Shutter Speed Evolution

Is anyone aware of any "logic" regarding the change in shutter speed markings in the 1950s/early 60's when 35mm took over from MF folders? I'm referring the the 25, 50, 100, 200 settings as opposed to the 30, 60, 125, 250 settings.

It would seem that the old marking system would be correct. Film is sold in 50ASA, 100ASA, 200ASA, etc, so a shutter speed of 125 should underexpose 100 speed film by 1/3 stop. But that obviously doesn't happen. On older cameras the aperture has to be closed down 1/3 stop for the correct exposure with the "even ASA" shutter speeds.

Instead of a "Sunny 16" rule, did photographers used to have a "Sunny 1/3 stop over 16" rule?

Isn't that more like a quarter stop? Anyway, I spent last Friday with my Argus C3 that has 10, 25, 50, 100, 300. I suppose I'm just too casual about it all, but I suspect a 1/4 stop is within the margin of error of many mechanical shutters -- not to mention apertures probably aren't +/- 1% either (especially after 50 years).

DaveT (whose Perkeo II with Prontor has that odd jump from 100 to 300 also)

My old Zeiss Ikon Nettar has 25, 75, 200. Try that one for mental arithmetics!

The logic behind the 30, 60, ... is in the rounding off: It starts with 1, then goes 2, 4, 8, 16 (marked 15), 32 (marked 30), 64 (marked 60), 128 (marked 125), 256 (marked 250), 512/500, 1024/1000 and so on.

The 25, 50, 100 and so on breaks down for longer times.

I think it has more to do with the relationship between f-stops and shutter speeds. With the newer progression of shutter speeds, the f-stop and shutter speed can be sychronized into EV values. For example, my Retina 3c has the newer shutter speed progression, and the shutter speed is coupled to the f-stops so once you have the correct exposure set, you can change shutter speeds and the f-stops change to ensure the correct exposure. My Rolleiflex T has the same arrangement.

Personally, I don't care for the coupled arrangement. I have a 80/2.8 Xenotar on my Century Graphic that has the old progression 1,2,5,10,25,50,100,200,400. There is enough latitude in most films that a half-stop or so is of no consequence. I find my brain compensates better than a mechanical coupled arrangement.

In the days when shutters were still marked as 5th 10th 25th 5oth etc, the film speeds were the old ASA/BS speeds anyway. Early 60's ASA/BS speeds were changed and a stop of latitude was removed, so Tri-X went from 200 ASA/BS to 400 ASA/BS, FP3 from 64 to 125 etc

Many films has been sold that don't fit the 50/100/200/400 progression such as 32 ASA, 64 ASA, 125 ISO, 640 ASA 18°DIN 21°DIN etc etc all in recent years 70's onwards.

On that note ASA - American Standards Authority BS - British Standards ISO - International Standards Organisation DIN - German

Current ISO 100/21° actually means this is an International Standard based on 100 ASA/BS and 21°DIN

Ian

DWThomas said:

Isn't that more like a quarter stop?

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Actually, it's closer to 1/3 stop, .322 stop to carry it out 3 places. The number of stops between two shutter speeds is the log (in base 2) of the ratio between them.

Ole said:

The logic behind the 30, 60, ... is in the rounding off: It starts with 1, then goes 2, 4, 8, 16 (marked 15), 32 (marked 30), 64 (marked 60), 128 (marked 125), 256 (marked 250), 512/500, 1024/1000 and so on.

The 25, 50, 100 and so on breaks down for longer times.

Click to expand...

Hmmm, that's an interesting theory, but the old numbering system would appear to work better with faster shutter speeds. It's at the slow end of the dial that it breaks down. E.G. 1, 1/2, 1/5, 1/10, 1/25 requires some creativity. But 1/50, 1/100, 1/200, 1/400, 1/800, 1/1600, 1/3200 works out perfectly and match standard (albeit modern) film speeds. Granted, Kodachrome is 64ASA, and 25ASA and even 10ASA used to be common speeds, and medium format folders didn't much worry about 1/800, 1/1600 and 1/3200 shutter speeds.

The real question comes down to why is there a 1/3 stop difference between the shutter speed and the film speed? If you do have an "off speed" film, e.g. Portra 160, the "standard" shutter speed would still be 1/3 stop faster. In this case, 1/200 second.

Mike Richards said:

Actually, it's closer to 1/3 stop, .322 stop to carry it out 3 places. The number of stops between two shutter speeds is the log (in base 2) of the ratio between them.

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Whoops, right -- my bad!

But I'm still back to the idea I'd be pretty thrilled to get every exposure within a third of a stop.

DaveT

thuggins said:

But 1/50, 1/100, 1/200, 1/400, 1/800, 1/1600, 1/3200 works out perfectly and match standard (albeit modern) film speeds.

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What's this about "matching" film speeds? Matching numbers are only important if you're using the sunny-16 rule (or something similar), and that's just a rough guide to exposure, nothing more. In the real world, light levels vary continuously, and 1/125s is no better or worse a match to get perfect exposure than 1/100s, 1/150s, or any other value you care to name. Of course, to get "perfect" exposure, you'll need a meter and the knowledge to use it properly.

IMHO, it's more important that the shutter speeds fall at reasonable approximations of 1-stop differences throughout their range. That makes finding equivalent exposures easier and more precise -- for instance, 1/500s and f/16 is very close to being equivalent to 1/60 and f/5.6. Both the aperture and the shutter speed change by 3 stops in this example. On an old-style camera with 1/500s and 1/50s rather than 1/60s, using 1/50s and f/5.6 will result in more of an exposure shift compared to 1/500s and f/16. Of course, all this assumes near-perfect calibration of both the shutter and the aperture. My impression is that there was substantial slop in most older cameras. Even the modern system introduces some rounding error into the equation, as noted in earlier posts.