Measuring Threads for Photographers Who Are Also Machinists
Precise measurement of thread and diameter is often necessary in various cases. While most modern equipment is standardized, precise measurements are required for niche and older equipment. This page provides some essential information to guide the process of measuring threads.
Metric or Imperial Measuring
There are two commonly used systems today - the Imperial and the Metric systems. Most foreign-made lenses and shutters produced after 1945 use the Metric system for threads and measurements. The Metric screw thread is specified by the distance it advances in millimeters in one turn of the screw. When a filter is turned once, and it moves 1mm towards the lens, the thread is labeled "M1.0." This number is known as the pitch and refers to the distance between two peaks of the thread. M.5, M.75, M.9, M1.0, M 1.25 are among the most popular metric threads with a filter thread of M.75 being typical for medium-sized filters. Filters with a diameter smaller than 40.5 are generally M.5.
On the other hand, Imperial threads are defined by the number of peaks in one inch of length, known as "Threads per Inch" (TPI) and the diameter is expressed in "thousandths of an inch." The most widely used English threads are 50 TPI, 40, 36, 32, 30, and 25 TPI.
The male thread's outside diameter, commonly referred to as the "Major Diameter," is the specification for a thread. If you measure an outside (male) thread with a caliper and the result comes up as 57.85mm, it is a 58mm thread and for filters typically M.75. The specification would be "M-58 X 0.75". In some cases, the ".75" part may be referred to as "A fifty-eight millimeter thread."
To identify an inside thread (such as a filter ring on a lens), first guess whether it is a metric thread based on its age and origin, then measure the diameter of the female (inside) thread, also known as the "Minor Diameter." Next, add the pitch number to the measurement. For instance, if the inside measures 57.2mm, add the M.75 to get 57.95, meaning "58mm."
In the Imperial system, "thousandths of an inch" are used. For example, the mounting thread of a #4 Ilex shutter is specified by "2.495-30" (probably intended to read "Two and a Half by Thirty"). To determine the inside diameter, measure the inside diameter, which in this case is 2.465,” and add the corresponding metric pitch number (in this case, .9mm or 0.035"). Therefore, the inside of the flange measures 2.465," add .035 (for the Imperial approximation of the thread pitch) to get 2.5-30 for the specification.
To determine the thread pitch of a screw, there are two methods: using gauges or comparing with known screws as references, regardless of their varying diameters. By aligning the known screw with the one to be measured, a slight mismatch can indicate the use of a wrong measurement system, such as mixing Imperial and Metric. It is not uncommon to stumble upon older European made items that follow Imperial specification for the American market.
Although Imperial and Metric threads are not interchangeable, the following correspondences can be helpful for identification: M-.5 : 50tpi, M-.75 : 32tpi, M-.9 : 30tpi, M-1.0 : 25tpi. These may not provide the closest matches, but they represent the most commonly used Imperial threads.
In the world of photography equipment, it is rare not to encounter a thread outside the mentioned eleven standards. Therefore, the aim of thread measurement is to determine which of these standards apply to the screw at hand.
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