Home of Professional Cinematography since 1996

Calibrating Lenses For Focus

Published : 3rd September 2003


Hi everyone,

I'm currently pulling focus for a 16mm SRII freebie/non-commercial/personal project for someone. I'm far from a Focus Puller by trade. But given the nature of the shoot, this is my way of getting such experience.

We've been shooting today with 8mm, 12mm, 16mm, + 25mm Zeiss lenses, plus a 10-100mm zoom. On tape-measuring distances, finding focus through the view-finder, and creating new marks on the lens, I am surprised at how far out they can be from the barrel markings.

I don't have anything near the experience of a quality Focus Puller that most of you are used to working with, but am a little amazed that I cannot measure the distance and set the focus ring, or the white follow focus ring, without having to start-from-scratch via the view-finder, and disregarding the lens markings.

Is this common? Or just common with the cheap, old lenses I seem to work with?

Jim Cox
Lighting Cameraman, Camera Assistant
Surrey, UK



Jim:

Before you do anything, run a viewfinder focus check test.

This is the only way you know if what the operator is shooting is what you'll get on the film. Take a dollar bill (or a pound note - that stuff has a picture on it doesn't it? ), tape it with double stick to an insert slate. Cover the dollar with 2" clear tape (like Scotch tape, just bigger). Put the insert slate on a C-stand somehow.

Take your widest lens and put it on the camera in question. Position the slate at minimum focus for that lens. Pan the slate away from the lens axis until its about 45 degrees away from the camera. Arrange the slate so the focus, again, is at its minimum. You should see a decent shift from fuzzy to sharp to fuzzy. Light the chart without glare and at the LAD color temp of the film stock (I like to use reversal film. It's really sharp, very little grain and I can see a nice positive without needing to go to a work print).

Lighting the chart at the LAD color temp will allow the best resolution and balance of all the dye layers in the film (thus guaranteeing you the sharpest possible image - no loss of res. due to blown out layers). Scrim the light down until your at the widest stop on the lens possible.

By now, you should have a good change from fuzzy to sharp to fuzzy. If not, then pan the slate away until you get a good shift area, but not so much that the face on the dollar bill is un-recognizable. Mark the centre of the focused area with small tape arrows (top and bottom). Take time with this, making sure you hit the exact centre of the sharp area (usually the bridge of the face's nose).

Shoot this arrangement at 24 fps and process. If you're using neg stock and not rev., don't get a work print. View the negative directly. Look for the area of most focus and see if its shifted. If it has, uh oh. If not, you now know that what you shoot is what you'll get.

Normally, this test turns out fine, but its great to check things out, 'cause that way you can do what has to be done to fix things (i.e., re-scale lenses, trade lenses out, have the back focus adjusted, etc.,). But if the camera goes into the shop for a depth change or any other work, re-test.

It's the only way you'll know with out a doubt that what you shoot is what you'll get.

Best wishes,

Michael Bratkowski
First AC, Los Angeles, CA



Can't beat Michaels excellent advice, but I'll add :

You are using pretty wide lenses. It's possible that your getting a sharp image through the finder that is not on the mark because of your depth of field. Set the lense to the mark and see if it is sharp at that distance, then shift the focus a hair in either direction and see if it's any sharper, softer or unnoticeable. The important thing is that when you hit the mark, the lense is sharp at that distance. The lense may also be sharp off the mark, but that is irrelevant as long as it is as sharp or sharper (sharpest) on the mark.

Test the 10-100 at 100mm and see how it comes up. If it hits the mark, the camera is probably okay. If none of the lenses hit the mark, it is probably a camera issue.

Good Luck,

Anders Uhl
cinematographer
ICG, New York



There are a few potential problems when the focus scales don't match what you're seeing. Either the lens is wrong, the lens mount (flange depth or back focus) or the groundglass depth could be off; or even a combination of all three (what I like to call an Ebola camera problem, where one is bleeding from every orifice).

The quick test for your flange depth is to put on the zoom lens. Zoom all the way in, focus, then zoom out. It the image falls out of focus then your back focus is out. This is a service call to have the depth reset with a collimator.

