Focusing Distance Headaches

[Charles Barreto]

I am a little thrown off by my Letus Extreme...I can't quite figure what to do.

I took a measuring tape and measure 2 feet away from the supposed ground glass marking as noted in the instruction. I put an object and perfect focus..sharp as can be. I go 2.5...still sharp focus. But as I measure out 4, 5, 10 feet..the tape shows one thing but the image shows something different. If the tape says 5 feet should be in focus.it is more like 4...if it says 10..its more like 8.

So what do do? Do I just live with it or is there something I could do to make every spot perfect focusing according to the tape measure?

[Bob Hart]

Charles.


Setting the focus is a dark art camera and lens manufacturers spend lots of money buying equipment to do.

For most practical purposes, providing I have infinity focus available and very close to its correct witness mark position on the lens barrel, this would satisfy me. Most lens witness marks are fairly accurate but not all lenses were made equal.


The following comments are not factory authorised so you follow them at your own risk.

My personal preference for a more precise set up, would be to use a measured 1 metre or 4 ft as a kick-off point for setting up with a good 50mm lens. The aperture of your lens should also be set wide-open when you make your adjustments to keep your lens depth-of-field shallow.

I also use a focus chart which has four siemens patterns, one in each corner. When you set the chart absolutely centred and square-on, you can then adjust the mount in all axes by selectively adjusting the three spacer screws in back of the mount.

If you have attempted to move the groundglass carrier itself and disturbed it, the best sequence for adjusting the whole system will be :-

Reset the groundglass to ex-factory position. (have forgetten the distance). If you cannot achieve infinity focus, then bring the groundglass forward about 0.5mm.

Precision calipers should be used to reset the groundglass position however it can be done without by using the camera squarely assembled to the Letus with a potentially better outcome.

Use your camera to check for squareness of the groundglass by inspecting the texture with the motor stopped. It should be even across the field-of-view. You need to do this adjustment with camcorder iris wide-open.

If a sharpness zone travels across the groundglass as you pull camcorder focus, you need to make subtle adjustments to get the groundglass square-on to the camera.

You may observe a travelling sharpness zone in the form of a circle travel from centre to periphery. There is nothing you can do about that. It is a product of relay optics having to please a great number of different camera types.

A hint. You cannot make repeatable subtle adjustments to the groundglass by pulling the pillars forward in the holes by fingers alone. You really need to get a good set of tweezers with good grip to grab each pillar in turn. Use a piece of roast skewer as a lever and the Letus body as fulcrum and gently lever against the tweezers for controllable fine movements.

If you choose a metal object for a lever, keep it clear of the condenser element in the front of the flip enclosure and away from the groundglass as you may scratch them

Don't apply any force to the groundglass carrier itself as you may move force its position on a pillar and it may then spring back when you run the motor and throw your adjustment off.

Once your groundglass alignment to camera is square-on, then it is time to adjust the lens mount. The correct forwards-rearwards position of this mount is controlled by the three tiny screws in axial holes in back of the mount and which face backwards. They seem to have no purpose sitting on their holes doing nothing.

Call them adjustable spacers or limit stops. They simply adjust outwards to hold the rear face of the mount forward of the inner front face of the Letus body.

Use the four siemens stars in the corners of your focus chart and adjust each screw to set the mount square-on to the groundglass. Once you have this set up, then adjust each screw a measured turn to bring the mount forward until you have sharp focus which matches the witness mark on your lens.

Once it is correct, you can then lock off the mount in the Letus body with the three allen screws on radial centres around the front of the Letus body. Take care to maintain reawards pressure on the mount as you tighten these screws as they may walk the mount forward as you tighten them.

P+S Technik use meticulous camera collimation methods to set up their adaptors. This involves using a reverse projection device and spacer shims instead of the Letus screws in the final adjustments.

Within practical limits and extreme care, you can use this method yourself for greater precision if you have access to a 35mm slide (transparencies) projector. It requires that you remove the achromat from the back of the Letus and that your groundglass has already been set square-on to the camcorder view.

Take the lens out of your projector. This leaves you with a bright light to shine in through the back of your Letus. With the achromat removed, the light through the path should be fairly direct and even onto the groundglass.

Position the Letus and projector shining in through the back of the Letus so that the Letus is square-on to a wall or daylight projector screen. Use a 50mm lens on front of the Letus with the iris wide-open. Position the Letus so that it is a measured 1M or 4ft back from the screen and set the lens focus of a known good lens to the matching witness mark on the barrel. If a longer distance suits you, use it. You could also use a wider lens but you may find the projected image a bit dull.

Use the three small screws in back of the mount to make the adjustments as described above. Because of the scale of the magnification, very subtle adjustments can be made.

Do not run the projector light in back of the Letus for more than one minute at a time and leave plenty of time for the Letus to cool down. Why?? ---

Optical glass does not have tolerance for rapid temperature changes and you risk cracking the composite prism, melting the plastic shroud around the condenser element or melting the plastic groundglass carrier and maybe cracking the groundglass. The aluminium case will also expand if it gets hot.

[Charles Barreto]

thanks...good stuff

[Leonard Levy]

Charles,
I need to go over Bob's instructions when I haven't been drinking to understand them though I'm sure he does know what he's talking about.

My own method might be easier though and it would begin by checking your lenses a longer distance away.

2' sounds kind of dicey to me for testing a lens as its pretty close to a minimum distance anyway. Those markings are probably more accurate further away. I found 5' to be a good distance to check on a wide variety of lenses because they all had that footage mark. If you only have a few lenses you might also try somewhere between 5' and 10'.

Even at 5' I found that of seven Nikon lenses some fell right on the mark and some hit it just before or just after so I'm not sure how accurate those numbers are. Still lenses aren't designed for focus pulling.

Also its critical to test your infinity focus. You should hit infinity ( try really far away 300 yards or so if you can. the moon is great.) just at or just in front of the end of the barrel movement.

Remember of course to do you testing with the 35mm lens wide open. Its best to have a variety of lenses as well since it is always possible that one of your lenses may be off.

Lenny Levy

[Bob Hart]

Four feet or one metre is not set in stone and what Leonard says is valid. I should have mentioned an infitinity focus check.

My choice of four feet or one metre and the 50mm is simply bone laziness, convenience and confined workspace on which to do the testing indoors.

I also test across a number of lenses afterwards to see if the story told by the witness marks versus the tape measure varies too much.

[Chuck Wall]

Hi
I have spent lots of time, too much to keep backfocus. I rent out the letus adaptors and each time before it goes out I would verify gg focus and optimze when needed. I found that the gg mounting distance can change whether its the vibration of the glass or the hostilities of shipping. Trouble is move the glass and now you need to redo slr lens back focus.

Any way I spent the 600 to get the "elite" back focus and in a few minutes I was done...

Granted I have only spent about an hour mounting it to the extreme it looks like a real time saver.

I will post more results once it goes out again early next week. I may also take it out and get some test shots.

Chuck

[Charles Barreto]

Originally Posted by Chuck Wall

Hi


Any way I spent the 600 to get the "elite" back focus and in a few minutes I was done...



Chuck

what is this? where can i find more info on it? you mount it directly to the system?

[Chuck Wall]

Hi
The elite backfocus update can be purchased from letusdirect.com or zacuto.com
It replaces the slr side of the extreme.

Chuck