The easiest way to document field-of-view of a macro lens, is to turn off Auto-Focus, then MANUALLY turn lens focus to longest length (closest focus). This is called Minimum Focusing Distance (MFD). Any true macro lens should be able to capture 1:1 (life-size) at MFD.
This is also the perfect time to physically measure your Minimum Working Distance (MWD) between lens front element to subject.
Set a metric ruler on a level, flat surface, then shooting straight down, focus on ruler by physically moving camera/lens combo up-&-down, and take photo at critical focus.
Compare image of ruler to your know sensor width. This math formula will determine your magnification ratio: width of sensor in millimeters divided by width of view in millimeters. See image #1.
Repeat process with Extension Tubes. See image #2.
Another method to document field-of-view is to photograph common coins. A U.S. dime is 17-mm diameter. A U.S. quarter is 24-mm diameter. A Nikon APS-C sensor is 23.6-mm x 15.8-mm (other manufacturers will vary slightly). A true 1:1 (life-size) macro photo of a dime laid atop a quarter, taken with a Nikon APS-C sensor will clip about 1.2-mm from dime, top-to-bottom, and just capture the quarter side-to-side. See image #3.
True macro photos taken with smaller sensors will see LESS of dime and/or quarter, and larger sensors will see more, but macro captured images of quarter and dime are exactly the same size, no matter the size of sensor (or film).
105-mm MWD field-of-view on DX sensor = 1:1 magnification (life-size)
105-mm MWD field with 68-mm tubes on DX sensor = 2:1 magnification
1:1 magnification (life-size) macro photo of Dime & Quarter on a Nikon (Sony) APS-C sensor. Captured field on a Canon APS-C sensor will be just slightly more narrow.
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