Confused About Image Quality?
DPI, PPI, Pixels, Resolution, Image Size, Document Size, Print Size, Quality, Argghhhhh!
Someone wrote to me recently and asked me to clear some stuff up for them. They
had a large volume of images to edit, eventually wanting to print out the results.
As they so rightly pointed out though, it would be silly to edit all those images,
and then get to the end and find that they all printed terribly!
So what makes the difference when it comes to print quality? Well ask many people, and they'll
immediately pull out phrases like PPI, DPI, resolution, and so on.
But if you don't really understand these terms, that doesn't help much does it? And many
people don't! Hopefully that's about to change...
First some basics though, without any terminology:
Some General Rules First
The general rule for modifying photos with the intention of printing them is that you should
keep them at the highest quality you can. This might be the quality they came off your digital
camera at, for example.
How do you avoid reducing the quality? Firstly, never resize the image so it is smaller. But
don't try and make it larger either, just keep it at the original size. (This will NOT limit
your printing options!)
File Formats and Quality
Secondly, always save at the highest possible quality level. When you save in some image formats,
such as *.jpg, you can choose from a range of quality settings. What you're actually doing here
is making a 'compression trade-off'.
When an image is saved in certain formats, it is compressed to save hard disk space. The problem
is this also degrades the image data, losing quality. You're choosing here between a highly
compressed image (small size, low quality), or a less compressed image (larger size, higher quality).
So when given this option, always save at the highest quality possible. It'll take more disk space,
but it's worth it if you are later going to print the image.
Some image formats don't have a quality option though. In some cases, that's because they are
'lossless' formats (such as *.bmp or *.png). This means that they are either uncompressed (large file
size, but retain 100% of the image quality), or they are compressed to achieve slightly smaller file
sizes, but in such a way that they still retain 100% image quality.
Don't feel forced to pick a lossless format because it sounds superior. For photographic images, a
high quality *.jpg file should usually be adequate. Lossless formats are often better suited to smaller
files with less colors.
So What About All Those Words?
Ok, so you're still wondering about all those words. I haven't mentioned printing yet either have I?
Let me try and explain how it all fits together. Here's the first core definition that's essential:
A Pixel - A pixel is the single, smallest unit of color in an image. All images are like a
rectangular grid of colors, and each cell in the grid is one pixel.
That means that when you read that the dimensions of an image are 1024x768, there are 1024 pixels in
every row of the grid, and there are 768 rows.
Now, whatever else we talk about, you have to keep coming back to this core fact:
An image is made of pixels. A pixel is a pixel is a pixel. Regardless of 'resolution' or 'size' or
any other measurement, an image is just a certain number of pixels.
So, keeping that in mind, let's discuss some other terms you'll hear for how 'big' an image is.
Image Size - This is measured in pixels, and is what we've already mentioned. The image size
might tell you that the image is 512 pixels wide by 256 pixels high.
Document Size - This is how big the document should appear when printed. It's measured as a
distance. This means it can be measured in standard units like inches, millimeters, or centimeters.
There are also specialist distance measurements for printing, such as points, picas, or columns.
Print Size - This is just another term for Document Size. They're exactly the same.
Now, here's the second key point for you to note. It's about the two separate size schemes we have here:
Image Size is directly related to the amount of image data, and the file size.
Document/Print Size is a measurement of the size the document will appear at on paper.
Image Size and Document Size are not linked. You can print the same amount of image data
at a certain size, or twice that size, or half that size. You can squish it and stretch it
as much as you like. This will affect how good the image looks when printed though.
Linking Image Size and Document Size
So are there any measurements of the link between image size and document size? Yes! That's
what PPI is. It stands for 'Pixels per Inch'.
Imagine you take all the image data, and spread it over a piece of toast. The number of pixels
that fall onto each square inch of toast is what this value tells you about.
If your square inch of toast contains 10,000 pixels (in a 100x100 square), then that's a PPI of 100.
100x100 pixels will make up each square inch of image that is printed on the paper.
If this value is very low, then the image will appear blurry when printed, because the printer
doesn't have very much detail to fill each square inch with. You can increase the PPI value by
spreading the same image over a smaller piece of paper.
So what's a good PPI value then? Well Photoshop Elements shows an alert if the PPI falls below
220ppi, so perhaps 300ppi is a good value to aim for in most cases.
Difference Between PPI and DPI
Most people's next question is, "What's the difference between PPI and DPI?". Well, here's one: DPI
has nothing to do with the image! It only concerns the printer.
Your printer needs to mix together smaller dots of ink in just three colors (usually cyan, magenta
and yellow) to make the printed image appear to show the colors you have used. The number of these
tiny ink dots in a square inch is related to the DPI.
If an image is printed at 1200dpi, it has 1200x1200 dots in each square inch, 1,440,000 dots in all.
You make think that's an awful lot for just a square inch of paper, and indeed 1200dpi is quite a
high value, but even at 300dpi you are still looking at 90,000 dots per square inch.
So if you print a 300ppi image at 1200dpi, each pixel will be printed using 16 tiny ink dots. This
is because 1,440,000 / 90,000 = 16.
Okay, enough maths. If you didn't get all that, don't worry.
The important thing to remember is that DPI should generally be higher than PPI, because the printer
needs more than 1 color dot to produce the color of the pixel.
That leaves resolution and quality...
These last two terms are very general, and don't really mean much. The 'quality' of an image is
not measurable, because it doesn't correspond to any particular value.
You might talk about a photograph taken in bad light conditions as being of 'bad quality', even
though it has exactly the same number of pixels as another image taken in good light conditions.
This term is often used to talk about the overall effect of an image, rather than its technical
As for resolution? Again, people throw this term around when referring to several different things.
Sometimes resolution means the total number of pixels in an image. For example in a 1000x1000
pixel digital photo, there are 1,000,000 pixels, which might cause someone to say that the resolution
of the photo is '1 Megapixel' (1 million pixels).
Sometimes people talk about the DPI of a printed image as the resolution, saying things like
"The resolution of that image is 300dpi".
Bottom line - Don't rely on the term 'resolution' to refer to any one particular thing either.
No more Argghhhhh!
Hopefully that has cleared up some of the confusing terminology for you. Here are some final
guidelines about printing.
Imagine once again that you have your image, taken on your digital camera, that has a certain
number of pixels. As long as you don't decrease the number of pixels in the image, you won't
limit the available printing sizes that you can opt for. You've kept all those original
pixels to spread over whatever the final page area turns out to be.
If you find that the image is blurry when you print it, this may be because the PPI is too low.
By decreasing the print size from, for example, 9"x6" to 6"x4", you would raise the PPI and improve
the sharpness of the print.
If the PPI is high, but the image still appears blurry, or the colors appear to be altered, then
you could try raising the DPI of the printer. This can usually be done using the Print Settings
dialog box. Higher DPI values will use more ink, and be slower to print, but should improve the
quality a little.
Don't go crazy on DPI however. Once the DPI exceeds a certain value, you really won't see any
quality improvement by continually increasing it. 1200 is probably the maximum value I'd use normally.
I hope you've benefited from reading about Image Quality and the various terms involved.
If you have any comments on this article, get in touch using my Contact Page.
Robert Redwood - Bio|
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