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ALEXA back focus


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#1 christian mann

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Posted 30 March 2011 - 05:31 AM

hi there,
this probably seems like a silly question but since i'm new tto the ALEXA i was wondering how one checks the back focus? can anyone advise me please?
thanks!
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#2 Mike Panczenko

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Posted 30 March 2011 - 09:41 AM

It's a PL mount camera, so you would check it the way you do with a traditional film camera- throw the lenses up, and see if they tape out. If they don't, then you will need to get the rental house to fix it and then you repeat the process.
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#3 Stephen Williams

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Posted 30 March 2011 - 12:56 PM

With a traditional film camera you measure the FFD. You are checking the lenses to the ground glass not the focus on the film. Using an autocollimator you can check the GG & Film with the camera running.


It's a PL mount camera, so you would check it the way you do with a traditional film camera- throw the lenses up, and see if they tape out. If they don't, then you will need to get the rental house to fix it and then you repeat the process.


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#4 christian mann

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Posted 30 March 2011 - 01:06 PM

okay, that's what i had in mind...another 1.AC confused me when he was talking about the ALEXA's back focus...busines as usual!
thanks!
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#5 Mike Panczenko

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Posted 30 March 2011 - 01:29 PM

With a traditional film camera you measure the FFD. You are checking the lenses to the ground glass not the focus on the film. Using an autocollimator you can check the GG & Film with the camera running.


I will say that every checkout I have ever done, we (the assistants) have never used an auto collimator. That's not to say that the techs have or haven't, but never once have I seen an auto collimator brought out during a prep. It does sound like it is a more scientific way to do it, but if the lenses are not taping out, and it is the ground glass being off, when they take the camera back for the FFD check this would reveal itself to be the problem. I've had it happen more times where the FFD is off and the ground glass good, rather than the other way around (I actually don't know if I've ever had it where the FFD is right but the ground glass distance is off.)

So, using the autocollimator sounds to me, as far as the assistants go, to be more of a way to check what is already suspect, or to confirm a fix, rather than the first step you would take.
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#6 Stephen Williams

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Posted 31 March 2011 - 01:16 AM

I've had it happen more times where the FFD is off and the ground glass good, rather than the other way around (I actually don't know if I've ever had it where the FFD is right but the ground glass distance is off.)

So, using the autocollimator sounds to me, as far as the assistants go, to be more of a way to check what is already suspect, or to confirm a fix, rather than the first step you would take.


Taping out does not chect the FDD which is whats important, your just checking ground glass & lens collimation combined.

Using an autocollimator is very quick set the lens to infinity, look through the eyepiece & then look through the autocollimator, 30 seconds & you know the lens, ground glass & FFD are all correct.
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#7 Mitch Gross

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Posted 31 March 2011 - 10:30 AM

The NULL Lens & Target kit work excellently with the ALEXA.

http://www.abelcine....mator-Kit/#tabs
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#8 Mike Panczenko

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Posted 31 March 2011 - 11:09 AM

Taping out does not chect the FDD which is whats important, your just checking ground glass & lens collimation combined.

Using an autocollimator is very quick set the lens to infinity, look through the eyepiece & then look through the autocollimator, 30 seconds & you know the lens, ground glass & FFD are all correct.


Well, yes, you are correct, but again, oftentimes if the FFD is too long or short, the ground glass will still be correct, so you would see it in the GG. Yes, there can be an instance where the flange to ground glass distance is correct, while the flance to film gate is not, but I am just saying I have not run across that, so often times taping out the lenses will reveal if the mount is off.

You are absolutely correct in saying that the taping out does not check the FFD- but most of the time if the mount is off, it will still be revealed because either A) the ground glass won't be or B)both will be, and when the techs do a true depth reading, it will be revealed. This is, though, 100% in conjunction with the rental house actually checking it with the dial indicator before you show up, and, on a feature, shooting actual film tests to confirm.

One more clarification- this is for non-Panavision cameras. With Panavision cameras, I would check the FFD every morning, like a lot of assistants do. The movements inside can float much more, but can also be easily adjusted. So with those, I would say, just because it is sharp in the eyepiece, it may not be and may not stay that way. With Arri/Moviecam, etc, I am more under the belief that you check it at the checkout, and if it's sharp, you are probably good, and you leave it.

One more thing- the taping out does only check the mount- not the movement. So, yes, taping out does only check for an improperly shimmed mount. If the movement has floated, you will not see that by taping out.

Is it common-place in European rental houses to use an autocollimator at the checkout? Maybe we are just behind- I certainly can't argue with the fact that it is more scientific.
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#9 Stephen Williams

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Posted 31 March 2011 - 02:32 PM

Is it common-place in European rental houses to use an autocollimator at the checkout? Maybe we are just behind- I certainly can't argue with the fact that it is more scientific.


I generally piss off rental houses by insisting every lens goes on a colimator & the depth is measured. I also like to use an autocollimator. There was somenody making a pocket autocollimator which he sold on Ebay for $175, unfortunately he has retired!

In some countries in Europe it's an insurance requirement that a film test is done & every lens is tested. The results looked at on a microscope. However most of the productions I work on don't have the budget to test or for insurance!
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#10 Mike Panczenko

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Posted 31 March 2011 - 03:07 PM

I generally piss off rental houses by insisting every lens goes on a colimator & the depth is measured. I also like to use an autocollimator. There was somenody making a pocket autocollimator which he sold on Ebay for $175, unfortunately he has retired!

In some countries in Europe it's an insurance requirement that a film test is done & every lens is tested. The results looked at on a microscope. However most of the productions I work on don't have the budget to test or for insurance!


There was one NYC 1st AC I worked for who kept a collimator (auto?) in his kit on the truck- probably about a foot long, but he never pulled it out. It certainly sounds like the way to do it, and if it's 30 seconds a lens, it would save all the time 2nd guessing your eye focus on the chart on the wide lenses!

I'd snatch one up in a heartbeat!
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