Next question is the groundglass depth.

The really quick way is to take to a rental house to check, but short of this just shoot a quick test. Take your longest lens and shoot a close-up of the prettiest PA on set with her head full frame. Set the lighting for the widest aperture the lens will allow. Focus on her eyes using the viewfinder and shoot the test. Project the result and see if her eyes are still in focus or if the sharp point has drifted back towards her ears or forward to the tip of her nose. If focus is not still on her eyes then you need to have the groundglass depth adjusted.

Once you're sure the back-focus and the groundglass are correct, you can set focus with your eye in the viewfinder and re-mark the lenses. Or you can go to a rental house and use their lens projector to test and reset the lens marks, which is by far the easiest and most accurate way to do this.

Mitch Gross
NYC DP



>On tape-measuring distances, finding focus through the view-finder, and >creating new marks on the lens

If you are having more trouble with the wider lenses than the tighter lenses you may have a back focus problem. If you are having problems with ALL lenses, then you may have a back focus problem.

Are you getting eye focus marks with the aperture wide open?

When I was an assistant I discovered that there was usually some "play" in what I could see as being in focus, especially on wider lenses. You'll probably experience that, especially with the wider lenses you are working with. You'll probably have less trouble at the long end of your 10-1 zoom. When you get an eye focus on an object roll the focus in first from near-to-far and note where it falls; do that again far-to-near.

If you end up with two spots on the lens barrel that are a short distance apart you may simply be seeing the depth of field of that lens as you approach critical focus. In that case your lenses may be fine, and your eye is simply telling you to stop too soon as you roll the focus towards where it should be.

The viewfinder, or old soft lenses, may not be showing you all the subtleties of sharpness available. Make sure you are getting all your eye focus marks at wide open. And double check your lenses against a chart that you have measured out at specific distances to see if your lenses or back focus are whacked or whether you are just looking through some older and softer lenses that are hard to eye focus.

I'd recommend using a tape measure as much as possible when starting out as a focus puller. I had a couple of operators who wouldn't let me eye focus for quite a while when I was assisting unless I was on a really long lens.

They said it was good for me. I think it was. Make sure those lenses are focusing where they should be first.

Art Adams, DP
Mountain View, California - "Silicon Valley"
http://www.artadams.net/



My strongest recommendation would be to have your camera's flange depth and groundglass tested and calibrated professionally. Only then should you use your camera as a back-focus reference for your lenses.

Mitch Gross writes:

>The quick test for your flange depth is to put on the zoom lens.

I'd be cautious about this approach for two reasons:

1) - A back-focus problem can be a matter of flange depth (on the camera) or mount adjustment (on the lens). If your camera isn't calibrated first, you may end up working to a false standard and then mis-calibrating your lenses.

2) - Zooming out can some times be deceptive because details naturally become fuzzier as you zoom out.

IMHO all normal-to-short professional lenses should have accessible back-focus adjustments the way HD lenses do.

Dan Drasin
Producer/DP
Marin County, CA



>IMHO all normal-to-short professional lenses should have accessible back-focus adjustments the way HD lenses do.

Begging to differ, the last thing you need on set is another variable to be set incorrectly...that is what checkouts are for...unless AC's are all going to go out and buy autocollimators and flange depth kits to use on set and on location so they can do the job properly (and learn how to use them correctly)

I think that we would all be better served if lens mounts and optical block/chip mounts on HD cameras were made out of something other than aluminium -like something that would be dimensionally stable enough over a wide temp., range - so that we wouldn't have to keep adjusting back-focus on HD cameras every damned time we changed lenses...at least one camera rental house has gone to great trouble and expense to replace the lens mount/optical block/chip mount assembly on their HD cameras with something that IS dimensionally stable precisely to address this weakness in video camera design and manufacture.

I have not worked with the modified cameras yet, so I can't speak personally to the improvement, but confidence is high.

Mark Weingartner
LA based
(yes, some HD jobs have VFX too)