letus35 flip. a week in. thoughts and problems

[Phil Bloom]

A week of playing with my Letus35 flip and am only now starting to get ok results.

problems, vignetting. do i have to zoom in on the camera to cut them out, will i not see more grain etc by doing this, i thought i should zoom in the very edge of the image projected by the letus35a, is this not the case? Bob Hart has mentioned in previous posts "max zoomed in"...you mean at the end of the 10x zoom on my a1? surely not. also i don't need a special anchromat or anything for this camera, it is all built in isn't it?

i have a few different lenses. With my zoom lenses i find it incredibly hard to get focus. It doesn't focus as it should, infinity on the barrel isn't etc. one of my lenses has a macro so i have to use that a lot to get focus. Surely that isn't right? they are fast lenses f2.8. My experience is mostly with 2/3 inch broadcast cameras as it feels like a backfocus issue, but i can't imagine where or how i can adjust that?

i read somewhere that someone has a special letus made by quyen for their sony a1/hc1. How is this different and why? I am using my letus35 flip 72mm with my a1 with a step up ring converter. My main camera is a z1 but that is in england and for the next two weeks i will still be using my a1. I know there have been tips and help for the a1 but i just can't find them again! Will the results be better with the z1? The annoying thing i have having with the a1 is the lack of complete manual camera control for iris and shutter at same time and the complete lack of gain control.

The other thing is somewhere the rig isn't level. The bubble in my tripod says i am but the horizon is askew! I wonder where it is going wrong? It must be the camcorder itself somewhere as the letus position is irrelevant to the horizon.

I still am yet to get a shot that I am really really happy with. It all looks a lot like 16mm at times, which is great but I need it to be able to cut in with my non letus shots and they won't currently without grading my dv stuff in the edit. Silly question, but is there a filter that will mimic the slight softness of the image for my normal dv stuff?

Phil

[David Delaney]

Phil,

I am finding alot of the same problem with an older 35A. Sharpness is a huge issue, I can't seem to get a focus. It looks good in the LCD, but when I capture it, it looks terrible. The Infinity is off and I do get vignetting even with a 50mm 1.4 Canon. Ben Winter has been giving me some tips, but I can't tell you anymore then that we seem to be in the same boat. I don't know if I am going to have to mod. this letus or just purchase a newer version to get it to work where I can even begin to seriously shoot something. Here is a grab from today, infinity focus (or close as I can get) - it is unusable in my opinion.

http://davidd.250free.com/STILL.JPG

[Bob Hart]

1. LENSES.

SLR lenses preferably f1.8 or wider so you can close aperture back for better sharpness at say f2.8 or f3.5 among other things.


2. SLR LENS BACKFOCUS.

Wides and zooms especially won't be ideal unless this is correct.

With Letus35 reversable Canon/Nikon mount, set up a focus chart a measured distance from point on the Letus front cylindrical body which houses the groundglass, about 40mm rearwards from front of Letus body, large Siemens chart is best target probably, commonly used for ENG cams.

Set a known accurate SLR lens to the distance measured. Loosen off thumbscrew on front of Letus35, then gently slide the mount forward in the body from its most rearward position forward about 1mm. This is approximately where it should be. Adjust back or forward until chart is sharpest. Resecure thumbscrew.

The thumbscrew will tend to move the mount when you retighten it and you will need to hold it firmly as you tighten the thumbscrew. The mount may also not sit squarely so you need to pay attention to this as well. Once you have it right, scribe a mark around the mount adjacent the front of the Letus body with a pointed tip of a dart or scriber or jeweller's screwdriver so you can quickly find the position if the mount becomes disturbed in future.

[Phil Bloom]

thanks Bob,

Will definately try the backfocus bit. With the canon and the nikon flippable mount it makes the lens sit in a different position doesnt it, so surely the backfocus must be out for one of them at least when you first take the letus out of the box. Is there a way to make the letus hold the lens mount more solidly?

Also, one more, sorry Bob, but you the fountain of all knowledge! what about when you talk about how much you zoom in on the camera into the letus?

Thanks,

Phil

[Bob Hart]

Phil.

I can only speak for the PD150 family, FX1 family, DVX100 which as far as I know use 1/3" CCDs.

For these a very general and inaccurate rule of thumb is approximately :-

4+ achromatic dioptre + approx 6" gives a frame on 24mm x 18mm at about 54mm.

7+ achromatic dioptre + approx 4" gives a frame on about 32mm x 24mm approx at about 32mm zoom.

Other factors come into play like how much the mirror or prism path is going to allow you to zoom wide.

Wider than a 24mm frame is going to get you in strife with corner or edge falloff AKA hotspot to varing degrees depending on the texture/transparebcy of the groundglass.

[Phil Bloom]

thanks bob,

i am slightly confused though with what you are saying. I don't really understand all the principles behind the adaptor as I am not hugely technical when it comes to them, especially with what you are saying with the achromatic dioptres. Let's say I am using a z1/fx1 all i need to do is just screw on the adaptor zoom in past the vignette, focus the camera then away i go?

