Jump to content


Photo

Back Focus


  • Please log in to reply
6 replies to this topic

#1 John Adolfi

John Adolfi
  • Basic Members
  • PipPipPipPip
  • 342 posts
  • Producer
  • Fulton, N.Y.

Posted 30 June 2006 - 11:49 AM

Perhaps this has been discussed before if so please point me to it.
My awareness to create a sharp professional looking super 8 frame includes correct back focus.
Do all Super 8 cameras have a back focus adjustment feature. How to you do it? Diopter adjustment?
Does the C mount vs. the M or bayonet have diferent considerations when mounting the lens and back focus?
  • 0

#2 Stephen Williams

Stephen Williams
  • Sustaining Members
  • 4708 posts
  • Cinematographer
  • Europe

Posted 30 June 2006 - 03:29 PM

Perhaps this has been discussed before if so please point me to it.
My awareness to create a sharp professional looking super 8 frame includes correct back focus.
Do all Super 8 cameras have a back focus adjustment feature. How to you do it? Diopter adjustment?
Does the C mount vs. the M or bayonet have diferent considerations when mounting the lens and back focus?


Hi,

Film cameras have their Flange focal length set and lenses collimated in a workshop. It is not a D.I.Y job.

Stephen
  • 0

#3 John Adolfi

John Adolfi
  • Basic Members
  • PipPipPipPip
  • 342 posts
  • Producer
  • Fulton, N.Y.

Posted 30 June 2006 - 04:48 PM

Are you saying if I take my camera to a specialist and he/she collimates the specific lens with the camera then the back focus is taken care of? Or are these two seperate things?
  • 0

#4 Douglas Hunter

Douglas Hunter
  • Basic Members
  • PipPipPipPip
  • 356 posts
  • Other
  • Los Angeles

Posted 30 June 2006 - 06:11 PM

What kind of camera do you have?

Since the majority of Super 8 cameras have fixed lenses there should not be a problem with the back focus unless the camera was dropped or something like that, but that would probably just mean you should buy a new camera.

I have never seen a super 8 lens with a back focus flange. I wouldn't think there is such a thing.
  • 0

#5 John Adolfi

John Adolfi
  • Basic Members
  • PipPipPipPip
  • 342 posts
  • Producer
  • Fulton, N.Y.

Posted 30 June 2006 - 07:21 PM

The two I'm interested in most is Leicina and Beaulieu.
  • 0

#6 Sam Wells

Sam Wells
  • Basic Members
  • PipPipPipPip
  • 1751 posts
  • Cinematographer

Posted 30 June 2006 - 08:31 PM

Are you saying if I take my camera to a specialist and he/she collimates the specific lens with the camera then the back focus is taken care of? Or are these two seperate things?


Literally collimation would be to assure the optical axis of the lens is lined up & centered - in effect perpendicular to the image plane (collimate as in "column") but in popular useage, yes.

You are right about specific camera, specific lens i.e. best done on the same camera (not just model) you are using.

-Sam
  • 0

#7 Alessandro Machi

Alessandro Machi
  • Sustaining Members
  • 3318 posts
  • Other
  • California

Posted 02 July 2006 - 09:32 AM

Are you saying if I take my camera to a specialist and he/she collimates the specific lens with the camera then the back focus is taken care of? Or are these two seperate things?


In theory any lens that is removeable can have the backfocus adjusted.

The Beaulieu can have it's lens collimated because the lens is removeable and therefore the ideal position can be found for best backfocus.

A lens specialist uses a collimater to both view and bounce an image through the lens and then via a mirror at the film plane the image bounces back and the backfocus of the film camera can be set by then adjusting the distance of the lens to the actual lens mount.

Professional Video cameras have a backfocus feature built in.

For Super-8, most Super-8 cameras probably cannot have their backfocus adjusted unless the entire lens can be removed as one piece. (I don't recommend trying this).

