Jim Trahan 
, Jan 05, 2012; 12:40 a.m.
First off, I am new to film. Please excuse me if this is a daft question. For the holidays my wife gave me a nice IIIc with an Elmar 90mm f/4 lens. What has me puzzled are how the stops are called out on the lens...but I suspect these will be familiar to you. The stops are: 4, 4.5, 6.3, 9, 12.5, 18, 25, and 36. These do not match modern stops that are powers of the square root of 2: 4, 5.6, 8, 11, 16, 22, 32, 45, etc. Because the modern stops are geometric each 1 stop change increases or decreases the light by 100%. Do the stops on my Elmar follow the same geometric sequence? For example, will a change from f/9 to f/6.3 double the light entering the lens? Thanks in advance, Jim
Mukul Dube 
, Jan 05, 2012; 12:51 a.m.
Yes, Jim, the "old scale" is also a geometric progression.
Zane Johnson , Jan 05, 2012; 01:09 a.m.
Do the stops on my Elmar follow the same geometric sequence?
With the exception of 4 -> 4.5, yes. This thread has more info:
Helen Bach: "The official f-numbers for the 'Continental scale' (common before the 1940's) are:
1.1, 1.6, 2.2, 3.3, 4.5, 6.3, 9, 12.5, 18, 25, 36, 50
These are nominally a third-step slower than the closest 'International' whole-step f-numbers that we are now familiar with. No matter whether the lens in question uses the Continental or International scale, the maximum aperture need not be a full step on that scale of course."
Jim Trahan , Jan 05, 2012; 01:41 a.m.
Wonderful! I was beginning to get concerned when even my Sekonic L-38 was using the modern sequence; it's only 10 years older than the lens. Thank you both for helping me.
Mukul Dube , Jan 05, 2012; 02:14 a.m.
The meter may have been bought ten years before the lens but it is not likely to be ten years older. If you give the serial number of the lens, I or someone else will tell you when it was made.
Winfried Buechsenschuetz , Jan 05, 2012; 06:35 a.m.
Both f-stop sequences are "geometrical". In both sequences any f-stop value is sqrt(2) - the square root of 2 - times the previous value.
BTW on french cameras (with a "modern" sequence" you sometimes find an f-stop value of 23 instead of 22, which is a bit closer to the precise value.
In the early days of photography there were some other (non-geometrical) f-stop values but unless you do large format photography with antique lenses you will never run across such lenses.
Charles Stobbs 
, Jan 05, 2012; 09:30 a.m.
Jim Trahan , Jan 05, 2012; 10:35 a.m.
I've noticed that some people are cagey about sharing their serial numbers. Why is this?
The SN on the lens is 675652. The light meter dates to 1960 and I assumed the lens is around 1950. I'm curious to hear how old the lens actually is.
Thanks again,
Jim
Mukul Dube , Jan 05, 2012; 10:46 a.m.
The serial number puts the lens at 1948. As it has the Continental aperture scale, the distance scale should be metres rather than feet. I was puzzled by this: "my Sekonic L-38 ... [is] only 10 years older than the lens."
Jim Trahan , Jan 05, 2012; 10:57 a.m.
Thanks, Mukul. The Sekonic L-38 was only produced in 1960. I just guessed that it was around 10 years older than the lens. Pretty close guess!
Thanks again,
Jim
