Bolex H16 RX Light metering

[Lunar07]

If it is of any help at all - Bolex has an exposure incident-reflective light meter which was made by Gossen for Bolex Reflex cameras.
The Bolex manual specifically states:
When using this light meter with Non-Reflex H8 and H16 cameras, adjust the ASA setting on the light meter by a factor of 1.3.
For example, if ASA of film is 25, set ASA on meter to 1.3x25 = 33
Which translates to less light for Non-Reflex cameras.

As to Carl's observations. What does above mean. I am too tired and I am at work. It can actually mean anything hehehehe
Just wanted to share that bit of info.

[carllooper]

If it is of any help at all - Bolex has an exposure incident-reflective light meter which was made by Gossen for Bolex Reflex cameras.
The Bolex manual specifically states:
When using this light meter with Non-Reflex H8 and H16 cameras, adjust the ASA setting on the light meter by a factor of 1.3.
For example, if ASA of film is 25, set ASA on meter to 1.3x25 = 33
Which translates to less light for Non-Reflex cameras.

As to Carl's observations. What does above mean. I am too tired and I am at work. It can actually mean anything hehehehe
Just wanted to share that bit of info.

Thanks Lunar07. All information helps to get an angle on what standards Bolex were adopting. Based on what you've provided, the Bolex Reflex specific light meter (as distinct from other light meters) would be taking into account the light lost to the viewfinder, which means it must be assuming the lens markings are in f/stops rather than t/stops, otherwise you would have a doubled correction, one done in the light meter, and one done by t/stops on the lens - the net effect being an error.

The effect of setting the light meter to a faster ASA setting (for non-reflex cameras), is so the light meter will recommend an f/stop that lets less light through for such cameras (for non-reflex cameras). To put it another way, it turns the reflex specific light meter back into a conventional light meter, which is what you would otherwise use for a non-reflex camera. Reciprocally, without such a "fooling of the reflex specific light meter", ie. normal use of such a reflex specific light meter, the f/stop it would recommend would let more light through (compared to the f/stop recommended by a conventional light meter), ie. to accommodate the light lost to the viewfinder, that being the purpose of the reflex specific light meter.

So this is further evidence in support of the position that Jean-Louis and Dom have adopted. I'm tending towards this same position. If nothing else, it says that if any lenses made for the Bolex Reflex camera ever were marked in t/stops (calibrated at the film plane), they would have to be the exception, rather than the rule.

Carl

[carllooper]

Here's a response from Bolex, which can be interpreted as lending further support to what Jean-Louis and Dom are saying - that the lens in question is marked in f/stops.

Given the specificity of the question I submitted it's not a very detailed response unfortunately. What would have been a better response (if true) would be to say that no lenses for the Bolex were ever manufactured with t-stops. Instead it treats the question as if it was one about how to use a light meter for an f/stop lens on a Bolex - which is sort of beside the point ie. already obvious and implied in the question.

QUESTION:

Hi,

I have a Bolex H16 Reflex, and a Kern Switar 10mm marked RX. Can you tell me
whether I should use 1/65 sec, or 1/80 sec for the light meter, when
shooting 24 fps.

Some sources say 1/65 sec because the RX lens is marked in T Stops (rather
than F stops), while others argue one should use 1/80 sec because (they
argue) RX lenses are marked in F stops (rather than T stops).

Could you confirm, either way, which is correct and point to any documents
that might satisfy others with the same question?

yours sincerely
Carl Looper

ANSWER:

Dear Mr. Looper,

Thank you for your message of 28.01.2013.

At 24 fps, the real time is 1/65, but in order to compensate the loss of
light due to the reflex prism, you have to set the lightmeter on 1/80, not
on 1/65.

Best regards.

Bolex International SA
Marc Ueter

www.bolex.ch (16mm and Super 16 film cameras, repair and service on 8mm and
Super 8, Bolex and Eumig)

[cameratech]

Carl, I don't really feel like continuing to bash my head against this wall, but there's already so much misinformation about this issue on the net that I feel obliged to smack my forehead on the bricks one more time, and try to kill this puppy.

