Student Asks, Was The Lens Out Of Calibration? (#1)

Student Asks, Was The Lens Out Of Calibration? (#1)

This is the first in a series of e-mail posts. The next post can be found here: 16 MM Camera Lens Out Of Calibration? (#2) http://mclarenblog.com/?p=1035

On 11/16/10 7:43 PM, “A. B.” wrote:

Hi I am a new filmmaker still in school and I recently read your blog from 3
years ago here http://mclarenblog.com/?p=196
My question is what if you have done all these steps and the feet marked with
measuring tape are not matching up.  For instance my subject is 5 1/2 ft away
but image is sharp at a setting of 3 ft?  Is the camera out of calibration is
it possible for it to be this way?  Not sure what seems to be the problem, my
professor told me to just trust my eye not the measuring tape.  But I feel
like the camera focus distance should coincide with the actual distance.
These cameras are 16mm Ariflex’s from the late 70’s.

Thanks for the help
A. B.

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On Nov 17, 2010, at 11:24 PM, Score Card wrote:

Mr. A. B.,

Let me put some ideas out for you to consider.

Ideally when you rent a camera from a rental house, you should expect the camera to be in near perfect running order.  Schools can slip from that due to a number of factors.  I am somewhat impressed that you found that this can be a problem.  Your concern and the measurements we are discussing should also be used when renting from a rental house.  Rental houses always have a setup area for the customers to set up and test the equipment before it goes out.

When you shoot an object close to the camera I have noticed that the lens makings are not all that accurate on some lenses.   There are a lot of reasons for this but they all come from being used by a lot of students, not being properly maintained, and not always being matched with the correct camera.

Please be aware that lenses MUST be collimated for each camera.   A really bad idea is use a lens intended for one type of camera in another camera.  If you do this, the lens markings might (will) be off.

Keep this in mind.   The lens forms an image on to the film.  The camera intercepts the lens image on its way to the film and throws it into the camera person’s eye using some trick:

Bolex = beam splitter is used but robs a portion of the light for viewing.
Arri = rotating mirror intercepts the light briefly to throw it to the viewfinder.

The eye then looks at the light image on its way to the film either as an aerial image formed in space (Bolex) or on a clear lens or on a ground glass (Arri).

When I troubleshoot any problem, I almost always look at what can go wrong but in order of most probable cause.  Of the three components listed above, the lens being out of calibration and the camera person not setting their eye piece focus properly are tops on my list.  In other words, the two devices that people touch and can screw up are truly the most prone to failure.  Those two problem devices are typically at either end of the camera.

You are correct that if you shoot a Siemens star chart and then taped the distance from camera film marking (line though a circle) to chart, the best focus should read on the lens pretty close to the tape reading.  If it does not, you should check the following:

For all the following tests, make sure you have the lens iris set to wide open.  This diminishes the depth of field.  This will make all lens measurements very accurate.
Look closely at the lens you are having problem with.  Are any rings loose?  Does the lens move side to side in any way?  Rotate the focus ring on the lens you are concerned about to both extremes, that being, infinity and then the closest focus marking on the lens.  Of particular interest for me is to see if the focus for infinity lines up EXACTLY.  If the infinity marking does not line up with the marking for the exact focus line on the rotating barrel, the focus ring has slipped and would then make the lens markings for focus be incorrect.   Resetting that ring is very easy and I had to do that many times.
Typically the camera has a viewfinder to see the image “through the taking lens”.  You must appreciate that you MUST focus your eye to the plane where the image is formed for your eye.  On the Arri S camera, they added a ground glass so the lens image stops and is rendered on a ground glass like a small theatre screen.  The camera operator then views this image from the opposite side of the ground glass.  Problem is that the camera operator, being you, must focus your eye so that the grain (NOT TO THE IMAGE).   That ground glass should look as sharp  as you can make it.  In other words you want to shoot a blank surface with the camera so you concentrate on that ground glass and rotate the diopter lens adjustment until that ground glass looks as grainy as possible.  It is best to rock the diopter lens adjustment back and forth until the ground glass “pops with definition”.  If you did not do that correct, this would easily create a lens focus error.  Another tip:  Some cameras come with a turret of three lenses.   If your viewfinder is not set for proper diopter, all the lenses on the camera should show focus ring errors.  Or you can screw on other lenses and see if they all have the same error.  If they do, I would suspect a diopter or lens viewfinder error.
If you have a Bolex camera you do not have a ground glass.  This method am going to tell you will also work for Arri.  Set the taking lens to infinity.  Next, focus on power lines or a building edge far far away.   Next, set your viewfinder dipoter so those power lines and/or building edge look as sharp as you can make them.  If this does NOT work properly; you do not get good results, the camera optics are suspect.  Now, take a Siemens Star Chart and set it to some close in distance. I look at my lens and look for a good convenient marking and tape that distance out from the camera and place the chart.  You set the diopter for infinity so do NOT change it!!!  Shooting the Siemens Star Chart, rock the camera lens focus back and forth until the center of that chart looks as best as it can get.  Bad is fuzzy.  Good is sharp pie shaped wedges.  (Note: Arri ground glass focus cameras will never look terrific).  If your tape distance to chart does not match pretty close to the lens markings, I would suspect the camera might be out of calibration.
A very telling test is to shoot some film with the camera.  I would shoot the Siemens Star Chart at different distances and put up a large label under the chart that can easily be read as to the distance from camera.  If you find a consistent error, jot it down and use that figure to compensate future shoots or send the lens in for repair if it is way off.
If you have access to an autocollimator that device is used by professionals to confirm lens focus to the FILM plane.  These devices are time consuming and tricky to use but the ultimate tool.

You have to get your hands on a Siemens Star Chart.  Use your computer and Google it.  Find a FREE one.  Print it out.

Next read up about the chart: http://en.wikipedia.org/wiki/Siemens_star <http://en.wikipedia.org/wiki/Siemens_star>

Print out one of the charts and glue it to a stiff board.   Tip: you can use dry mount tissue and a clothes iron will work on cardboard and smooth wood.

The Siemens Star chart is intuitive and thus simple to use.  Just focus your camera lens until the chart snaps into focus at the center.  An out of focus image rapidly appears bad when the center of the chart goes blurry.  Some smart camera operators sometimes use this chart to find or confirm correct focal distance for difficult camera shots.

I hope this helps.
Score Card

(Parts of this blog post revised Dec 7, 2017)

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