tape measure for focus

dlang

dlang

Well-known member
Ok I just watched the great tutorial on fresh dv about pulling focus and it lead me to a couple of newb questions.

1. When trying to use a tape measure with my brevis setup the distance on the tape does not match up, or rather have accurate focus when I focus my lens at that distance...why? If I measure 8ft then focus my lens at 8 ft it's not in focus...am I missing something here? Does the distance from the HVX lens to the brevis ground glass have to be calculated as well? on the tutorial the 1st AC was measuring right from the ground glass? (focusing plane)

2. Because the lens only has ft and not inches marked on the barrel, how do you know precisely where you need to be for critical focus? Say the subject measures 8ft 3 inches away, How do I set the lens right down to the inch? by the way I'm using nikon AI lenses (50mm, 85mm, 28mm)


I may be just confusing myself with this, but hey I'm scrambled at this point so(:
 
A couple of questions-

Is it focused at infinity?

Is it just one lens, or do others have the same problem?

How far off is it, and are the others the same distance off?

If it is not focused at infinity for all lenses, then it sounds like the focal flange distance is off. I don't know how to adjust it on the brevis, but the distance from the lens to GG must be adjusted.

I don't know how to focus for inches, I would like to know what others do for this too.
 
Your film plane is the ground glass or disc. The DOF adapter should bear a mark for the plane. The camera, regardless of what camera you're using, once properly focused on the screen / DOF adapter plane, has nothing to do with your focal distance from that moment forward.

So, you don't have your camera properly focused on the screen. Or, your lens markings are not calibrated properly to the focal elements in the lens (less likely). Or, your tape measure is defective (least likely).

Remember that your focus work, especially when your camera and lens are set to wider apertures, becomes much more critical. You need a 1st AC for a DOF set-up. You should be setting marks for the camera and talent anytime either is in motion. Your 1st will then hit these marks.

Playing with low light, wide apertures, long lenses or in very close with any focal length lens is going to challenge your focus abilities.

e
 
Thanks for this great post. I'm gonna go mess around some more. I know my focus is spot on as far as the ground glass...I took Dennis' advice awhile back and stopped my lens down with no vibration and focused to the grain on the ground glass, works quite well. I have no problem being in focus when using my monitor (marshall), but when I don't use a monitor and everything is markings and #'s that's when things go wrong. The 1st AC on fresh DV mentioned he believed monitors to be one more distraction for him, so this kind of motivated me to practice his route for diversity sake. At this point I always use a monitor, but if I ever stumble into a 1st AC gig outside of my element I want to be ready for that work flow. Still wondering about inches?? with the markings on the lenses themselves I can only get approximate #'s, is that sufficiant? Seems like for critical focus I'd need a more precise way to measure. Can someone explain how they do this. pleeeaaaz(:
 
Ok I went and did a test to see what was going on and I'm still puzzled. I literelly measured an object at 15'4" away, focused my lens and it was in perfect focus at the 8' mark on the lens? What am I missing here?
 
Dlang, I think I got a few answers for ya.

First of all, you're getting all these wrong marks because the distance from the lens to the ground glass is off, as others have already said. The only problem is, this distance is so precise that 1/100th of an inch could be the difference between spot on and totally off. Not exactly something you can really adjust yourself...unless of course you have the right tools.

There is a bit of a fix though. Get some very thin tape and wrap it around your lens, right where the witness marks are (the distance numbers). Get a focus chart or something of high contrast and then walk it out from say, 1 feet to 20 feet. Giving yourself how many intermediary marks as you'd like. Just to clarify a bit more, you'd set it up at 1 foot, find the focus, mark it on your lens, put it at 3 feet, find the focus, mark it on your lens. Then do this so on and so forth until you go as far out as you'd like. Just remember though, there's less of a turn from 20 ft to 23 ft than there is from 1 foot to 3 feet, so as you get further away, start marking less.

That little trick will sort of solve your whole "inches" problem, as your marks can be a lot more exact than what is on the lens. To get critical focus you can also just look in the eyepiece, as there's not much of a better way than just guesstimation. Honestly though, if you make those marks, you should be quite alright.
 
Before you mess around with what Ryan is talking about you need to get your 35mm lens to GG back focus correct. If your lens markings are anywhere near that far off your back focus is way off. This is typical with adapters but with your Brevis (unless its quite old) you have a very good way of setting back focus with the microcollimation ring behind the lens adapter ring.
I'm tired of explaining how to do it on these boards but try doing a search on backfocus on these boards and checking the Brevis forums. Its not very hard.

Lenny Levy
 
Ok looks like incorrect collimation was the answer. Back focus is fine and has nothing to do with my focus marks being off. Thanks Dennis for the great tutorial on how to set this up. Thanks all for your time
 
The word back focus gets misused on these boards a lot. Collimation, back focus and focal flange distance all refer to the same thing. This leads to confusion here.

It think what gets mislabelled is focus of the video camera on the screen. There's nothing "back" about this - its simply the focus of the video camera, biut it is often called "back focus" here.

I think you meant to say the 35mm lens back focus was incorrect and you fixed it with the Brevis micro collimation. The video lens focus on the screen was OK though.

I'm finally writing a sticky about all this stuff so we don't keep reinventing the wheel here.

Lenny Levy

Lenny Levy
 
Lenilenapi said:
The word back focus gets misused on these boards a lot. Collimation, back focus and focal flange distance all refer to the same thing. This leads to confusion here.

It think what gets mislabelled is focus of the video camera on the screen. There's nothing "back" about this - its simply the focus of the video camera, biut it is often called "back focus" here.

I think you meant to say the 35mm lens back focus was incorrect and you fixed it with the Brevis micro collimation. The video lens focus on the screen was OK though.

I'm finally writing a sticky about all this stuff so we don't keep reinventing the wheel here.

Lenny Levy

Lenny Levy
Click to expand...




That's a good idea Lenny. I figured back focus would be the furthest piece of glass "back" towards the camera. The confusing part about this is that adjusting the micro collmation doesn't put you in further focus but only aligns the lens to where the marks on the lens are accurate.
 
Lenny is spot on here. I was having similiar issues. I took about 8 hours one day and moved my GG up and down by the width of a PIECE OF PAPER to get my collimation spot on.
Now I have the tape pulls and my witness marks equaling the same measure.

Ryan's work around is cool for temporary set use, but when you get a spare day you should adjust that GG. It will help focus and edge to edge issues too.

Without a very expensive piece of gear you will just have to move it a bit then try it then move it a bit and try it....etc.


The best way to focus based on inches is to look at the barrel of the lens and note how close the different focal lengths are to each other. You can see the relation to the 10 foot mark and the 7 foot mark and the 4 foot mark. How close or far those are will give you an idea of the spacing you will need to keep in mind. The 7 is in the middle of 4 and 10, but on the lens it is MUCH closer to the 10 than the 4.

So lets say you have a tape pull of 3 feet 6 inches. You would NOT put the focus dead center between the 4 and 3, You would place it between the 4 and 3 using the same ratios you found in the 10, 7, 4 comparison. So 3 feet 6 inches is much closer to 4 than to 3.
I hope that makes sense.
 
anyone notice different collimation distances between different lenses of the same brand? I'm noticing some inconsistancies amongst some of my canon fd lenses.

Also, has anyone ever tried to make a custom follow focus marking disc for each of their lenses. In other words, have inches and feet marked on the disc for each prime lens in your kit? That would seem to be pretty ideal since focus pullers use the disc a lot.
 
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