Flange Back focus procedure.

Dennis Wood

Dennis Wood

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Just curious if anyone has a nice technical reference to proper back focus adjustment on B4 lenses? The Fujinon 17x manual procedure leaves a few important points out (at least in my mind).

1. They indicate to place a focus chart "approximately" 9 feet away from the lens. Measuring from what? I'd assume the lens surface based on their description, but normally I'd be looking for the equivalent film plane point on the HPX500.

2. I'm assuming that the focus mark on the lens should also approximate 9 feet, and that if back focus is correct, zooing in and out should retain sharp focus. This is what I used as the final determination as it's the only reliable way of setting focus, quick zoom or otherwise.

3. But....I cannot get infinity focus sharp at the physical end of the focus ring pull (regardless of back focus setting). I've got sharp infinity focus just before the focus ring physical stop. It seems I need a touch more adjustment than provided.

Any insights?
 
You could shoot everything at f16 or

plug into your 1080p monitor, zoom all of the way to a distant object @ infinity, set the lens to infinity, adjust the back focus, zoom all of the way out, check to see if your image is in focus then. That should work. Why are you using the 17x lens anyways, shouldn't you be making 2/3" relays!? = )
 
That's the funny thing Hunter. At infinity, I can't get sharp focus unless I back the focus ring back a degree or two. It's like I need more back focus adjustment that isn't there. If there's an adjustable stop that would work too. You're right on the relay...but we need to baseline the camera first :-)
 
There actually *is* an image plane marking... little vertical white line
through a circle, just below the Focus Assist button.
 
Dennis: Some zoom lenses focus past infinity, and the Fujinon seems to be one of them.. I first encounterd this with Canon flourite zooms in the 70's, which were designed that way on purpose to compensate for temperature variations affecting the relationship of the flourite elements.

Why this lens does that, I don't know - perhaps it has elements composed of something like flourite that vary with temperature - would be nice if someone from Fuji would respond...

Since I'm somewhat used to the problem from previous experience, it doesn't bother me.. by the way, I use a Chrosziel FF with infinity permanently marked on the scale so I can see it from the viewfinder...
 
Guys, thanks so much for your replies. EFX, I figured there must be a film plane marker at this level...just couldn't find it after several paper manual reviews.

Mac, I'll just have to be carefull at infinity :-( Having just finished a collimation how-to video segment for our Brevis users, I'll admit I'm a bit surprised that an $8000 lens has this issue. Not cool.

I was staring at the sheer girth of this camera thinking why anyone would want to lug it around. Then we started evaluating footage from the camera. I've always been annoyed by noisy video, regardless of format, and particularly at lower light levels. Drives me crazy. Not any more :-)
 
Dennis: You will notice that the infinity mark on the lens barrel is actual a few mms before the stop, so, I guess it is intentional.

The Canon lenses I was using in the 70's (on a 35 Arri) were (relatively) much more, so it's not necessarily a result of cheaping out on construction - if it is like the canon, it is to allow some room for expansion and contraction of elements...

It does seem curious that Fujinon doesn't mention anything about it in the horrible manual that comes with the lens.. Canon went out of their way to warn everyone (they actually bragged about it because they were real proud of the flourite technology)...
 
I can appreciate building a little tolerance in. Some of our more technical users have reported that back focus adjustments with the Brevis unit change slightly with camera temperature. Given how sensitive the DOF relationship becomes when using an achromat, this makes sense to me.
 
Guy, thanks so much for that link. It's fairly clear that I was over analyzing the issue. In other words, regardless of how you do it, we're just making sure sharp focus is maintained throughout the zoom range. Given those parameters, the infinity focus issue is just an irritant.

It was interesting to hear the advise on when you should re-adjust back focus. Temperature differences were cited, which jives with what we've found using the Brevis 35 adapter. We've always advised that folks set back-focus ever time they mount up the adapter, check it every now and then, and never go "by the numbers".
 
I have the same lens on my 500 and I often use the back focus to tweak the lens. I'm no optics expert but I think these lenses are averaged throughout the range and as I often find myself shooting at the extremeties using the back focus can help improve the image.
 
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