Bolex RX5 Flange Focal Depth

27th April 2005

Greetings,

How does one check the flange focal depth on a Bolex RX 5?

Whilst doing some testing at the rental place I decided to check upon my Bolex, but as I cannot put it on the collimator bench as I have no mirror to reflect the ground glass etc...my only option really is to check the flange focal length and the film plane.

Questions are : How do I go about it with a beam splitter in the way?

What should be the distance for the following mounts :

C-mount
PL mount

I haven't a clue, neither did the other technicians at the rental place, but I know there are some specialists on CML..

Thank you very much beforehand, appreciate your replies.

Regards
Emmanuel from Munich

Emmanuel from Munich wrote:

>How does one check the flange focal depth on a Bolex RX 5.

You need to use an autocollimator. The image will be reflected off the surface of a piece of film in the gate.

Wade K. Ramsey, DP
Dept. of Cinema & Video Production
Bob Jones University
Greenville, SC 29614

>You need to use an autocollimator. The image will be reflected off the >surface of a piece of film in the gate.

Wade,

I was under the impression that an autocollimator needed a front surface mirror to be put at the gate. I find it hard to accept that film can reflect an image. After all it is not even polished.

Best Regards
Argyris Theos
DoP
Athens Greece

Argyris Theos wrote:

>I was under the impression that an autocollimator needed a front >surface mirror to be put at the gate. I find it hard to accept that film can >reflect an image. After all it is not even polished.

You aren't actually reflecting the image. The autocollimator uses a beam splitter, so you can send the collimated target down to the film, and then look at it through a magnifier off the film. You can, with the right emulsion, see grain.

David Leitner used to use a cool trick which I hope he doesn't mind me revealing -- he'd use a strobotac as autocollimator light source, and could then watch the effects of the camera's movement, and even the mirror's flatness, on the image.

Jeff "working tonight with a 1903 American Mutoscope film shot with a Biograph camera that perforated the film as it exposed it" Kreines

>mirror to be put at the gate. I find it hard to accept that film can reflect an >image. After all it is not even polished.

Actually you are looking at the image from your autocollimator
projected on the film plane and viewing it through the beam splitter in the autocollimator - the cool thing is that autocollimator position with regard to the camera isn't even all that critical, infinity being what it is...

Mark Weingartner
LA based

>David Leitner used to use a cool trick which I hope he doesn't mind me >revealing -- he'd use a strobotac as autocollimator light source

Brilliant (if somewhat intermittent!)

I will try this tomorrow if my crystal pistol can summon up sufficient oomph

Mark Weingartner
LA

Mark Weingartner writes:

>I will try this tomorrow if my crystal pistol can summon up sufficient >oomph

It helps a lot if you really allow your eyes to adjust to very dim light and eat plenty of carrots.

Brian "Bugs" Heller
IA 600 DP

Mark H. Weingartner wrote:

>Actually you are looking at the image from your autocollimator >projected on the film plane and viewing it through the beam splitter in >the autocollimator

Greetings everyone,

If I am correct we did try the autocollimator. We took a Zeiss HS 9.5mm check it first to see if that one was ok, then put it on the Bolex. It was very difficult to see the two marks or whatever they are called.

The two vertical lines and the inverted brackets located in between. You could clearly see the one or the other but when both it became so dark. As the reading seemed so difficult to do the correction was not as accurate as the technician hoped, that is why he suggested measuring the flange focal depth. But he is not familiar with Bolex nor are the other technicians, this is Arriland after all.

So any explanation in checking the flange focal length would help?

Regards,

Emmanuel from Munich

Manny,

Andy Taylor at Arri Media is well versed with this as they used to have a PL Mount Bolex EL. I think that 9.5mm is too wide to work properly in this, a 25mm is probably better.

If I remember correctly once you have it set up you can see an obvious difference between the film and ground glass.

