Backfocus help on Nikon adapter for Let Extreme

L

Lenilenapi

Carbonite Member
Does anyone understand how the back focus adjustment is supposed to work on the newer Nikon adapter for the Letus Extreme?

I recently discovered my back focus was not as good as i thought it was, and I don't really want to open my unit up again and futs around with those adjustments.

I saw 3 screws inside of the nikon adapter but loosening them didn't seem to loosen anything else. Any help would be appreciated.

Lenny Levy
 
The screw are there to keep the back of the mount from going all the way back. You should have about 1mm off from the adapter.

Set your 35mm lens to infinity, point it to a place were it is past the focusing numbers and adjust the screws until you are perfectly focus then lock it.

It sounds difficult but it is actually very easy.
 
My Letus rig required some more intense internal adjustments to have proper back focus. If you cannot get it to focus at infinity after adjusting the 3 screws for back focus, especially with a wide lens, then you may need to contact Hein about the internal adjustments.


J
 
Hien sent me a copy of the new Extreme pdf manual and he confirmed my understanding of what that said. The 3 screws are actually there to adjust the back focus if you need top get further away from the screen ( which I do). The screws will emerge from the ring they are set into and end up pressing against the Letus unit determining the BF distance. You carefully turn each screw the same amount and keep rechecking untill the BF is correct. Its an odd solution but sounds like it should work fine. I haven't done it yet but I will soon.

If your screen is already too far forward then you have to open it up and futs around inside with the pins that hold the screen in place pushing the screen back. I already did that and didn't enjoy it. That part is a bad system.

Douglas - you are wrong about only needing internal adjustments on a defective unit unless you consider poor Back Focus defective. (though arguably that's true I guess.) Also there is more to proper Back Focus adjustment than "Set your 35mm lens to infinity, point it to a place were it is past the focusing numbers ". That's a decent start though.

Lenny Levy
 
Lenilenapi said:
Hien sent me a copy of the new Extreme pdf manual and he confirmed my understanding of what that said. The 3 screws are actually there to adjust the back focus if you need top get further away from the screen ( which I do). The screws will emerge from the ring they are set into and end up pressing against the Letus unit determining the BF distance. You carefully turn each screw the same amount and keep rechecking untill the BF is correct. Its an odd solution but sounds like it should work fine. I haven't done it yet but I will soon.

If your screen is already too far forward then you have to open it up and futs around inside with the pins that hold the screen in place pushing the screen back. I already did that and didn't enjoy it. That part is a bad system.

Douglas - you are wrong about only needing internal adjustments on a defective unit unless you consider poor Back Focus defective. (though arguably that's true I guess.) Also there is more to proper Back Focus adjustment than "Set your 35mm lens to infinity, point it to a place were it is past the focusing numbers ". That's a decent start though.

Lenny Levy
Click to expand...
You are just repeating what I said. The way you say it its easier to understand, but that exactly what I was trying to say.

I consider the backfocus problem in general as defective. I do expect an adapter with a lens mount to come with the back-focus corrected. The back focus adjustment is a nice feature to have if you have a lens that is out of backfocus and only for that reason.

Like I said when all of these back focus problems started to come up, "you don't have a backfocus adjustments on the still cameras that these lenses were made for" so why should you have to adjust back focus?

I know that someone somewhere is going to say."but the Arri has it" yes it does but for special effects with zoom lenses. (unless the lens is out of whack)
 
"I consider the backfocus problem in general as defective."

Well I won't argue with you a bit there. Its just so common that I haven't gone that far.
And I agree completely that I've never had to worry about it on a still camera and would consider it a defect there.

- Lenny
 
I am glad that someone finally pointed this out. When you have a 35mm or any camera other than a view or field 4x5 for example, there is no such thing as back focus. Why should there be a back focus problem unless the adapter has not been properly adjusted from the begining. The other problem that I often read about is the need to change focal length on the video camera when you change lenses on the adapter, this is also a sign of poor design and has nothing to do with the lens.

A Hasselblad or any other camera such as a Nikon, Canon etc, has any adjustment to correct for vignetting as you change lenses? Remenber all you're doing is photographing a groundglass and when the adapter is properly adjusted there should be no vignetting as you change lenses. Broadcast cameras all have back focus adjustment on the lens and this is for the sole purpose of making sure that the focus is maintained as you zoom from wide to tele or vise versa nothing more.

