Dumb-ish back focus question

[Mike 'Fireman' Ross]

Folks,

I guess the obvious answer to this should be 'yes' but would appreciate an answer from an optical guru or, better, RED!

I set the back focus on EPIC/Scarlet so it's spot-bollock perfect, with RED FOCUS or similar instrument.

I then pull off the PL mount and throw on the Canon.

Should my back focus still be perfect, assuming the Canon lens isn't out of whack?

Obvious reason for the question is lack of suitable instruments for calibrating back focus with the Canon mount... I'm assuming the mount system is made with sufficient precision to change mounts without changing focal distance.

Thanks

Mike

 

[Paul Ellington]

My best guess is that it's a lens to lens situation. How close the rear element is to the sensor...

 

[Jason Comparetto]

My best guess is that RED would have set the distance equal on the two mounts, but someone who wants to do a test can confirm this. When we were designing our lens mount we opted not to use a RED focus or similar instrument. We actually found the center position of the collimator and based our measurements off of that. This way, people would have maximum adjustment in both positive and negative directions.

 

[Scott Crawley]

With Epic and Scarlet, it seems that folks are using the term Flange Depth to refer to the distance from the lens' rear element to the imaging plane relative to the flange and mount while Back Focus adjustment refers to the distance from the lens' rear element to the imaging plane relative to the sensor. Is that correct or are we measuring flange depth from the rear element to some point on the mount?

Am I misunderstanding entirely?

 

[Terry VerHaar]

On my JVC, I could adjust back focus with a small adjustment screw while pointed at a star chart. Is the same process possible on my Scarlet? Otherwise, how does one set back focus with a Canon mount?

 

[Jacek Zakowicz]

That's a good question for the SALT guys. They are shooting the test today. I loaned them an Optimator with PL, Canon EF and Nikon adapters.
They also have Epic with PL, Canon and my ProLock mounts so they can easily test it...

 

[Scott Crawley]

I expect a Red Focus for EOS is in the works because even with the most rigid materials and outstanding engineering the distance to the imaging plane will vary minutely, from install to install and over time due to wear. I'm obviously no expert but my answer to your question would be no, don't expect your back focus to be perfect. There still needs to be a means to verify that the distance from the mount to the sensor has remained consistent once the new mount has been installed, and to fine tune that distance as needed.

Folks,

I guess the obvious answer to this should be 'yes' but would appreciate an answer from an optical guru or, better, RED!

I set the back focus on EPIC/Scarlet so it's spot-bollock perfect, with RED FOCUS or similar instrument.

I then pull off the PL mount and throw on the Canon.

Should my back focus still be perfect, assuming the Canon lens isn't out of whack?

Obvious reason for the question is lack of suitable instruments for calibrating back focus with the Canon mount... I'm assuming the mount system is made with sufficient precision to change mounts without changing focal distance.

Thanks

Mike

It sounds like it may be the same or very similar Terry. What model is your JVC?

On my JVC, I could adjust back focus with a small adjustment screw while pointed at a star chart. Is the same process possible on my Scarlet? Otherwise, how does one set back focus with a Canon mount?

 

[Douglas Underdahl]

Well, to go back to the original question, here goes:

If the PL and Canon mounts are, say, within .0005" of each other, then yes, you could swap to Canon after checking the flange focal distance with the PL, and you'd be fine.

They just might be. If they aren't, then the focus marks on your Canon lenses would be off. How much they would be off, you can test yourself by focusing them by eye with a chart and tape. If you eye focus your Canon lenses as your method of focussing, that is, you look at the monitor, do the zoom in thing, and focus, then you aren't using the marks on your lens, so the point is generally moot, unless the Canon mount is so far out that you can't reach infinity or the minimum focus distance. Red makes prettty precise lens mounts so I doubt this would ever be the case.

Also, any given PL or Canon mount can and will vary as they come off the CNC machines and are finished, which, along with the machine tolerances of the camera body, sensor holder, olpf holder, etc are the reason that the flange focal adjustment on the Epic, Scarlet, and Red One exists in the first place.

The flange focal distance of the PL mount is 52mm. This is the distance from where the lens meets the mount, to the sensor plane, somewhat modified due to the OLPF filter, but it is virtually 52mm. Camera techs call this depth. Back Focus is a term used for 3 chip digital cameras, and "adjusting" the Back Focus is done on the lenses themselves, not on the camera. So This term is not correct for Red cameras.

 

[Douglas Underdahl]

Now, I've seen some people worry that their camera isn't shooting in focus because the flange focal distance is off. This is most likely incorrect.

When you focus a lens, you are in effect changing the flange focal distance. Try it - grab that focus ring, turn it, and watch the lens move in and out in relation to the camera.

