# Pentax Spotmatic ASA/ISO question



## The Dread Pirate Robins (Jul 2, 2008)

So...  here I thought I knew everything about my camera, since I have been using these off and on for over 20 years.

I have a Spotmatic.  It has a film speed selector that appears to have "notches" at 100 ASA, 200 ASA, and two equally spaced "notches" in between.

(I have a photo of this selector but I seem to be able to upload any photos to this forum.  Maybe I am too new.)

Anyway, I just bought a roll of Kodak Portra 160 film, not thinking much about the ASA until I looked at my camera again.  I am thinking that I would be just fine to put my ASA selector switch at the "notch" that would be at about 166.66 ASA and just go with that.  I am wondering if anyone else has tried such a thing.

Intuitively, it seems like this shouldn't matter that much, but I thought I would check.

Thanks,
Adam


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## yellowjeep (Jul 2, 2008)

Second notch it and you will be fine.


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## deanimator (Jul 2, 2008)

Most cameras from this era used marking like this for film speed settings of 25, 50, 64, 100, 125, 160, 200, 250, 320, 400 and so on.

Depending on what you are photographing, it may be worthwhile getting a hand held meter (maybe borrow one) to see that everything is working as it should.  However, most print film is fairly tolerant, so it won´t really matter if you are a stop out, but just remember that over-exposing color negative film is safer than under-exposing.

Here´s an example...note the difference in the shadows...at 1 and 2 stops under the black is actually a washed-out dark grey only, whereas in the other pictures there is virtually no difference!


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## Helen B (Jul 2, 2008)

deanimator said:


> Most cameras from this era used marking like this for film speed settings of 25, 50, 64, 100, 125, 160, 200, 250, 320, 400 and so on.



Those, with the addition of 32, 40 and 80, are the standard third-stop speeds that the ASA and later the ISO specified - along with lower and higher speeds. They are still in use, and still in the relevant film speed and light meter standards. A third of a stop is a realistic level of accuracy for film speed determination. Each third-stop speed number is about 1.26 - the cube root of two - times the previous one, so the sequence is not 100, 133, 166, 200 for example.



Best,
Helen


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## The Dread Pirate Robins (Jul 2, 2008)

Helen B said:


> ... A third of a stop is a realistic level of accuracy for film speed determination. Each third-stop speed number is about 1.26 - the cube root of two - times the previous one, so the sequence is not 100, 133, 166, 200 for example...



That's even better news than my guess, and now that I look at the rest of the dial it makes sense.

If I multiply 100 ASA x (cube root of 3) x (cube root of 3) to get the second of those "notches" I get about 158.74, which is really close to ASA 160.

Thanks to all... the mystery REVEALED.


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