on a scale-focus camera. If you want one that doesn't do this you have to pay a LOT of money, and then it should be used with the same maker's prime lenses

The focus shift isn't very great, and for most people it doesn't matter because they use the extender at places like wild life parks where the subject is quite a lot closer than infinity, and the shallow depth of field throws the infinity background out of focus even at medium apertures.

I've got a few reasonable quality M42 long lenses, 200 and 300mm, and I've also got a few mid-price extenders, all M42 x 1.0 thread, but it's a long time since I used them. At the moment I'm trying out a Fujica ST605 I got recently, so I'll use half a dozen or so frames to try some long focus lenses with a 3x Soligor extender and a 2x Helios extender (Japanese, nothing to do with Russian Helios) and see what sort of results I get.

Might be a fortnight or so, because of the weather we're getting and the limited time I've got at the moment to get out on my own during the day (looking after four-year-old grandson, taking him to, and collecting him from, school while Mum works extra hours leading up to Christmas - yes, the shopping rush with extended shop hours has already started).

BTW, re the T-mount to M42 x 1.0 adaptors, take the lens and camera with you when you shop for an adaptor so you can try it. I've heard some third party ones push the lens forward too much so the focus won't quite reach infinity. Some, but not all, T-mount lenses are designed to focus 'beyond infinity' to allow for this. Don't rely on the focus scale when you've fitted an adaptor, trust the viewfinder.

Regards,

Peter

[John Parry]

Hello again Peter,

Was lucky enough to pick up a Praktica Eyepiece Telescope, which gives quite a powerful
range of eyesight correction, so you can get the focussing screen really sharp to start
with, then the telescope part comes in to zoom in on a small part of the viewfinder image.
If you can find a straight edge, you've cracked it. It folds out of the way too, so you can
see the whole image.

Tried it out on the 300mm with extender. It won't go to infinity with a 2x but just creeps in
with 1.5x

Got it off that site that you don't like - 99p !!! Meanwhile, found a T2 to M42 converter (the
other thread), but got pipped at the post for that one.

Best regards - John

[PeterW]

Hello John,

Glad you got that sorted out. You're obviously getting a little focus shift but you know now what you can and can't do with the 300 and extenders.

Peter