So you shoot something with your Red camera, and it doesn't seem sharp. You think, hey, I wonder if my flange focal distance is wrong and causing this. Do this test:

Set up your camera pointed to a sheet of newspaper, or Siemens Star chart. Focus, using the monitor on zoom in mode, by eye.

Were you able to focus before, and then after the chart? In other words, could you get the chart out of focus by turning the focus ring to the near end, then get it out of focus by turning the focus ring to the far end, passing a point of sharp focus in between?

If you can, then your flange focal distance might not be dead on, but it's close enough so that the lens is able to throw the sharpest image it can on the sensor. So if at this point you still feel that the image isn't sharp enough, then maybe you've discovered just how ultra sharp the Red camera sensor is and how it can turn up flaws in lense that you thought were super sharp. Or, maybe you need to do a better debay in Redcine. Or mabye you are looking at a lower res version of the file when you thought otherwise.

I just see a lot of misinformation here about how incorrect flange focal distance can make lenses soft, when in reality this would just shift the focus point, unless it's REALLY out and you can't focus near, then far, past your chart.

 

[Jacek Zakowicz]

Well, to go back to the original question, here goes:

If the PL and Canon mounts are, say, within .0005" of each other, then yes, you could swap to Canon after checking the flange focal distance with the PL, and you'd be fine.

They just might be. If they aren't, then the focus marks on your Canon lenses would be off. How much they would be off, you can test yourself by focusing them by eye with a chart and tape. If you eye focus your Canon lenses as your method of focussing, that is, you look at the monitor, do the zoom in thing, and focus, then you aren't using the marks on your lens, so the point is generally moot, unless the Canon mount is so far out that you can't reach infinity or the minimum focus distance. Red makes prettty precise lens mounts so I doubt this would ever be the case.

Also, any given PL or Canon mount can and will vary as they come off the CNC machines and are finished, which, along with the machine tolerances of the camera body, sensor holder, olpf holder, etc are the reason that the flange focal adjustment on the Epic, Scarlet, and Red One exists in the first place.

The flange focal distance of the PL mount is 52mm. This is the distance from where the lens meets the mount, to the sensor plane, somewhat modified due to the OLPF filter, but it is virtually 52mm. Camera techs call this depth. Back Focus is a term used for 3 chip digital cameras, and "adjusting" the Back Focus is done on the lenses themselves, not on the camera. So This term is not correct for Red cameras.

I beg to differ Doug:wink5:
First of all we don't rely on the machining entirely.
Each of our mount is built to a very precise height that offsets the measured height of the Red PL mount.
We checked 5 or so different Red PL Ti mounts and they were very close and that's how we determined the optimum height of the ProLock. Once you have that constant all you need to do is adjust the camera with the PL mount correctly and all the other variables you've mentioned are irrelevant. I guarantee that Red does the same- it's the only correct way to do it. Otherwise you'be constantly adjusting.
That's of course assuming that the lenses are correct. If they are not all bets are off...

 

[Douglas Underdahl]

The flange focal distance of any camera with a PL mount is 52mm. This is measured from the point where the lens mount on the lens meets the lens mount on the camera, to the sensor plane. This is true no matter what PL mount lens you are using.

Some PL mount lenses protrude far into the camera and some don't. Others have large rear elements, and others have smaller ones. But the flange focal distance doesn't change.

Different lens mounts, such as Canon EOS, Nikon, C mount, Leica M, etc, have different flange focal distances, and most are shorter than PL.

 

[Douglas Underdahl]

I beg to differ Doug:wink5:
First of all we don't rely on the machining entirely.
Each of our mount is built to a very precise height that offsets the measured height of the Red PL mount.
We checked 5 or so different Red PL Ti mounts and they were very close and that's how we determined the optimum height of the ProLock. Once you have that constant all you need to do is adjust the camera with the PL mount correctly and all the other variables you've mentioned are irrelevant. I guarantee that Red does the same- it's the only correct way to do it. Otherwise you'be constantly adjusting.
That's of course assuming that the lenses are correct. If they are not all bets are off...

Right, you checked 5 Red PL mounts, but what about 500? 5000? You say they were very close, but not identical?

Of course, those who make lens mounts for RED (I am one) strive to make tolerances as small and consistent as possible.

But variations do occur, as you yourself say. Then there are other variations at each point between the sensor and lens, in the camera, and they can stack up or cancel, and that's why the flange focal distance adjustment on the camera exists.

It's great that you make your mounts to such high tolerances.

 

[Jacek Zakowicz]

For all practical purposes they were identical.
For all practical purposes the variations are irrelevant IMO.
Once the camera is set correctly at the factory they don't matter.
Most of the time it's an operator error - so far every case concluded that.
But the skill and the tools are still necessary-just like with film cameras.