First off, stop talking about T stops "calibrated at the film plane", there are conventions when dealing with optics which you're obviously oblivious to. A T stop always relates to the lens only. Sites like Filmshooting should be clarifying things, not introducing confusion and horseshit.

On top of all the arguments already put forward, I'll add a simple one. Longer focal lengths for reflex Bolexes weren't marked as RX, since it was felt that the aberrations introduced by the prism weren't noticeable above about 50mm. So you won't find a 75, 100 or 150 designated as RX. If the wider angles marked RX had their aperture scales adjusted to compensate for the prism, what would they have done with the longer focal lengths that could be used on both RX and non RX cameras? Surely they would have made a complete range marked RX, otherwise it would be ridiculously confusing to go between one lens where you didn't need to compensate for the prism light loss, to another where you did. Why stop at 50mm if it wasn't just about optical correction?

There are other arguments I could use to disprove your theory Carl, but if that one doesn't remove all doubt I have to assume you're just being willfully ignorant.

[carllooper]

There are other arguments I could use to disprove your theory Carl, but if that one doesn't remove all doubt I have to assume you're just being willfully ignorant.

Up yours Dom.

It's not my theory I'm talking about. It's someone else's theory (the University of Texas, etc) which I am investigating in an open and honest way. And if you have a number of arguments to disprove the theory then please share. This isn't a "whose the guru we should follow in blind faith" competition. I've been assuming that the theory has some merit but also looking at how to debunk it. I would like nothing more. It bugs the shit out of me that I can't definitively debunk it. But so far I think I have assembled a persuasive case that the theory is more or less debunked. Not entirely so but much more so than when I began.

And personally, if you actually bother to read what I've written, rather than assuming I'm a stick-in-the-mud, you'll see I've said I'm actually quite persuaded by the evidence in favour of Jean-Louis' position, and yours!

So once again - up yours.

Carl

ps. T-stops are calibrated at the film plane. To do anything else makes them useless as T-stops. That's the whole point of T-stops. Otherwise you'd just assign a single transmission number, to the barrel of the lens, for use in an equation for calculating a composite system - an altogether different purpose and methodology. However it does make sense to factor out the prism for lenses that are designed to be interchangeable between prism and non-prism systems - for obvious reasons. This doesn't alter the argument that a lens specifically designed for a prism system (and inadequate on a non-prism system) might very well keep the prism factored in. After all it factors in the chromatic correction - so why not the light loss as well? None of this proves anything. It's just speculation in both directions, in an attempt to see if there are any non-factual (rational) reasons why the lens might be one way or another.

Here's a factual argument in favour of the RX lens I've been testing being marked in exactly the same way as the non-RX lens. Both apertures are set at x/2.0 and they present as exactly the same size (assuming no optical distortion of course). So at least for me, if only with this particular RX lens, I now feel quite comfortable in assuming the lens does not accommodate for light loss to the viewfinder. [The Non-RX looks pretty dirty as well so that could easily account for the 0.4 transmission discrepency between the two]

[cameratech]

Up your Dom.

For a second there I thought you were raising my flag as a sign of my victory!

Honestly Carl, I'm not trying to insult you, I do actually like an inquiring mind that questions assumptions. But when enough evidence is stacked up to make a position untenable it becomes like a pride issue or something to keep expressing doubt. We're not talking about existence or perception or some other philosophical question where doubt is healthy, but a technical issue around particular filmmaking tools, which causes confusion and uncertainty for people trying to use them. I'm trying to clear up the confusion, based on a certain amount of knowledge I have gained after 15 odd years as a professional. As did Jean-Louis earlier, who has many more years experience.

I don't think you have made a persuasive case that a theory is debunked, I think other people have given you plenty of good reasons why a particular website has dodgy information. You're entitled to your scepticism, to believe it's a 'not entirely' debunked theory rather than just one more example of internet misinformation, but I'm equally entitled to shout "rubbish" when I hear it.

[carllooper]

I don't think you have made a persuasive case that a theory is debunked, I think other people have given you plenty of good reasons why a particular website has dodgy information.

