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View Full Version : Converting from old US f-stops?

Ari
10-Nov-2013, 11:45
I have been trying to find equivalents for the old US (Uniform System) f-stops on my Protar.

Here is what they look like:
http://farm6.staticflickr.com/5500/10391437963_d5b6d3c5ff_c.jpg (http://www.flickr.com/photos/zsari/10391437963/)
P1010932-2 (http://www.flickr.com/photos/zsari/10391437963/) by Ari4000 (http://www.flickr.com/people/zsari/), on Flickr

Is there a method to determine the equivalent in modern f-stops?
For example, to my eye, f256 on the lens makes a similar-size aperture as f45 on a modern lens.

I have seen a few charts here and there, but they are quite old and make little sense to me.

Any help is appreciated.

Mark Sawyer
10-Nov-2013, 11:54
16 is the same in U.S. or f/stops. Then U.S. stops halve or double every stop, while f/stops halve or double every other stop.

U.S = F/stop
1 = 4
2 = 5.6
4 = 8
8 = 11
16 = 16
32 = 22
64 = 32
128 = 45
256 = 64

Ari
10-Nov-2013, 12:06
Thank you, Mark; that explains the over-exposed film. :)

Jerry Bodine
10-Nov-2013, 13:51
Lacking the benefit of Mark's conversions, the f-stop values can be approximated by dividing the focal length by the diameter of the aperture (as measured when looking into the front element). I believe this is true for any lens design, but if not I'm sure someone will correct me.

jcoldslabs
10-Nov-2013, 13:52
I just remember that US16 = f/16 and then count up or down in whole stops from there.

Jonathan

Mark Sawyer
10-Nov-2013, 14:41
The formulas are simple: The f/stop system is based on the only two factors that determine the intensity of light hitting the film in any given lighting situation: how big the lens aperture is and how far it is from the film. For f/stops, f/16 means the aperture is 1/16th of the focal length, f/4 means the aperture is 1/4th the focal length, f/64 means the aperture is 1/64th the focal length.

The Uniform System was based on exposure times. If you needed 4 seconds of exposure at US 4, you'd need 16 seconds at US 16, 64 seconds at US 64, 256 seconds at US 256, and so on. Of course, the exposure was seldom so simple; when the exposure needed was 1/8 second at US 4, it was easier to ignore the numbers and just double or halve exposure times as you counted off stops, (which is what most people still do in the f/stop system).

AtlantaTerry
10-Nov-2013, 14:50
f/stops halve or double every other stop

I disagree, f/stops change by half or double with every stop.

If I change the aperture from f/5.6 to f/8, I have cut the light in half.
If I change the aperture from f/5.6 to f/4, I have doubled the amount of light.

If this were not true then shutter speed changes (also half or double) would not work to balance out the exposure.

Mark Sawyer
10-Nov-2013, 15:00
f/stops halve or double every other stop

I disagree.

f/stops change by half or double with every stop...

Excuse me, I should have said "the f/stop numbers halve or double every other stop."

You can't get away with anything on this forum! :rolleyes:

Ari
10-Nov-2013, 19:16
The formulas are simple: The f/stop system is based on the only two factors that determine the intensity of light hitting the film in any given lighting situation: how big the lens aperture is and how far it is from the film. For f/stops, f/16 means the aperture is 1/16th of the focal length, f/4 means the aperture is 1/4th the focal length, f/64 means the aperture is 1/64th the focal length.

I get 12 focal lengths out of this lens; does that complicate things? :)

Bill Burk
10-Nov-2013, 21:27
I just remember that US16 = f/16 and then count up or down in whole stops from there.

Jonathan

https://archive.org/stream/secretofexposure00fraprich#page/44/mode/2up

I noticed U.S.16. is f/16 too...

Mark Sawyer
11-Nov-2013, 00:36
I get 12 focal lengths out of this lens; does that complicate things? :)

Well, it means it's an f/12 lens. Oh, look, below the Uniform System scale, it's engraved "f-12.5"! I'd call that within the margin of measuring error! :)

Ari
12-Nov-2013, 08:55
Got it; thanks, Mark.
I'll go and find that extra half FL right away!

goamules
12-Nov-2013, 16:19
Stand back! Someone has just used SCIENCE!

Jim C.
12-Nov-2013, 17:14
Nice find !
When I was trying to figure out my Volute shutter scales this book would have come in handy !

https://archive.org/stream/secretofexposure00fraprich#page/44/mode/2up

I noticed U.S.16. is f/16 too...

Tracy Storer
17-Nov-2013, 16:17
I get 12 focal lengths out of this lens; does that complicate things? :)

I believe the aperture scale on that barrel is repositionable for two or three different aperture scales index marks. Unscrew the front cell, pull and twist the part of the barrel with the scale and reposition to other indicators. There will likely be three index marks at the most on any one barrel.

Ari
17-Nov-2013, 18:57
Thanks, Tracy; the lens is on its way to be re-balsamed; when t comes back, I'll have a shutter for it and accurate f-stops.