If the Super-8 camera lens positioning was adjustable via a shim, you would need a collimater and a super-8 cartridge with a thin mirror attached to the cartridge film plane strategically cut to the perfect size so as not to touch the claw or other delicate parts of the internal camera. Put the film cartridge with the perfectly sized wedge of thin mirror inside the film camera, set up the collimater to both project and then view an image that will be bounced from the Super-8 film plane back through the camera lens and viewed though the collinmater. Find the ideal point where the camera lens properly focuses onto the collimater image with the camera zoomed in????(not sure if the camera is in telephoto or not). Then zoom out the super-8 camera lens and see if the focus holds while the collimater image that is being bounced back via the Super-8 cartridge mirror is viewed through the collimater. If the focus does not hold from telephoto to wide angle, then the back focus is out of alignment and the entire lens would need to have it's positioning readjusted by installing the right sized shim. This technique will not work on most super-8 cameras unless the entire lens can be removed as one piece, and then the back lens focal distance adjusted by use of a shim. (note-It's been several years since I actually had a collimater available to me so I'm probably not explaining everything on a technically perfect level.)

On a Beaulieu, the backfocus adjustment most definitely matters.

----------------------------------------------------------------------------------

You can probably tell if your backfocus is off on any Super-8 camera BEFORE sending a camera in for a backfocus adjustment by following these steps....

First set the focus on your viewfinder. Put the Super-8 camera in wide angle and find an object, like a building, that is located far away, at least an 1/8th of a mile away or farther. Adjust the viewfinder for maximum sharpness while in the wide position. Zoom in onto the distant building and focus, and then zoom out. If the object goes out of focus, recheck your viewfinder adjustment in the wide angle mode until the distant object is sharp. Zoom into telephoto and refocus, then zoom out to wide mode and see if the viewfinder stayed sharp.

If you can get the focus to stay sharp all the way through the zoom range, you're ready to start your test!

Find a time of day when proper exposure means your super-8 camera is set to the widest f-stop aperture setting possible, focus in telephoto mode, and zoom out. Film this procedure a few times in which you properly focus on something in the distance, and then zoom out to wide. After you get the film back from the lab, see if the lens held focus or not. If the lens held focus the entire time, the back focus is properly set.

If the lens does not hold focus, you may be able to avoid having the backfocus adjustment made IF you can determine where on the f-stop range the lens does hold focus when zoomed from telephoto to wide.

You may discover that any f-stop setting from 2.8 and up, (aka 4.0, 5.6, 8.0 etc) yields a sharp picture throughout the zoom. Now you know you can achieve proper back focus as long as your f-stop is set to f-2.8 or higher, of course, this means in low light situations you will be limited in how you use this particular camera. Some cameras may go soft even if the f-stop is set to 4.0 or higher, these super-8 cameras definitely need a backfocus adjustment.

If you have a fixed lens, (meaning no zoom), then do an f-stop test in which you set a shot up and shoot it through out the day so as to get various f-stop readings to see if the sharpness holds throughout the entire f-stop scale.
  • 0


Technodolly

The Slider

Visual Products

CineTape

Tai Audio

Paralinx LLC

Media Blackout - Custom Cables and AKS

Willys Widgets

Rig Wheels Passport

Wooden Camera

Broadcast Solutions Inc

Abel Cine

Metropolis Post

Gamma Ray Digital Inc

CineLab

rebotnix Technologies

FJS International, LLC

Glidecam

Ritter Battery

Aerial Filmworks

Opal

FJS International, LLC

Aerial Filmworks

Media Blackout - Custom Cables and AKS

CineTape

Glidecam

Opal

The Slider

Broadcast Solutions Inc

Wooden Camera

Paralinx LLC

rebotnix Technologies

Visual Products

Abel Cine

CineLab

Metropolis Post

Ritter Battery

Willys Widgets

Gamma Ray Digital Inc

Rig Wheels Passport

Tai Audio

Technodolly