I remember a discussion about this in another group now. I had the
problem once with my C-Series camera, and I just switched to (.) spot
lighting and no longer had a problem. This was hardest when the moon wasn't
very large in the view finder.
Thanks to both of you for the educations.
Scott
----- Original Message -----
From: "Chuck Norcutt" <chucknorcutt@xxxxxxxxxxxxxxxx>
To: <olympus@xxxxxxxxxx>
Sent: Monday, March 19, 2007 3:48 AM
Subject: [OM] Re: 50/3.5 odd sighting
> Coma is an abberation that can afflict all lenses. It is worse at
> larger apertures and also gets worse the further away from the optical
> axis of the lens. It's not limited to astrophotograpy; it's just that
> star images can make it painfully evident that the optical system
> doesn't handle coma well. Instead of nice round dots the stars may look
> like mini comets and grow to larger comets the closer they are to the
> edge of the field of view.
>
> I will disagree a bit here with Moose when he says you needn't worry
> about it until you get a nice long lens. In fact, the nice long lens
> will not likely be a fast lens. So a 500mm f/8 will not likely exhibit
> a great deal of coma but a 50mm f/1.4 might have significant coma when
> wide open. But the coma should be significantly reduced by closing down
> a couple of stops.
>
> The typical 500mm f/8 mirror lenses (those with a steeply, inwardly
> curved front element) use what is called a Maksutov corrector lens. The
> primary mirror is typically a spherical mirror which would suffer badly
> from coma but the Maksutov corrector front element is specifically
> designed to correct coma. If your long lens is a catadioptric system
> (both lenses and mirrors) and employs either a Maksutov or Schmidt
> corrector as the front element the optical system should be essentially
> free of coma. Coma free Schmidt designs have been constructed down to
> at least f/2 and possibly faster.
>
> Chuck Norcutt
>
> Moose wrote:
>> Scott Peden wrote:
>>> I gotta ask, what's a coma?
>> http://en.wikipedia.org/wiki/Coma_%28optics%29
>>> I assume this is a term that has to do only with
>>> astro photography and won't interfere with me shooting the moon?
>>>
>> Coma is only an off-axis phenomenon, so anything in the central portion
>> of the frame is not affected. So until you get a really long lens that
>> can fill the frame with the moon, you needn't worry.
>>
>> It is also reduced by smaller apertures and the moon is bright enough
>> that you needn't be using your lens wide open. Remember to spot meter on
>> the moon itself or stop down 3-4 stops from an average reading.
>>
>> Not a problem for the Moon.
>>
>> Moose
>>
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