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Old 16-10-2010, 02:21 PM
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Octane (Humayun)
IIS Member #671

Octane is offline
Join Date: Dec 2005
Location: Canberra
Posts: 11,158

The 85mm f/1.8 USM is an excellent portrait photography lens, however, it's not designed for astrophotography.

The longer focal length L-series lenses such as the 200mm f/2.8L II USM, or 300m f/2.8L IS USM, and even the 300mm f/4L USM are better suited to flat fields.

And, yes, you must stop down these lenses, unless you wish to use an external aperture mask -- the benefit of the external mask is that it prevents diffraction spikes caused by lens diaphragm.

Here's a field that I did with the 200mm f/2.8L II USM on my old 40D (now sold; same sensor size as your 550D):

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