Originally posted by ezechiel Does this mean that you loose less light than using 3 tubes?
In this case, it would be more interesting.
But what about DOF then? I'm not an optical expert and don't have the courage now
to calculate it
But what solution would be more interesting then?
3 extension tubes or lens+reversed lens?
Light loss is tricky to calculate. At the same magnification, you'd probably lose a bit more light with stacked lenses than with tubes. The basic rule is, your loss in EV or f-stops is M+1. At 1:1, you lose 2 stops. But closing the aperture of a stacked lens changes that, complexly. It's best to take test exposure readings.
The DOF would be razor-thin with either multiple tubes or stacked lenses. You could apply the same M+1 as the Effective Aperture difference. So if you're at f/16 and use 1:1 magnification, you're effectively at f/32. But I'm not sure if that applies to DOF as well as EV.
Extension tubes are cheap and straightforward. To get 1:1 magnification with a 50mm lens, add 50mm of tubes. To get 2:1, add 100mm of tubes. For 3:1, use 150mm of tubes. But that's an awful lot of tubes.
Stacking lenses is also cheap and simple. Use a thread-reversal ring, or just use gaffer's tape to hold the lenses together, front-to-front. The lens on the camera is the PRIMARY, and its aperture must be wide open or your image will be a little circle. The lens stacked on it is the SECONDARY, and its aperture controls the exposure and DOF. That's why a lens designed for stacking might contain a threaded socket to stop-down its iris with an external cable.
Stacking magnification is easy to calculate. It's the ratio PRIMARY : SECONDARY. So with a 105mm primary and a 35mm secondary, magnification is 105:35 or 3:1. Stack a 24mm secondary onto a 135 primary and magnification is over 5.6:1. And you lose about 8 f-stops. And at great magnification, the working distance is VERY VERY CLOSE. Use a sturdy, steady tripod.