Light loss using a TC

[sagebrush]

Will I still lose half a Stop when using a Kinko 1.5TC on a 24-105 f/4?

[casaaviocar]

Yes, and it's actually a full stop of light.
There is no magic bullet, you will end up with a 36-157 f/5.6, not really worth it IMO. You are much better off buying a longer lens, TC's were never really designed to be used with shorter lenses and are mostly used to increase the length of a telephoto lens.

[DrPablo]

Light intensity is lost exponentially as the distance increases from source to target. If you increase the physical length of your lens 1.4-fold, you will lose one stop (which is one doubling of light, or 1.4 squared). Look up the inverse square law for more detail.

[crn3371]

You'll loose one full stop. Plus, I'll second the other poster, why bother with a tcon on a short lens.

[DrPablo]

The one time it's been nice on a short lens is with the TS-E 24 f/3.5L.

The reason is that first of all the lens is reasonably fast, secondly it's a specialized prime that doesn't have an equivalent at 35mm (with a 1.4x TC), and thirdly its image quality is pretty mediocre already and its results are much better stopped down anyway, so there's nothing sacred about the f/3.5.

[PacAce]

Light intensity is lost exponentially as the distance increases from source to target. If you increase the physical length of your lens 1.4-fold, you will lose one stop (which is one doubling of light, or 1.4 squared). Look up the inverse square law for more detail.

I'm don't think the inverse square law has anything to do with the 1 stop of light loss here although you are right about the loss being due to the increase in the focal length (not the physical length) of the lens. Remember, the inverse square law has to do with the intensity of light falling on a subject (ore reflected from one) at a given distance from the light source (or the observer of the subject). Using a TC, in itself, does not change the distance of the subject, the light source or the observer.

The max f-stop number of a lens is determined by dividing the lens focal length by the max diameter. So, if the original f-stop number is 4, then adding a TC which has a multiplication factor of 1.4 will result in the the focal length increase by a factor of 1.4 and, hence, resulting in a new f-stop number of 5.6. And that is why there is a 1 stop loss of light. :)

Another way to look at it is that by increasing the focal length of the lens with the TC, the relative area of the aperture opening has been decreased by half. And decreasing the area by half also decreases the light going through it by half. Halving the light results in a one stop loss of light.

[DrPablo]

The f/stop change with increased focal length makes sense now that I think about it; I realize that the max aperture doesn't change. I have a convertible large format lens that is 300-f/5.6 convertible to 500-f/12 by removing the front lens cell, so it's probably the same FL/aperture diameter equation.

I was presuming (and maybe there is some truth in this) that the TC causes a similar phenomenon to a bellows factor. You'll lose light with macro extension tubes, for instance, and they don't alter focal length (or even contain any lens elements).

[PacAce]

The f/stop change with increased focal length makes sense now that I think about it; I realize that the max aperture doesn't change. I have a convertible large format lens that is 300-f/5.6 convertible to 500-f/12 by removing the front lens cell, so it's probably the same FL/aperture diameter equation.

I was presuming (and maybe there is some truth in this) that the TC causes a similar phenomenon to a bellows factor. You'll lose light with macro extension tubes, for instance, and they don't alter focal length (or even contain any lens elements).

With a bellow or extension tube, you are moving the lens away from the "film" plane so the image circle projected by the lens gets bigger. The light intensity of that project image circle is still the same brightness as it was before, only bigger. However, since the size of the frame did not change, the frame is now looking at a smaller area of the image circle. And a smaller area translates into less light. The image on the frame is magnified but, at the same time, the intensity of the light of that image is decreased.