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Re: [OM] (IMG) Deep Purple

Subject: Re: [OM] (IMG) Deep Purple
From: "Carlos J. Santisteban" <zuiko21@xxxxxxxxx>
Date: Wed, 10 Jun 2009 15:52:59 +0200
Hi Fernando, Chuck, Moose and all,
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
>Subject: Re: [OM] (IMG) Deep Purple
>I'm still surprised to see that much obvious diffraction around the
>blades.  I've never seen that before.

It's because of the extreme overexposure. Please note that diffraction on
_star_ images will be negligible with a focal as short as the 21mm --
planetary objects' exposure depends, as usual, on the _relative_ aperture pf
the lens (the F-number) whereas star objects are exposed according to the
_absolute_ aperture.

From: Fernando Gonzalez Gentile <fgnzalez@xxxxxxxxxxxxx>
>Fact is that the amount of diffraction you're seeing is a little less
>than in the Provia.

From: Moose <olymoose@xxxxxxxxx>
>Could this be an example of the halation, or whatever it is that Ken
>natters on about, in some Fuji films, spreading out the intense brightness?

I can't tell about Fuji's halation, but severe overexposure will spread out
in _any_ photographic material.

From: Fernando Gonzalez Gentile <fgnzalez@xxxxxxxxxxxxx>
>Just found that the scan is a little too dark, and darkens the longer
>and faintest strikes of light, those which cross the vertex of each
>triangle and fade progressively.

A similar effect is usually seen when correcting levels in PS.

>I could give a couple of possible
>causes: 1- the exposure was long (I may have override 1/3 to gather
>shadow detail, but don't remember) - ~10 sec. IIRC.

Indeed. With ASA 100 @ f/5.6, the universally accepted exposure for the moon
("moony-11") would be around  1/400... so about 10 seconds is 4000 times
more, roughly 12 stops -- BIG overexposure.

>2- I must confess, I'm
>not sure if I left the protective filter screwed.

Shouldn't be. Sometimes, a dirty/scrtched filter may render similar streaks
at bright points, but usually only an aligned pair, probably rotating
slightly depending on the situation of the bright spot...

>Ok, this does not increase diffraction but a larger image of the moon,
>with its borders lacking sharpness.

That's the expected behaviour, here combining the film's overexposure with
digital clipping...

>I didn't trust my eyes only while doing curves and anything else: I
>consulted my daughter, who has young cells and pigments in her retina -
>she consistently told me that it was not blue enough ... I trusted her.

Your original TOPE scan, besides less saturated, seems much more blue than
the later -- esp. the latest!

>Now, it's obvious to me that some fair amount of my hatred magenta hue
>is missing !, a very difficult color to tame for me (it falls into the
>A, B, C triangle, named a non-spectral color. Am I right?).

Being non-spectral is not an issue... but colour perception is a very
complex matter, especially when dealing with electronic devices, which
usually behave much differently than the eyes!

For a start, the usual RGB model is severely mismatched to the eyes'
response: the short cones have their sensitivity peak at around 430 nm,
which is not blue, but _violet_. The 'green' cones are OK, at 540 nm... but
the long cones peak at 572 nm, which is GREENISH-YELLOW -- nowhere near to
red!!!

Some films (e.g. Agfa Vista) have tried to mimic this response by shifting
the red response towards the green. This renders the film almost useless for
astrophotogaphy, where the eye's already poor response to Hydrogen-alpha
line (656 nm) is most desired -- emission nebulae.

>The appearance of surface colors is defined by the product of
>the spectral reflectance curve of the material and the spectral
>emittance curve of the light source shining on it. As a result, the
>color of surfaces depends on the light source us

Spectral density of the dyes used does matter a lot. If the "sources"
(combined spectra of the illuminant and the dyes) were fully monochromatic,
spectral lines at the peaks of the cones, AND the "receiving" device
mimicked the eye's response, we _might_ have accurate colour rendition...

From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
>Metamerism is a property of reflected light.  If you are talking about
>what you see on your screen then it's not metamerism.

Although not reflective, light thru a slide (which has substracting dyes)
may suffer from metamerism also. Some screens (not only LCD) may shift
colours depending on angle... and internal temperature -- don't make any
adjustments until it has worked for half an hour or so! ;-)

Cheers,
-- 
Carlos J. Santisteban Salinas
IES Turaniana (Roquetas de Mar, Almeria)
<http://cjss.sytes.net/>
-- 
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