[Bob Hart]

(I am slightly confused though with what you are saying. I don't really understand all the principles behind the adaptor as I am not hugely technical when it comes to them, especially with what you are saying with the achromatic dioptres. Let's say I am using a z1/fx1 all i need to do is just screw on the adaptor zoom in past the vignette, focus the camera then away i go?)

--- If an adaptor is tailor made for the camera, then that is all you should have to do. Don't forget to set the camcorder aperture to about f5.6, shutter speed 1/50th or 1/60th of a sec is best for the Letus35.

[Phil Bloom]

my adaptor isnt tailor made for my camera. are any of the letus35 or flips? obviously the xl and jvchd 100s are.

I am using a sony a1/hc1 for the next two weeks. I am starting to get better results now. My back focus is definately out. I moved the lens forward out by a mm and its great, but it is nowhere near as robust feeling now, feels like the lens will drop out. how can i make a 1mm cushion for the mounting to slip into? bluetack?

thanks

phil

[Bob Hart]

Phil.

I think some of Quyen's adaptors are generic in the sense that they fit a filter mount diameter size. Cameras with similar filter mount sizes as a general rule of thumb have similar optical performance. Some camcorders can do without an added close-up lens. Some cannot and require the close-up lens. Quyen is by far the best to consult on this.

The lens mount does become unstable once you move it forward away from snug fit against a small shouder inside the hole it sits inside of as you observed.

This is definitely a confidence killer especially with heavier more valuable lenses. You can help stabilise the lens mount by dragging the nearest breakfast cereal packet out of the cupboard and cutting very precise washers from the thin hard cardboard. The roundness of the inner hole does not have to be all that precise as long as it does not get in the way of light as it passes by. The outer must be very precise.

Inside the blue plastic front housing, you will observe there is a smooth machined area where the mount slips in. Behind that there is a slightly narrower section which is not smooth finished. There is a small shoulder face about 0.7 mm thick. The cardboard washers fit up against that, which is why they have to be precisely the same width as the hole where the smooth area is.

Two cardboard washers and maybe one of two made out of printer paper or thinner hard paper will help stabilise the mount and restore firm conact between the back of the mount and that shoulder face.

Once you have the thickness right and it tests okay, glue all the washers together. Don't glue it into the Letus as different lenses may not be consistent as to their flange to focal-plane distance, especially if they have been obtained second-hand and you may need to make up another layered washer for a different lens.

Another little trick you can use to help stop the mount from walking off position as you tighten the thumbscrew is to find a little hard bearing ball out of a pushbike, which is a close but not tight fit in the screwhole and place this between the end of the thumbscrew and the mount where the thumbscrew presses down against the mount.

Don't put this in by taking the thumbscrew right out as there is an internal relief machined into the plastic and the ball may end up rolling around inside the guts of the Letus. Just screw the thumbscrew out just far enough that the ball will fit snugly in against the screw end from inside when the mount is removed, use a tiny bit of vaseline to hold it in place, then re-insert the mount.

Be aware, that this will cause the thumbscrew to be very smooth as it tightens and you will not be as aware of overtightening. If the mount slides on the ball which it likely will, use a permanent felt marker to place a black mark on the mount, refit the mount in the hole, tighten down the screw, dismantle it and observe where the ball makes a mark in the mount and then use a smal three cornered file to file a small groove in the mount of the ball to locate into. Once that is done, confidence the mount will stay in place may be restored.

I am in the throes of designing a mount which has provision for backfocus without turning the lens itself for a client. Once I have this sorted out I will publish. It would have to be machined by a workshop. The challenge is to fit all this into the available space.

[Phil Bloom]

sounds great Bob,

thanks for taking the time to reply to all my questions. Finally got some good footage today. With the lens out by 1mm I finally am able to achieve proper focus. Got the right balance between shutter, exposure on the a1 and f-stop on the 35mm lens now. Low light is still a problem but I knew that. But the details doesnt appear to be lost using the letus with HDV. I won't know for sure until i get back home and plug it into my 37 inch flat screen but on my macbook pro looks lovely.

my next challange is balance the rig with the a1, 50mm lens and letus flip on my steadicam merlin. now that will be fun, and keeping focus through a walking shot, will look stunning if i can pull it off!

let me know about the back focus project you are doing, is it something i could buy off you at some point?

phil

[Bob Hart]

Phil.

The backfocus mount will be unlikely to be an earner for me as there are ten parts including screws involved for genuine Nikon mount.

At a fair Aussie living cost for my time for hand made work, the price is unlikely to be attractive but once I am done, I'll pass the design on to Quyen if he is interested.

The design itself is fairly simple. The final hurdle is proving the most difficult, how to retain the pin and release system for the Nikon lenses.

For your walking shot outdoors, try f4, f5.6, even f8 apertures. With lighting from behind you, as long as you camera has enough light, the Letus GG image should tolerate tighter aperture with relative ease. This will give you a wider depth of field for you subject to move in.

