Foremost, here is some common theoretical knowledge.
Metric threads are described by capital M (stands for 'Metric') followed by major diameter (in millimeters), 'x' sign and thread pitch also called a thread step sometimes (in millimeters). So, if you see M42x0.75, you know this is metric thread with 42mm major diameter and 0.75mm pitch.
To measure a thread correctly, you should understand few basic things.
- All metric threads (external and internal) are designated by major diameter.
- Threads with the same diameter may have different pitches. They are not compatible then.
- Pitch is distance between adjacent thread tops.
Therefore, if you need to determine external thread, you should just measure external diameter (D) and pitch (P).
If you need to determine internal thread, you should measure internal diameter (d), pitch (P), and add pitch to internal diameter - this will give you approximate external diameter (D). d + P = D.
The best tool to measure diameters is a caliper. Rulers are not precise enough.
Measuring pitch is more tricky. The best way is to use special thread gauges. They provide profile pattern for most common thread pitches (metric and Imperial). If you don't have a thread gauge, you can press a piece of paper to the thread and measure the distance between imprints. You can achieve higher preciseness if you measure the distance between first and last thread imprint, and divide it by the number of spaces.
For example, you got imprints of 5 thread tops on paper. Distance between tops 1 and 5 is 3 mm. You should divide this number by 4 (quantity of spaces) - this will give you 0.75mm thread pitch.
Let's proceed to measuring with tools:
Necessary tools are digital caliper and thread gauge. Total cost is about $15, so our advice for anyone stepping into adapting lenses/filters is to buy these useful things.
Turn your digital caliper ON either by pressing ON/OFF button, or by sliding its jaws.
Slide jaws till they contact each other and make sure that caliper shows 0.00mm value. If value is different, please make sure that there is no dirt on the jaws. If the jaws are clean, slide the jaws together and press 'Zero' button on the caliper. You are ready for measurements now.
External thread is most simple to measure. Place the caliper over the thread as shown on the photo below:
Correct positioning of the caliper jaws is the key for valid results. Below are two samples of incorrect placement to let you avoid these common mistakes.
Once you measure thread diameter, you should use the thread gauge to determine the thread pitch. This is just a guess game. Most common pitches in optics are in 0.5 - 1.5mm range, so you can start with 0.75mm or 1mm plate and check others if initial guess was wrong. Correct plate teeth should match examined thread teeth perfectly.
Now you know that this is a thread with 52mm major diameter and 0.75mm pitch. Correct name for such thread is M52x0.75. Unfortunately, most manufacturers specify only a thread diameter as you can see. This incomplete specification may lead to purchase of incompatible accessories, so always pay attention to both diameter and pitch.
Measuring internal thread is basically the same, though a bit less convenient.
Lens manufacturers often specify diameter sign with numeric value, as you can see on this lens - ⌀52.
From technical point of view this is completely wrong since such mark means smooth 52mm diameter, not a thread. But we have to live with this.
Summary: each thread should be designated by major diameter and pitch. Please provide us these parameters if you need a custom adapter. Thank you.
