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Old December 31st, 2008, 05:07 AM   #1
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35mm focus problems

I bought a Letus Extreme a while back, but have j recently been putting in some serious practice with it, and trying to integrate it into my jobs when possible. I have some questions,and problems, and hopefully someone can give me some feedback.
I love the look I'm getting, but am struggling to get sharp focus for an entire shoot. Here are my questions.

1- I've read conflicting advice on using the camera zoom when you need to reframe and get tighter. Some say that you should shoot with the lens as wide as possible, short of seeing the inside of the adapter, and others have said you can zoom in with the camera zoom. One of the reasons i'm asking this is an experience I had at a shoot yesterday.
The producer and I wanted to use the 35 rig for some stand up interviews at a golf course. It was really bright out, and i couldn't get much help from my monitor, so had to depend on my fu 1000 monochrome vf on my Canon H1. I used a 105mm f1.8 Nikon prime set at 2.8, framed and focused the first shot, and it looked good (according to the peaking setting). Then he asked me to come in tighter. This shot looked good in the vf also. When I got home and viewed the footage on my hd monitor, I saw the following:
A- the first shot was right on and the focus was sharp as a tack.
B- The beginning of the tighter shot looked good, but then got soft after a couple of minutes.
A local guy who does a lot of 35 stuff told me last night that I shouldn't have zoomed in, and that I should have chenged lenses or moved my sticks and camera in closer for the tighter shot.

2- This experience also leads to another question about back focus issues. If my subject didn't move and I merely zoomed in with the camera, shouldn't he still be in focus ( assuming i checked the peaking on my vf)? Or might i have a back focus problem? Or did I create shallower dof by zooming in, and therefore make focusing more difficult?
The guy I spoke to last night also said that I should have gone with a 50mm or 85mm, and that the dof on the 105 was so minimal that I was asking for trouble. He said that the reason he doesn't zoom in with the camera is that you're adding additional short dof to the situation by combining shallow dof of the lens with additional shallow dof from zooming the camera in. Another thought I had was that maybe I should have stoppoed down to f4, to get a little deeper dof.

3-Does anyone know of any good tutorials that deal with these questions (focus, lens choice, etc.)? I've searched on google under 35mm tutorials, but almost all are about diy building an adapter.

4- Monitors- I have a 7" high res monitor which I bought a couple of years ago from varizoom (swit). I us it on cam sometimes, and also on a stand. I was thinking thata better monitor might help, maybe the new Marshall that's around $1000. Also, I'd like to connect to a monitor from the hd/sdi out of my canon H1 as opposed to the hd component cables. I've never tried this bnc connection, so really don't know what it looks like yet.

Like I said, I really love the look of the 35 stuff, when it's in focus. I don't have the luxury or budget for an ac./ focus puller, so I have to find a system where I can get my own more prdictable sharp focus. I'd really appreciate any help, advice or feedback.
Thanks
bruce S. yarock
Yarock Video and Photo
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Old December 31st, 2008, 06:24 AM   #2
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Bruce, the most important thing to remember about 35mm DOF adapters is that your original zoom lens is now only used to focus on the ground glass inside the adapter.
If you zoom in with your camera's zoom lens, you must re-focus on the ground glass.
If you zoom out with your camera's zoom lens, you must re-focus on the ground glass.
The only time I generally zoom in tighter with my camera's zoom lens is to get rid of any porthole effect from some of my widest lenses. To get a tighter shot of a subject I would either switch to a longer lens, or I would move the camera closer to the subject.
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Old December 31st, 2008, 09:19 AM   #3
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Bruce.


I'm going to buck the trend by suggesting you can re-compose the frame with the camcorder zoom. You may soften the image slightly by going in closer on the groundglass because the "grain" size off the groundglass is scaled larger relative to the frame size.

That is something you would observe along with a possible need to re-focus the camcorder as soon as you zoomed in. Because you are zooming through a close-up lens, focus may not hold through the zoom movement and need trimming.

Focus drift after the zoom is something else again and that one has me a little perplexed.


The possibilities :-

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Auto-focus on camcorder was left on after relay focus was corrected and the camera has gone hunting for detail on the groundglass and called close-enough good-enough. The PMW-EX1 will do this to you. It is easy to mismanage the manual and autofocus on this camera - Usually if this happens, the camera may take off and relay focus crash entirely.