If by chance you place a lens adapter over the existing lens mount in order to use nikon lenses for example on a Canon mount, then there may be a need to adjust the back focus, in fact what you're doing is simply adjusting the focal distance from the rear element of the lens to the groundglass which is basically your film plane because it has been changed by placing more distance between the rear element and the groundglass. Don't get fooled by all this nonsense from some manufacturers. You want to test an adapter for alignment zoom back so that the camera is filming the full frame on the groundglass , there should be black borders all around the fame, now change lenses and see how much vignetting you get. If in fact you're getting vignetting there an adjustment problem. Lenses are made to cover a circle larger than the film area that the lens is designed for and when the lens is properly alignned there will be no vignetting. It's that simple.

There is one important factor to consider, since you're filming a ground glass the glass must have a fresnel lens usually inbedded in the ground glass for light distribution, this is where the problem comes in, an improperly designed glass will have vignetting, that's why some ground glass cost $30.00 and others the same size will cost you $180.00.. That's where the problem is and no where else.
 
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I agree that back focus should be set at the factory (like a Canon or Nikon SLR) and should not require a user adjustment. One point which I disagree with Douglas is that it is all too common for the adaptors to arrive needing a more course adjustment that cannot be accomplished by anything except for an internal adjustment. I am one of many who have needed to open up the front end of the Letus Extreme and adjust the back focus by carefully re-positioning the ground glass. In fact, I was instructed to do so by Hein himself after all other 'normal' calibration attempts failed.

Forums like this really help the whole community become better filmmakers. This includes sharing tips, hacks and other information that helps overcome the lowest common denominator - lack of time, money and experience.

While I really wish companies that cater to indi-filmmakers would get their act together and not sell poorly designed products or send out defective units, they are doing a huge service to our community. They bring cost effective, creative and somewhat guerrilla solutions to real world problems, bridging the gap between hollywood and my indi shoot.
 
Winston,
While I agree with you about the back focus issue ( though I think they should all have adjustments for backfocus like any professional video camera.) I think you are biting off way too many issues when you make all those statements about vignetting. Adapters have completely different optical properties and problems from cameras that image on a simple film plane or chip. Vignetting issues are specific to different video camera/ lens/ and adapter configurations. Fresnel lenses for diffusion screens may not be appropriate at all in these cases. In spinners they are completely irrelevant .

Vibrating adapters have their own issues regarding what makes a good imaging screen that I would not hazard to assume I understand.

In short- its not that simple.

Dennis at Cinevate in particular has done an enormous amount of work in matching and designing adapter screens for specific kinds of lenses and video cameras. I have no understanding of what he's doing, but I wouldn't accuse him of throwing smoke in people's eyes unless you can really back up what you are saying technically. By the way he put in a back focus adjustment a year ago.
 
Just show a sample of what back focus is on professional video cameras.

You can focus on an object far away and pre adjusting back focus to zoom out to reveal another object that it is now in focus. So in essence you have zoom in focus and when you zoom out the focus changes to a closer distance without readjusting focus..
 
Thanks for your point of view on this. Don't misunderstand I am not accusing anyone or any company of any wrong doing just making a point on the many discussions about the back focus probem. It's really not as simple as people think to manufacture a good adapter, there are many hours of trial and error and expense.

Unfortunetely many of the statements made are from individuals that have never even attempted to make one, I respect those of us that have actually made them and are continiuosly trying to improve them, however the discussions are helpful.
 
Douglas, Do you really do that?
As far as I know that is a novel way to manipulate backfocus. Generally one would adjust backfocus properly so that focus remains consistent throughout the zoom. If I wanted to change focus as I zoomed out i think I would rather just rack my focus during the zoom with a properly adjusted lens. Intentionally throwing the back focus out to catch different focus points at different zoom lengths sounds pretty wild, but must be really hard to set up for a specific shot.

- Lenny Levy
 
It is no different that adjusting BF. Try pulling focus while you are on rails zooming with out an AC. It just eliminates the need to pull focus.
 
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