"analog lenses" "digital lenses" backfocus

[Manolo S. Navazo]

i read here that some people are verifying/checking the backfocus every morning on his red, and some check on every change of location and change of temperature. this sounds to a very broadcast style. http://www.reduser.net/forum/showthread.php?t=8899

i mean if i got a collimated prime set(all lenses on 52,000mm)
1) does somebody had problems with the variable backfocus on red? in different temperated locations or just by moving/working with the camera. so does the pl-mount from red with his variable backfocus is rock solid? is it in pre-production test once collimated(also the lenses) neccesary to collimate it every day like in broadcast? or is like normaly with filmcameras.

what ever the answer is, i know an AC must keep in mind that something could move in micrometers, innerside.

2)somebody showed me an article in the newest "TV und Filmkameramann" an article about si-2k camera.
this production which was reported about, said that if you got a collimated pl-lens prime set (all collimated to the same 52,000mm distance) you have to collimate/change the backfocus on camera everytime you change the lens.

all those lenses for analog fotocameras or cine-pl-lenses got a focus distance for filmemulsion. with digital cameras we got now a glass in front of the chip/sensor.
so what the articel says is, that with every different focal lenght lens, you would get different distances in backfocus (also if all lenses are really collimated correctly by 52,000mm).
this should happen by the glass in front of the sensor which breaks the light, so every lens with different focal lenght should get different angle to the chip.
the article said, that "digital lenses" like zeiss digiprimes(only 2/3 chip) and fotolenses for digital fotocameras got it solved, in manufacturing a more parallel light pass trough the last lens elements. so you don't have to adjust the backfocus by changing the lenses.

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i don't know if i should trust this article, i mean all "digital lenses" got an backfocus adjustment on lens, not on camera. in fotolenses it doesnt sound so important, because fotographers decide the sharpness by the eye, not the printed marks on lens.

so perhaps somebody could explain me, afirm or negate me, if i have to correct my backfocus on the red-one camera by every change of lens.

thx

a) does the variable backfockus on red-one camera is rocksolid?
b) does i get problems in different temperated locations with the backfocus?

c) does i have to correct the variable backfocus by changing a lens(if all lenses are collimated at same distance, perhaps 52,000mm)
d) does i get different flange back of the lenses because i got a glass in front of the sensor?

e):sorcerer: what would this mean for the new prime lenses from red

[Andrew M.]

I tested few PL lenses on focus test charts (see my tests under the Lenses) and some lenses like Master Primes are very sensitive for back focus and some not, like super Speeds.
I can’t talk to the extreme temperature change -10F to 110F but from my experience the old worn out set of lenses may go bit deeper in to the mount (2 microns) also the mount itself (not RED mount) can be already worn out so add another (1-2 microns).

Also the tolerances the lenses are manufactured +/- 1.5 microns, so you can accumulate -5 /+2 microns of difference. MP have to be adjusted with 2-3 microns accuracy and most other will take up to 5 microns. So it is good to check BF once you rent the lenses for each and given set.

Did anybody rough handle your camera with the lenses on, lately?

And here you go why some people check BF every morning before shooting.

[Manolo S. Navazo]

I tested few PL lenses on focus test charts (see my tests under the Lenses) and some lenses like Master Primes are very sensitive for back focus and some not, like super Speeds.
I can’t talk to the extreme temperature change -10F to 110F but from my experience the old worn out set of lenses may go bit deeper in to the mount (2 microns) also the mount itself (not RED mount) can be already worn out so add another (1-2 microns).

Also the tolerances the lenses are manufactured +/- 1.5 microns, so you can accumulate -5 /+2 microns of difference. MP have to be adjusted with 2-3 microns accuracy and most other will take up to 5 microns. So it is good to check BF once you rent the lenses for each and given set.

Did anybody rough handle your camera with the lenses on, lately?

And here you go why some people check BF every morning before shooting.

sorry andrew,
but what you say doesn't make sense to me to recollimate the red-one backfocus after changing a lens on set.
and to check the lenses and the camerabody in rental is fundamental, we are pro's not honky donkeys, right :)
so if you got differences by worn lenses you have to adjust it with shims on lens, or adjust it by the camerabody.
a good camera rental will give you all lenses in 52,000mm collimated, so there are still all my question i startet up.
thx

[Brigham Edgar]

In my opinion i am going to check my backfocus on my RED everytime i go out on a shoot. I found out the hard way on my first paid gig with RED that the backfocus was out.

The RED's backfocus ring is awesome in the way it is easy to adjust B4 'tv' lens style but...once bitten twice shy'...it can obviously be knocked out.

[Manolo S. Navazo]

In my opinion i am going to check my backfocus on my RED everytime i go out on a shoot. I found out the hard way on my first paid gig with RED that the backfocus was out.

