Lex wide angle infinity focus issue

[Arthur Liou]

Hi,

There's an issue that I don't quite understand and can't resolve since I got my Letus Extreme. With wide angle lenses (I have a Nikon 24mm/2.8 and a Nikon 20mm/2.8), I can't seem to focus past 6 feet. With the focus distance set at infinity, the longer range image would still appear to be out of focus. Focus within 6 feet is sharp, though. Had this happened to anyone?

Both of my wide angles are quite old (at least 15years), but they're clean and undamaged. I tried to shoot them w/o filter, but no luck. Stepping down on the aperture would bring the focus further, but increase the DoF. Other lenses (Nikon 50mm/1.4 and 80-200/2.8) do not present such issue to my Letus .

I'm waiting to buy a Nikon 17-35mm/2.8 and a 28-70mm/2.8, and will have something else to compare to. They're back-ordered for a long time. Till then I hope to hear about your thoughts. Thanks.

 

[robbz]

Sounds like a back focus issue to me... This happened to a forum member over at dvinfo. If I recall correctly he sent it back to Letus for a fix.

 

[Lenilenapi]

In my experience the back focus of every sub $2000 35mm adapter has been woefully inconsistent.

However it does not make sense that some of your Nikon lenses are fine and some aren't. The lenses could be inconsistent but this would be unusual. probably you are just noticing the problem because it is more critical on the wides.

My experience is that when the back focus is correct that your Nikon 80-200 should maintain ( track) focus perfectly as you zoom when the iris is wide open. If it doesn't you back focus is probably off.

How to adjust is another question. You can send it back to Letus and pray it comes back correct. You can ask them how to correct it in the field but Letus has a pretty wanky method. If you have their new improved Nikon ring though, that has correction screws for backfocus Plus its a better ring anyway.

To adjust properly you will hear different methods most of which are unreliable in my opinion (e.g. setting it to focus on a barrel footage marking ). The lens should be just hitting infinity at around the end of the barrel movement, but notice that's a vague statement as still lenses sometimes overshoot infinity. the most precise way IMO is to use your 80-200 and get it to track properly. When it does the other lenses should just hit infinity around the end of their movement. If you don't hit infinity ( measure by focusing on the moon or something very far away - at least 300 yards or so) or you go way past it something is wrong - could be your 80- 200.

The best method is if you know a place with an optical bench that will check it with a collimator - an optical device used to check backfocus for still and cine lenses on the camera.

Lenny Levy

 

[moldcad]

Sounds like a back focus issue to me... This happened to a forum member over at dvinfo. If I recall correctly he sent it back to Letus for a fix.

I guess it would be me

The adjustment procedure - while not difficult - is extremely unreliable. Not only does it involve a lot of trial and error, but even if you goal (focus in infinity) seems to have been reached, it may introduce some very nasty side effects, including:

- changing the amplitude of GG assembly vibration, hence its frequency and increased noise
- making the GG not planar, which can manifest in softness of one of the corners or edges

I wish you luck - you will need it! But the effort will pay you back... For how long, it's another matter altogether.

 

[Arthur Liou]

The adjustment procedure - while not difficult - is extremely unreliable.

After a few days of reading and tweaking my adapter, I confirm that it was a back focus issue. As I tried to follow the Letus instruction, moving the Nikon ring didn't improve the focus. But it made me pay more attention to the focal plane. I then found out that the front side of the Letus was slightly loose. I locked it back tight, and the focus is corrected!

Now the infinity can be focused with the wide angle. Even better, the 80-200 will not lose focus while zooming back and forth. (thanks for pointing it out Leni, I thought this was inherent issue of Letus) Not sure if it's 100% precise though. I'm glad that my lenses are fine, but it kind of irks me that a $1200 adapter will rely on such crude procedure to adjust. Since an extremely minor angle shift on the lens mount can cause inconsistent focus from side to side, it's hard to tell whether our equipment is set up correctly.

Thanks for your input folks. A lesson well learned.