# Thread: Extension tube question on fstop

1. ## Extension tube question on fstop

Let say a 50mm f2-f16 len when I add a 50mm extension tube to get 1:1, my actual aperture will be about equal to f4-f22? Is this correct? If not can anyone explain the aperture relationship when I add extension? Ofcause the above assumption may be wrong.

2. There should be some reduction in the effective aperture because the field of view is slightly reduced when you add an extension tube between the lens and the camera.

The actual reduction in the effective aperture is dependent on the length of the extension tube added, so it is not as simple as multiplying the f number by 2, as in using a 2X teleconverter. It should be less than that if the amount of extension added is considerably shorter than the focal length of the lens.

When using extension tubes I simply rely ont he camera metering to determine the exposure setting and use DOF preview instead of trying to calculate the DOF using some mathematical formula.

3. Originally Posted by FIR
Let say a 50mm f2-f16 len when I add a 50mm extension tube to get 1:1, my actual aperture will be about equal to f4-f22? Is this correct? If not can anyone explain the aperture relationship when I add extension? Ofcause the above assumption may be wrong.
The aperture does not change, but exposure meters assume that the lens is focused at infinity. If the lens gets extended significantly, you have to correct for this. In the example you gave, the light intensity at the image sensor drops by a factor of 4, or 2 "f-stops".

A note on apertures: it is not "f2" but "f:2" of "f/2", where "f" is the focal length of the lens. For f=50mm, the aperture f/2=25mm, independent whether you use extension tubes or not.

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