There are many cases where accurate measuring of a diameter or a thread is necessary. Most equipment today has been standardize, but older equipment and niche items require accurate measurements. This page provides some guiding information for measuring threads.
Metric and Imperial Threads
There are two systems in use today, The Imperial and Metric. Most foreign made lenses and shutters produced after 1945 use the Metric system of threads and measurement. A metric screw thread is specified by how far, in millimeters it advances in one turn of the screw. For example, if one turn of a filter brings it 1 mm closer to the lens then it is called “M1.0”. It is the distance from one peak of the thread to the next one. This number is referred to as “The Pitch”.
The most popular metric threads are: M.5, M.75, M.9, M1.0, M 1.25. The usual filter thread in medium size filters is M.75. Filters 40.5 in diameter and smaller are generally M.5.
Imperial threads are specified by how many peaks there are in one inch of the length. They are specified as “Threads per inch” written “TPI” The diameter is specified in “thousandths of an inch” The most popular English threads are: 50 TPI, 40, 36, 32, 30, and 25 TPI
Measuring Instructions
Threads are specified to the outside diameter of a male thread (The “Major Diameter”) So, if you measure an outside (male) thread with a caliper and it comes up “57.85mm” then it is a 58mm thread and, if its a filter, it’s almost certainly M.75. This is specified: “M-58 X 0.75”. In the case of filter threads, the “.75” part is often left off and it is called “A fifty eight millimeter thread”.
To identify an inside thread (such as a filter ring on a lens), first take your best guess from age and origin to determine if its a metric thread then measure the diameter of the female (inside) thread. Take (The “Minor Diameter”) then add the pitch number to the measurement. For example if the inside measures 57.2mm add the M .75 to get 57.95 which means “58mm”.
In the Imperial system thousandths of an inch are used: Such as the mounting thread of a #4 Ilex shutter : “2.495-30” is the way this is specified (and its probably intended to be “Two and a Half by Thirty”) The inside diameter is read by measuring the inside diameter, in this case 2.465″ and adding the corresponding metric pitch number _In this case .9mm or 0.035″ So 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.
The thread pitch can be determined by using gauges or by using known screws as gauges (even if they are different diameters). Hold the known screw against the one to be measured to see if it fits. A very slight mismatch means you are probably using the wrong measurement system. Metric instead of Imperial, or vice versa. It is a common surprise to find older items of European manufacture made to Imperial specification for the American Market.
Although Imperial and Metric threads are not interchangeable, for the purposes of identification they correspond M-.5 : 50tpi, M-.75 : 32tpi, M-.9 : 30tpi, M-1.0 : 25tpi. These are not necessarily the closest Imperial equivalents but the closest commonly used Imperial threads.
In the case of Photo, equipment it is most unlikely that you won’t find a thread other than the eleven mentioned here. So the goal of your thread measurement is to find out which of these you have on hand.
