Astro Modding Lenses

[Stub Mandrel]

If you modify a DSLR by removing a thick glass filter and do not replace it, and it is (like a Canon 10D) not possible to adjust the position of the sensor then it becomes impossible to achieve infinity focus with some lenses. I also have one lens that has an unusually long M42 thread  so it won't screw all the way in and focus to infinity with a standard Canon adaptor.

For those who don't blanche at the idea of modifying a lens, it is practical to adjust the focus travel. This is what I did with three lenses, noting that although only about 1mm of extra travel is needed, these measures can sometime add rather more and the focus graduations can become inaccurate. As we tend to do trial focusing for every session anyway, all we need to do is note the approximate position of the new infinity position.

This is not something to do with valuable lenses, and different versions of the lenses below exist! This is just to give you the gist of what to do. The main things to realise are (1)

Carl Zeiss 135mm

Remove the screws holding the mounting plate in place and lift it off. The next layer down has two alignment brackets screwed to it and a tiny stop screw. Remove this screw and, taking care to re-engage both parts of the iris operating mechanism, replace the back plate. The lens will now focus a good quarter turn past infinity, but be careful not to focus it past the near point on the scale as the lens can now be unscrewed and may fall out!

Hellios 44M (58mm)

Remove the mounting plate, then remove the plate with the iris operating mechanism and focus fidudicial mark on it. You can then remove the small metal bracket that limits the focus travel to achieve the same result as with the Carl Zeiss lens and reassemble.

It is possible to be more sophisticated. I next removed the metal alignment brackets and screwed the lens right out on its multi-start thread.  then re-engaged it a 1/3-turn out (by one 'start' of the thread). This allows it to focus past infinity but once the limit bracket is reattached it can't unscrew, though you have lost close focus.

When refitting the iris mechanism (etc.) take care to engage the upright rod with the 'claw'. And on fitting the back plate don't lose the little pin that operates the iris mechanism.

There is at least one other version of this lens (I have one) that assembles differently - I've kept that one for normal use.

PrinzGalaxy 400mm

The focus stop mechanism is obscure, but there's another way. Remove the shroud with the focus marking on it - this needs a very tiny screwdriver. Take this off and unscrew the focus until it come out, move the multi start thread around by one 'start' and screw it back in. It will either stop too soon or screw in rather further than before before stopping. If it doesn't go far enough in unscrew and move the opposite way by two 'starts'. Re-attach the focus shroud.

So, these are not detail instructions, but if you have the confidence to take lenses apart, it isn't impossible to put them back together and achieve a shifted infinity focus.

Don't do this with lenses you don't mind damaging, but that said all the things I have done are reversible (if you keep hold of any focus stops that you remove).

[melsky]

I was disappointed when I first had a dslr modded to find I couldn't focus on infinity but soon realised it wasn't a problem, when shooting I either use a CLS or HA clip-in filter behind the lens and the thickness of the glass brings the focus point back to the sensor again.

Mel

[Stub Mandrel]

I was disappointed when I first had a dslr modded to find I couldn't focus on infinity but soon realised it wasn't a problem, when shooting I either use a CLS or HA clip-in filter behind the lens and the thickness of the glass brings the focus point back to the sensor again.

Mel

My IR filter is less than 1mm thick compared to the 3mm one that's been removed.

[Psychobilly]

Useful advice

When I had my 7D full spectrum modded by Astronomiser , the sensor position  was adjusted to compensate for the removal of the glass... I am also lucky that L series lenses I use all  tend to have plenty of travel beyond "infinity"

Peter...