Lens test due to change in camera

Mathew Collins · March 6, 2017

Hi,

I was working as a camera assistant for one movie. We used Redone-MX and Carl Zeiss CP.2 lenses for almost 90% of the movie. Later there was a fault in the power chord of the camera and we started using Red Epic-MX.

Before using Epic, The focus puller tested the lenses with RedEpic.

He measured the distance from the lens mount to a subject by holding the tape near to lens mount. That was 10ft.

There he tested each lenses on Epic. Following are the focus mark for each lenses

15mm - No issues

21mm - Focus at 8ft

35mm - No issues

50mm - Focus at 9.6ft

85mm- Focus at 9.9ft

135mm- Focus at 9ft

I have few questions.

1. What is point on the camera to hold the tape?

2. In above case, is the subject distance measurement correct?

3. Is this testing is necessary while changing the camera? Is it due to the change in flange focal distance?

4. What are the other tests required while changing the camera?

5. Is there any specific charts available for tests?

Please share your inputs.

Satsuki Murashige · March 6, 2017

This should go into the Camera Assistants section of the forum.

1. The place to measure on the camera is called the focal plane or film plane. It is usually indicated by a circle with a vertical line through it. The focal plane is exactly where the sensor or film sits within the camera, and where the lenses are designed to focus. On some cameras, there is a little tape hook installed there for a measuring tape.

2. Basically, the focus marks on the lens should match the measured distance. If they do not, then either the lenses are out of adjustment (collimation), or the camera's lens mount (flange focal distance, or FFD) is off. You would need to use special tools to determine which one is the problem.

3. Yes, it's always necessary to test this on every camera body and every lens when prepping a shoot. Usually, for a long job the 1st AC would check this periodically. The rental house usually makes all the adjustments.

4. There are many tests to be done, depending on the camera system. You should keep working with this Focus Puller and learn by observation. A lot of these things require training. Check out the books on camera assisting here for starters: http://www.cinematography.com/index.php?showforum=107&prune_day=100&sort_by=Z-A&sort_key=last_post&topicfilter=all&page=1

I recommend 'The Camera Assistant' by Doug Hart, Samuelson's 'Hands on Manual for Cinematographers', and 'The Camera Assistant's Manual' by Elkins.

5. A Siemens Star focus chart is good thing to keep in your kit. You should be able to download one on the Panavision website and print it out on good photo paper. A Putora focus chart is a different type of focus chart.

Satsuki Murashige · March 6, 2017

Looks like the focus chart is no longer up on the Panavision site. Shoot me a PM and I'll send you a link to download one. I'd make it public, but it's on my personal Dropbox at the moment.

Mathew Collins · March 6, 2017

Thanks much Satsuki.

>Basically, the focus marks on the lens should match the measured distance. If they do not, then either the lenses are out of adjustment (collimation), or the camera's lens mount (flange focal distance, or FFD) is off. You would need to use special tools to determine which one is the problem.

What is the meaning of 'flange focal distance is off'?

Where do I have to sent PM?

Mark Dunn · March 6, 2017

Assuming you are sure that the lenses are not out of collimation, then the distance between the lens flange and the sensor may be incorrect. It is critical to a few thousandths of an inch and needs to be checked.

But the lenses must be checked first. It's less likely for primes to be out than zooms as they have fewer moving parts.

The focal plane mark is clearly visible in this photograph above the "R".

https://www.google.co.uk/search?q=red+epic&source=lnms&tbm=isch&sa=X&ved=0ahUKEwj6iLnMssLSAhVkJMAKHUb2ABwQ_AUICSgC&biw=1600&bih=756#imgrc=kWPcNAvOWW0e9M:

If focus was measured from the mount then all the distances would be a few inches short. Since they vary it suggests a collimation or FFD problem.

Edited March 6, 2017 by Mark Dunn

Satsuki Murashige · March 6, 2017

What is the meaning of 'flange focal distance is off'?

Every type of lens mount has a specified FFD to which it must be set very precisely. For PL mount, the FFD standard is 52.00mm, sometimes actually set to 51.98 or 51.99 depending on the camera.