Justin Pentecost Focus Puller London

Emmanuel from Munich wrote:

>...It was very difficult to see the two marks or whatever they are called. >The two vertical lines and the inverted brackets located in between.

Sounds like you need a brighter light source on the autocollimator. The physical flange focal distance is 20.76mm.

For more than you probably want to know about this, see:

http://www.city-net.com/~fodder/bolex/truth.html

Wade K. Ramsey, DP
Dept. of Cinema & Video Production
Bob Jones University
Greenville, SC 29614

Dennis Couzin's site says 20.76mm, I'm guessing from reading that this is physical distance, not "in air" distance....(which would be 21.86 if I've calculated right but I'm sleepless don't count on it..)

http://www.city-net.com/~fodder/bolex/truth.html

Sam "although not in Seattle" Wells

Emmanuel from Munich writes:

>If I am correct we did try the autocollimator. We took a Zeiss HS 9.5mm >check it first to see if that one was ok, then put it on the Bolex.

That sounds more like an MTF machine than an autocollimator.

Brian Heller
IA 600 DP

In any case what you need for the SuperSpeeds is the rear focal distance, that's what counts....

You need to ask Bolex themselves maybe....

It would also seem you would have to optimise focus for a particular stop, strictly speaking. (see the web page).

However I've eye focussed a Switar 26mm @ f1.1 got away with it...

An AC friend of mine worked with a DP who had Bolexei w/ PL mounts & used SuperSpeeds, he claimed they worked quite well but there are issues. (Now since Digiprimes are designed to work w/ prisms, maybe you should just adapt them, plus they make a collimator too; the cost is worth it, right?

Sam Wells

Argyris Theos writes:

>I was under the impression that an autocollimator needed a front >surface mirror to be put at the gate. I find it hard to accept that film can >reflect an image. After all it is not even polished.

As I recall from my film days, an autocollimator projects an infinity-focused test image onto the film as if the film were a projection screen.

I've never used an autocollimator on a beam splitter Bolex, so I don't know whether an autocollimator can distinguish the image projected on the film from the image projected on the ground glass. As I recall, the prism swings out on a hinge (for cleaning), so it's possible that you could slip a bit of black paper in front of the ground glass to block it. Then, when you've adjusted your lens mount, block the film gate instead, so you can check the ground glass.

(Of course, you must close the viewfinder shutter, or cap the viewfinder, when using the autocollimator.)

Dan Drasin
Producer/DP
Marin County, CA

It's can 'o worms time!!!

Ok, why do you need to check the depth of your Bolex?? If you were to find that it's out, what will you do??

In my experience of Bolexâ€™s ( Boli?? )the lenses were always set to the camera. If you are using RX lenses, they are easily adjustable by slackening the grub screws on the last ring, then just screw the lens body in or out to adjust. If it's a RX zoom, then the adjustment is usually in the rear optical group, there should be a locking ring that, when slackened, will allow you to screw the rear optics in/out to get your adjustment. This will have to be checked to film with a collimator.

If you are using non RX lenses, then the beam splitter becomes very important. If you are using say, a Zeiss T1.3, you cannot use these lenses beyond T4 ( Justin Pentecost's 3rd Law of Bolex, or something like that...)

OK, assuming that you want to use non RX lenses, try this:

Use a 25mm, set at T4. You will need film in the gate, to make it more reflective, you can use a soft pencil on the emulsion to leave a layer of shiny graphite. Another way is to polish the emulsion with a metal polish wadding ( Dura-Glit in the UK ) be gentle. This sounds strange but does actually work!!! Using the collimator, the lens should just go through infinity ( to film ). The next check is to look through the eyepiece to see what is happening on the ground glass. The lens should reach infinity, but not go through. This may be hard to see...

In my opinion, the best collimator for this is the Chrosziel video collimator ( loads of luvley light!!! )

Make sure you shoot tests. ( Justin Pentecostâ€™s 17th law of Bolex )

Hope this helps, If you should end up in the UK, bring it in to me and I'll check it here....