The RED's backfocus ring is awesome in the way it is easy to adjust B4 'tv' lens style but...once bitten twice shy'...it can obviously be knocked out.

hi bris,
and do you think you can make a 100% decision on set, by adjusting everytime your backfocus?, we still haven't 4k monitors. i am not talking about operators who pulling his own focus, i am talking about AC's who have to trust his measures and marks.

so if i am taking hands on this variable backfocus on set. i would never know if i am right on my marks or not. you could also do every time a measure verification, make a test shot, load in redalert or redcine, ... but this takes a lot of time.
nobody on set would give an AC the time to verificate after every lens change this procedure.

[David Wyatt]

We set our back focus when the camera turned up & it hasn't drifted since (the mount is fairly bullet-proof). IMHO the Red's totally unlike an F900 or 750 where the back focus can regularly drift, requiring time-consuming checking & monitoring. It's a good idea to double-check your Red camera's back focus on your camera test day before it goes out on a job. We found a good way of checking back focus was to find a reliable lens that looks spot-on on a lens projector and put it on the Red at a certain distance away from a star chart and match the distance on the lens barrel (measuring the distance from the focal plane to the star chart with a Disto is the most accurate way). If your back focus is good the distance from the focal plane to the star chart should match the distance on the barrel and you'll have a nice sharp image on the monitor (only 720p of course). It also helps to use the zoom-in function on the camera. Shoot a little test and watch it zoomed in full-res in Red Cine - you probably won't be able to actually play it but you can view it as a still frame (if your back-focus is out, as ours was slightly when the camera first turned up, it should look fairly obvious). If it is out it's fairly time consuming to get it spot on and I wouldn't want to be doing that on set with everyone breathing down your neck - the best way is to check it on your test day and rely on it not to drift throughout your job. If you want to be super-keen at the beginning of each shoot day you could shoot a star chart at a fixed distance and check it in Red Cine at the beginning of the day but to be honest I think that's overkill - with experience you learn to trust the reliability of the mount.

This is all fairly time-consuming (much longer than checking back-focus on a F900) but the upside is that it doesn't drift (ours is 4 months on & still rock-solid!)

[Hans von Sonntag]

The RED PL-Mount is solid and by no means to be compared with a B4 mount. Nevertheless it's prone to temperature changes, meaning after 3 hours permanent running in a warm set (studio for instance) the backfocus starts drifting a bit. 1m is now 1.10m. A skilled 1st AC can compensate that though.

With a 35mm camera this does not happen since the mount does not get heaten up by the cameras electronics.

Hans

[Manolo S. Navazo]

with experience you learn to trust the reliability of the mount.

This is all fairly time-consuming (much longer than checking back-focus on a F900) but the upside is that it doesn't drift (ours is 4 months on & still rock-solid!)

thats good to hear

Nevertheless it's prone to temperature changes, meaning after 3 hours permanent running in a warm set (studio for instance) the backfocus starts drifting a bit. 1m is now 1.10m. A skilled 1st AC can compensate that though.

With a 35mm camera this does not happen since the mount does not get heaten up by the cameras electronics.

Hans

holy shit, thats even better to keep in mind.

i like to see the AC who can compensate this, if you got in 1 meter differnce to 1,10 meters, than you would get in 10 meters a difference to infinity. it depends on the lens and aperture, i watched my kelly table with a 25mm lens and 1f (for example).

and if you got even a wider lens, there would be no way to compensate this, you will get a softer image, so no way for the ac to compensate the focus by pulling.

-

for me the only way to compensate would than be to recollimate the variable backfocus. and in 4k on set without 4k monitor or macbook to have a look, this would be a godforsaken situation for the AC.
and after all it would take time

[Finner]

The RED PL-Mount is solid and by no means to be compared with a B4 mount. Nevertheless it's prone to temperature changes, meaning after 3 hours permanent running in a warm set (studio for instance) the backfocus starts drifting a bit. 1m is now 1.10m. A skilled 1st AC can compensate that though.

With a 35mm camera this does not happen since the mount does not get heaten up by the cameras electronics.

Hans

Hans have you ever shot outside in cold -20C and then a few hours later move into a warm +20C interior? If you have I would love to hear your experience.

If others want to test this problem set your PL mount back focus at a typical room temp, then turn your camera on for four or more hours in a reasonably warm location. I am hearing multiple cases of the flange focal depth changing throughout the day with cameras throwing lenses off. Aluminum is appearing not to be the best mount choice. Not sure if it is just the mount or if the aluminum body is also attributing to the problem?

[Florian Stadler]

..have you ever shot outside in cold -20C and then a few hours later move into a warm +20C interior?

If you're planning on doing that you'd better have 2 separate sets of lenses. One acclimated to the warm, the other to the cool. You'd be unpleasantly surprised to find your lenses completely fogged up for several hours when going through the temperature change you are describing. If you're smart you schedule to go from the warm to the cold environment as the lenses can handle that easily. Sorry to drift off topic...