 

[Domenic Barbero]

from My RED rep regarding backfocus on canon mount

"In regards to adjusting your back focus. First, double check to make sure that it isn’t the lense you are using, I would test it on a few lenses before adjust anything on the camera yourself.

If you would like to adjust the back focus yourself, I attached the Scarlet Ops guide to this email, on page 223 it describes the process for adjusting the back focus on top of the camera. The subsequent pages describe the process using the RED Focus but it is possible to do this without it. Mount the Scarlet on a stable surface exactly 5 feet from a focus chart, measure this to the sensor mark on the left side of the camera. Set the lens to focus exactly 5 feet away and on your LCD you should see the out of focus image if it is indeed soft.

Using the T10 Torx driver, insert it into the backfocus adjustment screw on top of the camera as shown on page 225 of the ops guide. Turning it clockwise moves the sensor forward, and counter clockwise will move it rearward. Carefully adjust the screw as necessary until the image comes into focus.

I hope I answered your questions. Feel free to contact me directly should you have any other questions. Thanks and have a great day! "

Best,
Andrea

 

[Douglas Underdahl]

Yes, that's the way to do it. But I would add

A) use a wide angle Canon lens, like a 25mm or so. The Depth of Focus on wide angle lenses is shallower than on telephoto lenses so using wide angle lenses to set the depth results in tighter tolerances;

B) since you are using a wide angle lens, set the chart at 3 feet away'

C) the Canon lens you are using had better be correct, or all you are doing with this procedure is setting your camera to work with one lens only. This is where the Red Focus or Jacek's Optimator come in - they are "correct".

D) see my youtube videos:

http://www.youtube.com/watch?v=IhmolBMtDzg

http://www.youtube.com/watch?v=pRicc1HU44c

 

[Terry VerHaar]

I expect a Red Focus for EOS is in the works because even with the most rigid materials and outstanding engineering the distance to the imaging plane will vary minutely, from install to install and over time due to wear. I'm obviously no expert but my answer to your question would be no, don't expect your back focus to be perfect. There still needs to be a means to verify that the distance from the mount to the sensor has remained consistent once the new mount has been installed, and to fine tune that distance as needed.



It sounds like it may be the same or very similar Terry. What model is your JVC?

JVC GY-HD110 - back focus works pretty darn well

 

[Domenic Barbero]

good call

 

[Terry VerHaar]

Now, I've seen some people worry that their camera isn't shooting in focus because the flange focal distance is off. This is most likely incorrect.

When you focus a lens, you are in effect changing the flange focal distance. Try it - grab that focus ring, turn it, and watch the lens move in and out in relation to the camera.

So you shoot something with your Red camera, and it doesn't seem sharp. You think, hey, I wonder if my flange focal distance is wrong and causing this. Do this test:

Set up your camera pointed to a sheet of newspaper, or Siemens Star chart. Focus, using the monitor on zoom in mode, by eye.

Were you able to focus before, and then after the chart? In other words, could you get the chart out of focus by turning the focus ring to the near end, then get it out of focus by turning the focus ring to the far end, passing a point of sharp focus in between?

If you can, then your flange focal distance might not be dead on, but it's close enough so that the lens is able to throw the sharpest image it can on the sensor. So if at this point you still feel that the image isn't sharp enough, then maybe you've discovered just how ultra sharp the Red camera sensor is and how it can turn up flaws in lense that you thought were super sharp. Or, maybe you need to do a better debay in Redcine. Or mabye you are looking at a lower res version of the file when you thought otherwise.

I just see a lot of misinformation here about how incorrect flange focal distance can make lenses soft, when in reality this would just shift the focus point, unless it's REALLY out and you can't focus near, then far, past your chart.

This seems to be a pretty important comment for this topic. I'll admit that, personally, I don't rely on my Canon still lenses to be "accurate" when it comes to focus marks - to me, that just doesn't seem feasible; it's never been my impression that still lenses are that precise and accurate. (Am I wrong?) So, is the "back focus" issue about getting the lenses accurate, or about getting them to focus properly at all - as some have been saying, or at least inferring? If I can get my lenses to focus by using visual checks and any tools on my monitor, then I guess that is as good as I expect for this level of gear. If I were working in the PL world, I imagine my expectations would be much higher. :smiley:

 

[Domenic Barbero]

terry, i couldnt agree with you more.

 

[Scott Crawley]

JVC GY-HD110 - back focus works pretty darn well

That adjustment is happening in the lens, so it must be a different procedure than on the Scarlet. It's a different procedure, same effect. It must be a flange depth adjustment, no?