(meteorobs) Fisheye lens misunderstanding and photography update

[Leo S]

Thanks for explaining that Chris, but surely if you use a longer focal 
length to essentially get the same crop as Thomas posted (2nd image down 
on the page he put up) as a "full frame", then no aliasing artifacts 
would be visible without once again cropping?

I agree, selective defocussing via a filter in post-processing seems 
like a much better idea.

Leo

Chris Peterson wrote:
> Somewhat counter-intuitively, the aliasing is dependent only on the pixel 
> size of the camera and the focal ratio of the lens. The field-of-view 
> (essentially, the focal length) doesn't matter at all. The physical image 
> size of a point source is the same for all [perfect] lenses of a given focal 
> ratio. For instance, any f/4 lens will produce an Airy disc size of 5.4 um, 
> which is about the same size as a typical DSLR pixel. So you'll usually see 
> aliasing, at least until the meteor becomes so bright that the tails of the 
> PSF are above the noise floor of neighboring pixels, and then you see the 
> characteristic wide flare of imaged fireballs.
>
> I wouldn't defocus the camera to reduce filtering. You can achieve the same 
> thing by the careful use of blurring filters during processing, and that 
> gives you the chance to experiment with different methods- an opportunity 
> that is lost if the raw data is already filtered by the optics.
>
> Chris
>
> *****************************************
> Chris L Peterson
> Cloudbait Observatory
> http://www.cloudbait.com
>
>
> ----- Original Message ----- 
> From: "Leo S" <l.stachowicz at btinternet.com>
> To: "Global Meteor Observing Forum" <meteorobs at meteorobs.org>
> Sent: Monday, December 07, 2009 12:54 PM
> Subject: Re: (meteorobs) Fisheye lens misunderstanding and photography 
> update
>
>
>   
>> Thanks Thomas,
>>
>> The "spiral" part threw me. I see what you mean now. I get this on all
>> my bodies as well.
>>
>> The best way to avoid it is to not crop so much. Not going too wide with
>> the lens is going to help with this obviously.
>>
>> I suppose that slightly defocussing the lens might help, acting just
>> like a "poor quality" lens or AA filter as Chris mentioned. I'm not
>> sure, but it might be worth trying? Chris?
>>
>> Leo
>>     
>
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