Welcome to our community

Be a part of something great, join today!

Log in
  • Hey all, just changed over the backend after 15 years I figured time to give it a bit of an update, its probably gonna be a bit weird for most of you and i am sure there is a few bugs to work out but it should kinda work the same as before... hopefully :)

Back Focus Fixing

C

cinemano

Banned
Joined
Jan 15, 2008
Messages
376
Reaction score
0
Points
0
Hello

Could anyone explain me like im a 5 year old (a line from Philadelphia) how to do a back focus correction. step by step.

I never touched the back focus so perhaps thats why things look a tad soft.

Thanks soooo much
 
Watch Douglas Underdahl's video about swapping lens mounts on YouTube. Part two covers setting back focus:

http://youtube.com/watch?v=pRicc1HU44c

Caveat: Doug's procedure depends on having a lens with properly calibrated focus ring. Be sure your lens has been collimated by a professional shop before proceeding. Alternatively, use a wide zoom lens to adjust back focus. Simply assure that your focus remains tack sharp at both the longest and shortest focal lengths. Set your back focus at one end then check at the other. Keep adjusting until both are good.

Michael
 
Why Red don't document these features in their manual I don't know? Any way I'll add my 2 pence worth.

The following is a quick way of setting the back focus in the field when using a zoom lens & pulling focus with reference to the image (i.e. not measured). It will allow you to use a lens with a collimation error but the focus marks will be wrong hence you would not be able to reference these when pulling focus. Also this won't guarantee a good back focus if changing lens. if a lens change is done you would need to go through this again.

Start by opening the iris fully and then adjust the light level by using ND's this minimises the depth of field.

Then zoom in tight to a specific object (or chart) and rock the focus backwards & forward to focus the object

Then zoom wide

Now you adjust the back focus mechanism to bring the object sharp

When you've done that zoom in & out checking to see that the object stays in focus over the full zoom range.

when happy relock the back focus mechanism but make sure you don't move it in the process & once locked check the back focus again over the zoom range.

The two important points are:

Wide open iris & the largest monitor you can find.

Regards

Olly

ps I would welcome any comments counter to this advise as I'm always happy to learn from other peoples experience.
 
We are also creating a video tutorial for swapping mounts and setting backfocus. They will be posted on REDUSER and RED.com soon...

BC
 
Brent, great news!!

Could you include in the videos how to take the mid adjustable part off (the one with two screws), in case if one of the PL mount screws (8 screws) are stuck.

Looks like the best way is to take the mid part together with PL mount ring off the camera when handling the stuck screws extraction.
 
So you are asking for a reshoot? Haha. Both videos are in post right now...

BC
 
You are fast, from concept to the execution!

On the second thought, if the screws are stuck it is better to send it to RED to get it unstuck.

Except Acube, he is far away, I was laughing loud, reading his encounter with the stuck screw:-)
 
  • Thread starter
  • Banned
  • #8
had stuck screws too.. and they were breaking like butter. Scary thing.. but then i bought a regular screwdriver pen style grip and was able to unscrew them Just dont use the L shape tools.. in my view they suck cos you cant firmly push down before turning. lol
 
  • Thread starter
  • Banned
  • #9
i did some focus tests with the LG 42" Plasma.. looks very sharp before recording.. i focused a piece of paper with text.. but the playback seems washed down a little.. not realy a blur.. more like a pastel effect on the edges. I noticed doing sharpen just once in photoshop to a TIff from it looks much better to me. Can Red do that 1 time Sharpen during filming?
 
does the back focus really make a huge diference if you can compensate with the lens anyway? (speaking focusing by eye not measuring tape)
 
cinemano said:
does the back focus really make a huge diference if you can compensate with the lens anyway? (speaking focusing by eye not measuring tape)
Click to expand...

Hi,

Many modern (read complex design) lenses cant be focused if the FFD is out by very much.

Stephen
 
cinemano said:
i did some focus tests with the LG 42" Plasma.. looks very sharp before recording.. i focused a piece of paper with text.. but the playback seems washed down a little.. not realy a blur.. more like a pastel effect on the edges. I noticed doing sharpen just once in photoshop to a TIff from it looks much better to me. Can Red do that 1 time Sharpen during filming?
Click to expand...

In RecdCine if you set high detail and 100% sharpen it all will be very sharp.
As to the BF yes if BF is just bit off and you do not need infinity then you can compensate with the focus ring, providing that you do not use zoom lenses. For zoom lenses you have to be right on with BF.

Also when testing, hit record and move focus ring a bit left and right.
Then pick the best frame to estimate focus.
Also remember playback is not a TIFF.
Test all in TIFF file.
 
One of the problems with an incorrect back focus is the image will not be entirely in focus across the film plane. With the center in focus, the corners may not be. Proper flange depth assures the entire film plane is in focus.

Using multiple lenses on a shoot:

When shooting with your own lenses, you should have them all collimated so they are accurate as a group. That way, the distance markings all work once back focus has been properly set.

When renting lenses, you should test for focus accuracy and have the rental house collimate them if possible.

Michael
 
If you had access to a collimator you can pop a 35mm lens on and check your back focus that way.

Short of that, people have given great advice. I hate having to check back focus on ENG lenses. For some reason that's hard for me. Thank goodness for RED!

Kristina
 
misskrissy said:
If you had access to a collimator you can pop a 35mm lens on and check your back focus that way.

Short of that, people have given great advice. I hate having to check back focus on ENG lenses. For some reason that's hard for me. Thank goodness for RED!

Kristina
Click to expand...

Assuming that the lens itself is properly collimated.
 
"Assuming that the lens itself is properly collimated."
-Chuck

Yes, good point. Something to double check as well.
 
You must log in or register to reply here.
Back
Top