Modern digital cameras (digicams) can easily produce an image which is larger than 15Mb in size.
What happens if you want to use that image in, say, a Power Point presentation? If you put in a whole bunch of such pictures into your PPT file, the presentation will become very sluggish and slow.
What happens if someone asks you to e-mail them a picture and you send that 15Mb image to them? You will not be popular because it will take nearly forever to send it and it would be even worse for the recipient who is on the receiving end of a 15Mb e-mail and doesn’t know why it is coming down so slowly.
Or look at the optimum creation of web sites. Want your web site to be as snappy as possible? The image file size becomes absolutely critical. The smaller the file, the faster the page loads. BUT… The smaller the file the lower the quality. So it’s also a matter of compromise.
Want to make a large blow-up print? Say A4 or A3 size? If there are not enough pixels, the picture will look blurred and lack detail.
To create more pixels artificially is a very difficult task to do convincingly.
So we need to control the size of our images and know what we can and can’t do with our images.
Images are made up of lots of electronic dots called pixels (picture elements) arranged in wows and columns.
An image dimension is quoted as “a large number of pixels wide” x “a large number of pixels high”
Now get this clear:
- The more pixels, the higher the quality (usually, but not always).
- The more pixels, the larger the file size (but how much larger, depends on the type of file)
- Different file types (see later for file type details) with the same number of pixels in them can have vastly different file sizes (in KB or MB).
Ooooh – this can get bit complicated…
No, down boy… let’s keep this simple…
For an image on a monitor, the pixel resolution of the monitor defines how big the picture will appear.
For example if we had an image with a pixel dimension of 1200px x 1000px and displayed it on a on a 1280 x 1024 monitor at 100% scale the image would nearly fill the screen.
(A web browser, for example, by default will display images at 100% scale. An image viewer may well display an image at different “zooms” so as to fit into a certain area on the monitor screen.)
If we used an 1050 x 1680 monitor, the image would nearly fill the screen vertically at 100% image scale, but there would be quite a bit of spare space in width. The overall impression would be a slightly smaller image.
If we used a small LCD monitor (say of size: 600 x 800), the image at 100% would be too big and we would get scrollbars.
The waters get a bit muddied if we used a physically smallish CRT (Cathode Ray Tube) monitor and increased its resolution – the higher the resolution the smaller the image would appear. (But let’s not go into CRT monitor characteristics – they are disappearing from the market quite quickly now .)
The main thing to grasp is “More pixels would give a bigger picture on a monitor at 100% scale. The image quality is constant “.
This is not the case with a print. In printing, the rule is simple and logical: the more pixels, the better the image quality for any given size. A small number of pixels will provide an acceptable image at a small size, but for a big enlargement, many more pixels are needed.
The rule of thumb for a decent print on glossy paper is that the image resolution should be 300 pixels/inch
Let’s try a practical example.
Let’s say we have an image and we want to make an A4 enlargement but we don’t want to pay for the print unless our image quality is good enough. Well, Photoshop can tell us…
Download this ZIP file and once downloaded, extract it to your computer. Then view it in your favourite viewer and check that it’s file size is approximately 750Kb. Minimise this picture but don’t close it.
Now open the file up in Photoshop.
Go to the “Image” pull-down menu and choose “Image size…”
(Alternatively just simultaneously press [Ctrl] [Alt] [I] )
Look at the first Dialogue box and notice what the resolution is. In my case it says 72 pixels/inch.
Photoshop Image Size Dialogue Box
Now make sure that the check-box next to “Resample Image” is NOT ticked. Change the resolution to 300 pixels/inch and the document size Width and Height immediately changes.
Photoshop Image Size 300px Dialogue Box
The image doesn’t change at all, but the document size values in this dialogue-box change. It now tells us that at 300 pixels/inch the width is 32.78cm and height is 21.95 cm. That’s slightly bigger than A4 so yes we can make a print that large. An A3 print would be out of the question, however.
Reducing File size
Now if we wanted to e-mail this image to someone, to send them a 773Mb file by e-mail would be discourteous. It’s just too big unless the recipient is expecting such a large attachment. If possible you want to keep your image sizes down to 100kb or smaller.
So we have a choice:
- Tell the recipient that a large file is on it’s way or
- Reduce the file size by reducing the number of pixels, or
- Reduce the file size by lowering the quality of the image, or
- Both 2 and 3
Reducing the file size by reducing the number of pixels.
The first thing to do to reduce the number of pixels is to crop the image if the composition allows it.
Imagine a rectangle which is smaller than the picture but contains all the pictorial information that you want. Make a mental note of where the corners of this rectangle would be.
Click the CROP tool
Crop Tool
The cursor changes to a cross-hair.
Click and drag from one corner of the imaginary rectangle to the diagonally opposite corner and click.
(Depending on which version of Photoshop you are using, this CROP command can have quite a few extra bells and whistles.) An indication of the cropped area appears. You can use the arrow keys to move the psition of the rectanular crop area and you can drag the corners unitl you are happy with the crop. Press [Enter] to actually perform the crop. If you change your mind, [CTRL] [Z] undoes that step.
Once cropping is complete, the next step is to go to the “Image” pull-down menu again and choose “Image size…”
Or just use the shortcut [Ctrl] [Alt] [I]
This time, ensure the “Resample Image” check box has a tick in it.
We want to reduce the image size to one which is 800 pixels wide. This is a good compromise between low resolution and reasonably visible size in most e-mail viewers.
Change the number of pixels in the “Pixel Dimensions: Width” to 800 and the height will also reduce automatically.
The “Pixel Dimensions:” will give you an indication of how much you have reduced the image by showing you what it was and what it now is about to become.
Note: The “Pixel Dimensions:” indicator is a bit confusing because it indicates how large the file size will be (In Bytes or Megabytes) when saved as an uncompressed native Photoshop file (a .PSD file) which is much larger than the file we started off with. Don’t worry about this for the moment, I’ll come back to this in a later lesson.
That’s about all we can do with pixel-size reduction.
Reducing the file size by lowering the quality of the image
We can reduce the quality of the image to a large extent with the effect being almost imperceptible to the eye by using what is called JPG compression.
We save the image as a JPEG (Joint Photographic Experts Group) file by choosing “Save As” from the file pull-down menu (or type[CTRL] [SHIFT] [S])
We get a dialogue box similar to this:
The Save Dialogue Box
Next to “File name:” type in “2 Boeings-email.jpg” and check that the format shown on the next line is indeed JPEG.
Press “Save” and a JPEG Options dialogue box appears:
JPG Options Dialogue Box
Move the slider left and right between “small file” and “large file” and it will show you the file size that you will get (in Kilobytes or Megabytes).
An image that is only 800px wide and a bit smaller in height will have a pretty acceptable compression quality if you choose a file size of between 50K and 100K. You will not notice any appreciable loss of quality due to the JPEG compression.
Compare this image with the one already in the image viewer
Now your picture is ready to send to someone by e-mail.
Until the next exciting episode…
