Re: C is for Christmas

[Robert Erickson <cirkut8@xxxxxxxxx>]

Merry Christmas to everyone on the list. For some
holiday cheer please visit:

http://christmas.omni-webhosting.com

Bob Erickson

--- ADavidhazy <ANDPPH@xxxxxxxxxxxxxxxxxx> wrote:
> > Did you notice that every panoramist has problem
> with there camera.  
> > ... Everybody as a story about banding.  What
> causes banding? "The slit"
> > somebody told me. The slit? OK. But i'm sure there
> is more than that. Why 
> > does the slit causes banding? Is the slit
> manufacturer the common cause of 
> > all panoramist headaches? Could the film
> manufacturers one of the cause of 
> > banding? ...
> 
> Well, in "Cirkut"-type cameras, banding that
> manifests itself as "lines" of
> density extending perpendicular to the direction of
> motion of the film are due
> to changes in the speed at which film moves past the
> exposure slit. Thise that
> are oriented in the same direction as the film moves
> are due to uneven slit
> edges.
>                                                
> Uneven speed of the film past the slit also may
> cause unsharpness or blurring
> of the image recorded on the film. To minimize such
> blurring it is advisable to
> make the exposure time, or the time over which the
> image can move with respect
> to the film, short. This calls for narrow slit
> widths.
> 
> To minimize banding one can use large slit widths
> thus minimizing the effect of
> a discrepancy in film speed as it moves from one
> side of the slit to the other
> and consequently on the total exposure. Another way
> to do this is similar in
> concept and that is to make the edges of the slit
> fuzzy by locating the slit a
> significant distance from the film surface. 
> 
> As you can see, there is an direct relationship
> between lack of banding and
> evennes of exposure but an inverse relationship with
> image sharpness. It is
> impossible to make the film move exactly at a given
> speed. Slight fluctuations
> in speed are always present. In the audio industry
> associated with transporting
> tape past a recording head they talk about flutter,
> wow, etc. ... these are
> factors that the moving-film type panoramic cameras
> also exhibit but in visual
> form. To make really sharp panormaic pictures you
> must use a really narrow slit
> witdth. If you do so, however, the likelihood of
> picking up banding increases.
> So, what to do? - well, compromise! 
> 
> In the case of rotating lens cameras the variations
> in density paralell to the
> sweeping slit are caused by uneven sweep rate caused
> by various factors. With
> these cameras there is no image blur introduced by
> uneven sweeping (assuming
> correct camera construction) as the image does not
> (is not supposed to!) move
> while the lens rotates on its rear nodal point. A
> density band perpendicular to
> the slit orientation is caused by uneven slit width.
> 
> 
> I thought that going into the digital realm the
> problems of banding would be
> gone forever. Not so. Banding-type artifacts seem to
> be present there too. My
> cheap, hand-scanner based panoramic camera project
> is coming along nicely. I
> will be making a presentation at the next SPIE
> (Optical Engineering Society)
> meeting in San Jose in late January. I am looking
> for a hand-scanner with a
> direct USB connetion to a computer. Know of one?
> 
> regards,
> 
> Andrew  o o  0 0 o . o  Davidhazy, Imaging and Photo
> Tech
>          \/\/\/\/\/\/         
> http://www.rit.edu/~andpph
> __________|       
> |_____________________________________
> 
>  
> 


=====
Robert Erickson, cirkut8@xxxxxxxxx
http://members.tripod.com/cirkut8/links.html
http://www.panoramic.net


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