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Old February 15th, 2011, 09:34 AM   #1
Join Date: Apr 2010
Location: Chicago, IL
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HPX300, Letus Ultimate, Relay, HELP!

Hey all ... trying to setup a relay and a letus ultimate with a HPX300. Thought I had it setup but the when I put on the Zeiss it is way out of focus at infinity. Can use a bit of know how if anyone can provide it to me!
Steve Alsip is offline   Reply With Quote
Old February 15th, 2011, 08:39 PM   #2
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Join Date: Dec 2003
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On the surface, it sounds like your 'front lens" backfocus is off. There should be a blue ring on front of the Letus to adjust this. There will be some sort of lockscrew, thumbscrew in the ring, maybe more than one.

Okay. Firstly, I will use the term "back-focus" when talking about setting the correct focus distance from the Zeiss lens to the groundglass. I use the term "relay focus" when talking about setting the camcorder-to-groundglass focus.

Actually the term "back-focus" gets used to describe both which can be confusing. So from here on, what I am referring to is the Zeiss lens to groundglass distance when I use the word "backfocus". I am assuming you have the "relay focus" already sorted.

Indoors. Hook your camera output to a very big LCD TV screen - very important if you want to do a good job of this. Camera LCDs are not so good for this.

If your Zeiss lens is a good one and the focus witness marks are known to be accurate, set up a siemens chart (focus star pattern). Set your Zeiss lens focus to about 1.5metres or to whatever is the nearest witness mark inscribed in the lens ring, ie., 1.2 or 1.6 metres. Place the chart 1.5 metres away or whatever the nearest number is on the focus ring, ie., 1.2metres or 1.6 metres.

Measure from the focus chart to the focal plane mark on the Letus and adjust the position of the chart until the distance is exact to the witness mark setting you chose on the Zeiss lens.

Light the chart so that your Zeiss lens has to be wide-open. This will make the DOF shallow and ensure an accurate backfocus setting.

Rotate the loosened blue ring until the chart becomes sharpest. There may be a bit of lost movement in the ring mechanism. Once you have a sharp image, lock the blue ring with the thumbscrews. Keep a close look on the image. It may creep off focus a fraction and you may have to trim focus again a few times until you are satisfied.

This is not just a Letus trait. The Mini35 and Mini35C direct relay focus also does this and I expect most other adaptors, as well as many ENG lenses. As the locking screw bears down on the inner barrel, it kicks the mechanism ever so slightly sideways within its working clearances and a really tiny refocussing movement may occur. If there was no working clearance, you would not be able to adjust focus.

Unlike most other adaptors, the Mini35 and Mini35C front lens "backfocus" is set precisely in the factory by projection from the groundglass plane through a calibration lens and packing the mount with shims as is practice with film cameras and lens collimation.

In practical terms it probably does not matter - "unless" you want to work to the lens witness marks and tape measure from camera to subject in low light conditions. Why not go for perfection when with care and patience, it is achievable.

Outdoors. Again, use a big LCD screen. ND the camera down until the front lens has to be wide-open.

You might have to wait for a suitable time in the day if you don't have ND gel or mattbox filters.

Pick a distant object like a radio tower or tree on a hilltop. Set your Zeiss lens to its infinity focus mark, not to the end-stop of the lens movement. Depending on build quality across brands, the end stop can be past true infinity focus even from new. Pre-owned examples may have an uncertain history.

Trim the back-focus as recommended above for indoors.

If you are viewing outdoors on an infinity target through a window, make sure the window glass is opened. Practically, it probably makes no difference, but if you are up close to the glass and using a wide lens, it might be enough to put your focus off just a trace, as window glass is not optically true.

If your lens is a zoom, set your zoom movement to about halfway through its travel. Set your initial back-focus in that position, then check it at the wide and tele ends of the movement. It should hold fairly true but may require some additional trims to get it right.

Some stills zoom lenses do not hold sharp focus through their zoom range at all. In that event, you are best to choose a zoom setting you will most often use, probably about 85mm for framing on face, or about 75mm for head and shoulders, set the backfocus for an accurate reading on the lens barrel at the distance you choose. At least then your close-ups and reverses will be true as long as you stick to the same distances from camera to subject.

If you got your Ultimate second-hand, I think you may still be able download the manual from Home or Letus Corporation - Manufacturer's Website.


Last edited by Bob Hart; February 15th, 2011 at 08:54 PM. Reason: error
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