I wasn't claiming ownership of what' was presented. What I meant there was that I've assembled a persuasive case for myself, ie. from your information, from that of Jean-Loius, from Lunar07, plus various other sources, and the various tests that I've done. What I was trying to say there, and in earlier posts (if anyone bothers to read it) is that I've come to agree with you.

You're entitled to your scepticism, to believe it's a 'not entirely' debunked theory rather than just one more example of internet misinformation, but I'm equally entitled to shout "rubbish" when I hear it.

Fair enough. What's better is to put the reasons out as well. I don't mind the claims of "rubbish". It's just irritating when there isn't any information to back up the claim. Just someone's say so, and that I should just listen to Jean-Louis rather than testing out what he says, or what you say. What is that all about?

The skeptism I have isn't personal. I consider Jean-Louis a friend. Not sure about you though

Carl

[cameratech]

The skeptism I have isn't personal. I consider Jean-Louis a friend. Not sure about you though
Carl

Well that's gonna conflict with my desperate need to be liked.

Remember, only friends tell you when there's a big leaf of garnish stuck in your teeth.

[carllooper]

The skeptism I have isn't personal. I consider Jean-Louis a friend. Not sure about you though
Carl

Well that's gonna conflict with my desperate need to be liked.

Remember, only friends tell you when there's a big leaf of garnish stuck in your teeth.

cheers

This was only ever about making sure whether the lens I had should be used in the way I was going to use it, ie. setting the light meter to 1/80th sec, based on my calculations, to account for the light loss to the view finder. I then stumbled onto the University of Texas website. I assumed it was correct. That was a mistake - not because the website was or is necessarily mistaken - but because I just accepted it. But what was presented, following from that, was no different: just statements (claims) of the opposite position. Having made the first mistake (blind acceptance of one claim) it was hardly a solution to make the same mistake again (blind acceptance of the opposite claim). I had no choice but to be skeptical, of both positions, and give both the benefit of the doubt.

What followed from this was not in anyway me arguing that the University of Texas was correct (or not correct) but me just working through what was subsequently presented. Jean-Louis followed through and provided his source but that didn't clear it up for me. It still doesn't. But there's enough information to say, that as a rule, RX lenses do not accommodate for light loss. You or your Bolex light meter have to do that. As to whether there are any exceptions to this rule, nobody can say (not yet anyway). All we can say is that it looks unlikely, ie. according to the information available, and some thinking about it.

I too have a lot of experience in this area. I started shooting film in the mid seventies, last century. But it doesn't mean anything. It doesn't mean I'm necessarily any wiser when confronted with a camera system I haven't used before. If someone says the lens is marked in T-stops (calibrated for light lost to the viewfinder), who am I to argue? If someone else says they are not - I don't know. And that's been my position though out: that I don't know. In this situation one has no choice but to explore both sides of the argument. To investigate the claims. Which is what I was doing. I wasn't advocating one way or another. I was being simultaneously skeptical of both and giving both the benefit of the doubt - as difficult as that might be to comprehend. It wasn't about who was right (who was to be awarded "victory") but what was right.

To frame this as "tenacious" is interesting but really, this should be what anyone interested in this sort of thing should be doing as a matter of course. It should be the norm. It's always been the norm for me. Doing things yourself is the way I've learnt everything I've ever learnt. I listen to what people say and follow through and test it, in theory and in practice. I enjoy it. I certainly don't do it to just piss people off, even if that is a side effect. It's all part of the process.

This was only ever about making sure that when I use this camera as I was about to do, with the lens I have, that I will be closer to getting a good result. As it is I'll need to run test film anyway. Even if I was to set the light meter correctly, my light meter could be out!

Also I don't think this thread will confuse anyone. If anyone with the same questions I had (or have) comes across this thread, I think they'll see here a good working through of the questions, better than anywhere else that I've found. They'll be in a much better position to work out what their own conclusions might become. Better than just blind faith.

Carl

[Lunar07]

LOOOOOOOOOOOOOL
I just love these conversations.
I have always thought the f-stops on the RX lenses were actual f-stops not T-stops.
One reason is that lenses from the same time period were marked as T-stops when the need arose. Like those of Taylor, Taylor and Hobson. So, I did not think Bolex on a professional line like their RX lenses would go against the grain.

