Earl Dunbar wrote:
>
Sorry to hear about the scratches. It's part of what drives me to
digital. I haven't processed my own B&W in many years but could if I
wanted to. But I really can't do color.
> 1. What accounts for the variable file sizes of the XP2 scans? I mean,
> every frame is a full 35mm frame. Is this related to overall negative
> density, or with scanning technique or settings?
The variable size is just the nature of the JPEG image format. JPEG is
a "compressed" format and, further, is a "lossy" format. The actual
processing is quite complex but a simplified example would be something
like this:
Across the 3000 pixel width of a single row of blue sky pixels let us
say that 2995 are all the identical color of blue which, for simplicity,
I will designate B#1. Randomly scattered across that line are 5 pixels
that are different which I'll call B#2. A simple compression algorigthm
is called run length encoding. So the software records this line of
pixels as something like (1000, B#1), (2, B#2), (800, B#1, (1, B#2),
(600, B#1), (2, B#2), (595, B#1).
As you can see, the representation of the data and the space required to
store it is significantly reduced from simpy repeating B#1, B#1, B#1,
etc, etc, 3000 times. You can also see that, if the pixels in a given
area of the image are of very uniform color and brightness that the data
can be encoded in a small space. If there is a lot of variation the
space required will be much larger.
This type of space reducing data encoding is common to all compression
algorithms. The final twist in "lossy" algorithms such as JPEG is that
the software might determine (based on the JPEG "quality" level) that
B#1 and B#2 are really so close in color and brightness that no one will
ever be able to detect that it just ignored the difference and stored
the entire line of pixels encoded as the very short line (3000, B#1).
From this you can also see that as the quality level is reduced (the
"lossy" part) the required storage space is also reduced.
> 2. I assume that since I got JPGs, and from a non-custom service, I
> really can't judge the dynamic range of the files as being best
> possible. One some of the shots with wide tonal range, the highlights
> are blown. Even on the negs they look like they may be blocked under a
> low power loupe, but I can't be totally sure. Any insight?
If the highlights are blown on the JPEG file they're probably blown on
the negative as well but that's certainly not a given. Depends on the
quality of the scanning. The JPEG file is not capable of reproducing
the full dynamic range of the film but the values should be adjusted so
as to preserve the highlight and shadow detail as best as possible
within the range of brightness values allowed by the JPEG data. In
short, you shouldn't have blown hightlights.
> 3. If I find that the highlights are actually blown on the negative,
> might this be an indication of exposure error? Does anyone have
> experience of XP2 doing better when rated higher than 400, which is what
> I used?
That's certainly how I would interpret it.
Chuck Norcutt
>
> TIA, Earl
> Now practicing the zone system, as in "no political sniping zone"
>
>
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