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Old December 7th, 2007, 01:08 AM   #1
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Letus Extreme v. Brevis?

One of the things that drew me to the Brevis was the interchangeable imaging elements and, most specifically, the fact that one or more of those elements would allow you to really stop down without fear of vignetting.

Denis posted a video on the cinevate forum showing a slow zoom lens which was viable at all fstops and all focal lengths, if I remember correctly.

There are some downsides to the construction of the Brevis that it appears the Letus Extreme would trump. But beyond the quirks, there are a couple technical aspects that I really want in a DOF adapter and one of those is the ability to shoot while stopped down to a 5.6 or an f-8 on my 35mm lens. Does the Letus Extreme realistically provide this option?


The other thing I'd like to know is how you set the backfocus with the Letus. I was exremely disappointed with the trial and error method of setting the backfocus with the Brevis. Pulling out the entire mount, holding it in some unsupported position while measuring with a mm ruler and then trying to hold it steady while you "lock down" the mount with set screws is, in my opinion, an absurd way to do it.

Even the new and improved micro-collimation TM thing the Brevis has going on leaves a lot to be desired IMO, and still involves attaching the mount, checking your lens, removing the mount, tweaking the inner-ring slightly, then replacing the mount, checking your lens, removing the mount and on and on and on.

So, how easy or difficult is it to set the backfocus for the Letus Extreme mount? I've read about how the flippable Canon/Nikon mount is more problematic than just going with one or the other, but I'd like to know if there is a precise way to accurately set the backfocus.

Thanks
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Old December 7th, 2007, 02:21 AM   #2
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Originally Posted by David Garvin View Post
The other thing I'd like to know is how you set the backfocus with the Letus. I was exremely disappointed with the trial and error method of setting the backfocus with the Brevis. Pulling out the entire mount, holding it in some unsupported position while measuring with a mm ruler and then trying to hold it steady while you "lock down" the mount with set screws is, in my opinion, an absurd way to do it.
I found the answer to this one. The Letus direct site has publicly available instructions for their DOF adapters which describe how the backfocus is set and it sounds very similar to the old style of Brevis backfocus setting.

Oh well. I guess the good news is that according to the site "**The back focus for your lens mount is factory pre-set, so it should not need adjusting.**" so maybe it's not as big of a problem as it could be. I guess I don't understand why the lens mounts couldn't simply be threaded so making the micro-fractions of a millimeter adjustments would be easier.

I'm still curious about the ability to use slower lenses and stop down with the Letus, so if anybody has some first hand info or images, I'm all ears and eyes.

Thanks
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Old December 7th, 2007, 02:54 AM   #3
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David.

A while back, I did a little design exercise.

In regard to a user installable genuine counter-threaded keyed backfocus adjustment accessable from outside the appliance, with Letus for Nikon, there would be enough space - only just.

There would remain no space to provide a releasable positive pin lock for the Nikon lens mount.

It would be a difficult and expensive piece of work, requiring three layers of perfect cylindrical machining, two requring precise reversal in the chuck, only two layers if you were to cut a working thread directly into the Letus housing.

Needless to say, I did not build one.
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Old December 7th, 2007, 08:29 PM   #4
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Originally Posted by David Garvin View Post
there are a couple technical aspects that I really want in a DOF adapter and one of those is the ability to shoot while stopped down to a 5.6 or an f-8 on my 35mm lens. Does the Letus Extreme realistically provide this option?
No, and neither does the Brevis. For stop-down-ability you'd be better served by a spinning adapter such as the SG Pro.
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Old December 8th, 2007, 05:57 PM   #5
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Quote:
Originally Posted by David Garvin View Post
there are a couple technical aspects that I really want in a DOF adapter and one of those is the ability to shoot while stopped down to a 5.6 or an f-8 on my 35mm lens. Does the Letus Extreme realistically provide this option?
No, and neither does the Brevis.
That is a completely inaccurate assertion on your part. The Brevis absolutely has this ability.

Not only does Dennis have a video clip which shows a 28mm-200mm zoom at all focal lengths shooting at an f-8 with zero vignetting, but I also have first hand experience with the Brevis and shot at 5.6 many many times with various lenses and zero vignetting.

I have a big handful of complaints about the Brevis, but I will give credit where credit is due and the fact is that the Brevis completely has the ability to shoot stopped down.

And if you still don't believe me, here is a low rez clip of the Brevis shooting at an f-8 with the 28mm-200mm zoom all the way from wide angle to telephoto with no vignetting. The entire imaging element is visible so there's no question that it's not vignetting at all:
http://www.cinevate.com/images/protovignette.wmv
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Old December 8th, 2007, 06:03 PM   #6
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David.

A while back, I did a little design exercise.

In regard to a user installable genuine counter-threaded keyed backfocus adjustment accessable from outside the appliance, with Letus for Nikon, there would be enough space - only just.
Thanks for the reply, Bob. I find this interesting because the new Brevis "micro collimation" feature is ALMOST what I'd like to see.

The newer collimation feature involves a threaded/rotating ring that acts as a "stop" for the lens mount. The problem is that you still have to put the mount&lens all the way in, check it, then remove it, then poke around inside the barrel of the Brevis with a screwdriver to twist the threaded-stop-ring. Then put the mount back in and repeat until it's set.

Since this kind of threaded system is already implemented, I'd just like to see the mount be attached to the threaded-ring-stop. If the mount were simply welded to the threaded ring, then you could spin the entire mount WHILE the lens is attached, be sure that you were perfectly set (basically "focusing" the backfocus while looking through the lens) and then lock it down with set screws or whatever.

