 Photo
Editing
Getting it the Right Size
Although there are now photo library programmes that will adjust
the size of your photo automatically when you select print, a
little knowledge will put you in control of the process, enabling
you to be much more precise.
Photos for the Screen
When publishing photos on the web, it is best to make the photo
the exact size, in pixels, that you need. This way the download
time will be minimized. There is no advantage in having a greater
number of pixels in the picture as they will not show on the screen
and often the picture will have a strange 'squashed in' look about
it. Minimising the size of the file is not so important if you
are just going to make a slide show for your computer or a CD
but you will find the whole operation will be slicker and work
faster if you do.
The first and most important thing to
realise is that reducing the number of pixels in a photo to make
it smaller is a destructive process, the pixels are discarded
and cannot be reclaimed. So always make a new file by choosing
'save as' and giving the photo a new name. This way you will still
have the original file as well.
In Photoshop, once you have your new file open, select 'image'
then 'image size' from the menus which will open the window below.
Other programmes will have a similar window.
 If
we are dealing with a picture destined for the web, we are interested
in the top section headed 'Pixel Dimensions'. The section below
deals with sizes for printing which we will come to later.
At the bottom of the window we have two tick boxes, the top one
'constrain proportions' keeps the horizontal and vertical proportions
of your picture in ratio and, unless you are looking for a special
effect, it is important to always keep this box ticked.
The second box 'resample image' just means that we are changing
the actual file size as opposed to just the dimensions of the
photo for printing. Keep this box ticked when working with photos
destined for the web because, as I mentioned above, we want to
make the picture the right size in pixels, which means
we do want to change the file size.
Resizing pictures should only ever be
done downwards. If you made the photo too small and now it needs
to be larger then go back to the original and start again. (see
'resampling' for an explanation)
The drop down box next to the 'Resample' tickbox allows you to
choose a resampling method. These different methods govern how
the new pixels are created when you upsize your picture or how
they are selected for deletion if you are downsizing. Bicubic
is the default setting and this setting gives you the best quality
result, it is also the slowest method but that shouldn't be a
problem on modern computers.
When designing web pages, like this one, the unit of measure
we use is the pixel, we are not really interested in how many
pixels there are to an inch or a centimetre because that differs
depending on the screen resolution set by the viewer on his monitor.
Most screens are capable of showing either 72 pixels per inch
or 96. So that gives you a guide to how big your photo will look
on most screens.
The JPEG Format
The JPEG format is the most popular file format for photos by
far. It has a variable system of file compression which can make
files up to ten times smaller than the full TIFF or RAW files
or more. All digital cameras have an option to store data in the
JPG format and it is a very good compromise between file size
and quality. The method of compression used is fairly complex,
it basically squeezes the colours a little to give you a slightly
smaller palette but one that contains 'almost' all the colours
in your original photo. The important thing to know is that the
compression system is 'lossy'. This means that when you decompress
the file, some of the original information has been lost and if
you compress the file again, after making some changes in Photoshop,
some more information will be lost and so on.
Most of the time the effect of this 'lossyness' is not too apparent
on a first or second generation file but after that you begin
to notice the deterioration especially when you resize the picture.
Resizing a JPEG can often result in a softening of the sharpness
of the image. Sometimes you can fix this easily enough with the
unsharp mask in Photoshop but other times this quick fix will
not look right.
Always keep the original file safe and
do your editing on a copy. The best thing to do, especially
with your best pictures, is to save the file straight from the
camera to TIFF or PSD format. Even though it may have been saved
in the camera as a JPEG it is still only undergoing one level
of compression/decompression and will look fine. Then you can
work on the TIFF or PSD version, resizing it or correcting the
colour, and then save the finished photo as a JPEG if it is for
use on the web or keep it as a TIFF for printing.
When you save a JPEG file for the first time an extra window
will appear with a quality select slider. I have found that the
best setting for the web is 8 and for print I use 12.
Photos for Printing
Photos for printing need to have a much higher resolution. 300
pixels per inch is considered the minimum for printing in a magazine
and seems to be a good optimum for printing on a colour inkjet
printer. When sizing a photo for printing refer to the bottom
of the two areas in the image size window headed 'document size'.
Here you can select the unit of measure inches, centimetres etc.
that you are happiest with and choose your print size. If you
untick the box at the bottom marked 'resample image' then the
file size won't change. If you reduce the print size, the resolution
(number of pixels per inch) will increase. As the resolution for
printing is largely a matter of the more the merrier there is
no point in reducing the file size unless you need the space on
your hard drive.
Do
not be tempted to increase the resolution by resampling.
It is possible to make your file size bigger by resampling to
a higher resolution. The way the programme does this is to fill
in the new pixels based on the colour values of the pixels around
it. This normally makes for a fuzzier picture, it will never get
better. If you have a photo that you want to make a large print
of and the resolution is lower than 300 pixels per inch you will
be better off printing at whatever resolution you have rather
than trying to increase the resolution. I have printed pictures
with a resolution as low as 100 dots per inch and they don't look
too bad.
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