For macro nuts, extension tubes and magnification.

[Dave in Wales]

How do Kenko tubes 12, 20, and 36 wide, relate to magnification on DSLR's?

Back in the old days when I was doing proper photography, film that is, a
50mm lens on 50mm extension gave 1:1

How does this relate to Nikon with it's 1.5 crop?

What tube width, with what focal lenght, gives what magnification?

Is there some sort of formula, in the good old days it was....

Focal length/extension = magnification.

Dave

 

[Hairyduck]

There's a claculator here: http://www.mystd.de/album/calculator/

When you enter the focal length you will need to multiply it by the crop factor, it also shows all the equations used

 

[Canon Bob]

The crop factor doesn't come into the equation. When magnification is specified, it's just that....magnification.

At 1x then the image is lifesize on the sensor....larger sensors will display more of the image. A 1cm object will be 1cm on your sensor whether it's
full frame, cropped or even medium format.

Bob

 

[jomike]

The formula still stays the same, 1:1 is still 1:1 except that on full frame (35mm film) an object 36mm X 24mm would fill the frame, now with the 1.5 crop an object 24mm X 16mm will fill the frame.

That formula works for simple lenses, but once you put tubes on a macro lens that can focus to 1:1 on its own it gets more complicated, because the lens' focal length changes at these distances. Easiest is to take a photo of a ruler and compare it to your sensor size.