Speaking of exposure tables...

ChrisN

ChrisN

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While we're talking about exposure tables, has anyone seen one of these? It's labelled as the "Johnson Standard Exposure Calculator". It consists of a notched dial or circular card which rotates within the outer sleeve. The instructions on the face say:

1. Set at "Start".
2. Select Scene notch and dial clockwise to stop.
3. Turn over - repeat for weather, time and film speed.
4. Read off exposure against f/ratio.

I've been playing with it, and I go alright up to the film speed setting - there are two scales for film speed; one headed BSI with values 34, 31,28,25, and 22; and the other headed "SCH" with values 35, 32, 39, 26, and 23.

Is anyone familiar with these scales for film speed? Possibly a British system that pre-dates the current ASA / DIN standards. I've tried working backwards from "Sunny 16" and I think 100 ASA might be equivalent to BSI 25 / SCH 26 on the scales on this device.

An interesting little gadget - it even makes allowance for filter factor!
 

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I don't know this system, but can deduct how it works.

Since it is used in the same way as the weather scale, it means that each step halfs the sensitivity. Since the steps are in 3's this means it is a db scale.
(a factor of 3 means double the power); just like the DIN scale.

Now, no notch should be at 100 ASA = Din 21.
the first notch should then be at Din 24 (if the scale was in Din).
OR at DIN 18 if the scale goes the other way

So it seems that either
Din 24 = BSI 22 = SCH 23
Din 18 = BSI 22 = SCH 23
I presume the first is correct, since the numbers are likely to go up just like the DIN scale, and thus that 100 ASA = Din 21 = BSA 19 - SCH 20.

Now if the first notch would be at the high end (34 BSI and 35 SCH) then the numbers should be adjusted accordingly.

By the way,
DIN = Deutsche Industrie Norm (German Industrial Norm)
ASA = American Standard Association.
BSA I guess is Britisch standards association.

Mad_boy
 
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Resurrecting an old question: I saw a few other people mention they had one of these. Does anyone know how the film speed scales (in "BSI" or "SCH") relate to the DIN or ISO scales?

Not discounting Mad_Boy's suggestion, above, but the lowest value on the scale is BSI 22, so I'd expect iso 100 to be above that figure.
 
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We need a book from '50s, from memory I thought

BSI - 10 = DIN

But my meter did not do DIN, so I never had to recall the conversion, in '50s.

Noel
 
Having bought one of those, I was also puzzled by the filmspeed markings. However, I have found http://photography.about.com/library/glossary/bldef_bs.htm.

As to what "SCH" stands for, I've no idea. Time to dig into those old books, methinks...

In the meantime, I'll rely on the Weston V. 🙂

(Edit: I've just compared the filmspeed settings to the Weston Master V... those filmspeeds are ASA equivalents).
 
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Dave

Sch is described if you push the button that says film speed...

Noel
P.S. Kodak did a nicer circular slide rule.
 
350D_user said:
As to what "SCH" stands for, I've no idea. Time to dig into those old books, methinks...
Click to expand...

I think that was Scheiner, if I remember correctly. I've got an old film speed conversion table (pretty sure it was printed in 48) here somewhere. I will post again if I find it, but it's in a moving box somewhere...
To confuse matters even more, I think there was both American and European Scheiner, which were different.
 
Here we go.... Tables attached.

I posted a thread on the Johnson calculator a few months ago 🙂 I have three (black and white, colour transparency and indoors) from the early 60s, thus with standard ASA speeds. Each should come with tables which can be stuck onto the calculator allowing correction for specific latitude/longitude anywhere on the planet, which I will happily provide, if needed.

The Johnson calculator was the last form of the Welcome Calculator, which was originally produced in the 1890s and refined over 70 years of use. It thus reflects the empirical experience of generations of photographers. Used with moderate intelligence it is at least as accurate as TTL metering and takes little more time - the transparency version is miraculously good.

BS (British Scheiner), later renamed BSI (British Standards Institute), was a short-lived speed system based on the same mathematical principle as DIN - one table below also features the completely forgotten APEX, a first attempt at a global standard from 1962.

All the best, Ian
 

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Jocko said:
My pleasure Chris - This should enable you to correct your calc. for Australia! 🙂

Cheers, Ian
Click to expand...


Spot on! The correction gave me sunny-16 for this afternoon's session, which was what I was shooting at in the open. 🙂
 
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For those who may be interested, I recommend "Photographic Facts and Formulas," by E.J. Wall and F.I. Jordan, revised and enlarged by John S. Carroll. It has been reprinted and expanded upon in numerous editions. Mine is from Amphoto, copyright 1976, ISBN 0-8174-0580-1.

It is often available inexpensively on eBay, and it contains a wealth of information, from developers to printing-out methods, to lens types, to exposure, and so on. It includes all types of conversion tables, including those seen above and guide numbers for flash bulbs, conversions, adapters, and so on.

I collect and read old books on photography and photographic chemistry and all of them are useful and entertaining in one way or another, but this book has become a guide for me. If I could choose to keep only one, this would be it.
 
mad_boy said:
By the way,
DIN = Deutsche Industrie Norm (German Industrial Norm)
ASA = American Standard Association.
BSA I guess is Britisch standards association.
Click to expand...

since this is a bit of a technical thread, i thought i'll add a little pernickety correction:

DIN actually stands for Deutsches Institut fuer Normung (German Institue for Standardization) and BSA stands for British Society of Audiology.
 
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