"analog lenses" "digital lenses" backfocus

[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...

[Prem Edpuganti]

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?

Well Finner, aluminum has one of the largest coefficients of expansion for metals, meaning it expands and contracts more than most metals. From a theoretical point of view, over a focal distance of 50mm, and for a 100 degree Fahrenheit change in temperature, since the body is also made of aluminum, expect a change of 5.6 microns in the focal distance. Is it bad. I don't know. You are the expert, you have to tell me.

Besides, what about the lens barrel. Doesn't changes in temperature cause changes in relative distances between lens elements, in which case the problem is not limited to just Red in this regard.

Obviously, you want to let the camera reach the temperature of the surroundings and then adjust back focus. If the temperature is changing constantly throughout the day it seems you have two choices: Control temperature of the camera, short of that, adjust back focus periodically. Focal distance change is linear: 5.6 microns per 100 degrees F. You can observe temperature changes, decide how many microns change you can live with and decide when you want to check back focus.

Hope it is helpful.

[Finner]

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...

I can go from outside shooting -30C to shooting interiors in a very humid pool in about 40-50 minutes. When you live in Canada you learn many cold temp tricks.

[Jorge Díaz-Amador]

MANXLX,

That article got it wrong. You don't have to re-calibrate with each lens change. I discussed this issue with Alfred of P+S. But you might need more than one PL mount for the IMS, with each one optimized for a range of focal lengths. This is due to the digital cinema camera's focus sensitivity. Nothing to do with analog or digital lenses - there is no such thing.

On this feature we're using Zeiss T2.1 primes, optical designs that are decades old, and there are zero problems with the RED/Zeiss T2.1 combo. In fact the footage looks gorgeous.

I personally have experienced a change in the flange focus of a RED camera due to temperature change. The camera needs to be set up at the temperatu e it will be working at. A change of 20º F will cause a detectable shift in the flange focus.

That's all for now, I've got to get some rest, this movie is not in the can yet. I will post this weekend in more detail.

Till then...

[Jannard]

Checking the back-focus is a standard procedure on ALL PL mount cameras (which before RED meant mostly film cameras). One would assume, wrongly, that all lenses are set correctly from the factory. Actually, we have checked several hundred lenses on a locked down RED with no temperature change and verified that many lenses are out, due to age, wear and tear or whatever. This is not just a camera issue. It is good to know exactly what you have, camera and lenses, prior to a significant shoot... just good practice. This is professional shooting, right?

We understand how important this practice is... which is why we made it so easy to do with our new mount. But don't think the RED mount drifts around easily. I would beg to differ on this point. I have checked my cameras many times and none have them have moved the slightest. But I still check.

Jim

[Shawn Nelson]

I have checked my cameras many times and none have them have moved the slightest. But I still check.

Jim

How do you check? Coming from the digital world I have never done this. I understand the concepts but am unsure how to go about it. Has Red published a how-to sheet?

[Finner]

Hi Jim

Main complaint I have been hearing is that the reds FFD will change from the begining of the day after the camera is on for 4-6 hours. Seems like a slightly warm interior is where it is commonly being noticed. Hence the thought that the heat generated by the camera is causing the problem with the aluminum on the mount and possibly the camera body. If this is indeed happening then this is quite a problem. Checking FFD every 3 hours on the same set is not a good answer.

Please note these are complaints I have heard from trusted pro's. Maybe you have tested this or would be interested in trying it? I would really like to check this myself but my camera is stuck in this hold pattern thing (You wouldn't by chance have contacts or pull with a red employee or someone that could get me my camera quicker would you Jim?).

[Jannard]

The best way is to do both a chart test and an infinity collimation test. But if you don't have one of those... the easy way is to mount your lens(es), set up a high contrast chart 6' and 10' away from the camera and light it so you are close to wide open on your lens. Loosen the screws and twist the mount until the image is sharp (should be at both distances). Lock it down. If you have two lenses that are different, you have a problem with one of your lenses that you need to know about.

If you focus by monitor (instead of tape) you will still be OK except at infinity if your back-focus is off.

Jim

[Jannard]

Hi Jim

Main complaint I have been hearing is that the reds FFD will change from the begining of the day after the camera is on for 4-6 hours. Seems like a slightly warm interior is where it is commonly being noticed. Hence the thought that the heat generated by the camera is causing the problem with the aluminum on the mount and possibly the camera body. If this is indeed happening then this is quite a problem. Checking FFD every 3 hours on the same set is not a good answer.

Please note these are complaints I have heard from trusted pro's. Maybe you have tested this or would be interested in trying it? I would really like to check this myself but my camera is stuck in this hold pattern thing (You wouldn't by chance have contacts or pull with a red employee or someone that could get me my camera quicker would you Jim?).

I have never had this issue on any of my 5 cameras. And I have run two of them all day in the sun at the races.

Care to tell us who your trusted DPs are? Sounds like they are guessing...

Jim

[Finner]

Like I mentioned I did not experience this but is something I have heard from two well established pro's that I trust their judgement that own cameras. Check this thread out to read about one.

http://www.reduser.net/forum/showthr...850#post213850

From your response Jim I can tell that you would agree that if this is found to be more then just an isolated problem that it will be a major issue. I for one hope it is isolated to just a few cameras.

Please note this is not intended as an attack or red bashing by any means. It is simply bringing up a concern with what may turn out to be an odd problem with as little as 2 cameras.