Best Method for ensuring Flange Focal Length is adjusted

rbirnbaum

rbirnbaum

Well-known member
I feel like my PL mount needs to be adjusted for infinity and I am looking for steps on how to perform this.

What lens to use? I have primes.
Best way to go about it?

Any help would be really appreciated.
 
My advice take it or leave it.... Start with your lenses. Make sure they are collimated properly first before you start adjusting your back focus.
 
dhessel said:
I would think a prime lens might be better but ultimately the lens with the best collimation should be used. I ran across discussions on a special lens red made for just this purpose and luckily there is actually one on eBay now.

https://www.ebay.com/itm/IBE-IB-E-O...ON-DEVICE-RED-CAMERA-MAKE-OFFER-/223114689568
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Not made by RED, but meant for use on their cameras. Will work on the F35/F65 as well. You want a wide angle lens at a fast aperture. That's what this sucker is. Do a Google search for the NULL Lens and you'll find my videos on how to use it.
 
I’m pretty sure you can adjust backfocus on the F35 directly. My AC was constantly having to do it on a shoot we did with locations down some wild bumpy roads.
 
You need a wide lens. A wide zoom lens is what you need to get it perfect, go to rental house.

But if you only have primes, put your widest one on there, turn back focus screw all the way clockwise, open iris up and focus to infinity. The back focus should be way out when it is all the way clockwise. Use magnification on 7q, Spin the back focus screw until you see infinity start to come into focus, go past it a little bit if you want.

If you want to use zoom lens, zoom in and focus on object and zoom wide. Turn screw until wide is in focus.

Or put the lens you don't think goes to infinity and spin the screw until you see it go past infinity. As long as you have a short allen wrench, the back focus is so easy to adjust.

Is this by chance the F35 you bought with Panavision Mount? Perhaps you kept the shims from PV mount under the PL mount? This would prevent the F35 from ever getting to infinity on wides.

I assume you know where the "back focus screw" is. It's behind the little black port cover close to the lens mount.

I wouldn't worry about getting the perfect calibrated lens as others here suggest. Instead, understand how to quickly manipulate the back focus because in this day and age, cheap PL lenses vary a lot, I love my catcan 18-35 but I have to adjust F35 every time I use it. There is no point to having a perfectly calibrated F35 according to the PL specifications, what's more important is having your F35 set to your lens set, and knowing how to adjust if you see it is out.
 
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yachacha said:
You need a wide lens. A wide zoom lens is what you need to get it perfect, go to rental house.

But if you only have primes, put your widest one on there, turn back focus screw all the way clockwise, open iris up and focus to infinity. The back focus should be way out when it is all the way clockwise. Use magnification on 7q, Spin the back focus screw until you see infinity start to come into focus, go past it a little bit if you want.

If you want a much better set, use zoom lens, zoom in and focus on object and zoom wide. Turn screw until wide is in focus.

Or put the lens you don't think goes to infinity and spin the screw until you see it go past infinity. As long as you have a short allen wrench, the back focus is so easy to adjust.

Is this by chance the F35 you bought with Panavision Mount? Perhaps you kept the shims from PV mount under the PL mount? This would prevent the F35 from ever getting to infinity on wides.

I assume you know where the "back focus screw" is. It's behind the little black port cover close to the lens mount.

I wouldn't worry about getting the perfect calibrated lens as others here suggest. Instead, understand how to quickly manipulate the back focus because in this day and age, cheap PL lenses vary a lot, I love my catcan 18-35 but I have to adjust F35 every time I use it. There is no point to having a perfectly calibrated F35 according to the PL specifications, what's more important is having your F35 set to your lens set, and knowing how to adjust if you see it is out.
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I absolutely disagree with this advice. Who has time on set to be adjusting backfocus every time you switch lenses? And with many lenses you simply won't be able to judge this critically until you've ruined a shot.

You will want all of your lenses set to a matched calibration. They need to be at a constant, otherwise that critical shot at 50mm T2 of the actress at five feet away will be soft because the lens was really set to 5'2".

Don't trust a zoom lens to set the camera's backfocus. These can have some drift in the tolerance -- in fact they're almost sure to. And if the lens doesn't open to a very fast aperture then the backfocus is not super accurate.

Use a wide angle lens that has a fast aperture. Set it to infinity. That's when it has the narrowest backfocus. The range of focus in front of a lens is called Depth of Field. The range behind the lens is called Depth of Focus. They function as opposites of one another. So a wide angle lens has a large Depth of Field but a tiny Depth of Focus. And by tiny I'm talking of a tolerance of a few microns.

