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Old October 5th, 2007, 07:50 AM   #1
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Question for Letus HD100 users

Where do you set the exposure? Because there's two places to set it, either on the built in lens or on the nikon lens you stick on the front - I've been playing around with it and I find the best results are by opening the nikon lens up totally, and closing down the built in lens, so I'm focussing on the nikon and adjusting exposure on the built in lens. However I've not really had ideal conditions to do a test (its been raining a lot recently) so I'm wondering if this is the right combination, or if anyone else does it differently?
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Old October 5th, 2007, 11:50 AM   #2
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My personal preference with groundglass based iimage relay of the Letus flavour is to set the Nikon lens in the range f3.5 or wider, no tighter than f3.5, try to keep the relay lens ("built-in" lens you refer to) in the ballpark of f5.6 and preserve those settings as much as I can with what is left of the camcorder's built-in ND filters and exposure controls after the standard lens is replaced by the Letus35.

If your Nikon lens can be sharp right to the widest aperture, then by all means use this aperture however you may find after the intial novelty of shallow depth-of-field works off, that you may begin to prefer a slightly less shallow depth of field to enable dynamic camera moves in concert with movements of the subject.

Please note that this is a personal preference and not gospel according to good usage of Letus.

It all shakes down to your own personal preferences in the end and practice-practice-practice until you become intuitive with this much more complicated image gathering process.

People pick up a groundglass device, stuff it on a cam and expect to shoot miracles. - Won't happen.

A little more organisational discipline is needed and a little addition to the check calls before you call "action" -- "Letus35 ON?" -- "ON". "Roll camera?" -- "Rolling" etc..

There also needs to be a greater vigilence on both the prime lens focus and relay lens focus for consistent results. A full page backfocus chart of the siemens star type is a must.

Without the value adding in composition, framing and lighting, video will still look like video with a softer tonality to it.



Outdoors at 32 degrees south in bright clear summer sunlight which comes out at about 11 degrees overhead, with the Mini35, ND2 on the JVC provided the best result. In your conditions this may not be valid. If you encounter a "best" JVC camera ND setting which is sweet for the Letus/SLR lens combination but otherwise too dark, try using a round piece cut from a sheet of camera grade ND filter gel and put that in behind the SLR lens on front of your Letus, then open up your camera settings to enable more light.

Last edited by Bob Hart; October 5th, 2007 at 09:24 PM. Reason: spell-error
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Old October 6th, 2007, 08:32 PM   #3
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Hi Bob
Thanks for clarifying this. I haven't had the opportunity to shoot outdoors or shoot any living (human) subject yet, and also the interiors of my home are too small to get any useful test setups, so hopefully I'll be able to try out some of your settings soon.

Just a couple more questions if you don't mind? How do you set back focus on a non-zoom lens using a star chart? I'm used to the zoom in/zoom out method of back focus setting - is it just a question of moving the camera closer to the chart? Also, I turned the detail setting up on the camera in order to increase the sharpness of the focused part of the image - is this good practice?
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Old October 6th, 2007, 11:28 PM   #4
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The term "backfocus" might have been slightly misused in my previous comments. It is the common term relating to setting the detachable lenses correctly with ENG cameras.

The term "collimation" I understand also relates to this process. Sometimes misinformation is the outcome of commentaries I get into.

The still-image camera lenses we use on the groundglass devices have no facility for user-adjustable "backfocus" built-in. Neither do the camera bodies the lenses fit to. It is established by very precise build qualities in the camera body and in the lens.

As I understand things, "collimation" is the process of adjusting the lens to film plane relationship correctly and then fixing it.

As I understand things, the larger image formats like 35mm motion picture and still-image are more forgiving of very slight failures of "collimation" due to flange wear or reflex mirrors going out of adjustment or cameras being dropped and disrupted.

The much smaller video image formats require more build precision. With lens-in-camera style video cameras, this is established and maintained by the lens being integrated with the camera and kept away from prying hands and protected to some degree by being surrounded by camera body.

