R5 lunar photography with stacked EF extenders

Larry Rexley wrote:

Bigger wrote:

Yes, but what I was trying to clarify was that the same amount of light is hitting the front element, the tele-extender just spreads it out over more pixels. Whether you crop the image in post, or crop it in camera with a tele-extender, you still have the same amount of usable light, and the same amount of lost light--the only light that is lost is the light that is cropped out (ignoring the small light loss in the tele extender air/glass). So, you are correct that less light hits the sensor, but that doesn't matter because we will be using more of the sensor area in the final image.

For practical purposes, final image noise doesn't depend on how many pixels you break that fixed amount of light up into. With more pixels, each pixel is noisier, but since there are more of them, it averages out in the end.

The change in aperture ratio (f/number) is not really relevant when you are cropping the image, only the aperture area is, and that doesn't change. So yes, the f/number goes up with a tele-extender, and you need to increase ISO (keeping ss constant). That increases noise per pixel, but you have more pixels, so overall image noise is roughly the same.

The more glass (lens elements) the light has to go through, the more scattering occurs within the glass and at the boundaries. This reduces the sharpness of the image and reduces the contrast of the overall image, washing the image out and wiping out detail.

True, but spreading the image over more pixels increases the apparent sharpness of the sensor, which counteracts the loss in optical sharpness. Final IQ is a tradeoff between these opposing forces. The optimal configuration matches the lens resolution to the sensor resolution, and that could be none, one or two TC depending on the combo. So, an R6 could use an extra 1.4x TC over an R5, for example.

Once you get beyond 20 or so elements of glass in the total light path, there doesn't seem to be much benefit in the added magnification --- the image is too degraded.

That's an arbitrary rule of thumb. It depends on what type of elements (especially whether they are simple or compound) and coatings, as well as the base lens magnification and the added teleconverter magnification. So, more or less elements could be better, depending on the situation.

Also, stacking teleconverters may bring you pretty quickly to the diffraction limit of the configuration, and diffraction will also wash out detail and contrast.

The diffraction "limit" is not a hard limit at all. Yes, diffraction will also degrade the image similar to scattering above. But the point of diminishing returns in final IQ is often beyond the supposed diffraction limit. If you are cropping the image, then the limiting factor is the aperture area (i.e. front element diameter), not the aperture ratio.

Light transmission of lenses is also not 100%, for lenses and teleconverters it is reasonable to assume 10-20% of the light is reduced by the glass and the air-glass interfaces. This multiples, so stacking a couple teleconverters with a lens with a lot of elements/groups could reduce light transmission by as much as 50%.

I disagree with this framing. Yes, there is some light loss in the extra elements of the teleconverter(s), but they are fewer and thinner than the elements in the base lens, so this is a relatively small issue. I doubt that even stacked TC would reduce the light transmission by 50%, i.e. an extra f/stop.