Understanding a 35mm negative/slide scanner spec

jim wrote:
Please can someone help me understand the basics of a current advert.

Ignoring all questions about quality, software, aliasing, etc etc, it
is the very basic arithmetic that doesn't seem to add up.

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http://www.aldi.co.uk/uk/html/offers/2827_11713.htm

Turns negatives into digital photos/files. Enhance or edit your
favourite pictures.

5 mega pixels
Film and slide strip scanner
48 bits
3600dpi optical resolution
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BUT how do you connect the numbers together?

35mm negs are 36x24mm or 1.417322835ins x 0.94488189ins - at 3600 dpi
that is 17.35million dots overall.

Even if the claimed 3600dpi reduces, as is common in scanners, to
(say) 1800dpi in the other direction that becomes 8.7m dots.

But, either way, the spec states 5 mega pixels! ignore that & carry
on

Each dot contains 3 values - one for each primary colour.

The combined colour value at each dot is stored in 48 bits = 6 bytes

So 17.35 million dots therefore needs 17.35m x 6 bytes of storage =
104MB!

Which, again, doesn't seem to square with a stated 5M pixels.

There must be something very basic I am misunderstanding, but what is
it?

No doubt the machine will make heavy use of data compression. To what
extent does that explain the sums?

No, because compressed files are always measured in so many bytes, not
pixels. To take another example, my digital camera takes 6.3 megapixel
images. At 24-bit depth (the usual) the image file is 18 megabytes. At
48-bit depth the file is 36 megabytes. Both files are still 6.3
megapixels. When the files are saved as a Jpeg compressed file, the
file size is around 2 to 4 megabytes.

I would guess that the 5 pegapixels mentioned in the ad should be 5
megabytes, which makes more sense, and implies a fairly heavy jpeg
compression.

However, the bottom line is, as Alan intimated, that at 40 pounds, the
scanner has to be junk. Prices for recognised good scanners are about
10 to 15 times that price. It's a toy. Be pleased you asked before
doing your 40 quid.

Colin D.

BTW the scanner looks cheap but in the UK ALDI special offers (there's
a new lot every week & when they're gone, they're gone) are often of
above average quality.

TIA for an explanation