(meteorobs) Meteor photography and fisheye lens advice please

[Chris Peterson]

Hi Tom-

There's no answer to the question of optimal focal length. It simply comes 
down to your imaging intent: how much of the sky do you want to cover? The 
focal length will determine the FOV. Once you've settled on this, increasing 
aperture increases sensitivity (or more indirectly, you'll want the fastest 
lens you can afford or which you can rent). Many fisheye lenses introduce 
distortion away from the center that can be severe for point sources (or, in 
the case of meteors, line sources). The usual fix is to stop the lens down a 
little, but that reduces sensitivity. It's a balancing act.

The ISO setting on DSLRs is largely bogus; the actual S/N is most images is 
dependent on the number of photons you collect, and it doesn't matter how 
much gain is applied. Most DSLRs are set up so that their best performance 
is around ISO 400 (which might be considered unity gain); less than that you 
are actually losing a bit of signal, more than that the signal and noise 
increase together, so the gain is empty. If you have a chance to experiment, 
try some sky shots at ISO 200, 400, and 800. Then compare the images for the 
best S/N. Most likely for meteors you'll either be at ISO 400 or ISO 800.

For exposure time, the longer the exposure, the lower your sensitivity 
(sounds backwards, but for meteors it is true, because the sky background 
washes out the meteor trail). So you want to make your exposures as short as 
possible, keeping in mind that you lose some exposure time between shots, 
and if too short you risk truncating meteor events. I'd consider 10-15 
second shots, taken continuously.

If it's cold, the battery life will be annoyingly short, so get an external 
power adapter if you plan on shooting over many hours. Make sure you shoot 
RAWs, of course.

Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com


----- Original Message ----- 
From: "Thomas Ashcraft" <ashcraft at heliotown.com>
To: "Global Meteor Observing Forum" <meteorobs at meteorobs.org>
Sent: Friday, October 30, 2009 3:13 PM
Subject: Re: (meteorobs) Meteor photography and fisheye lens advice please


> Leo S wrote:
>> What specific info are you looking for regarding fish-eye lenses?
>>
>>
> Hi Leo and all,
>
> I recently learned that my local camera store rents out fisheye lenses
> by the night and I am thinking of renting a lens for the Leonids to fit
> onto a Canon T1i dSLR camera. . But I don't know what types of fisheye
> lenses would be best suited for meteors.
>
> Any suggestions are welcome. For example, what mm lens is optimum?
>
> Thank you in advance again.
>
> Thomas