Focus, trusting eye or measure tape?

[Karl Eklund]

Hi, this is my first post here at Cinematography, so please be kind :)

 

I was shooting for my cinematography class with Arri 16st, using 16mm and 50mm lenses (arri lenses with no backfocus) and the value of the lenses focus distance did not match what my eye (and of two other guys in the group) saw.

 

We made sure the lenses where properly attached, that they were as tight as possible to the camera (sometimes the lenses will be off by 2-4 mm, which obviously would make them miss their target correctly, the film). Didn't matter really were the diopter was set to, it would just make it more out of focus by changing it from what we started with. However, there was no markings on the camera we used for guide frames and such, so a little bit harder than with, but still pretty close.

 

Anyways, the lens would look very soft focusing at an object 6 feet away with lens set to 6 feet, and when we set it by our eyes it was closer to 10-12 feet.

 

Just because there have been issues with the cameras, and they are not in top notch shape anymore, we decided to shoot at as closed down aperture as possible (instead of favoring using ND filters) to get deep depth of field.

 

We shot everything using tape measure, and using hyper focal distance chart to try to save ourselves, on some shots we did shot both lens value and my eye value.

 

So, right now I can only wait for 5 rolls to be developed and see what the verdict is, if the eye was good or the tape measure was good...

 

However, I still wonder, if someone knows what would cause the big difference between our eyes focus and that of the lenses?

Edited October 23, 2010 by Karl Eklund

[Edgar Dubrovskiy]

With older, student-used lenses always trust your eye. As 9 out of 10 times unis will not be looking after the lenses very well, unfortunately.

So as long as the viewfinder is set to your eye - you should be safe to do eye-sharps.

We had this problem with old bayonet-mounted lenses on Arri 16st as well - and were pulling the whole short by eye, as we had exactly the same problem - at 6 feet we are looking at about 12 on the barrel.

[Tom Jensen]

First the camera should be checked. Mainly the flange focal distance. That is the distance from the flange to the film plane. This is critical. Then the lenses should be checked out. Obviously, you can't do that now. If you encounter this situation again, eye focus. What you see is what you get, unless the flange of the camera is out. If you doubt a camera, stay with longer lenses because the depth of focus at the back end is greater for longer lenses. On wider lenses it is dangerous to use them because the depth of focus is so shallow, it may produce a soft image if the flange is out. Always eye focus on lenses 50 mm or longer.

[Dom Jaeger]

What you see is what you get, unless the flange of the camera is out.

 

Actually what you see is what you get even if the flange depth is out. The critical thing is that the ground glass is set to the same depth as the film plane. If that is good then no matter where the lens is seated or how out the back focus, the image on the ground glass will be the same as that formed on the film.

 

On most reflex cameras the ground glass is the least likely setting to be out, generally because it's harder to fiddle with or damage. So the advice to rely on eye focus is a good bet.

 

If only one lens is out it's probably the lens, but if every lens is out it's more likely to be the camera. You might have two lenses with back focus issues, but it sounds to me like the lenses might not be seating properly. The Arri standard mounts on an St are aluminium and prone to wear. The later S/B had one mount replaced with the much more durable steel bayonet mount.

 

Tim Carroll is the long-standing expert here on these cameras, so I'd defer to his judgement. But the camera and lenses should definitely go to a tech to be checked.

[Tom Jensen]

Actually what you see is what you get even if the flange depth is out. The critical thing is that the ground glass is set to the same depth as the film plane. If that is good then no matter where the lens is seated or how out the back focus, the image on the ground glass will be the same as that formed on the film

 

That's debatable. If the flange is out and you eyeball a wider lens, chances are you, it won't be sharp. That's why I mentioned that if you shoot with a camera that you suspect the flange is out, stick to the longer lenses. I see what you are saying and you make an important point. For the sake of argument and for those that are less informed, the main points are:(1)The flange focal depth is set at a distance with very small tolerances. So, when you put on a lens that is properly serviced, that image will fall on the film plane. (2) The flange focal distance must be the same as the distance to the ground glass. When the shutter is blocking the film plane the image from the lens it is reflected onto the ground glass. So, when your diopter is focused on the ground glass, you should be seeing the image at the exact same distance that the film sees when the shutter is out of the way. (3) The lens must be set correctly so that the image formed my the lens falls exactly, within very small tolerances, onto the film plane. (4) Depth of focus. Most people understand depth of field. Longer lenses have a shallower depth of field and wider lenses have a smaller depth of field. The exact opposite occurs on the back side of the lens where the image falls onto the film plane. On longer lenses, the depth of focus is greater than that of the wider lenses. When a flange is out, a longer lens may have enough depth of focus the compensate for the flange where a wider lens does not. (5)This is my preference and personal opinion but, tape off lenses 35mm or less and eyeball lenses 50mm and above. 40mm do both. You can tape off longer lenses but eyeballing will give you a much sharper image.

