Re: Picking a Mark IV Lens

[hjohnson@pluto.njcc.com (Herbert R Johnson)]

On Thu, 15 Jan 1998 20:23:11 -0800, Chris Albertson <chrisa@wavenet.com> wrote:
*>Herbert R Johnson wrote:
*>
*>> able to tell you right off.) Finer measures would require some effort
*>> to build a camera that would expose film across the entire width of
*>> the image field (I believe a standard 35 mm camera would not be wide
*>> enough.)
*>
*>Herb, you read my last post before I posted it.  I was about to
*>suggest using a 35mm camera and transfer to Kodak Photo CD.
*>
*>I think the diagonal of a 35mm frame (36 x 24 mm) is about 1mm
*>_larger_
*>then the CCD.  I think the chip is 30mm square.  So if you plot
*>the density along the diagonal of a 35mm frame you will get the light
*>fall off v. radius data you need.

[I think Chris meant 40mm square. - H]

*>If you did want to measure the whole image circle you could use a
*>view camera.  This would be easyer then making one.  An old "thrasher"
*>speed graphic would be perfect.  They sell for $200 and have a 4x5
*>inch focal plane shutter.  I'd just go with the 35mm camera.

Thanks for acknowledging my suggestions. The diagonal of a 35mm negative
is about 42 mm by measurement. I just did a massive search of my local
"database" on the Mark IV - surprisingly, I could not find an authoritaitve,
DIRECT mention of the size of the Ford/Loral CCD, but remarks suggest it
is about 44mm on a side. (All our discusions are either at the pixel
level or in fields of view). IF THAT IS TRUE (?) then Chris is right, but
barely so. (Tom, what's the number?)

So at best I'd recommend several 35mm shots with
the lens rotated, maybe with a wire or other indication across the field
appropriately explicitly showing the rotation of the lens relative to the
film. It's not a good test - but it is convenient. Of course, it's up
to Tom to make whatever setups he can, but if he can borrow or rent
some larger format camera as Chris suggests it would *really* tell the
tale with only a few shots.

As for the analysis: The human eye is pretty good at seeing
distortion - a good photo and a jeweler's loupe would give a hint. Tom could
make a simple transparent "ruler" based on pixel size to see if such
distortions are of multi-pixel length. See, no computers required (!).
Shots of grids, starfields, blue sky, etc. seem appropriate.

*>I do like your daylight projection idea.   It is quick and simpler.
*>Get a light meter and
*>cover the sensor with a pin hole.  Take reading along the radius
*>line.  My Minolta meter reads to 1/10 f-stop.  This is not so good as
*>a 12 bit scanner but would catch a problem.

I meant the above as a five-minute test, not quantative, but Tom is
pretty clever and could do something like the above if he liked.
I think Tom has a scanner by now. Any scanner would see a light dropoff
and reveal vignetting from a sky photo. Chris suggested a high-quality
scanner: it would resolve *far more* than the Mark IV could see. Remember,
2000 pixels across 40mm or more is pretty coarse, about 600 dpi.
In any case, as I said, a *visual* review of a photo would uncover
field distortions. Chromatic problems would require more thoughtful testing,
maybe photos with filters?

Tom, I hope all this kibbitzing is helpful.

Herb

  **** ------------------------------------------------------ ****

Herbert R. Johnson                      voice/FAX 609-771-1503 day/nite
hjohnson@pluto.njcc.com                 Ewing, in central New Jersey, USA

                 amateur astronomer and astro-tour guide
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