Sometimes however, autofocus finds a little speck or something it thinks is worthwhile looking at on the opposite clear face of the groundglass. Because the groundglass is moving, you won't necessarily see it in the recovered image. It may be apparent as a light freckle possibly hidden in an area of same brightness or colour.

There might be some later subtle movement within the focus mechanism of the camcorder lens after focus is found. In outdoors heat, draggy lubes in the camcorder zoom lens moving group guides may let go as things warm up.


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The 105mm lens focus may have shifted slightly with expansion outdoors in the sun, or viscous lube on the focus threads extruded and allowed the focus barrel to ease forward slightly - fairly unlikely.
----------------------


The 105mm lens may be loose in the mount and may have initially been in flange contact but may have drooped forward slightly when the motor has been running for while. You may see a slight image drop in the frame when this happens.


The lens mount insert may have drooped forward and down slowly with motor vibration and maybe a little bit of heat expanding the bits and pieces and letting one of the three compression screw-ends slip.

It will be top one if this has happened unless the little spacer screws have not been set against the inner face of the orifice the lens mount fits into.


In these events, there may be a softer focus apparent on the lower portion of the frame after it happens. The upper portion will remain closer, if not on true focus.

To check if this has happened, take the lens out, press firmly against the mount and loosen the compression screws in turn whilst maintaining that back-pressure.

If you feel a little backwards click as one screw loosens, the lens mount has moved forward or the little spacer screws were not used to add length to the mount when mount backfocus was adjusted.


--------------------------
A most unlikely cause may be that the groundglass panel itself may have moved during operation. If it is a brand new unit, then there might be a possibility the flexible components have bedded in slightly during the first runs, settling down after being hammered in shipping. If this has occurred, one more adjustment of the mount should see it right.

This is more likely to be observable as a developing soft edge on the bottom (12 O'clock), top left (10 O'clock) or top right (2 O'clock) positions in the recovered image as normally viewed.


--------------------------
The front tube may have drooped due to combination of motor vibration, outdoors heat expansion and the two pairs of screws at the rear having also become untensioned in violent shipping.

I doubt this will have occurred as the tube is a snug fit on the shoulder it slips over but it might be worth checking to see if a slight gap has opened up on the upper half of the junction and if the tube can be moved on the shoulder.

There would be unlikely to be enough forward movement from this cause to show as a focus shift but it is worth checking anyway.


FOOTNOTE:

These comments are all hypotheticals based on my own expeditions inside a Letus Extreme and from the experiences of constructing my own adaptors with three-way axis adjustment, so should not be interpreted as a critique of the design or build quality.

Last edited by Bob Hart; December 31st, 2008 at 09:41 AM. Reason: error
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Old December 31st, 2008, 10:37 AM   #4
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David,
I'm a bit confused about how I would re focus on the ground glass without taking off the lens. On the Letus extreme, you focus on the ground glass without the lens mounted (unless i'm doing it wrong). I use a white background, turn on the motor, and look for the darker pieces and get them sharp. Then I set a focus preset on my camera (H1). it's right at 1mm. Is there a way to re focus on the ground glass with the lens attached?
When I can , I plan to try to change lenses or move for re framing.

Bob,
I don't think that the lens was faulty or had a bad connection. I also don't use autofocus on my camera when using the Letus. I'm going to try to replicate the conditions and do another test with the same lens, though, and I'll let you know what my results are.
Thanks for the feedback.
Bruce S. yarock
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Old December 31st, 2008, 01:44 PM   #5
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Bob,
I forgot to mention that where the focus was soft, it wasn't any particular section of the frame; it was the entire image was slightly shifted to a softer focus.
Another question I had was regarding fast zoom lenses with manual aperture. My fiend just got the nikon 80-200 f 2.8 with the manual aperture ring. I plan to test it out. With that type of lens, would you be able to zoom in, pull focus, then zoom out without losing focus? Since you wouldn't be usimg the camera's zoom, would this type of lens avoid the problem I mentioned in my post (zooming in an losing focus)?
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Old December 31st, 2008, 07:44 PM   #6
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QUOTE:


"I forgot to mention that where the focus was soft, it wasn't any particular section of the frame; it was the entire image was slightly shifted to a softer focus."

