When doing extreme macro photography, one of the most frequently asked question is: What Is My Magnification Level? This question arises when there is no marking on a lens or in many situations that it is just very difficult to determine magnification. For example, a DIY'ed setup, a reversed zoom lens, a lens from garage sale, etc. I have been asked with this questions so many times that I think it is time to write this blog.
The answer to this question is actually very simple -- taking a picture of known measurement and compare it against sensor size of your camera. Hmmm, sounds good, but how? Well, the best way is to get a ruler with clear millimeter markings on it, then take a picture of it with the lens of question. After taking the picture, you can determine how many millimeter marks in the image. Then divide your sensor size by number of millimeters captured.
Here is an example. The image below is shot with Zhongyi 20mm super macro lens with extension tubes. The subject is a ruler with millimeter markings and the camera is a Canon 550D. It is clear that number of markings is about 3, meaning the camera captured 3 millimeters of the subject. This 3mm fill up complete width of the sensor, which in this case is 22.3mm wide. So, the magnification of the lens is 22.3 / 3 = 7.43.
To conclude, there are two factors in this approach: a known measurement, such as a ruler with mm markings on it, and sensor size. It is easy to find a ruler, but how to find the sensor size? A quick google will get you the answer. Here are sensor sizes for some of the popular cameras.
Full Frame : regardless make, the width of sensor is 36mm
Canon APS : most Canon crop factor cameras have width of 22.3mm, this include popular models such as Rebel series (T1i, T2i, T3i, or 500D, 550D, 600D, etc), intermediate level cameras like 50D, 60D, 70D, 80D, etc.
Nikon APS : most of Nikon crop factor cameras has sensor width of 23.5mm.
Olympus and MFT cameras: the imaging width of sensor is 17.3mm. Note these cameras have sensor size of 18x13.5mm, but actual imaging size is 17.3x13mm, so actual width of sensor is 17.3
There are other cameras not listed here, but Google is your best friend to find out.