 

Feel free to add anything Dom that you think is important or I left off but for you students and beginners, it is critical that you know and understand this.

 

If you have anything to add Dom, feel free

[Dom Jaeger]

If you have anything to add Dom, feel free

 

Nope, you covered it.

 

You're absolutely right that focusing wider lenses by eye can be tough. Unfortunately if the flange or the lens is out it's all you've got to go by. That's why a tech is a cameraman's best friend! :D

 

It's good to know there are still film schools that teach students with film cameras. I just wish they would get their gear checked regularly. There's nothing more dispiriting for a student than spending all that time and money only to have the equipment let them down. For the price of a single prosumer video camera a school could probably have their entire fleet of film gear working dependably for a couple more years. At which point the video camera would be obsolete..

 

There, I've had my rant, I can go back to meditating in the yoga room. B)

[Karl Eklund]

Thank you so much for the replies, it seems to be what I feared though, that we should have gone with our eyes. Well, we decided to use lens values and measuring tape because that would eliminate the "human error", but I guess we just inserted a different "human error" then :)

 

We, still have 7-8 more of 100f rolls to shoot, and I will shoot with the eye (which is tricky on the shorter focal lengths since the ground glass is so dirty and old), but at about 25 - 50mm it gets easy.

 

We will probably get a different camera, and different lenses, same models though...

 

Hopefully though we should get the rolls we shot back before the next shoot.

 

 

I am trying to figure out a workflow to determine if the eye is correct and or the measure tape is correct, and if get it right, most of the times it would be the eye that is correct, but if the flange is off, it would be the tape? So how do I check/correct those things?

[Phil Rhodes]

This is something I've never quite understood about film cameras and the people who talk them up so much.

 

You have a reasonably high resolution colour viewfinder, yet you insist on taping everything out, and the only person who can see down it is not in charge of focusing. Er...

[K Borowski]

I agree Phil.

 

The thinking behind the system is that the operator has too much to do to be worrying about adjusting focus, too. But focus is, arguably, just as important as framing.

 

While every lens is supposed to be tested in standard shooting, I wonder how many ACs check the accuracy of the focus ring on every lens. . .

[Stuart Brereton]

I wonder how many ACs check the accuracy of the focus ring on every lens. . .

 

All of my ACs do. It's their job.

[Tom Jensen]

All of my ACs do. It's their job.

Exactly and that's why they get paid a prep day. You can't rely completely on a rental house because they make mistakes as well. Your prep is the most important day of the shoot.

 

The operator's job is to see focus as well but not to focus the camera. He can't under most situations. Unless he grabs the and dials it in but that doesn't always look good on film. The AC has the best view because he can see the ground in front of the camera. He can see the actor move. The operator only sees what's in the frame.

[Dom Jaeger]

I am trying to figure out a workflow to determine if the eye is correct and or the measure tape is correct, and if get it right, most of the times it would be the eye that is correct, but if the flange is off, it would be the tape? So how do I check/correct those things?

 

No, most of the time eye-focus will be correct, unless the ground glass depth setting is out, which is less likely than the lens or flange being out. If either the lens or the flange are out, using a tape measure will give you a soft image, particularly with wide lenses.

 

To properly check everything a tech with a bench collimator needs to look at the gear.

 

But if you try the same lens in a different camera, and the focus scale is similarly out you can assume the lens needs adjustment. If it comes good then the first camera has a problem. It might be worth trying each of the mounts in the turret also.

[Karl Eklund]

No, most of the time eye-focus will be correct, unless the ground glass depth setting is out, which is less likely than the lens or flange being out. If either the lens or the flange are out, using a tape measure will give you a soft image, particularly with wide lenses.

 

To properly check everything a tech with a bench collimator needs to look at the gear.

 

But if you try the same lens in a different camera, and the focus scale is similarly out you can assume the lens needs adjustment. If it comes good then the first camera has a problem. It might be worth trying each of the mounts in the turret also.

