FreeLists / opendtv / [opendtv] Re: Red camera lenses

[Mark Schubin <tvmark@xxxxxxxxxxxxx>]

Tom Barry wrote:
> I have only been skimming the recent discussion of lenses since I know 
> very little about them.  But I'm somewhat enamored by the rumors of 
> the new RED 4K camera and today was on their forum reading the RED FAQ.
> I happened to notice it stated:
>
> "Since Mysterium™ captures all three color wavelengths at the same 
> depth [unlike a 3-CCD system], standard lenses designed for film 
> cameras [both cinema and still photography] can be used. More on this 
> in the lens section below."
> Dunno if this factors much into the lens discussion or not, but it 
> seemed it might, and the darned camera sounds quite nifty.
Here are the two ways it factors into the lens discussion.  Most 
professional video cameras use a prism for color separation.  That adds 
a back focal length of around 100 mm (it varies with imager-size 
format).  Movie (and still) cameras tend to have much shorter back focal 
lengths.  That prevents those lenses from being used directly on video 
cameras. 

Without changing the color-separation system, one can use some form of 
relay optics.  Some (e.g., P+S Technik, Red Rock Micro) create a real 
image from the film lens on a ground-glass screen and shoot the screen 
for the video camera.  They tend to rotate the ground glass so it 
doesn't create a fixed pattern in the image.  Others (e.g., JVC, 
Century) relay a virtual image from the film lens to the video camera.
It is also possible to change the color-separation system.  There have 
been three techniques for this: rotating color filters (used in the old 
CBS-color system, the Colorgraphics system, and the Apollo mission 
camera), stacked sensors allowing higher-energy colors to pass through 
the layers intended for lower-energy colors (the Foveon sensor used in 
Sigma cameras), and on-chip color filtering -- stripes (Abtography, the 
Panavision Genesis, old inexpensive color video cameras) or on a pixel 
basis (Bayer, diagonal, or the lower-ratio systems used in some of 
Sony's Clear-Vid systems).  The color filters are most popular: stripes 
in the Panavision Genesis and Bayer in the ARRI D20, the Dalsa Evolution 
and Origin, the EasyLook Modula HD, the NAC Memrecam fx K4, the Red One, 
the Silicon Optics SI-2K, SI-2K Mini, and SI-1920, the Vision Research 
Phantom HD and Phantom 65, the Weinberger Cine Speedcam, the Weisscam 
HS-1, and many smaller-format cameras as well as cameras not intended 
for TV use.
The other issue relates to longitudinal chromatic aberration.  In a 
prism-based system, it is possible to mount the three color sensors at 
slightly different back-focal distances; that is not possible in 
single-sensor systems, though it is at least theoretically possible to 
change the refractive characteristics of the color filters to 
compensate.  The adjustment for longitudinal chromatic aberration was 
much more common in the old days of tube cameras with wide variations 
between lenses.
> The whole RED FAQ on their forum can be seen at:
>
> <http://www.reduser.net/forum/showthread.php?t=1487>
>
> I think I'll have to learn more about Bayer patterns.
They're fairly simple.  In each square of four individual-pixel sensors, 
the upper left is green, the upper right is blue, the lower left is red, 
and the lower-right is green.  The order can vary, but all adjacent 
four-sensor squares must be the same.  Thus, there will be a 
green-blue-green-blue row and a red-green-red-green row.  When the 
adjacent squares swap blue and red, the result is a diagonal filter instead.
> Anybody know what the effective Nyquist sampling limit is on a Bayer 
> pattern.
It depends what you're measuring.  If you're looking at alternating 
white-black patterns, the color filter may be largely ignored (luma 
variations are compensated in post-imager processing).  If you're 
looking at color patterns, it would also depend on the pattern.  If you 
assume an unfiltered optical system that exceeds the resolution of a 
Bayer-filtered imager, you could have, say, a blue horizontal or 
vertical line that would disappear in one position and reappear one 
pixel-shift later.
> I'm guessing you might divide the H & V resolutions by 1.414 since it 
> is effectively a 45 degree rotated grid.
No (unless you're talking about Sony's Clear-Vid sensors or the ones in 
Fuji digital still cameras).
> But I'm only starting to look into this.
Enjoy!  Many of the great minds in the industry also haven't agreed on 
the results.  ARRI, for example, posits a significant resolution loss 
due to the color filtering; Dalsa doesn't.
TTFN,
Mark



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