Letus 35 focus...a dead horse

[Leonard Levy]

Well i just checked my Nikon lenses at 10' and 5' with my Letus Extreme.

This was the First thing time I checked my focus using the expanded focus on the viewfinder and I was disturbed to find out that my zoom lens didn't track as perfectly as I thought so now I know my Extreme back focus is slightly off which PO's me enormously as i tried hard to get it perfect already and don't want to open it up again. I thought my newer Nikon adapter ring was supposed to have a back focus adjustment but i can 't find it.

I adjusted it by moving the Nikon adapter in and out and I think I got it right, but its a lousy way to have to adjust an adapter.

That said i have to seat some crow, because the way I did it was by moving it until my 50mm lens hit 5' on the barrell marking while focused on a chart.
That was correct when I cross checked it with my zoom lens. However there was a lot of room for slack in this method. Exactly where my marks on the barrell? I tried 6 lens afterward at 10' and 5' and while all were close, some where a teeny bit in front of and some a teeny bit behind the marks.

The wide angle lens were more finiky and thus a better way to check. When my 35mm lens was measuring 9'3" or so , my 135mm was looked dead on on 10'. When it was corrected the wide was looking like almost exactly 10', but the 135 was a hair past it.

I did notice that when the back focus distance was too short, the barerell markings also read too close.

So the conclusion was that barrell markings were pretty close but not exact. Always double check that you are reaching infinity, and if you know your lens tracks that's the best test.

Lenny Levy




I know its close but I don't know how much the fact that it's slightly off will affect my observations which are that some of lenses focused at exactly 10' and some didn't

[Bob Hart]

"Can you explain the slide projector method in a little more detail. I have an old one laying around just waiting for something this nuts."


I used this to reset a loose barrel around a prime lens.

The Mini35 and mounted lens I positioned in front of a Hanimex 150 slide projector.The slide projector's own lens was removed.

The light from the projector lamp goes through a condenser which directs the light more or less straight ahead.

The relay lens was removed from the back of the Mini35. This leaves the rear prism as the first optical surface the project beam enters.

The Hanimex projector was positioned about 1" from the rear of the Mini35. It has to be moved about a bit until you find the centre of brightness. You do this with the prime lens on front of the Mini35. There is no need to take it off.

The prime lens aperture is set to wide-open for two reasons, wide is best for critical focus and wide means brightest image on screen.

The screen was postioned to be square-on to the focal plane of the Mini35.

The screen was positioned to be 1.2M from the focal plane for no other reason than it was convenient at the time.

The projector was turned on for only a minute or so at a time as I was anxious not to cause any cracks or chips in the prisms or the groundglass in the Mini35 from the heat. The Mini35 was never intended to be used or abused in this manner.

Whilst the projector lamp was on, I adjusted the prime lens focus until I had sharp focus on the groundglass texture as seen projected on the screen.

I then rotated the outer barrel until the witness mark matched and tightened the grub screws.

Depending on the focal length of the prime lens you wish to calibrate, assuming the barrel carrying the witness marks can be adjusted around the focus barrel, the area and brightness of the projected image will differ.

For the wider lenses, you may find it more helpful to use some sort of reflective surface like road marker or daylight screen material for your projection screen. I made do with a white wall.

That's it.


I am not inclined to try for P+S precision in setting up the Letus Extreme backfocus. Unlike the P+S appliance which is predominently metal components, the Letus contains some plastic pieces and one or more rubber wedgepieces for retaining the prisms.

If you are more obsessed than I and minded to use this method to set accurate backfocus for your Letus, be warned that you would need to dismount the achromat from back of the Letus, otherwise you may cause a concentrated hotspot withn the appliance somewhere and do irreparable damage.

The groundglass itself is mounted on a plastic panel, which looks like stable PVC. Cook the groundglass it may be fine, maybe not.

The panel which supports it however is likely to attempt to return to the last shape it remembers at the time of its birth, a molten blob. It may slump around the groundglass if the groundglass becomes too hot.

When you remove the achromat you also risk damage. Rockanahardplace mmmh?

[Leonard Levy]

Bob,
Thanks for the detailed description. I think I understand it but in no way would I ever try something like that. My hat is off to you though.

I cannot agree however when you say :

"I am not inclined to try for P+S precision in setting up the Letus Extreme backfocus."

I think correct back focus adjustment in an optical device that 's being use for professional production should be a bottom line assumption. Anything less is unforgivable and the number of units out there that are incorrect has to be substantial if my experience is any indication. It doesn't require a $10,000 piece of equipment to design something that simple. Thankfully all these manufacturers are listening at this point. I

The SGPro has always been adjustable and the Brevis has been for over a year. The new Letus Ultimate is adjustable also. I thought my new Nikon adapter for the Extreme was supposed to be adjustable but I couldn't figure it out. I'll have to email Hien at Letus or check their site.

Lenny Levy

- Lenny

[Bob Hart]

Leonard.


Perhaps my wordstuff is a little bit careless.

I have some reservation about encouraging people to attempt to do precision tech things which might be outside of their ability and inadvertantly cause them misfortunate damage and expense in repairs. My comment was intended to be along the lines of recommending against individuals doing this sort of precision work.

I concur with your efforts at having the alternative manufacturers do the technical value-adding at their end. As far as I can see between the lines, they all have the wit and skillsets to do so.

However for the average home hacker, achieving precision to the same degree might be extremely hard and risky. I would not like to have it on my conscience if somebody melted the guts out of their Letus or burnt their house down through sitting their projector on top of a stack of books, starting a projection backfocus operation then going off to answer a phone or doorbell. There it is, my paranoia in express. Books, stacked = please don't do it.

On reflection, I should have kept my mouth shut already but there it is now out in the world probably taking on a life of its own.

[Leonard Levy]

I don't think too many people are going to attempt your method so i wouldn't worry about it.

[Bob Hart]

To furthur clarify, I would not recommend people burrow into the guts of the Extreme to move the groundglass panel itself unless the burrower has fine dextoral skills, precision calipers and good understanding of the design.

Otherwise things are going to pop and snap if hooves with fingernails enter and gallop around inside.

The only reason to go there would be if the panel has become severely misaligned, has become dislodged, or is too far rearward for infinity focus to be possible at the rearmost position of the mount in the housing.

As mentioned in your other post, there is sufficient adjustment precision available in back of the Nikon and Canon Mounts by way of those three tiny Philips Head axial screws on the rear face of those mounts. These are not to be confused with the three Allen Key (Bristol Key) clamping screws on radial centres found around the front of the housing.