There might be a very good reason for no ISO25...... If it is so bright that you need it, you have a lot of light pouring into the camera.... and light is energy, and energy causes heating. What happens to your sensor on a long exposure? will the heating damage the sensor or will it just create a whole lot of thermal noise? Would you get a cleaner picture at higher ISO with a ND filter keeping the heat away from the sensor?
I don't know..... just asking....
You're not going to damage the sensor with any scenes even at EV 23, which is ISO 25 @ f/22 @ 1/4000, which you can also get with ISO 100 @ f/32 @ 1/8000 (which, depending on the lens, is the dimmest exposure you're going to get on a 5DIII).
very easily damage the sensor, at any
level of brightness, with or without an ND filter,
is prolonged direct exposure to the Sun. And the damage will be caused by the same method that causes blindness in humans who stare at the Sun. (Lasers can also fry a sensor as well as they can fry your eyes, but the damage is caused by a different mechanism.)
But there is a very good reason for no native ISOs below 100 (ish). The photosites have a maximum number of photons they can record to a single charge. Dump more than that many photons onto a photosite and it still records the same number for its full charge. The physics of it works in such a way that, if you shoot outside on a sunny day at 1/100s at f/16 with minimal / no analog amplification with a sensor that clips slightly above the brightest non-specular highlights in the scene, you'll have an optimal dynamic range for that imaging technology over most shooting conditions most people encounter.
Incidentally, no Canon camera actually shoots at ISO 50. When you set the camera to ISO 50, the meter changes, but the electronics still operates exactly the same way it does at ISO 100. If you shoot JPEGs, the onboard raw processor will divide every value coming off the sensor by two before converting it to a JPEG, thereby costing you a stop of exposure headroom. If you shoot RAW, the same RAW file gets created as if you shot at ISO 100 except that the metadata flag for ISO says "50" instead of "100." The RAW processor on your computer sees that, and it, too, divides every number in the RAW file by two before continuing with raw processing.
You might therefore wonder what the point is of ISO 50. The point is twofold. First, if you shoot JPEG, it's a wonderfully convenient in-camera way of doing ETTR when either your scene has no highlights or you don't care if they clip. And second, if you're planning on doing ETTR in the digital darkroom, you might as well shoot ISO 50 instead and get a much more useful preview image on the back of the camera, plus you'll save yourself a step in post-processing. Also, considering that most ETTR workflows I've encountered apply the exposure correction after
all the tone curves and color adjustments have been made to the RAW file whilst ISO 50 does that stuff before, you're much more likely to get better results with ISO 50 than with ETTR.
Unless, of course, you actually know what you're doing with ETTR, in which case you probably already know that there's no point to ETTR. I'll once again again note that, unlike in the early days of DSLRs, there's no noise in modern DSLRs at ISO 100 so there's nothing to be gained any more from an image quality perspective in doing ETTR or, similarly, shooting ISO 50.
P.S. Highlight Tone Priority is much like ISO 50 except in the opposite direction. It's baked-in ETTL, in other words. Considering the low noise levels of modern DSLRs, HTP is potentially much more useful in real-world shooting than ISO 50, especially in high dynamic range environments. b&