And since I have used only the Bolex light meter with their RX cameras, I never cared to question the f-stops

But I have learned to be careful in my assumptions in this business And let the big Tech guys have their word

But the info is good to know. So it is 1/80 at 24FPS if I ever use my super duper Sekonic with an RX camera

[carllooper]

Postscript

Notwithstanding the conclusions we've reached (or already maintained), here's an amusing postscript. I had written to Bolex with a follow up question from my previous post to them, which I repeat here:

Hi Marc,

Firstly, thanks for your reply. I understand the purpose of setting the shutter to 1/80th second, to compensate for the shutter. However the real question was really about the lens I mentioned.

The University of Texas claims that RX lenses made by Kern Switar for the Bolex were marked in T-stops rather than F-stops, ie. to compensate for light loss to the viewfinder. If this was the case then obviously you wouldn't want to set a generic light meter to 1/80th second, since the T-stop markings already do that.

So the question I'm really asking is if you know whether any RX lenses made by Kern Switar for the Bolex reflex were ever marked in T-stops (to compensate for the light loss to the viewfinder).

I haven't been able to locate any documents that would confirm one way, of another, whether such marked lenses were ever made.

yours sincerely
Carl Looper


And the response from Bolex:

Dear Mr. Looper,
To our knowledge, there are no T or F-Stops markings on Kern lenses. The only reference is the compensation chart (i.e. 1/80 to 1/65. If you have some photos of the lens with T and F Stops markings, please email them to us.
Thank you.
Best regards.
Bolex International SA
Marc Ueter

Funnily enough there are two things that could make this true.

1. An F-stop should really be written as "f-stop", ie. lowercase "f". Is this the point being made? Probably not.

2. Many lenses will be in T-stops anyway, and since most cameras don't have the "problem" of a prism, the corresponding T-stop refers to just the lens. Lens markings for a Bolex specific lens could still very well be doing the same thing, ie. accommodating for lens transmission (but only lens transmission, and not the prism) - but as such the markings can have difficulty being called T-stops (there being some ambiguity as to the definition of a 'T-stop' in the context of a Bolex reflex camera). We can call them lens-only x-stops instead, the definition of which requires factoring in viewfinder light loss for the lens: "real" for bolex light meters, and "adjusted" for generic light meters. As we've surmised. In other words it could be quite correct to say the lens is not marked in f-stops (or F-stops), nor in T-stops.

In all likelihood markings are factory calibrated for lens transmission (and only lens transmission), whatever you want to call that (or not call that): f-stop, T-stop, or X-stop.

Carl

[cameratech]

There are certain conventions that can help to decode lens markings. On the barrel or front ring you will very often find a ratio such as 1:1.4 or 1:2.8. The second number is the maximum geometric aperture, the widest f-stop. Occasionally it's written in combination with the focal length, eg 1.2/18 being an f/1.2 18mm lens. On very old lenses sometimes they wrote f/1.5 or f:1.5 for example, along with the focal length marked as F=1" or F=2cm (note the small f for aperture, capital F for focal length). At any rate, the barrel or front ring marking is always the widest f-stop.

If the widest stop marked on the iris ring matches the barrel or front ring number, the lens is marked in f-stops. If it's a bit slower, then it's a T-stop. (For example I'm currently working on a Zeiss 1.2/18 - f/1.2 18mm - which has 1.3 as it's widest iris ring mark, being T1.3) Some zooms had f-stops marked on one side of the iris ring and T-stops on the other.

All the Kern prime lenses I've come across (and most C-mounts and older lenses other than zooms) are all marked in f-stops. The Vario-Switar zooms (not sure if it's all of them) have a second index mark on the iris scale to indicate the T-stop (adjusting only for light lost through the lens, of course ).

