Steve Smith said 400 = Sunny 16, that's off by 1/3 stop.
Close enough to use Steve's rule of thumb in a pinch.

Steve's rule gives you 1/3 stop more exposure.

Here is how I would use the rule of thumb:

I typically take 1/3 stop off box speed when I select my EI.

I could use Box Speed with Steve's Sunny 16 = 400 equivalent.
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Found this thread which correlates Weston Speed to ASA...

Two of the links are dead. One of the links works, but shows a chart comparing OLD ASA to Weston.

http://www.apug.org/forums/forum48/1...eston-asa.html

For my own convenience, I converted to New ASA, maybe this will help.


DIN - New ASA - Weston
9 - 6 - 4
10 - 8 - 5
11 - 10 - 6
12 - 12 - 8
13 - 16 - 10
14 - 20 - 12
15 - 25 - 16
16 - 32 - 20
17 - 40 - 25
18 - 50 - 32
19 - 64 - 40
20 - 80 - 50
21 - 100 - 64
22 - 125 - 80
23 - 160 - 100
24 - 200 - 125
25 - 250 - 160
26 - 320 - 200
27 - 400 - 250
28 - 500 - 320