BackFocus

[Pedro Millan]

Hi Everyone,

 

I worked with a F900 with Panavision lenses.

 

Every time I changed the lens I had to adjust the back focus so that the real ft meassure between the camera and the actor could be marked in the lens. Otherwise there was any reference and it was impossible to pull the focus without them.

 

My question is what happens in the optical process when the back focus is adjusted? Does any other camera needs back focus too? why do the film cameras don´t need the back focus adjustment?

 

Thank you guys...

 

pedro

[Brian Drysdale]

Here's Michael Bravin explaining back focus

 

http://www.bandprodigital.com/cgibin/eDatC...atalogno=TEK-BF

 

With a zoom I'd check the focus marks after setting the back focus, rather than setting it to the marks. With some zooms the markings can be out when the back focus is correctly set.

 

Michael explains that film lenses are adjusted on the bench. However, if they do go out of adjustment, you'll have the same problems as you'll have with a video lens.

[Brian Drysdale]

Here's Michael Bravin explaining back focus

 

http://www.bandprodigital.com/cgibin/eDatC...atalogno=TEK-BF

 

With a zoom I'd check the focus marks after setting the back focus, rather than setting it to the marks. With some zooms the markings can be out when the back focus is correctly set.

 

Michael explains that film lenses are adjusted on the bench. However, if they do go out of adjustment, you'll have the same problems as you'll have with a video lens.

[Carl Brighton]

why do the film cameras don´t need the back focus adjustment?

Because film cameras don't heat up much while running.

A video camera has to be running all the time if you want to look through the viewfinder, and so it gets hot..

A film camera does not, because it has an optical viewfinder which works whether the camera is powered up or not. The only time the film camera uses much power is when it's actually shooting.

If film cameras heated up like video cameras they would need the back focus adjustment.

[Jess Haas]

Back focus tends to be more critical on video cameras because they use smaller imagers. The smaller imager means that you are shooting at shorter focal lengths which inherently have a shallower depth of focus. Depth of focus refers to the image being projected onto the imager and is NOT the same thing as depth of field.

 

Film cameras with smaller imagers such as Super8 cameras have the same issue which is part of the reason so few of them where available with interchangable lenses. On the interchangable lensed Super8 cameras you need to have the backfocus adjusted to the particular camera body or the focus marks will most likely not be accurate.

 

Backfocus also tends to be a much larger issue with zoom lenses than with prime lenses since if it is off the focus will shift throughout the zoom range. Zoom lenses are used much more often than primes with video camera.

 

Backfocus is adjustable on film camera lenses but it is usually not done in the field. Since backfocus is less critifcal they can be set on a test bench and throughout use they should remain within acceptable tollerances.

 

~Jess

[banba ban]

Anyone care to offer an explanation as to why backfocus tends to drift more on HD cameras than SD?

 

I am regularly readjusting backfocus on 750's and 900's in the field and very rarely had to do this with any SD cameras.

 

I don't think it's a simple explanation such as "focus is more noticeable", because the amount that the HD lenses are drifting on HD cameras is fairly substantial and would definitely be noticeable if it were on a SD camera.

 

Any thoughts?

[Brian Drysdale]

Anyone care to offer an explanation as to why backfocus tends to drift more on HD cameras than SD?

 

I am regularly readjusting backfocus on 750's and 900's in the field and very rarely had to do this with any SD cameras.

 

I don't think it's a simple explanation such as "focus is more noticeable", because the amount that the HD lenses are drifting on HD cameras is fairly substantial and would definitely be noticeable if it were on a SD camera.

 

Any thoughts?

 

Apart from there's a higher pixel density on HD, the cameras also run hotter and the mounts that Sony use are prone to thermal expansion. Clairmont have changed the mounts on their cameras using stainless steel to overcome this issue and I understand that the F23 mount has been changed by Sony.

[Michael Nash]

Also, you don't usually measure and pull focus "film style" with SD cameras; it's usually an "eye focus" done by the operator, so you're less likely to notice if the actual focus is off from the marking on the barrel. And the larger CoC does make backfocus errors less noticeable even when checking focus by zooming in and then widening back out again.

[Pedro Millan]

Michael Bravin's explanation is very helpful and says that one has to change the backfocus often: "at beginning of day, after shipping or changing locations, and whenever the temperature changes enough so that you have to remove or add clothing to stay comfortable".