[carllooper]

There are certain conventions that can help to decode lens markings. On the barrel or front ring you will very often find a ratio such as 1:1.4 or 1:2.8. The second number is the maximum geometric aperture, the widest f-stop. Occasionally it's written in combination with the focal length, eg 1.2/18 being an f/1.2 18mm lens. On very old lenses sometimes they wrote f/1.5 or f:1.5 for example, along with the focal length marked as F=1" or F=2cm (note the small f for aperture, capital F for focal length). At any rate, the barrel or front ring marking is always the widest f-stop.

If the widest stop marked on the iris ring matches the barrel or front ring number, the lens is marked in f-stops. If it's a bit slower, then it's a T-stop. (For example I'm currently working on a Zeiss 1.2/18 - f/1.2 18mm - which has 1.3 as it's widest iris ring mark, being T1.3) Some zooms had f-stops marked on one side of the iris ring and T-stops on the other.

All the Kern prime lenses I've come across (and most C-mounts and older lenses other than zooms) are all marked in f-stops. The Vario-Switar zooms (not sure if it's all of them) have a second index mark on the iris scale to indicate the T-stop (adjusting only for light lost through the lens, of course ).

Thanks Dom.

On the Kern Switar 10mm lens I've got it reads, on the rim:

1:1.6 and f=10mm

The small f is not a misprint on my behalf, but of course the mm tells us that f means focal length anyway. The widest aperture on the barrel is marked 1.6, so we can assume, if the markings are conventional, this means the lens is indeed in f/stops.

cheers
Carl

T-stops will have their origin in markings made by a cinematographer on a lens barrel, rather than a lens manufacturer in the lab. They will have been put there by the cinematographer to make exchanging of lenses less of a headache with respect to the same light reading. They are a convenience in the field. Instead of having to pull out a notebook and check which offset to use for each lens one could just use the T-stop markings on the lens instead. It is this activity, or "use case" which defines a T-stop. Eventually this will evolve into such being printed during manufacturing of the lens barrels themselves. In relation to this original definition of a T-stop, the Bolex provides us at first glance with something awkward. The prism. But if one thinks it through from the point of view of usage in the field, the original use case, the solution presents itself. Were we, as cinematographers, using both a reflex camera and a non-reflex camera, and to inscribe T-stops on the lenses, we would mark the lens on the reflex with T-stops that took into account both the lens and prism, and on the non-reflex camera, mark it's lens with T-stops that were lens only. Of course, if we are just using a reflex, we could also just do the prism offset in the light meter instead. Any lens only offset, on the lens, we could call an X-stop instead. Or to confuse our great grand children we could call these T-stops as well.

T-stops (or better: T-numbers) are not something you'll find used (required) in optics. They have no purpose in optics. What you will find in optics is something called "Transmittance", also represented by the letter T, and while related to a cinematographer's T-number (or T-stop), is defined very differently, and quite specifically, as the ratio of the transmitted light to the incident light, or "flux" as it is more specifically called. And along with other values, such as index of refraction and so on, will provide the means by which (amongst many things) optical components can be designed, one of those components being, for example, a camera lens, or where to mark T-stops (for the cinematographer) with respect to an aperture/diaphragm/iris on such a lens.

[S8 Booster]

ready, set GO....run your iPhones hook up to the app store an brows all lens calculator.
pretty sure they will rip the secret out of - and correct the lens values.

secondly, whynot use a light metering cell in front of then on the back (correctly mounted) of the lens (or beamsplitter) to read eV numbers directly?
differential loss - accuracy.

now, isnt film sensitivity also given in eV in the Kodak film specs...?

a little sixer shot from the hip here budd nevertheless,,,,should work - if lens loss accuracy is the goal regardless of the manufacturer spec.

how-ev-er, i do not think film materiels are truly accurate from production off to ageing so real tests can/must be done - if perfection is the original goal.

and if everything else fails: http://www.fredparker.com/ultexp1.htm (Ultimate Exposure Computer)

or:

http://www.newport.com/Technical-Note-O ... ntent.aspx



shoot .... more film.... I love it....

[carllooper]

To put a small spanner in the works, I've just realised both of the lenses I've been testing are RX lenses. The only difference was that one was a preset (the one with handles, for presetting an aperture). I had acquired the preset lens because it enabled the attachment of a wide-angle lens adapter, whereas the non-preset didn't.

Carl