back focus on the Extreme

[Paul Mailath]

In another section a comment was made about back focus

"The Letus Extreme for example doesn't come with the back focus adjusted from the factory even though they say it does and some people are having a heck of a time trying to adjust it themselves without any success."

I have no problem using the LEX, just bolt it on and away we go but I'm not sure what's meant by 'back focus' and how to tell if it's not correct and how to adjust it.

anyone care to enlighten me?

[James Hooey]

Back focus when it comes to lens adapters is the position of the taking lense from the focusing screen. This is a critical measurment which when correct, allows for the lense to focus its image onto the screen at all focal lengths for any given manufacturer.

My understanding of the Letus is that back focus IS set from their manufacturing facility...... in the following manner. The focusing screen is adjusted to a 7.5mm gap from the Letus prism housing (all screen corner edges must be the same or soft image edges could occur) and each lens mount is spaced different to allow the lenses to focus to this set screen height. This is why you should be able to put on a different mount and not have to make any particular back focus adjustment because the mount itself creates the different distances from a known focus screen height.

The way to know if your back focus is set accurately is to use a focus chart measured 3' from the focal plane (focus screen edge). Use whatever lense on the adapter and focus to 3 feet and see how accurate focus is to the lenses markings. If it's good then another indicator would also be checking that the lense also focuses to infinity crisply.

There are some caveats here....

Firstly...getting to, and adjusting the ground glass requires a bit of disassembly....and if done wrong may, scratch the screen, move a rubber holder, or knock out your back focus worse. That 7.5mm gap is critical...and it must be a perfectly flat plane (ie: all four corners same height)

Second.....it might void your warranty. But I have also heard on the forums that the instructions for correcting back focus have been walked through to various users by Hein. Best to talk to Letus before poking too much.

Depending on the mount there is another procedure available for Nikon users. The new Nikon mount has 3 screws on the back of the lense mount to allow for small adjustments of the distance of the taking lense from the focus screen. These 3 screws are meant to be all raised/lowered by the same amount (to achieve proper planar alignment to the focus screen) to adjust for back focus.

This information is around on other forums and I'm only word of mouthing it here. I've been waiting 43 days for my LEX to arrive (my first adapter!!), so I've had some time to do some reading/research. :)

Hope that helps.

James

[Bob Hart]

Smack me over the ears if I am wrong. (gently please). - My understanding is that what users have come to call "backfocus" and is achieved by fixed settings of a lens position is the process of "collimation". This definition is also applied to adjustments within individual lenses to make them comply with a fixed focal plane standard relative to their flange position.

In ENG style cameras with detachable lenses, there are too many things like grit on the mount, bent mounts, worn mounts, which can throw the relationship between the lens flange and the focal plane out. Adjustable compensation is provided in these lenses and this is called "backfocus".

A serious impediment to using video cameras for adjusting backfocus (collimation) is the poor resolution of the viewfinder screens. The guesswork practice of "soft this way - soft that way, therefore the midpoint must be sharpest", does not always work if the focus effect is not truly linear.

If you have access to a good daylight screen and a 35mm slide projector (projection lens removed) or a strong small camcorder spotlight, you can get good results by shining this in through the back of a 35mm adaptor and setting the lens position to its correct relationship using the groundglass texture to appear at its sharpest.

In fact this is about the only way you will get the precise backfocus (collimation) you need with ultra-wide lenses. I understand that such a process is part of the normal quality control with motion picture camera manufacture, with P+S Technik and Movietube. It is tedious, is high-skill labour-intensive and takes time to do right which is why you pay premium for these products which once assembled, do not move unless you tamper with them.

As long as the lens witness marks on the focus barrel are true, the lens is set to its correct witness mark and the distance accurately measured between the projection screen and the groundglass focal plane. I use 1 metre or 4ft, but this may not be correct practice.

The flange position of the mount in the adaptor is then moved until the groundglass texture is sharpest. The lens iris has to be wide open for this method which is standard industry practice, albeit with proper equipment.

WARNING-WARNING-WARNING-WARNING: This is not an authorised Letus owner-servicing procedure. YOu play this game at your own risk.

WARNING-WARNING-WARNING -WARNING: Do not shine these light sources into the back of any adaptor for any more than TWO minutes at a time otherwise things may melt or crack. Do not use 500w lighting kit lamps. These are guaranteed cookers.

[Paul Mailath]

Wow - thanks guys - don't worry, the last thing I'll be doing is taking a screwdriver to the LEX - that's a guaranteed disaster with my fine motor skills.

I'll certainly try those tests although as I said I don't think I have a problem, I just wanted to understand the term 'backfocus' or 'collimation'

If I discover a real problem you'll hear me knocking on your front door Bob.

Thanks again