With a special tool, a technician would check the mount. If it's off, then it needs to be adjusted back to spec. Otherwise, a properly collimated lens on that mount will not focus accurately.

Lens collimation is a whole other issue and requires more special tools. If lenses need to be adjusted, that can only be done by an experienced technician with proper tools. And that gets very expensive, unless the rental house which owns the lenses has a technician on staff. The good ones do, and the large ones have many of them.

Where do I have to send PM?

Write your request on the back of a $20 bill and mail it to my PO Box. Just kidding! ;)

Click on my user name and go to my profile page. If you're on a mobile device, switch to the Full Version of the website. Then select send a Personal Message from the menu on the left side of the page.

Gregg MacPherson · March 6, 2017

Mathew,

In case you are uncertain about what FFD means...there is a contact surface where the lens mount sits on the camera mount. The distance from that surface to the film/sensor plane has to be exact. For the camera and the lens.

Mathew Collins · March 7, 2017

Assuming you are sure that the lenses are not out of collimation, then the distance between the lens flange and the sensor may be incorrect. It is critical to a few thousandths of an inch and needs to be checked.

But the lenses must be checked first. It's less likely for primes to be out than zooms as they have fewer moving parts.

The focal plane mark is clearly visible in this photograph above the "R".

https://www.google.co.uk/search?q=red+epic&source=lnms&tbm=isch&sa=X&ved=0ahUKEwj6iLnMssLSAhVkJMAKHUb2ABwQ_AUICSgC&biw=1600&bih=756#imgrc=kWPcNAvOWW0e9M:

If focus was measured from the mount then all the distances would be a few inches short. Since they vary it suggests a collimation or FFD problem.

Thanks Mark.

The lenses were working fine with RedONE-MX. Before we start using RedEpic-MX, the focus puller performed the above mentioned test(not sure his procedure is correct or not, because he measured the distance from the lens mount to subject.).

Whether the FP done is right or wrong, I think the FFD is different for RedONE-MX and RedEpic-MX(by seeing the shape of camera body, RedOne-MX is longer than RedEpic-MX).

In that case, is lens collimation required?

Is the FFD is set by the manufacturer or rental company's technician?

Edited March 7, 2017 by Mathew Collins

Mathew Collins · March 7, 2017

Every type of lens mount has a specified FFD to which it must be set very precisely. For PL mount, the FFD standard is 52.00mm, sometimes actually set to 51.98 or 51.99 depending on the camera.

With a special tool, a technician would check the mount. If it's off, then it needs to be adjusted back to spec. Otherwise, a properly collimated lens on that mount will not focus accurately.

Lens collimation is a whole other issue and requires more special tools. If lenses need to be adjusted, that can only be done by an experienced technician with proper tools. And that gets very expensive, unless the rental house which owns the lenses has a technician on staff. The good ones do, and the large ones have many of them.

Write your request on the back of a $20 bill and mail it to my PO Box. Just kidding! ;)

Click on my user name and go to my profile page. If you're on a mobile device, switch to the Full Version of the website. Then select send a Personal Message from the menu on the left side of the page.

>With a special tool, a technician would check the mount. If it's off, then it needs to be adjusted back to spec. Otherwise, a properly collimated lens on that mount will not focus accurately.

I think the mount is already fixed on the body while purchasing the camera. Hence the FFD is fixed.

Later do we have to change the FFD by changing to another PL mount of different thickness or adding specific type of rings at the mount?

There are some documents given here in the pdf 'Mount Change Instructions CP.2'

https://www.zeiss.co.in/camera-lenses/en_in/cine_lenses/compact_lenses/compact_prime_lenses.html

Mathew Collins · March 7, 2017

Mathew,

In case you are uncertain about what FFD means...there is a contact surface where the lens mount sits on the camera mount. The distance from that surface to the film/sensor plane has to be exact. For the camera and the lens.

Thanks Gregg.

Is FFD measured from the extreme outer part of the PL mount(front) or from the inner part of the PL mount(back) to the sensor

| PL mount | |sensor

If count '|' s from the left side, Is the FFD measured

from the first '|' to sensor

or

from the second '|' to sensor ?