That event is more consistent with camcorder relay focus moving or the focus on the stills lens on front moving. Some camcorder lenses, although they have a "manual focus" option, remain electric servo lenses, even in the "manual-on" state. I think this applies to some Canon cameras.

Some camcorders draw mechanical power for the focus and zoom functions from a single electric servo-motor. One function may only work at one time.

To selectively split the power drive may involve some form of mechanical clutching device or differential gear with individual braking on the function not selected to force movement of the function selected. I would expect that braking would come into effect on both focus and zoom in "at-rest" states.

That braking may be spring actuated and released by small solenoids.

In any camera servo lens, single motor type or multiple motor type, if there is no braking during "at-rest" states and reliance placed only on the stiffness of the lubricants in the lens or friction devices like leaf springs sliding in the guides, moving focus groups within the lens just might creep after an activation.

Moving focus groups might also creep with vibration or movement of the camera. The vibration from the Letus might be sufficient to cause this.

This is a whole lot of speculative guesswork on my part.



QUOTE:

"Another question I had was regarding fast zoom lenses with manual aperture. My fiend just got the nikon 80-200 f 2.8 with the manual aperture ring. I plan to test it out. With that type of lens, would you be able to zoom in, pull focus, then zoom out without losing focus?"

Only if the lens will do this on the camera it is designed for. Some stills zooms will not hold true focus through the entire zoom movement although most modern lenses are pretty good. - If the lens is good, the flange-to-focal-plane distance (backfocus) must be precisely adjusted on the camera or 35mm adaptor it is fitted to.



QUOTE:

"Since you wouldn't be using the camera's zoom, would this type of lens avoid the problem I mentioned in my post (zooming in an losing focus)?"

With backfocus of the 35mm adaptor correctly adjusted and the settings of the camcorder lens itself remaining stable, it should.


FOOTNOTE:

With any adaptor, my personal preference is a two-step process.

Firstly, to set initial relay focus with the groundglass stopped and the "grain" forced into visibility by either removing the 35mm lens or closing the iris on this lens. The camcorder lens iris must be wide-open for this adjustment to be effective. Use in-camera ND to bring the light level down if need be or adjust the ambient lighting.

Secondly, to set the groundglass in motion, then establish the sharpest image with the 35mm lens, then trim the relay focus for the sharpest image. The only precise way I have found to do this is to use a focus chart with resolution blocks comprised of fine parallel lines. Even if they are too closely pitched for the sensor to see them, a moire pattern may become apparent when sharpest relay focus is reached.

A quick cheap trick if no focus chart is available is to hold a haircomb against a window or on a piece white paper, well lit so the gaps between the teeth are distinctly visible. It has to be sufficiently distant as for the teeth of the comb to be barely distinguishable or just beyond so a moire pattern "pops" at point of sharpest focus.

It is helpful for focus assist or peaking functions to be selected to "pop" a moire pattern when the camera LCD is used. A good high-definition monitor is the better way to go.

This is the dark side to the use of all groundglass based 35mm adaptors, every single one of them. The focus must be diligently, even obsessively managed for constistent results to be achieved.

Why be this obsessive to the point of perceived personalilty disorder? With some camcorders, the act of switching off then switching back on at a new location is enough to throw the relay focus off ever so slightly, because some camera types seem to undergo a mechanical zero-set or calibration procedure then reset to a last known state.

This is not to impugne the build quality or design of any camcorder. In normal usage, the camera would be unlikely to be left in one setup and powered on and off many times. Nearly always, the fresh battery goes in the back, - click thump, the power goes on, then the hands move to the lens. Shoving an adaptor on front pushes most cameras outside of their normal operating envelope. It is a therefore a positive testament that cameras continue to work as well as they do.


FURTHUR FOOTNOTE:

Another cause of an apparent focal shift may be the variable aperture of some camcorder zoom lenses. If the camcorder iris remains automatic after shutter and gains are locked off, then a zoom-in will cause the iris to open to compensate. If the camcorder lens iris was not wide-open when relay focus was trimmed, then a widening iris will cause an incorrect relay focus to become more apparent.

This would be immediately apparent to the operator on zooming in and not appear as a focus creep. However if there was a change in lighting conditions such as a cloud going over, then a camcorder iris compensation would be set off and an apparent focus creep then occur.

Last edited by Bob Hart; December 31st, 2008 at 08:36 PM. Reason: error
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