R5 lunar photography with stacked EF extenders

Marco Nero wrote:

Bigger wrote:

Marco Nero wrote:

PhotosFlight wrote:

A little help please. I have the 1.4 MII and 2x MIII. I'm missing something because it feals like I would be forcing it were I were to stack them.

The EF 1.4x II is not as sharp (optically) as and EF 1.4x III Extender because it's an older design that was produced for pre-digital lenses. Adding one to an EF 2x Mk III Extender would result in (A) softening of the image from the EF 1.4x III Extender and (B) a further reduction of contrast and loss of image resolving due to the longer focal length of the 2x Extender being coupled with it. We do get more actual magnification from stacking two extenders together but there's a cost. The trade-off is in the amount of light lost and the optical consequences of putting too many glass elements between the subject and the sensor.

Technically the light is not lost, it's just spread out over more pixels, or photo-buckets, if you will. So, the buckets still collect essentially the same number of photons, but each one has fewer photons in it. Each pixel is noisier, but when you take them together in scale, the overall image is not noisier.

Well, there's less light filling the light-wells of each photoreceptor, hence there's less light reaching the sensor (hence the reason I used the term "less light"). By using two extenders on say the EF 100-400mm f/4.5-5.6L II lens, you end up going from a lens that has a narrow aperture of f/5.6 at 400mm to getting an aperture of f/13 (the camera seems to round it down from f/15.68) - which is a significant loss of light hitting the sensor. This forces us to use slower shutter speeds and higher ISO settings, both of which contribute to noise, which results in less detail. If you just use this same lens with a 2x Extender you'll be starting with f/11.
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Some cameras have larger light wells than others. The EOS R6 (for example) has a larger pixel size than the R5, so it's slightly better at gathering light, hence the much wider extended ISO range from that model of camera. The difference between Full Frame and APS-C sensors is much the same in that Full Frame sensors tend to have larger light-wells.
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From the Canon White Paper on Full Frame sensors (which probably needs an update).

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But not all is lost. By shooting at slightly higher ISO settings, we can often capture details missed with a shorter exposure using lower ISO settings. Oddly enough, higher ISO settings can often capture subtle elements in an image missed otherwise.

The converter increases the focal length, and therefore the aperture ratio (f/number), but the overall aperture is the same. The diameter of the optical tube doesn't change, so the same flow of photons from the subject are captured.

Adding Extenders acts like a "choke" on the amount of light racing down the barrel and hitting the sensor. The extra elements reduce the intensity of the light so that the image being focused onto the sensor is slightly dimmer and less intense.

The trade-off is whether the extra glass hurts more than the extra pixels help.

There's a fine line there. It depends on the recipe behind the extra glass. Modern optical coatings can redirect light from the peripheral edges of the optical elements to keep it from creating Coma where the glass itself is thinner towards the edges. And modern coatings can increase light transmission to a small degree. Canon claims they used their finest optical-grade glass when they created the Mk III Extenders and they also claim no detail was spared with their Spectra coatings for ghosting and flare (which are simply a revised version of their 1980s coatings)... and Fluorine coatings to increase light transmission (compared to previous coatings) whilst repelling dust and oils and enabling the lens element on the Extenders to be cleaned without solvents or liquids.
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There are times when you may wish for more magnification to increase perceived or visible details. An example is the moon. You can's see any crater shadows or related details with the naked eye. Or with a 3x (77mm) zoom lens. But you sure can when longer focal lengths are used. The greater the magnification, the more your light transmission to the sensor drops. Add extenders and you cut down on the light even further. And you can calculate it quite easily using the magnification of the Extenders. A 400mm lens which offers f/5.6 x 1.4 = f/8 (7.84). The camera will round the aperture up or down slightly.

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.