 

But on location I had to change the backfocus every time I changed the lens no matter what. No matter if the camera was in the same position, same location or same temperature condition.

 

So I guess that what Carl says its possible because the HD "lens are made of non complimentary metals so dimensional stability is an issue and the mount geometry can change with extremes in temperature". quoted from Michaels Bravin article

 

http://www.bandprodigital.com/cgibin/eDatC...atalogno=TEK-BF

 

On the opposite lens and camera mount of film cameras are made of stainless steel that is very precise and strong.

 

I don´t know you guys, but it seems that makes sense to me.

Edited January 26, 2008 by Pedro Millan

[Matt Kelly]

HD "lens are made of non complimentary metals so dimensional stability is an issue".....

 

Really? Then why not just make it out of say...complimentary metals then?

 

I've been under the impression for a while that back focus is such a video-specific issue because to the smaller focal lengths (shallower depth of focus) in congruence with beamsplitter/prism thermal expansion issues. Aluminum lens mounts don't help either...

 

If the issue has nothing to do with sensor sizes/prisms, then why not just make lenses and mounts out of less temperature sensative metals? I can't imagine weight being the sole factor in making that decision. I don't get it! lol

Edited February 26, 2008 by Matt Kelly

[Chris Keth]

HD "lens are made of non complimentary metals so dimensional stability is an issue".....

 

Really? Then why not just make it out of say...complimentary metals then?

 

I've been under the impression for a while that back focus is such a video-specific issue because to the smaller focal lengths (shallower depth of focus) in congruence with beamsplitter/prism thermal expansion issues. Aluminum lens mounts don't help either...

 

If the issue has nothing to do with sensor sizes/prisms, then why not just make lenses and mounts out of less temperature sensative metals? I can't imagine weight being the sole factor in making that decision. I don't get it! lol

 

I tend to believe that Matt is right here. There is no reason the "complimentary metal" thing is right because, as Matt says, it could be fixed in manufacturing.

 

It is really just that video cameras get hot inside and film cameras don't. Combine that with extremely shallow depth of focus and you ahve yourself a nagging problem.

[Matt Kelly]

It is really just that video cameras get hot inside and film cameras don't. Combine that with extremely shallow depth of focus and you ahve yourself a nagging problem.

 

Hmm... I guess I've never taken the temperature aspect very seriously.  They really get THAT hot?  I would think a good cooling system isn't the hardest thing in the world to devise.

 

Maybe i'm just failing to realize that most of these cameras are still aiming for the ENG style, which would explain a lot of comprimises in design.  So my real question is...Are the D-20, Genesis, RED, and Origin REALLY all exempt from backfocus pains?

[Chris Keth]

Hmm... I guess I've never taken the temperature aspect very seriously.  They really get THAT hot?  I would think a good cooling system isn't the hardest thing in the world to devise.

 

Maybe i'm just failing to realize that most of these cameras are still aiming for the ENG style, which would explain a lot of comprimises in design.  So my real question is...Are the D-20, Genesis, RED, and Origin REALLY all exempt from backfocus pains?

 

That I don't know. I haven't been able to work with those cameras yet. I'm working a short next week with the RED so hopefully I'll find out about that.

 

The problem isn't really that they get too hot. It's that the lens temperature is different on the camera than it is off the camera. Then the cooling fan intermittently goes on and off, changing the inside temperature from time to time and I think the backfocus can wander because of that, too.

[Walter Graff]

The reason this happens with Panavised Sony cameras is because Panvision took off the front of the camera and made a large aluminum plate that holds the mount for the lens and it breaths. It is a poor design. Add ten ounds to it and you'll see how easy it bends. Back focus errors are three times front focus so it does not take much to throw off the short distance of the target to the rear element. No other HD camera should have this problem and if it does, you are doing something wrong. This back focus issue has become one of the biggest HD myths out there. A stock HD camera and lens will rarely if ever go out of back focus, even in 140 degree temperatures. I am such a beliver in the myth that I invite anyone to call me down to a set if they have the issue, other than with Panavision which does do it because of how they had to make a Sony camera into a Panavision camera, and I will show you that you must be doing something incorrectly. It will not be the first time I've shown this to be a myth. 917 217 9766. We had along discussion on this subject on the CML where I revealed Panavisions design error a few years back when they first came out with the camera and wrote in their book that you had to continuously had to adjust back focus because "it was part of how HD worked". NOT!!!! Look up the thread on the CML.