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1. ## Which extension tube?

So, I've been thinking about getting an extension tube for my Bronica ETR, and I was just curious which one people find most useful? Mainly I'm thinking I would use it on the 150mm for portraits to have a little wiggle room to get a bit closer to the subject, but possibly some light macro work too. I already have a 1:1 macro setup in 35mm, so I don't plan on doing pictures of insects or anything.

Of the 14, 28, and 42 it seems like the 28 might be the sweet spot, but i'm not entirely sure how much closer it would allow me to get, or what kind of exposure compensation to expect with any of them.

Thanks.

2. What is the minimum focusing distance of the lens itself?
That will allow a guesstimate of how much extension the lens brings along, i.e. how long/short a tube needs to be to provide a seemless transition.

Or better still, assuming the lens is a unit-focusing one, measure the difference in the length of the lens, front to back, when set to infinity and set to its close focus limit.
That will be the amount of extension it brings along, and will tell you what tube will be most convenient.

You can calculate almost anything given a few key data, like focal length and amount of extension.
Those two alone, plus frame size, will tell you what the field of view will be, and also how much exposure compensation is needed (assuming a symmetric lens - the result will not be quite correct, since the lens is not that. But close. We need to know the position of the exit pupil, or the sizes of the exit and entrance pupils, to get an exact figure).

The actual focusing distance is a bit harder, since it is measured from film plane to subject, and includes another unknown entity, the internodal distance.

The field of view is : Format size / (Extension / Focal length)
The exposure compensation in stops is: log(((Extension / Focal length) + 1)^2 ) ) / log(2)

"Extension" in these calculations is the extra extension you add by using the focusing mount or/and by adding tubes.
So when you know how much the lens racks out when set to its close focus limit, using only the length of the tube first, and the combined length of the tube and lens extension second, you find the range the tube provides.

The single steps you go through in calculating exposure compensation are written up here.
There is also a "pocket calculator routine", that presses the buttons in order for you after you have entered the extension and focal length (and pressed "=").

P.S.

I agether the minimum focusing distance is 1.5 m, which would mean the maximum built in extension is about 19 mm.
That would make the 14 mm tube the best to get, if you don't like to have a gap between lens-alone and lens+tube.

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