I don't know, it seems like the Brevis is 80% there already.
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Old December 8th, 2007, 06:16 PM   #7
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I've shot up to f11 with the Brevis. But it all depends on your lighting conditions and the lens you're using. This is why I stress that you should buy the fastest lenses you can afford.

The faster the lens, the bigger the rear element and optical projection <i>tends</i> to be. My 50mm F1.2 can be stopped down to f11 without seeing grain or vignetting. But my 50mm F1.8 (E Series) can only be stopped down to f4 before it will vignette.
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Old December 8th, 2007, 06:19 PM   #8
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Since this kind of threaded system is already implemented, I'd just like to see the mount be attached to the threaded-ring-stop. If the mount were simply welded to the threaded ring, then you could spin the entire mount WHILE the lens is attached, be sure that you were perfectly set (basically "focusing" the backfocus while looking through the lens) and then lock it down with set screws or whatever.
If the mount were attached to the threaded ring, the aperture ring and focus marks on the lens might not line up in a convenient position.
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Old December 8th, 2007, 06:25 PM   #9
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I've shot up to f11 with the Brevis. But it all depends on your lighting conditions and the lens you're using.
Exactly, but it's absolutely possible with the Brevis and for somebody who makes a living as a film shooter, the concept of being required to always shoot at an f-2 or a 2.8 is ridiculous.

So unless anybody else chimes in, I guess the answer to my question is "no", the Letus Extreme doesn't have this ability and the Brevis still rules that aspect of DOF adapters. That's a deal breaker for me and is all I need to know when I purchase or recommend DOF adapters.

If somebody has experience shooting with the Letus Extreme at 5.6, 8, 11 or higher, I'd love to hear about it.
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Old December 8th, 2007, 06:28 PM   #10
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If the mount were attached to the threaded ring, the aperture ring and focus marks on the lens might not line up in a convenient position.
Yeah, I get that part of it, but accurate backfocus is critical IMO and the tradeoff would be worth it. Instead, you have to remove-twist-insert-check, remove-twist-insert-check, remove-twist-insert-check, remove-twist-insert-check and never really be able to get it as perfect as I'd like.

I can also think of other ways to realign the witness marks wherever you'd like, but obviously would require even more re-engineering of the adapter.
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Old December 8th, 2007, 07:41 PM   #11
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The back focus on the Letus35 is user-adjustable if any tweaking is needed. It is quite a simple process should it need to be done for any reason. We'll be adding instructional videos to the LetusDirect.com site over the next few weeks to help users with everything from initial setup to advanced back focus adjustments.

Just thought I would let you know things are in the works to make life as easy as possible for the Letus35 users :)
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Old December 8th, 2007, 08:31 PM   #12
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Realigning the witness marks as a secondary operation requires some part of the adaptor to be unlatched or unsecured in some way for the sleeve or ring or whatever to be turned.

This introduces the possibility of an adjustment to be altered, however slight.

I realise the engineering can be done to defeat this probelm however it requires front access to the mount and the same old problem of available space arises.

The facility for re-aligning the witness marks, except for the threading requires the same number of parts and process as the counter threaded process which is intended to maintain the witness marks in place.
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Old December 8th, 2007, 09:04 PM   #13
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Realigning the witness marks as a secondary operation requires some part of the adaptor to be unlatched or unsecured in some way for the sleeve or ring or whatever to be turned.

This introduces the possibility of an adjustment to be altered, however slight.
I understand, but I can't believe we're more worried about a slight micro-millimeter shift during rotation than we are interested in being able to accurately set the backfocus while the lens is attached.

I mean, that just seems like odd prioritizing of concerns to me. If a user is worried that the non-threaded rotation of the adapter while re-locating the witness mark might possibly throw the backfocus out, then how can that same end user not be concerned that there is currently no way to set the backfocus while the lens is mounted at all?

I can't imagine taking a set of primes to be collimated and then having the tech do it with the lens caps on. Or, perhaps more aptly, I can't imagine having the flange focal depth checked on an camera without the precision equipment required to set it within micrometers. The fact that we're setting these extremely precise distances through trial an error instead of being able to actually look through the lens while it's happening seems amazing to me. Particularly when it seems that it's actually not an impossible thing to do.

Again, to me, not having the engraved witness mark in the perfect position is nothing compared to the inability to truly accurately set the backfocus. I can take some pinstripe tape and a fine-tipped sharpie and I have a new witness mark, and whenever I'm using a follow-focus rig, I could pretty much care less where the real witness mark is engraved on the lens because all the focusing reference has been moved to the ff rig anyway. I've spent HOURS trying to get the Brevis backfocus set as precisely as possible and it just seems uneccesary when a threaded solution (which would make it much easier) is almost completely implemented already.

EDITED TO ADD: BTW Bob, I'm not blaming you or anybody else about this, and I do appreciate your responses. It's just one of those things that I find a bit astounding and therefore I talk about it emphatically.
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Old December 8th, 2007, 09:18 PM   #14
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David.

I am well known for my elephant hide and lack of ability to read things into conversation until much too late so don't fret.

You have touched on something which people sometimes don't appreciate. The genesis of Quyen's product rests in amateur enthusiasm although it has matured and evolved since but is probably still very much under-resourced for more than one stream of R&D at a time. People want their Letus and they want it yesterday. Quyen may think and suggest otherwise.

Your suggested solution is valid and doable. I guess what you arre up against with the likes of me is that I don't like to take something on unless I can do it properly, compulsive obsessivism if you like.

My old man used to tell me, if you spent as much time getting the job done as scheming and planning how to get it done, it would be done by now - So there you have it.

Barebones innovation might be summed up in the joke of a boilermaker welder friend - "Me I can do anything. Gimme two bits'sa plywood and a couple of sticks out-a the rolled roast and I'll weld it tagether for ya."
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