Soft background is not so important as your subject is drawing attention by the act of moving and you following or leading.

[Bob Hart]

Phil.

In regard the washers, I've made a small error. I was thinking about the abortionate first attempt at an alternative mount I was trying and not the reversable one because mine of course did not have the Canon mount on back of it. The washers should still work but the centre hole will have to be cut a lot more precisely and much wider, to the diameter of the inside of the groove the Canon lens lugs fit in. The washers will have to be eased over the lip into the groove. You may find you have to glue them in place on the Canon face of the mount once you have established the correct buildup to thickness to maintain the mount forward where it should be for backfocus.

Except for the the more fiddling around, it should still be okay.

If the mount still wants to walk about, you could try two small flat drops only, each a matchhead in area, of yellow contact adhesive (Selleys Quick Grip if it lives and prospers in your neighbourhood), each about an inch or 25mm apart on the bottom side of the mount to key it from moving. Should you need to dismantle the mount later, it should be possible then to tear the adhesive.

[Phil Bloom]

finally got some good footage in the can today. Somehow dust has got in and landed on the GG, so there is that horrible circular type dot crawl in a couple of place. how can i clean them out of there, as when I take the 35a flip apart i can't access the front of the GG. Also the whole thing seems a bit loose, none of the screw tighten hard, is that normal? I also definately can see some grain, especially on the blue sky. The footage looks nice though, more 16mm than anything. Been using it handheld with my sonya1 and cavision rods with a sigma 2.8 28-700 zoom lens with support strap. Got more looks with this than I ever got with my full size Beta camera!!

Will post some footage tomorrow hopefully!

Phil

[Bob Hart]

Phil.

The thumbscrew on top which holds the lens mount into the body unscrews right out and this releases the guts of the Letus to be drawn out through the front. Take really good care when doing this as the screen might catch on the side walls of the tube and be damaged.

Don't wipe with fluid cleaners or cotton tips but use a dry fine lens brush, lots of care and patience because you do not in any way want to mark that groundglass.

Crooked horizon. Yes, I have that problem with the AGUS35 and my own mouting system, also the Letus35 on the teleprompter mount I have been strengthening.

It takes very very little to make this happen. We are used to using the horizon as a reference, I guess to stay on our feet, so we are visually sensitised to this. The 16:9 widescreen frame doesn't help.

There's several things which interact.

Our own eyeball usually can only see the ball bubble in the tripod head from an angle which introduces a parallax error. Check it from two directions if you can. I guarantee you will never set it in the same place twice.

The plane surface of the actual mount point on the bottom of the plastic bodied cameras is small.

I seem to recall, the HC1 has the cassette enclosure down there as well which must weaken the case structure.

Some tripods have soft nonslip surfaces. The added offset weight on front of the camera can pull the camera just that little bit off the horizon due to flex in the camera body itself and in the pad on the tripod.

Introduce a rods adaptor between the camcorder and tripod to fix this support issue and there is now another variable in the mix. I actually had to mill the base of my own homebrew bridgeplate arrangement to true it up. There was only a paper thickness in it across a 100mm face but it was enough to be apparent.

Build quality on some tripods themselves can be less than precise between the fluid bubble and the plane surface which the camera sits on.

The flip path needs very little variation at all in the ninety degree opposition between the two mirror or prism pairs, whichever flavour is used, to cause an image skew.

A hair's breadth or less and the horizon will skew slightly as the image twist is twice the actual degrees of misalignment. The image flip occurs at ninety degrees and two hundred and seventy degrees of a 360 degree physical rotation. The image rotation is actually 720 degrees.

This is why you rarely find a pair of prismatic binoculars which don't send you crosseyed.

With the HC1, I would be inclined not to sweat it too much. Maybe fold a piece of paper and put it under the side where the horizon is high before tightening the camera down to the tripod.

In relation to tightening the screws, yes it is a little bit fragile. To strengthen the whole thing I have added two more screws at 120 degree radial centres. Think Mercedes Benz star and you'll see what I mean.

Machining the ends of the screws to a cone point also helps with the walking problem when you tighten them up because most of the mechanical pressure is produced by the threading in the thin metal tube which is about 15mm out from the mount the screw ends bind down onto.

Overtightening is not recommended for two reasons. The threads may strip and under more pressure the screws will tend to skew and push the mount and the front section off alignment ever so slightly.

If you drill and tap these extra screwholes, take care not to catch the wires which feed through to the motor.

Cone-pointing the screw ends introduces another problem with the reversable mount. The groove for the Canon lens passes very close to the centre of the screw once backfocus has been found and the screws may walk off into the groove making accurate securement impossible.

Flat bottomed clearance holes then have to be drilled or milled where the screw centres fall. The typical cone ended hole of a common metal drill bit will cause the mount to always find a common point when screws are tightened. This is great if the mount is exactly on backfocus but useless if furthur adjustment is needed later.

[Phil Bloom]

the whole thing seem so wobbly! i have very little confidence in it's stability at the moment!