The detachable lens style video camera offers a new challenge. Flange faces wear, depending on design, some more than others.

The first journey on the floor of a vehicle on a rough road, or flight in the rear cargo hold of a 747, with the lens mounted but worked loose soon polishes the flange faces. It may not move any significant amount of metal but the possibility requires user-adjustment to be available.

The zoom lens requires collimation to remain precise for the focus to remain sharp through the zoom range.

A prime lens might not seem to require this same precision and in many instances, with the modern through the lens reflex film camera or video camera you may not need it providing the range of focus adjustment available in the lens can cover the altered state caused by flange wear or bending.

You will not even be aware things are off because you are focussing by eye. If you attach a zoom to your Letus, then like the camcorder's own lens, the backfocus must be adjusted.

If you have a known good still-camera zoom lens, you could use this same process to set the backfocus of the Letus mount. Not all still-camera zooms were born equal and some may not ever hold sharp focus through the entire zoom range as they were never intended for motion imaging.

My personal preference is to use a known good prime lens, and set the Siemens focus chart up at a measured distance from the Letus film plane which is 40mm rearwards from the front face of the front section.

I then set the lens barrel focus mark to that measured distance and then slide the mount forwards in the Letus body until the image is sharp then secure it.

As long as things don't move, then the focus marks will be telling me the truth and I can use a tape measure to set marks for the actors if I want to.

Prime lenses on the long end of the telephoto range are less sensitive. At the subject, the depth-of-field is very shallow. At the film plane end, the depth-of-field is relatively deep.

Wide angle lenses however have a very deep depth-of-field at the subject but very limited depth-of-field at the film plane. So if a wide-angle lens/camera body combination is not correctly collimated, it is possible for the wide-angle lens to be incapable of sharp focus.

My ramblings in the Clayton's Sticky post above give a hint on how to what I call "backfocus" but might be correctly called "collimate" the lens/Letus35 combination.

The reversable Canon/Nikon mount on the Letus35 is capable of being about 1.5mm out of correct backfocus or "collimation" when the mount is at its rearmost position in the housing. This is not a manufacturing error but a necessary trade-off for the convenience of a reversable mount.

To establish the correct relationship between the lens and the film plane which is the groundglass, the mount has to be moved forward in the housing and locked off.

For the average enthusiast using the most common lenses, 50mm and 85mm, the mount position is not very critical. If it is off, the excessive adjustmewnt of the lens focus to compensate might introduce some edge or corner softness but it will work.

To get into the last 15% of the lens's specified performance, the lens position must be correct.

If you intend to rely on the lens barrel focus markings for a camera assistant to pull focus, the lens position must be absolutely correct or the focus numbers will be meaningless.

My personal preference is to bring the detail level up a little but not to maximum.

The groundglass texture may become more apparent which may introduce codec compression issues as there will be too much fine detail variation between frames and the codec will throw away resolution to preserve frame rate.

The JVC HD100 family is understood to be more robust in this respect with a GOP of 5 if I am correct compared to the GOP of 15 for the Sony HDV family.

The JVC HD100 has a focus assist function which in Sony speak is called "peaking". This function makes the high contrast edges or the sharpest portions of the image shimmer or "pop" as some describe it.

This function enables the sharpest edges to be coloured to make it more apparent and the function with practice is very useful. You will find reference to this function on pages 14 asnd 41 of your camera manual.

Towards the bottom of the Clayton's Sticky for Letus thread, there are some links to YouTube which illustrate some of what I have been talking about here. It refers to the Letus XL but the principles for the JVC version are the same.

I hope I have not confused you.

Last edited by Bob Hart; October 6th, 2007 at 11:35 PM. Reason: added text.
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Old October 8th, 2007, 06:52 AM   #5
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Thanks again Bob for your comprehensive reply. Hopefully some time this week I'll have a chance to try it out
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