Re: Stacking Extenders & Light Transmission ...
Bigger wrote:
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.
I should clarify --- I wasn't referring to a loss of 50% light loss by adding teleconverters, I was referring to a 50% loss in the whole optical path (some lens reviews use T-stop vs f-stop). This is useful when comparing with telescopes dedicated for lunar or planetary use.
https://www.dpreview.com/forums/thread/4608842
If the point of this discussion is to see what Moon photos can be gotten with telephotos and equipment already owned --- that's a very fun thing to do and I've done it too --- even bought cheap vintage lenses for $20 and $30 in the hopes I could get good Moon shots, made my own teleconverters, tried a wide variety of teleconverters to see which performed the best, etc. If I happen to be out shooting in the evening and see cool Moon or a conjunction, I'll still shoot it with my telephoto and teleconverter.
I don't mean to spoil the party, but if the end goal is high quality lunar or planetary photography, I eventually realized that stacking teleconverters is not an economical way to do this and get the best results, for the reasons I mentioned above. One very high quality teleconverter does generally produce very good results with a high quality telephotos, but by the second I think you are at the point of diminishing returns already. Even if you still get some resolution benefit from another stack, you're still lagging further behind a good telescope's performance of similar aperture.
My earlier post in this discussion above with a very portable Meade ETX-90 ($100 used or ~ $500 new) on a night with average 'seeing' did about as well as some $2000 - $3000 combinations posted here. For that kind of spend, $1000 - $2000 will get you a reflector or refractor of 6" or more aperture which will put any Moon photo posted here to shame.
