# Weston Film Speed Formula

Discussion in 'Exposure Discussion' started by zemzem, Sep 10, 2006.

1. ### zemzemMember

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I ran across this film speed (value) for a Weston Master lll:

ISO / 5 x 8 = Weston Setting. Anyone heard of this? I have a working Master lll and I'd love to use it. Any other ideas for matching meter to modern ISO? Thanks

2. ### Nick ZentenaMember

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That sounds wrong to me. It's either 100 ISO equals 80 Weston or 100 Weston equals 80 ISO. The first one sounds right.

Take it outside and try a sunny 16 test.

Or just do a film speed test.

3. ### zemzemMember

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I've also heard the 100 ISO equals 80. Thanks, I'll give it a try.

4. ### DBPMember

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One of my older books around here has a table with Weston and ASA. Weston is 0.8xASA.

5. ### zemzemMember

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Thanks. It's looking like 100 = 80 is the right formula. Thanks for the math. No I just need a day off to shoot!

6. ### BrianShawMember

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I'm sure you guys have the arithmetic worked out, but my Master III's (all of them) has ASA in addition to Weston numbers. I know Master II and earlier are in Weston but never saw a III like that.

p.s. No matter what, it's a GREAT METER!

7. ### Jim NoelMember

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Here is the list.
Jim

Weston ASA
2 2.5
2.5 3
3 4
4 5
5 6
6 8
8 10
10 12
12 16
16 20
20 25
24 32
32 40
40 50
50 64
64 80
80 100
100 125
125 160
160 200
200 250
250 320
320 400
400 500
500 640
640 800
800 1000
1000 1280
1280 1600

8. ### Nick ZentenaMember

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I thought the III was still Weston and the IV was ASA?

9. ### BrianShawMember

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No, Weston III meter has ASA.

EDIT: Ah... I need to correct myself... my Weston III has ASA. Not all do. According to http://www.westonmeter.org.uk/ there was a European version with Weston Numbers and an American version with ASA. Mine (all of them) are the American version.

Last edited by a moderator: Sep 12, 2006
10. ### Roger HicksMember

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Add 1/3 stop to Weston to get ASA.

I'm not sure that the conversion is always accurate because I don't know the equations for Weston speeds, i.e. the criteria might be different, but it seems likely they used the 1940 Kodak criteria and the speeds are therefore strictly interconvertible, unlike (for example) old DIN, Scheiner or H&D.

Cheers,

Roger

11. ### bob01721Member

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That's how I use my Weston II. Much easier than trying to remember formulae and charts which are only restatements of "+ 1/3 stop" anyway.

12. ### zemzemMember

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thanks for the numbers