Ok, thank you!

 

Next time I check out gear I will try to see if I can test lenses/cameras quickly to see any differences.

 

We shot a test roll before filming, to check lenses and camera, but the camera we used then had a broken diopter, which obviously made the whole test sort of useless... Then we had another camera which would not run at the correct speed, it would be 30 fps without load and only 20 fps loaded (according to the tachometer), but you could hear how it ramped up in noise/speed when cranking it...

 

Well, you live and you learn...

[K Borowski]

All of my ACs do. It's their job.

 

So they check the lenses, on the camera(s) all the way from their closest focal point to infinity? I see guys check them at a certain distance, which is no guarantee they're accurate throughout the scale.

[Chris Keth]

All of my ACs do. It's their job.

 

I have been covered by Stuart's "my ACs" category at a couple different times. I spend a lot of my prep time taping out lenses and looking at them on projectors to check matching and check for problems in performance. Even at good camera houses I send things back for replacement or adjustment quite often. If a lens doesn't tape out and perform properly, I am not even going to bother putting it on a camera in front of the DP.

 

I don't know any assistant worth hiring that doesn't do the same thing to very high standards.

[Chris Keth]

So they check the lenses, on the camera(s) all the way from their closest focal point to infinity? I see guys check them at a certain distance, which is no guarantee they're accurate throughout the scale.

 

Yes I do. I check each marked distance as well as infinity. I'll also add marks if the lens is deficient in a particular important range.

[Tom Jensen]

I have been covered by Stuart's "my ACs" category at a couple different times. I spend a lot of my prep time taping out lenses and looking at them on projectors to check matching and check for problems in performance. Even at good camera houses I send things back for replacement or adjustment quite often. If a lens doesn't tape out and perform properly, I am not even going to bother putting it on a camera in front of the DP.

 

I don't know any assistant worth hiring that doesn't do the same thing to very high standards.

 

I agree. I would even take lenses outside to check infinity. It's a must to do these things at the prep. You get paid a lot of money to do this and there is little room for mistakes. Just remember that here in LA we have a huge parking lot full of DP's, A/C's, grip and electric just waiting for someone to screw up so they can get the call. And remember, when you work on big budget movies the standards are higher so when you get the opportunity to work with Vilmos, don't screw it up. Be the best, eff the rest.

[Dom Jaeger]

So they check the lenses, on the camera(s) all the way from their closest focal point to infinity? I see guys check them at a certain distance, which is no guarantee they're accurate throughout the scale.

 

All high-end cine lenses, especially modern ones, have very accurately marked focus scales. If they check at infinity and also at say 6 feet you can pretty much guarantee they will be right through the range. Angenieux for example hand mark the scales on each individual zoom in their Optimo series, Zeiss have a selection of focus scales to match the small variations in machining that are then individually fitted etc. But there are always exceptions to the rule.

 

I can't speak for other rental houses, but I regularly check all our lenses on a bench collimator at infinity and then on projection at 6 or 8 feet and minimum focus. Projection also gives you a good idea of how the lens performs optically and whether there are issues such as mechanical wear causing focus backlash or the image to shift when focus is pulled. The back focus is shimmed to a tolerance of plus/minus one hundredth of a mm for lenses under about 40mm.

 

Back focus is pretty stable unless the lens is damaged, but things like wear over time, temperature changes and frequent travel can cause a few hundredths variation. Zooms are more complicated, and need a bit more checking.

 

Obviously we can't check every lens for every job, and Murphy's Law works overtime in the film industry, which is why a prep day is invaluable. If an assistant picks up any issues we can take the lens straight to the projection room to have a look, fix it or swap it over.

[Jed Shepherd]

Did you tell brett or eve or anyone that their was a problem. When we shot with the ST's they were pretty problematic. I thought processing of the stock your using couldnt be done in australia anymore.

[Karl Eklund]

Did you tell brett or eve or anyone that their was a problem. When we shot with the ST's they were pretty problematic. I thought processing of the stock your using couldnt be done in australia anymore.

Yes, told them yesterday (wanted to confer with our teacher to see if he thought we made a mistake first, that would account for focus being off). We aren't shooting on BW anymore, it is Kodak Vision 3, 200T at 100f.

 

They must love me, I've only used the ST cameras one time out of four where there haven't been issues for me to report.