OK, that all makes perfect sense, and I did take a look at the PowerPoint
FAQ. However, I am not clear as to how I calculate the scanning parameters
from the screen size. Could you help me do the math? For example, let's say
the image will take up the whole screen, and the projector does 1024x768.
What does that mean in terms of scanning the original photo?
(plug in any numbers here, depending on size what you are scanning).
Example: If you are scanning 6 inches at 100 dpi, it will create 6x100 = 600
pixels in that dimension... that is what dpi means there (pixels per inch).
So 6x4 inches scanned at 200 dpi will create 1200x800 pixels, which is a bit
too large for a 1024x768 pixel screen. If you were scanning wallet size, you
may need 400 dpi. If scanning 8x10 inches, maybe 100 dpi. If scanning
small 35 mm film, maybe 850 dpi. The dpi concept is pixels per inch, the
inches being what you are scanning, and dpi being the rate that you create
pixels of image dimension from those inches.
The dpi concept is that you ==could== compute it exactly as the
(1024 pixel goal) / (6 inches being scanned) = 171 dpi.
Scanning at 171 dpi would work to give you 6 inches x 171 dpi = 1026 pixels.
However, it is a little better to scan at even divisors of the scanners
maximum dpi, for example scan at 300 dpi with a 1200, 2400, or 4800 dpi
scanner. 300 dpi is /4, /8, or /16 of those maximums, even integer divisors,
and the point is these divisors are easy for the scanner to resample well.
171 dpi is not an even integer divisor of maximum, which gets complicated,
and it is somewhat less desirable (assuming we are a little fussy and the
work critera exceeds casual).
It will probably be easier to just scan it a little larger than you need (or
even a lot larger is fine if you need or have a larger image for some other
purpose) but I am definitely excluding smaller than you need. 300 dpi is a
very good scanning value for 1200, 2400 and 4800 dpi scanners. Then resample
it to your smaller goal size, to the 1024 pixel dimension, which might be
around 1/2 of that size for example.
Before you resample, unless some kind of special case, crop the image to the
4:3 shape to match the 1024x768 pixel screen shape. If you have Adobe
Elements, it has a marquee toolbar option (Fixed Aspect Size) to make that
crop trivial to do. Then reample smaller to the 1024 pixel goal size
(bicubic choice if available). Follow the resample with sharpening (menu
Filter - Sharpen - Unsharp Mask), but modestly, not so much to ruin it.
The point of giving PowerPoint the right size image is to keep it from having
to resample it in real time during the presentation to fit the screen,
because the common complaint is that it doesnt do that part so well. We know
what size is needed by the projector, and we can do the resampling job best
ahead of time at the resample menu of the photo editor. In the same way, the
scanner can resample 2400 dpi to 300 dpi better than it can resample to 171
dpi.
The photo editor resample menu, followed by USM sharpening, can handle any
case rather well (assuming you are resampling to be a smaller image). In
these cases, someone has to do the resample, and the editor resample menu is
the best case with the greatest resources, better than the scanner, and
better than PowerPoint, better than anybody.
Hope that helps.
