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Re: TTL VS Flash meter - was Re: [OM] In the studio (of sorts)

Subject: Re: TTL VS Flash meter - was Re: [OM] In the studio (of sorts)
From: Mark Marr-Lyon <o9938156@xxxxxxxxxxxx>
Date: Tue, 4 Apr 2000 09:43:03 -0700
>I started out creating a page to compare TTL and Flash meter shots
>but wound up with a problem of my own.
>Why do my web pic's come out all funny looking?
>I especially have this problem with high key and with higher res 
>scans reduced to 72 dpi JPEG.
>This didn't always happen.  I have some clown pics that I scanned to 
>a huge file and printed on an ink jet printer that look great.
>Now I just get junk no matter what I do.
>http://www.geocities.com/soho/hall/7283
>
>
>Charlie L.

Hi Charlie,

What you're seeing in those photos is an artifact of the GIF format.
The GIF format actually uses lossless compression, i.e. the decompressed
data is identical to the data before compression.  The problem lies in the
fact the the GIF image format only allows up to 256 colors.  If your image
contains more than 256 colors (like a 24 bit image, with up to 16.7 
million
colors), then the computer must decide which 256 colors to use.  This
process is known as color quantization.  It often produces effects like 
you're
seeing there.  Since the background is nearly one color, it only picks a 
few
colors to represent the background, leaving more to represent the 
foreground.
If, on the other hand, you start with an image with only 256 colors in it
already, like an 8 bit grayscale image, then the decompressed image will 
be
identical to the uncompressed image.  There are no settings in the GIF 
format
to select the level of compression - other than the number of colors in 
the
image.

The JPEG format is a lossy compression, but it allows up to 24 bit images.
JPEG compression is more sophisticated than GIF compression, and allows 
changing the compression level.  There is a tradeoff between image quality
and image size.  Very small images (1:100 compression ratio isn't 
uncommon),
can be made, but they may not look very good.  If the compression level is
too high, then artifacts can be introduced, especially at sharp intensity
boundaries in the image.

JPEG images are usually best suited for "real world" images, i.e. those
with lots of continuous tone color variation.  GIF images are more suited
(aside from the icky legal problems) for things like animations with few
original colors in them.

My advice is to use JPEG compression set to medium or medium-high for
your images, and you should see a dramatic improvement, while still having
relatively small image files.

Mark Marr-Lyon.

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