How to use backfocus adjustment

[Alexis Hanawalt]

I had some trouble getting any results when turning the backfocus adjustment recently. I had a focus chart at about 5', my lens wide open, etc., etc. I loosened the 2 hex screws on the backfocus collar and turned it as far as it would comfortably go each way, but I couldn't discern any change in focus. (I was using a panasonic 17" monitor.) I loosened the hex screws a bit more, and was able to turn the collar more, but still couldn't discern any difference. I loosened them until the collar turned freely all the way - and still nothing. I gave up and tightened it all back down. On a previous shoot a few weeks ago, we adjusted back focus on set and there was clearly a change every time we turned the collar.

My guess has been that the entire backfocus collar is like one big threaded bolt - so it slides back and forth slightly as it's turned - if I'm right that these are the mechanics, I have a hard time imagining that something is broken. Is there something I'm missing here?

 

[Andrew M.]

Did you adjust the focus on the lenses each time you turned the collar?
What lenses did you use?

 

[Alexis Hanawalt]

I was using my Cooke 20-100. Collimation is a bit off on the lens, but it was clear that I was as focused as I was going to be with it at about 5'.

 

[Andrew M.]

And what was the FL when you were doing the test 20 or 100?
When I test on MPrimes it takes 1/4 turn and I am out of focuss.
But don't touch anything on the lenses when you are adjusting the mount.
Also check the focus in the TIFF file not on the screen.
On the screen you will not see any difference even if you make maybe 3 turns

 

[Conrad Hunziker]

Couple of tips:

- Make sure you are using the wide end of your lens. The wide lenses have a smaller backfocus depth of field.

- You should use a prime lens if you are doing this process.

- Use the zoom feature of the red (user 2 button) to check your focus.

- And as suggested above, check it with a tiff export.

 

[Michael Ragen]

Mike Prevette adjusted the backfocus on my camera and the magic focus assist seemed to be useful when shooting the focus chart at 6 feet. I havent had any issues since. He used a 16 or 18mm Ultraprime( can't remember which) and cross checked with a wide Cooke s4.

ps- we just checked out his nikon mount with my evf yesterday. pretty awesome. i think i'm going to have to get one.

 

[Alexis Hanawalt]

I'll try the TIFF file technique. I'm still surprised that with my 1st AC's eyeball right up against the 17" monitor there wasn't even the slightest hint of change in focus when I turned the collar 180 degrees.

Yes, I was wide open, at the 20mm setting on the lens.

I feel like my lens is inherently pretty soft, but I would expect to see different kinds of "soft" when adjusting back focus.

I'll also try it with a prime the next time I get my hands on one.

 

[Jorge Díaz-Amador]

This doesn't seem possible. Each full turn on the RED One adjustable PL mount collar changes the FFD by 2mm. Perhaps the weight of the big zoom lens was causing it to bind?

 

[Alexis Hanawalt]

Perhaps the weight of the big zoom lens was causing it to bind?

Oh, you know - we had a hell of a time taking the lens off afterward - the PL mount was stuck really hard. Are you saying it's possible that the back focus adjustment might have "bonded" itself to the back of the lens?

 

[Mitch Deoudes]

I know this is an old thread, but something caught my eye:

Yes, I was wide open, at the 20mm setting on the lens.

I'm not sure if this is accurate - but since a wide lens is used due to its shallow *depth of focus* (and hence large *depth of field*)... is it correct to assume that opening the lens up all the way, which decreases the depth of field, would therefore *increase* the depth of focus? (Which is the opposite of what you're trying to achieve.)

In other words, wouldn't you want to stop the lens down to adjust the back focus?

EDIT - thinking about it again, I suppose opening up the lens decreases the depth of field, which exaggerates the effect you get from adjusting the backfocus... But I'm still curious as to whether it's having the "opposite" effect inside the camera.

 

[Peter Repich]

Back Focus Instrument

Back Focus Instrument

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