Get all your lenses collimated at a rental house. It doesn't matter if that rental house has a slightly different standard than another facility, as long as all your lenses match one another. Do this once and your lenses shouldn't drift for years. They'll only change due to temperature expansion/contraction over time, break down of the grease, and general shift with usage. If you treat your gear nicely then you probably only need to bother having them checked every couple of years.

Once your lenses all match, you set the backfocus on the camera to that widest lens. Then you stop playing with that thing or you'll go blind! Seriously, the backfocus on a camera is something that should not need to be constantly touched. If you haven't used it in awhile if you move it from extremes of cold & heat or high/low humidity, and if it's just been under heavy wear, then yes, by all means check the thing every now & then. But otherwise it is something that shouldn't need adjusting.

I'm not gonna toot my own horn here, but I have some serious firsthand knowledge of this stuff. I actually helped develop some of the technology. There's a right way and a lot of wrong ways to do this.
 
Have to agree with Mitch.

Which is why I suggested by starting with your lenses.

Time is money on a shoot.

Do it right and be done with it.

Or not.
 
This is great advice guys! Thanks so much for all the different input. I have never done this type of adjustment before. Yachacha, I'll check see if I left the shims in there, and yes, the manual shows where the adjustment screw is.

My widest is a 24mm. Let me start there. If I see a huge difference between all my primes, then clearly I am going to have to send them in for service. On the other hand, I never use tape for focus marks in the shoots I am accessing, so hopefully I can get them all to focus to infinity.

Will let you guys know how I do. Wish me luck.

Another question that I have. The F35 has a menu that allows you to select different shims... I apologize for the ignorance, but does this mean that to calibrate different lenses, shims may be added and therefore you have to account for that in the menu? Doesn't make sense, unless it is only metadata.

Thanks again!
 
Even if you’re not pulling by marks, the backfocuse can still be off enough to not be able to achieve focus at the distance you may need. Especially on wider focal lengths.
 
I'm actually not quite sure why manufacturers put in the flange adjustment, or why did they advertise it so widely. It has created more trouble than good IMO.
I get that the sensors are getting hot , the cameras are getting hot and expand, the PCBs can move. But the adjustments are not really understood by the majority of the end users. And get them in trouble most of the time.
There are two options here:
Either you are willing and able to comprehend this concept entirely or you should not touch it at all and let a professional that understands this technical aspect of image acquisition do the job for you. If I only had a penny for each time owner operator screwed themselves by adjusting the flange depth....lets say I wouldn't be posting here---unless it was from a tropical island with a drink in my hand...;-)
 
I did it!
Thanks everyone. My infinity was at 16 feet... and all my primes seem calibrated and focusing within scale from infinity to close focus. Happy about that. I have done one of my cameras. Now I have to work on the other.

FYI for anyone who wants to try this, the torque size was 15 I believe... was fiddling with allen wrenches for a while before I got the one I needed.

Also, I wanted to say that a lot of the screws on the camera are M3 .5 thread. In case you need to replace any.

Thanks again, you guys gave me the confidence to go ahead and do it myself.
 
Agree to disagree. The problem is your philosophy, i saw the same thing happen to film cameras. There is the million dollar production company philosophy and i understand this way and follow it in my professional life.

The once 100k film cameras dropped in price and so did it’s users. We do things that disgust the pros.

I’d rather learn how to adjust MY adjustable sensor, i’m not taking my lenses to rental house that cost $$.

Personally i like that i can make $300 nikon zoom lens hold focus by adjusting that screw.
 
yachacha said:
Agree to disagree. The problem is your philosophy, i saw the same thing happen to film cameras. There is the million dollar production company philosophy and i understand this way and follow it in my professional life.

The once 100k film cameras dropped in price and so did it’s users. We do things that disgust the pros.

I’d rather learn how to adjust MY adjustable sensor, i’m not taking my lenses to rental house that cost $$.

Personally i like that i can make $300 nikon zoom lens hold focus by adjusting that screw.
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Good luck on doing that with a lens that's not parfocal.
 
https://vimeo.com/183530536

I use this lens, works the same as shimming the lens. Focus holds fine through-out.

I'm not doubting you Mitch, but the whole "don't mess with it" philosophy I only agree with when the F35 was a rental camera. Now it belongs to "hobbiest" who spend their own personal cash on the camera. It is up the the end user to understand what they are doing.

I agree that the understanding of simple concepts like "back focus," are a vague idea to many. Our industry has suffered greatly from beginner filmmakers starting on DSLR's instead of 16mm film. Film demands a basic understanding of the science that DSLR does not. This is why the RED is the ultimate camera to so many, it is the ultimate DSLR.

But I am also not going to withhold information because it can be mis-used. The question was asked "how do you adjust the back focus on F35," so I answered.

Everyone should read Cinematography once a year by Malkiewicz and Mullen, our industry would be a much better place.
 
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