Back Focus - Different for all lenses?

[Bob Hart]

Francois.


Mea culpa. In my haste I failed to read your post correctly. It seems that the achromat lens in back of the Letus came out of its barrel. I misread your post to believe it was the front glass behind the Nikon lens in front. My suggestion ust therefore seemed to be ridiculous nonsense and I thank you for letting me off the hook so diplomatically.

I apologise for this misleading info I have sent you in the sense that you may have interpreted it as the fix for your achromat.

I did also encounter this issue. It came about however due to a violent event. I had taken it out of the back of the Letus Extreme in order to do a centering mod on the compound prism and enclosure to enable the lateral adjustment you get as standard on the Ultimate. Like a fool I left it on a table top and dragged it off onto the floor with the trailing corner of a castro jacket as I stood up and turned away to get something.

I managed to get my foot under it but it then flew across and hit the skirting board at the wall and came apart there.

Anyway. - I did the same cheap trick with the bathroom sealer but in this instance, where there were chips in the very edge of the glass, I sealed these with black parcel marker and then reset the achromat back in the barrel after cleaning out some small glass flakes. It has worked fine since.

If that achromat has been floating loose, the focus issue may have been due to that. Using the Ultimate without the achromat fitted would not have worked at all. However I wold still check the Nikon lens fit into the Letus-Nikon mount.

[Bob Berg]

When I use the Elite on an EX3 with Nikon primes, the ground glass generally disappears around Z69-Z71ish, which seems to be fairly typical according to posts I've seen on Phil Bloom's site and others. The new RED family of lenses works well on the Letus, but I've found I have to zoom in to somewhere around Z84 to "crop" out the circular vignetting around the edges of the RED image. The further you zoom in, though, the less light you're seeing since you're using less of the ground glass and prism. The 18-85 is a T2.9 already, so add that to the loss through the Letus (especially at Z84) and the the falloff is fairly considerable.

I have noticed that my ground glass image isn't perfectly square to the EX3 image sensor; it's a bit low and a bit right. I've done all I can with the mount adjustments.

Can this be adjusted inside the Letus?

[Bob Hart]

The zoom-in to Z84 is to the groundglass area which represents the 35mm motion picture frame.

The Letus Elite as you will observe from this has more than enough viewing area of the groundglass for faithful representation of 35mm motion picture film framing for most practical purposes.

The Letus Ultimate enables the next step up to precisely centering the camera optical axis to the optical axis of the prime lens on front of the adaptor.

The Extreme/Elite can be modified for a lateral centering adjustment but no vertical adjustment can be sensibly implemented.

To add precise microadjustment requires total dismantlement of the appliance and machining work. Modding is not something I would recommend unless you possess good dextoral skills, experience in handling and cleaning optics and access to a metalworking machine shop.

What is involved can be found in this thread.

http://www.dvinfo.net/conf/let-us-di...g-extreme.html