Canon 300 /f 2.8 IS L

[smr]

Hi,

I have myself this lens and am wondering whether I should give it a go for Astrophotography - I could use at 300mm but realise I may have to stop it down to /f4 (?) with respect to chromatic abberation. 

I'm thinking if it's suitable I could buy the 1.4 Extender to make the Lens 420mm and f/4. A nice widefield focal length with a fast aperture.

A couple of things - it's an expensive lens, and therefore I'm a bit worried about dew on the front lens element - I'd use a dew heater band but is there any reason to worry about the front element when it comes in from the outside and the lens might fog up?

Is there a way of autofocusing this lens? I think I read a while ago someone mentioning about how, with the optics and design of the lens, it needs refocusing every 30 minutes. 

Any other advice would be appreciated.

Thanks

Joel

[Nik271]

I use a Sigma 70-200mm F2.8 for occasional astrophotography and I'm very happy with the results. I use mine wide open to save time unguided. I would say that if you stop down yes it will get sharer but then longer exposures need good guiding.

Teleconverters usually ruin the sharpness and introduce aberrations, you may be better off with a camera with smaller pixels for the same result.

Fogging up is not an issue for the coating, just leave it in a warm place for a few minutes to evaporate. Don't wipe it when its moist, because any bits of dust stuck on it will create mini scratches. Once dry you can clean it in the usual way blower, brush, lens cloth  (if it looks very dirty). The main issue to watch out for with dew is  not the coating but if you cap the lens with dew on it, the moisture  will gradually work its way into the insides. Otherwise L-lenses are build to last so a bit of dew during the night should be OK. Just dry it thoroughly before storing.

Refocusing is potentially needed with all fast lenses, because the tolerances are so fine. I check mine very hour or so - just look at the size of the stars. Most of the time it's fine unless there is a big temperature variation or I move to a different area of the sky.

Anyway you should definitely give yours a go! Must be quite a beast at 300mm and F2.8

Edited August 19, 2021 by Nik271

[Andrew_B]

Definitely give it a go because it's a superb lens that you've already got!

Turn off autofocus because it won't work except maybe on a bright subject like the Moon, and turn off IS as well. You don't need to worry about dew other than to try and prevent it forming while you're shooting; these L series pro telephotos are designed to be used in the pouring rain so a bit of dew won't harm it in the slightest.

A good teleconverter shouldn't significantly degrade the image quality but I'd hold off buying one until you work out whether you really need it.