Redrock M2: Where to measuring focal-distance from on camera?

[John Kary]

I was on set yesterday where the production was using an HVX200 with the RedRock M2 using Nikon lenses. There was some confusion from the crew on where on the camera rig to measure the focal distance from camera to subject in order to set focus correctly on the 35mm lens.

For instance on an Arri 35mm camera there is a little notch to attach a tape measure to and measure to the subject where the film runs through the gate. Where is the equivilant part of the HVX rig using the RedRock adapter? Would you measure focal distance from the ground-glass in the RedRock adapter, the CCD in the HVX or some other part?

 

[Steve Holmes]

You would have to measure to the ground glass as that is where the image is being focused, you are then simply recording that image with your camera.
Steve

 

[John Kary]

This raises an interesting point, then. The ground-glass obviously resides inside the black box of the M2 adapter. To my knowledge, there is no exact marking on the outside of the box to mark the depth of the ground-glass within the box. One might say, just mark it with some line-tape on the outside of the box.

Is there any official word on where exactly on the box to measure from? Should we measure the depth of the ground glass inside the box and replicate that marking on the outside? I'm curious what other people do using the HVX + M2 setup.

 

[siniarch]

Hi John,
Well, the question is not as clear as you put it. There are two measurements that you want to be concerned.

1. From Camera to Ground Glass
This one in my opinion is the most crucial one. If this one is wrong, EVERY SHOT WILL BE WRONG (blurry) There is a really good tutorial at the Redrock Website of how to set this up. But from memory:

• you take the lens off the redrock
• you point the camera to an even white bright area.
• you open the iris on the camera
• you engage the ND filter on the camera if its too bright
• with the M2 turned off and not spinning
• you zoom in and out until you get rid of vignetting
• then you focus until you see the grain of the GG clearly

(keep in mind the camera will not let you zoom in all the way and focus as close. so if you cannot focus close enough, zoom out a little and you'll be able to focus more. hope that made sense) Also knowing the distance in mm from the camera to the GG will not help because the readings on the camera are not too accurate, and there are even 1-3 different levels of zoom 17 and 3 levels or so of zoom 16, etc... this one is just a pain in the butt setting that you have to continually keep checking from time to time.

Once that is done pop the lens, turn the m2 and pan and till and make sure a fixed point is crisp all throughout.
I recommend using the Hardmount for the M2. if you don't know what it is, do a search on this site or send an e-mail to Redrock. I think they are selling the stepdown rings to be able to hardmount the m2 to the camera.

2. From Lens to Subject
This one works as if you had an 35mm SLR camera. I believe you just measure from the front of the lens to the subject. and that goes along the settings on the lens. Though you probably want to use a Follow Focus and set your Hard Stops and not go by the numbers on the lens itself.

Hope this helps and makes sense.

​

 

[Nathan Beaman]

This raises an interesting point, then. The ground-glass obviously resides inside the black box of the M2 adapter. To my knowledge, there is no exact marking on the outside of the box to mark the depth of the ground-glass within the box. One might say, just mark it with some line-tape on the outside of the box.

Is there any official word on where exactly on the box to measure from? Should we measure the depth of the ground glass inside the box and replicate that marking on the outside? I'm curious what other people do using the HVX + M2 setup.

The reason there is no marking on the box is that the ground glass is adjustable.
For example - when mine arrived I had no problems getting razor sharp focus staight from the box - though the markings on the lens in noway actually correlated to the distance in focus. I've been able to tweak this a bit, but it is never dead on to the lens markings, especially in the short focal distances. The admins at the M2 forums claim there is a tutorial on adjusting this on the site, but there really isnt anything specifically on adjusting the ground glass distance and the exact distance for the different mounts.

The best practice I've found is to just find and mark your focus points for each shot.