Help with pulling on RED and using DOF calc

Michael Jasen · June 17, 2010

Hi everyone,

I'm hoping someone can clear up some questions I have as to how a DOF calculator works.

Here's the skinny:

Today I was camera assisting on a testing day for a shoot that is happening tomorrow. We are shooting on the RED at 4k 16x9 using Cooke Pancro lenses.

The only thing we wound up shooting was a jib shot that was minimal. The distance from the focal plane to the subject was between 14' and 16' at all times.

I used my trusty pCam software (DOF Calc) and set it to RED CAM 4k 16x9 (COC .00075") with a focus distance of 15', focal length of 32mm and apreture 2.8 (this info was all correct)

We did the move and I didnt pull focus at all because pCam told me that the range of sharpness was from 12'4" to 19'2". I set the lens to 15" as it had a mark for this,

Unfortunately, some of the shot was soft.

My question is: Do I understand how to use pCam and a DOF calculator properly? If I input my data and it tells me the near and far distances, shouldnt I be ok to work within those range? Should I still be pulling from 14' to 16'? I was always under the impression that at a specific lens, aperture and distance you had a certain DOF that was "acceptable sharpness" and wouldnt render as obviously soft in playback. The DP seemed to think that that much depth of field (for that lens etc) was a lot and we were probably working in a space about half that.

The back focus was verified to be in the calibrated position.

We found out it was soft by viewing a high res proxy of the shot on the DIT station.

Any tips? pointers?

Thanks guys,

Michael Jasen

William Coss · June 17, 2010

Next time focus.

Michael Jasen · June 18, 2010

Thanks for that. Really.

I appeared in focus on my monitor.

Next time focus.

Chris Keth · June 18, 2010

Thanks for that. Really.

I appeared in focus on my monitor.

Well, you made a couple mistakes there. First of all, William is right. Unless there are physical limitations, always pull the shot. Even if it's wide and you think your depth of field should hold it just pull the shot anyway. Second, you can not trust the monitor to tell you what is sharp when you're shooting on the red. You are recording more than three times more pixels than the monitor can show you. You have to treat it like film and in some ways, it's tougher. The way focus falls off on the red is more abrupt than with film. You could have probably got away with what you did on film.

The pcam info sounds plausible to me and you do understand it correctly. That wasn't the problem.

Corey Steib · June 18, 2010

Well, you made a couple mistakes there. First of all, William is right. Unless there are physical limitations, always pull the shot. Even if it's wide and you think your depth of field should hold it just pull the shot anyway. Second, you can not trust the monitor to tell you what is sharp when you're shooting on the red. You are recording more than three times more pixels than the monitor can show you. You have to treat it like film and in some ways, it's tougher. The way focus falls off on the red is more abrupt than with film. You could have probably got away with what you did on film.

The pcam info sounds plausible to me and you do understand it correctly. That wasn't the problem.

Use your tape measure that is and always will be your best friend on set. Focus pulling is a bit touchy with the red even with a 16mm and just a hair out of focus on one shot I did and then DP/Camera Op saw and and I was like wtf but that's the RED for you, so weither or not you use the DOF cal or not just always use your tape measure just to be safe.

Michael Jasen · June 18, 2010

Thanks guys!

Michael Kubaszak · June 18, 2010

I've used the Pcam app and despite putting the info in correct and pull was soft because I didn't pull to that actual distances and just tried to keep it within the DoF the Pcam told me.

What's really interesting is that I've compared different DoF calculator apps, Pcam, Kodak, Pocket DIT, ISee4K. put in the same information in all of them and they each gave me different results. I've since decided to just be 'on' and just use it as a reference and not a crutch.

Evan Luzi · July 15, 2010

I agree that alot of the iPhone DOF calculators are a bit iffy on their accuracy. It's usually best to use them as an estimation and to pull whenever possible. Even though the DOF may be large enough to be acceptable focus, it's usually best to keep it on point. Focus tends to soften towards the edge of the DOF and your subject could float towards that. Also, if you aren't pulling alongside and something unexpected happens you'll be unprepared to compensate.

Ravi Kiran · July 15, 2010

You have to treat it like film and in some ways, it's tougher. The way focus falls off on the red is more abrupt than with film. You could have probably got away with what you did on film.

How does the Red affect focus fall-off? I would have thought that would be dependent on the lens rather than the camera.

Chris Keth · July 16, 2010

How does the Red affect focus fall-off? I would have thought that would be dependent on the lens rather than the camera.

That's a difficult question to answer and I've never been able to do proper tests to find out. I have noticed, and it's been supported by other ACs experiences, that focus just seems to fall off faster on the RED as opposed to film and other digital formats where the sharpness is often dialed down to look more like film. It should be a function of lens, but the high edge sharpness definitely affects our perceived focus. The next time I have a red in hand, I'm going to do a quick newspaper test and see if the depth of field the eye sees is the same as what is calculated.

Edited July 16, 2010 by Chris Keth