ISO ,Lens and filter help, please

[Piers127]

Hi,

I'm a little puzzled. I have a Canon 20d and want to photo things like M42 etc. Someone once told me to go for a 50mm focal lenght lens at the highest ISO to get good images. This would be approx 35mm (due to the upscaling from the CMOS sensor), which I can do. I don't, however, like the noise at 1600ISO. Imaging planets and nebulae I'd like as much detail as possible, so I've tried at 100ISO but with limited success - even then the stars are too bright!

- Would a filter help? If so, which one?

- I'm new to imaging, would a longer focal length give better pictures albeit at shorter exposures?

- Is ISO1600 the best speed to shoot at?

Please help, any ideas gratefully received.

[dph1nm]

Re: ISO

Firstly, note that changing ISO does not change the sensitivity of the camera to light - this is a fixed property of the sensor.

Secondly, don't be fooled by the appearance of the noise, what matters is the signal-to-noise ratio, and this is usually slightly better at high ISO due to the lower read noise (certainly on Canons). Hence the general advice to use high ISO for faint galaxies/nebulae etc.

However, you do lose dynamic range at high ISO which means stars become saturated and loose some of their colour. This is also a problem for M42, as it is so bright in the center. Often people take both short and long exposures and combine them in software to get round this problem.

If you are imaging with the camera lens why don't you just stop down the aperture to avoid things being too bright? Or just use a shorter exposure and then stack lots of them?

NigelM

[Piers127]

Thanks, Nigel. I'll try your tips if we get clear skies tonight.

Cheers!