Leica M5 ISO Dial intermediate stops
Old 08-14-2019   #1
shorelineae
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Question Leica M5 ISO Dial intermediate stops

Hi there,

I have a Leica M5 and I'm trying to figure out the exact ISO that each intermediate click stop indicates. For example, Between 200 and 400 there are two intermediate click stops. I looked at the M5 manual and couldn't find this information there, nor elsewhere on the internet.

My calculation would be: (400 - 200) / 3 = 66. So that makes it:
  • 200
  • 266
  • 333
  • 400

Basically, I'm trying to rate Kodak Vision2 500t at ISO 325.

Any help would be appreciated.
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Old 08-14-2019   #2
CharlesDAMorgan
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The dial is also in DIN with 24 25 26 27. On the normally accepted scale that is 200 (24), 250 (25), 320 (26), 400 (27).

Your calculations assume the progression in ISO is arithmetic, when in reality it's geometric. A one stop increase is always a doubling of the Iso/ASA.

The closest you'll get to 325 is 320 Iso / Din 26.
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Old 08-14-2019   #3
rhl-oregon
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This is logical, but consider that the intermediates might correspond to more typical Tri-X exposure reductions—iso 320 and 250. That’s how I treated it on my M5. The differences are trivial for results, in any case.

Good observation by Charles—the M5 comes from DIN world, not ASA, and ISO appears to have been an Anglo triumph rather than a true international compromise in speed measurement.
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Old 08-14-2019   #4
Sarcophilus Harrisii
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Quote:
Originally Posted by shorelineae View Post
Hi there,

I have a Leica M5 and I'm trying to figure out the exact ISO that each intermediate click stop indicates. For example, Between 200 and 400 there are two intermediate click stops. I looked at the M5 manual and couldn't find this information there, nor elsewhere on the internet.

My calculation would be: (400 - 200) / 3 = 66. So that makes it:
  • 200
  • 266
  • 333
  • 400

Basically, I'm trying to rate Kodak Vision2 500t at ISO 325.

Any help would be appreciated.
They would be for ASA 250 & 320 respectively. You can't "rate" an ISO 500 film at "ISO" 325. The ISO is 500 as it was, presumably, set in accordance with the relevant standard? You can however expose it at any particular "Exposure Index" you like.
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Old 08-14-2019   #5
shorelineae
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Thanks to all who answered. 250 and 320 it is.

@Sarcophilus Harrisii: This particular film is supposed to be rated at ISO 320 when using it with a 85b filter.
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Old 08-14-2019   #6
Sarcophilus Harrisii
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Quote:
Originally Posted by shorelineae View Post
Thanks to all who answered. 250 and 320 it is.

@Sarcophilus Harrisii: This particular film is supposed to be rated at ISO 320 when using it with a 85b filter.
Kodak's published data sheet for the film refer to a range of EXPOSURE INDICES for the film. The only EI referenced for the 85b filter you mention is 200.

https://www.kodak.com/uploadedfiles/.../H-1-5229t.pdf

I suggest you educate yourself on the difference between the terms ISO and Exposure Index.
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Old 08-14-2019   #7
Gregm61
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If this is some form of print film, be it color or black & white, the visible difference in the negatives between exposing at 200 and 320 is going to be non-existent with the hyper-availability of latitude in the highlights. I would simply rate the film at 200 and fire away.
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Old 08-14-2019   #8
mike rosenlof
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as Charles said in post #2, it's a geometric progression. To get the next value in the progression multiply by the cube root of 2, which is approximately 1.25992105 and you'll get (rounded)


200, 252, 317, 400 and the normal ASA/ISO progression rounds a bit more roughly to give you 250 and 320 as others have stated.
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