McLarenblog

This is the second in a series of posts.  The prior post can be found here: Student Asks, Was The Lens Out Of Calibration? (#1) https://ypw.wrq.mybluehost.me/website_12972da0/?p=1032

On 11/18/10 9:57 PM, “A. B.”  wrote:

All this information is very useful and I have been told since emailing you many of the same things.  All these things are definitely key in my future filming.  I was told by another about the ground glass being specifically important as well.  Although I new about the ground glass, for my previous shoot I set the diopter based on opening up the f-stop and setting the focus to infinite and then adjusting diopter so that the buildings a couple hundred feet away were as sharp as possible, focusing on the lines and edges of the building.  I’m not sure if the grain was as focused as possible because I hadn’t learned that information yet and didn’t think to check.  Although my future focusing will definitely involve the grain on the ground glass, is the method I used incorrect?  I have to send all my rolls in at once due to budget constraints so theres no way to see if I should reshoot or not. Budgeting also is an issue for reshooting with this new knowledge.  Everything was focused to my eye and then rechecked by my DP.  Should I be worried or am I being overly concerned?  I have never shot on 16 and want my first project to be a success in my academic journey into 16. Again thank you for the detailed response and it is very appreciated.

Best,
A. B.

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Dear A. B.,

I just found your reply in my junk e-mail folder.  Entourage, my main e-mail software, can be a pain at times.  Most sorry I did not spot this e-mail earlier.

Your method of placing the lens to infinity and opening up the iris all the way impresses me that you seem to understand the principal of an open iris will diminish the the area of acceptable focus.  Your next procedure of using infinity lens focus as a “trick” to make sure the lens is in calibration, is correct but might be premature.  (By the way, opening up the iris all the way has ruined many a student project.  Make darn sure you reset the iris to proper setting before filming.)  The process you followed worries me a bit.  The eye focus adjustment (diopter) must be properly set so the grain you see in the viewfinder achieves best focus before you do anything with the camera.  I tell students to shoot a blank wall or even a sheet of paper so they and YOU are forced to concentrate on the viewfinder screen grain.

AFTER you can guarantee that the ground glass is in proper focus, THEN shoot objects at infinity with the lens and shoot building edges and power lines to see if the lens is in proper calibration.

What you can do if you find a perfect lenses:  You can do the Siemens star chart tests for closer than infinity to make sure that the lens is accurate all along its markings.  If you know for sure that the lens is dead on perfect for focus at every mark on the focus ring, you can then run a “TAPE” (tape measure) out from the camera to the subject and then rotate the lens focus ring to the corresponding mark.    Every professional camera has a film plane marking.  It looks like a circle with a line through it and the line is ALWAYS vertical.  I personally would viewfinder focus the shot and then tape it to check that my eye had found the correct lens focus plane.  The reason for this is my eyes are getting a bit old and that ground glass just does not do it for me every time.  In other words you can viewfinder the camera for correct focus.  You can actually tape the camera for correct focus (only if the lens is perfect for the camera you are using). Best of all, use both methods and you should NEVER get back out of focus film.

Tip: Proper focus requires LINES.  I defy you to find proper focus shooting a black curtain with no wrinkles.   I love shooting women with all that eye make up that approaches a Siemens start chart.  LOL.  Always look for lines at the plane (from the camera) that you want to look the best.

Tip: When shooting a person, focus on the eyes!   If the camera person can not determine accurate focus, for any reason, put a Siemens star chart at the SAME distance as the object you intend to shoot to make the setting of perfect focus more easy.

Tip: Just as I told you to make sure that you reset your iris to the proper setting before pushing film, ALSO make sure that the camera person sets viewfinder focus for his or her eye before they start the camera.  You would not believe the number of student films that were shot slightly out of focus because the director viewed the scene, adjusted the viewfinder and then the camera operator took over and did NOT reset the viewfinder focus.

Another tip: If you go to college in a large city, you just might have a good film equipment rental house that you can use.  Check their prices for equipment rental.  If you think the college cameras are a problem, consider renting out a camera.  Now here is the trick.  Get other students to do their projects the same WEEKEND as you do yours.  This ONLY works if the rental house closes Saturday and Sunday.  Often times the rental house will consider equipment rental ONE DAY if you take the stuff out Friday and bring it back Monday.  If you find a rental house that does offer this, fine tune it a bit by asking how early Friday can you pick up the equipment and how late you can return it Monday.  You then split the costs with the other student groups.  If YOU pay for the rental and check out the equipment, I suggest that YOU attend every shoot, guard and protect that equipment, and return it back to the rental house because you are financially responsible for loss or damage.  Consider paying for insurance to further protect yourself.  That insurance is typically offered by the rental house.

You asked, “Should I be worried or am I being overly concerned?  You told me you shot the footage; it is done.  Get over it.  If it comes back out of focus, you will have more deeply and painfully learned some pretty basic and very important technical processes that need to be understood when using a film camera.

By the way, what college do you go to?

Extra credit question 1: why should you NOT set your lens ring to infinity to get the greatest (most) area in the scene in acceptable focus?
Extra credit question 2: where should the focus ring be set if NOT at infinity?

I hope this helps.
Regards,
Score Card

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) https://ypw.wrq.mybluehost.me/website_12972da0/?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 https://ypw.wrq.mybluehost.me/website_12972da0/?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)