aram535 wrote in post #8539069
This was my thought, that yes there is a loss, but not in any way shape or form is going to effect me. So is it really a loss of light? I'm still getting what I want at the DOF that I want without having to sacrifice shutter speed or higher ISO.
Ok, so maybe it's my point of view, that enlarging doesn't mean loss of light. The amount of light isn't falling off, you're getting less light because you're looking at small amount of space. No?
Yes. It is a loss of light. Precisely how much loss is a function of the amount of additional magnification afforded by the use of the tube.
A really simple way to think about it is to imagine a slide projector casting an image on the wall. If you move the projector away from the wall, the image will increase in magnification. But the total light output remains the same (you have only changed the position of the machine). Therefore, the light intensity per unit area must be lower, which means the image appears dimmer as it grows in size, and brighter as it shrinks (if you were to bring the projector closer to the wall).
The relationship between the distance of the projector to the wall and the light intensity, however, is an inverse square one. That is to say, if you move the projector twice as far away, the light intensity has decreased by a factor of 4.
This is why even a seemingly minor increase in the separation of lens and body (via an extension tube) can lead to significantly less light gathering capability, because the effect is squared. With a small tube, it may be less than a full stop, but since the increase in magnification is directly linked to the amount of light loss, it is unavoidable.
You are correct that the total light coming in has not changed, but since the only light that is captured is that which falls upon the sensor--everything else is either absorbed by the mirror box or the interior of the extension tube--the fact remains that at the sensor, the light intensity per unit area has decreased.