Edited March 7, 2017 by Mathew Collins

Satsuki Murashige · March 7, 2017

The flange focal distance is a technical specification of the lens mount itself. They are manufactured, assembled, and adjusted to maintain that specification. This means that any camera with a PL mount must have the same spec. Otherwise, correctly maintained lenses will not focus the same on different cameras.

I'm guessing that in your case with the Red cameras, that was PL mount? Those cameras can come in a variety of different lens mounts.

With the Red One, the flange adjustment (or as they call it, 'back focus' adjustment) is done by loosening the two screws on the mount collar and slowly rotating it. This was a pain to do because it was very imprecise, and when you tightened the screws the focus would shift slightly. One of the many reasons ACs hated that camera.

With the Epic, Red introduced an adjustment screw on the top of the camera body for this. Much easier to do. You need a Torx driver to adjust this, T5 if I remember correctly. In the newest Red DSMC2 camera bodies, the adjustment screw has been moved to the front of the body under the lens mount.

Other cameras have different methods. For Arri-style lens mounts, you need the appropriate set of shims. A Denz collimator or similar optical device is useful for checking the flange on digital cameras. You cannot measure flange distance with a depth gage and steel plate on a digital camera as you do with film. There are additional glass surfaces between the lens mount and the sensor that you should not touch.

Again, this is the type of adjustment that should be done by a qualified technician, or at least a knowledgeable person. You can do more damage than good if you don't know what you are doing.

Gregg MacPherson · March 7, 2017

Mathew,

To undestand Satsuki, I think it will help to clearly visualize what the "flange is. Here's a Pl camera mount with the red dot scribbled on the "flange"...

So meauring from this flange surface to the film plane or sensor plane there is an idealized number, 52mm I think. The same notional flange surface is used when collimating the lenses. The surface is at the rear of the lens mount.....

Dylan Tidmore · March 7, 2017

Excuse me for asking a silly question, because I'm trying to wrap my head around the OP's measurements.

Are you saying that when the subject is 10ft away from the lens mount, the lens has a reading of 9.9ft at perfect focus of the subject?

I suppose then that warrants the discussion of where to measure the focus from to ensure the readings are accurate on the lens.

Stuart Brereton · March 8, 2017

Excuse me for asking a silly question, because I'm trying to wrap my head around the OP's measurements.

Are you saying that when the subject is 10ft away from the lens mount, the lens has a reading of 9.9ft at perfect focus of the subject?

I suppose then that warrants the discussion of where to measure the focus from to ensure the readings are accurate on the lens.

You should measure focus from the focal plane. Measuring from the mount (on a PL camera) would mean you were 2 inches too close.

Dylan Tidmore · March 8, 2017

You should measure focus from the focal plane. Measuring from the mount (on a PL camera) would mean you were 2 inches too close.

So even measured from the focal plane. If the lens achieves perfect focus at 9.9ft when the subject is 10ft away, that means the lens should be checked and sent in?

Satsuki Murashige · March 8, 2017

So even measured from the focal plane. If the lens achieves perfect focus at 9.9ft when the subject is 10ft away, that means the lens should be checked and sent in?

I think a lot of private lens owners may not bother in this situation, which is a whole other issue. If they are just focusing the lens by eye on their own camera, the footage is sharp, and their clients are happy, then great.

If on the other hand they are working with an AC who focuses by numbers, or they are renting additional (properly collimated) lenses or a camera body for a job and get back soft images, then there's going to be a lot of anger and finger-pointing after the fact.

This is why a good AC will always perform their due diligence and spend the time to check the lenses in prep. And why rental houses have technicians to maintain their gear up to spec. No one wants to be liable for ruining a shoot with soft focus.

Satsuki Murashige · March 8, 2017

Not to mention, the problem could be the camera's mount. Without a known reference standard, you're pretty much just guessing where the problem is. That's why tools like collimators and depth gages exist.

Where this may really come to back to bite you is when you suddenly need to add an additional camera or set of lenses on an existing job, only to realize that the equipment you've been shooting with was not up to spec.