[Not a Number]

There's the bellows extension factor to consider. Basically exposure changes by the factor given by dividing the focal length by the extension. and squaring the result ( (FL ÷ extension)² ) At some point the won't be enough light to take practical images (high ISO detail loss) or the length of exposure too long for moving subjects.

[Michael Piziak]

You get 1:1 magnification on any lens when the lens-to-subject and the lens-to-sensor (or film) are both twice the infinity focal length (that which is normally listed for a lens). On a 100mm lens, the lens-to-sensor distance would be 200mm for 1:1. The macro lens itself will provide some of that (actually all of that if it's capable of 1:1 on its own), but if left at the infinity setting, 100mm already exists so a 100mm extension is needed. If you make use of the macro lens close focusing abilities, less than 100mm may be needed.

1:1 magnification is irrelevant of sensor size and simply means the image on the sensor is the same size as the subject. This happens when subject and sensor distances from the lens are equal.

On my Asahi Pentax m100mm macro f4, I generally shoot with the lens fully extended (opposite of infinity to get maximum macro shots) - I think it gets 1:2 macro at that. Fully extended, would 50mm of extension tubes give one 1:1 then ? Or does it not matter (is it still 100mm even then?)

[RICHARD L.]

You should play with your equipment and take notes and a few measurements. Make your own experiences and learn with your hands-on.

Regards

[Bob 256]

On my Asahi Pentax m100mm macro f4, I generally shoot with the lens fully extended (opposite of infinity to get maximum macro shots) - I think it gets 1:2 macro at that. Fully extended, would 50mm of extension tubes give one 1:1 then ? Or does it not matter (is it still 100mm even then?)

As I mentioned, if you use the close focus ability of your lens, the lens is moved away from the image plane so less extension is required than with the infinity focus. To get 1:2 magnification, the subject distance would be 300mm and the sensor distance 150mm, hence your lens would have moved 50mm further from the sensor (relative to the infinity position). To get to the 1:1 magnification, the subject will get closer (200mm from the lens) and the sensor distance should grow to 200mm, or you would need an additional 50mm provided by an extension tube (and the lens in the 1:2 magnification focus position). So, the answer to your question is "yes", 50mm of additional extension should get you to 1:1 magnification.

Note: the basic formulae here are 1/FL=1/s+1/i; and magnification = i/s (the latter formula not valid at s = infinity)
Where s is subject-to-lens distance and i is image-to-lens distance (FL is the infinity focal length of the lens)

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Last edited by Bob 256; 03-31-2021 at 09:58 PM.