From: Peter Marshall (petermarshall@cix.co.uk)
Date: 02/09/01-02:28:36 PM Z
> Hi Gang,
>
> I've done a lot of experimenting with my Microtek Scanmaker 4. I'm now
> getting scans from 4x6" prints and 4x5" film that appear to be
> acceptable
> for webpage use. If I scan at 72 dpi and save as a JPEG to a final
> size of
> about 40kb, the quality is OK.
>
> I'm planning to construct a website loaded with many photos. Since I
> suffer
> terribly from a crappy internet connection here in NW Wyoming, I'm very
> sensitive to slow downloads. I'm struggling to determine what's the
> best
> compromise between quality and speed. Since I'm about to spend
> countless
> hours scanning hundreds of photos, some tips and guidance would be very
> helpful, and much appreciated.
>
> Questions:
>
> Is 72 dpi the 'optimum' for webpage use?
Dave,
What you really need is the course I teach on web design for
photographers! But here are some of the more relevant parts:
Web browser software takes *absolutely* no notice of file dpi - they will
display at the resolution of the user's screen - some of my web images say
4000dpi resolution and it makes no difference. 72dpi is nominal screen
resolution, though most screens are now not 72dpi - mine is over 100 dpi.
The important thing is to get the size right in PIXELS as that is what web
browsers use. The median browser window size (according to figures from a
very popular site) is currently around 750 x 520 pixels. How big you want
your pictures in that is a matter of design, but I seldom go above
550x450, and often use a 500 pixel limit.
Incidentally, browsers don't throw anything out. They are not particularly
good at displaying images, but they do display all they are sent as best
they can, so long as you don't give wrong values for height and width in
the html.
>
> Is 40kb a good compromise between quality and fast downloads?
Yes, though when you jpeg images they will have a range of sizes at the
same 'quality' settings - I normally work to a range of 25-45Kb for images
around 500 pixels wide. You are better off scanning to about 700-900
pixels wide as tiffs, working on the images as required (rotating,
retouching, cropping, adjusting colour and tonality) and then sizing them
down to the required size before saving as jpegs. Black and white images
can be converted to gray scale before saving as jpeg. One of the most
common image faults on the web is overdone unsharp masking by the way. The
best rule on this is that if you can see it you have gone too far. Make
sure you are working on a high quality screen display. You might find it
better to use specialised software such as Jpeg Optimizer (from xat.com -
the free version is fine) to get the jpegs as small as possible without
visible quality loss. (If you are not on a PC there are other similar
programs.)
However you will see that if you are scanning 4x6" prints, 72dpi will not
give you enough for best results - you need to scan at around 120dpi,
while 72dpi is about right for 8x10, but if you are scanning 35mm slides
you would want to use around 600 dpi. I sometimes have much higher
resolution figures as I tend to scan for printing at 4000dpi and then take
a reduction from that file to use on the web. Basically resolution is only
important for images at the scanning and printing stages - and you can
change it for an image file without in any other way altering the file or
file size - it is really just an instruction to the input/output devise
that is embedded in a file that has no intrinsic size except in
pixels/bytes.
Don't forget it is the total load per page that is important. I'd suggest
never putting more than 80kb on a page in total - a max of 2 pictures at
that size, though I'd normally only put one on.
> In Photo Paint, I have the option to select encoding as either
> "progressive"
> or "optimize". I like the thought of doing progressive JPEGs, so
> should I
> save the JPEGs this way as a matter of course?
There are a few older browsers still around on some platforms that have
problems with progressive JPEGS, but not enough to worry about. I prefer
not to use them, but to make sure I specify height and width in the image
tags. That way you get an empty rectangle that fills. Progressive jpegs
come up first in very low quality, and on a slow connection I feel this
gives a poor impression of your work, as people are looking at this for
longer than your final picture.
> Is there any real advantage to making 48bit scans, or should I just
> speed up
> the process by doing 24bit scans?
I don't think there is normally much advantage unless you are going to do
a lot of work on the images in Photoshop, when 24bit images can sometimes
get sort of posterized. I think my scanner actually always does 48bit
scans, but then throws away the extra data if you ask for less. With the
small sizes you are wanting for the web I don't think it makes much
difference in time. You will need to convert to 24bit in Photo Paint
anyway as jpeg is a 24bit format.
Peter Marshall
Photography Guide at About http://photography.about.com/
email: photography.guide@about.com
_________________________________________________________________
London's Industrial Heritage: http://petermarshallphotos.co.uk/
The Buildings of London etc: http://londonphotographs.co.uk/
Also on Fixing Shadows: http://www.people.virginia.edu/~ds8s
and elsewhere......
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