Using a macro bellows

[kwhunter]

I'm new to using a bellows for macro photography and I have a question on the setup:
the lens (I'm using a 50mm prime in this setup) should be focused at infinity or at the minimum focusing distance, or does not matter?
I took comparative shots using my EF 100mm macro@1:1 and the bellows fully retracted with the lens focused at infinity and the bellows setup magnification was less than the macro lens, which is odd because I believe it should be 1:1 or more.
My macro bellows does not have a scale (I'm using a Pentax bellows), how can I determine what magnification I get? It is the distance from the focal plane of the lens to the sensor, divided by the focal length of the lens?
Where is the focal plane then, at the iris plane?
Help would be much appreciated.

[LordV]

Bellows are similar to using extension tubes where the magnification = existing lens mag + length of bellow mm/focal length of lens mm.
You would need at least 100mm of bellows length to get back to 1:1 with a 100mm macro lens focused at infinity. I suspect I would probably use the lenses at minimum focus to get the max magnification (the focused on infinity applies to lenses reversed onto another lens).
Easiest way to measure the magnification you get is to take a picture of the mm scale of a ruler. Then mag= sensor width mm/mm across pic.

Brian V.

[kwhunter]

Thanks LordV.
I'm using, as I previously wrote, a 50mm prime lens; the bellows extends, I believe 120mm, so going with the theory magnification should be 120/50=2.4
I suspect it is more than that, but I will check shooting a ruler and comparing the distance between minor units.

[LordV]

Thanks LordV.
I'm using, as I previously wrote, a 50mm prime lens; the bellows extends, I believe 120mm, so going with the theory magnification should be 120/50=2.4
I suspect it is more than that, but I will check shooting a ruler and comparing the distance between minor units.

Yes you should get around 2.4:1. These type of calcs are normally fairly good for non- macro lenses but fall apart slightly when you are using macro lenses at near minimum focus as the focal length of macro lenses changes quite markedly at minimum focus.

It is always worth checking the mag use the ruler picture method

Brian V.