I have a 90mm 1:1 macro lens, which I use on a 2/3 frame mirrorless camera. I have a total extension tube length of 65mm. What magnification might I achieve?
Magnification achieved with an extension tube is determined by the total length of the tubes used. The longer the total length, the greater the magnification, though this also depends on the working distance and other factors. Also, with a big tube in place focal distance will be quite short which might affect how much light gets on to your subject, the aperture available and so depth of field, and the composition you might be able to achieve.
The new magnification is equal to the native lens magnification added to the ratio of extension tube length to lens focal length.
For a 1:1 lens, like the 90mm. the native lens magnifaction is 1.
The ratio of extension to lens is 65/90.
So, that’s 1 added to 0.72 which gives a new magnification for this combination of extension tube and lens of 1.72 times life-size.