Andy Taylor
Arri Media
UK

Andy Taylor wrote:

>It's can 'o worms time!!!

>Ok, why do you need to check the depth of your Bolex? If you were to find >that it's out, what will you do??

Greetings Andy,

The readings on my lens do not match the eye focusing. On the wide angle lens that focus becomes critical. Now, I use a Bolex with a Bayonet lens mount and a PL mount adapter on top of it. I want to know if the adapter is slightly of or if the original bayonet mount is of.

We did test it on a Chrosziel collimator ( the latest model), checked that information today with the technician. However we did do this with a 9.5, as I believed the focus is so more critical. Obviously that was a wrong starting basis.

I will have a go again at the collimator at the end of the month as for now I have no time due to shooting.

I will be in London from September onwards so I will be very happy to come along. I am not to much in a hurry with this and have planned plenty of testing as my project really start taking pace next winter, spring.

Appreciate very much your reply, as well as the others.

Regards
Emmanuel from Munich

SUYS Emmanuel wrote:

>However we did do this with a 9.5, as I believed the focus is so more >critical. Obviously that was a wrong starting basis.

The non-Rex lenses under 50mm in focal length are not going to perform well with the prism in a Bolex's optical path.

Use Rex mount lenses like the 10mm Switar instead of PL lenses.

The 10mm and 26mm Switars are fine lenses.

Read Dennis Couzin's paper about Rex mount optics at:

http://www.city-net.com/~fodder/bolex/truth.html

Jeff Kreines

Emmanuel,

I think there's also an Aspheron option (can't remember the size)-Switars are indeed amazing. Of course Bolexes are great for confined areas/handheld etc...

I was just on an LL Cool J video-they had a 435/Zeiss, and the directors wanted a Bolex/Switar 10mm/ B&H 2709 (amazing...) for funky looks

John Babl
Miami

Jeff Kreines wrote :

>Use Rex mount lenses like the 10mm Switar instead of PL lenses. The >10mm and 26mm Switars are fine lenses.

Greetings Jeff,

It's a great lens.

Have a 10mm Switar RX but in S16 I have vignetting.

But on some shots would like to use other lenses, maybe I am just looking for something to complicated.

Regards
Emmanuel from Munich

Jeff Kreines writes:

>Read Dennis Couzin's paper about Rex mount optics at :
> http://www.city-net.com/~fodder/bolex/truth.html

Thanks for this superb reference, Jeff. Having read it, I take back what I said about slipping something opaque between the prism and the ground glass. It can't be done, because (duh...) the ground glass is the (frosted) top of the prism itself. How soon (maybe 35 years?) we forget...

That article is also a sobering reminder of the risky compromises inherent in a beam splitter-type finder system. IMHO it should be required reading for all Bolex users.

As I recall, some 35mm Eyemo reflex conversions used a similar prism-beam splitter system (albeit with a vertical ground glass on the side, not a horizontal one on top). I wonder how many of these same caveats would apply to those converted Eyemos...

Dan "consider a used Arri, and sleep better at night" Drasin
Producer/DP
Marin County, CA

>Have a 10mm Switar RX but in S16 I have vignetting.

Well you could keep the Switar 10mm and swap the Bolex for a Kinetta, right ?

Really this issue is one keeping me hesitant from Super 16-ing my Rx 4, altho I'd go with the mod that let me switch back and forth. Bolex is pushing Kern's 12.5-100 Switar zoom, with _its_ Aspheron you can get fl's between - what - ~6.5 & 12.5mm (can't zoom in that case).

But, to my way of thinking, once you've started festooning a Bolex with lotsa stuff, it can start loosing its "subversive charm" (although a reflex, I kind of look at the Bolex as a motion pic equivalent to an old Leica or Nikon rangefinder camera....)

Then again I play trix with the 10mm & Century 6mm adaptor, even thought of buying the Optex Diopter holder, but geez it's as much as I paid for the camera...

Sam Wells