What do we mean by Pixel Resolution?
Digital images have two sets of dimensions – physical size or linear dimension (inches, centimeters etc) and pixel dimensions (long edge & short edge).
The physical dimensions are simple enough to understand – the image is so many inches long by so many inches wide.
Pixel dimension is straightforward too – ‘x’ pixels long by ‘y’ pixels wide.
If we divide the physical dimensions by the pixel dimensions we arrive at the PIXEL RESOLUTION.
Let’s say, for example, we have an image with pixel dimensions of 3000 x 2400 pixels, and a physical, linear dimension of 10 x 8 inches.
Therefore:
3000 pixels/10 inches = 300 pixels per inch, or 300PPI
and obviously:
2400 pixels/8 inches = 300 pixels per inch, or 300PPI
So our image has a pixel resolution of 300PPI.
How Does Pixel Resolution Influence Image Quality?
In order to answer that question let’s look at the following illustration:
The number of pixels contained in an image of a particular physical size has a massive effect on image quality. CLICK to view full size.
All 7 square images are 0.5 x 0.5 inches square. The image on the left has 128 pixels per 0.5 inch of physical dimension, therefore its PIXEL RESOLUTION is 2 x 128 PPI (pixels per inch), or 256PPI.
As we move from left to right we halve the number of pixels contained in the image whilst maintaining the physical size of the image – 0.5″ x 0.5″ – so the pixels in effect become larger, and the pixel resolution becomes lower.
The fewer the pixels we have then the less detail we can see – all the way down to the image on the right where the pixel resolution is just 4PPI (2 pixels per 0.5 inch of edge dimension).
The thing to remember about a pixel is this – a single pixel can only contain 1 overall value for hue, saturation and brightness, and from a visual point of view it’s as flat as a pancake in terms of colour and tonality.
So, the more pixels we can have between point A and point B in our image the more variation of colour and tonality we can create.
Greater colour and tonal variation means we preserve MORE DETAIL and we have a greater potential for IMAGE SHARPNESS.
REALITY
So we have our 3 variables; image linear dimension, image pixel dimension and pixel resolution.
In our typical digital work flow the pixel dimension is derived from the the photosite dimension of our camera sensor – so this value is fixed.
All RAW file handlers like Lightroom, ACR etc; all default to a native pixel resolution of 300PPI. * (this 300ppi myth annoys the hell out of me and I’ll explain all in another post).
So basically the pixel dimension and default resolution SET the image linear dimension.
If our image is destined for PRINT then this fact has some serious ramifications; but if our image is destined for digital display then the implications are very different.
Pixel Resolution and Web JPEGS.
Consider the two jpegs below, both derived from the same RAW file:
European Adder – 900 x 599 pixels with a pixel resolution of 300PPI
European Adder – 900 x 599 pixels with a pixel resolution of 72PPI
In order to illustrate the three values of linear dimension, pixel dimension and pixel resolution of the two images let’s look at them side by side in Photoshop:
The two images opened in Photoshop – note the image size dialogue contents – CLICK to view full size.
The two images differ in one respect – their pixel resolutions. The top Adder is 300PPI, the lower one has a resolution of 72PPI.
The simple fact that these two images appear to be exactly the same size on this page means that, for DIGITAL display the pixel resolution is meaningless when it comes to ‘how big the image is’ on the screen – what makes them appear the same size is their identical pixel dimensions of 900 x 599 pixels.
Digital display devices such as monitors, ipads, laptop monitors etc; are all PIXEL DIMENSION dependent. The do not understand inches or centimeters, and they display images AT THEIR OWN resolution.
Typical displays and their pixel resolutions:
- 24″ monitor = typically 75 to 95 PPI
- 27″ iMac display = 109 PPI
- iPad 3 or 4 = 264 PPI
- 15″ Retina Display = 220 PPI
- Nikon D4 LCD = 494 PPI
Just so that you are sure to understand the implication of what I’ve just said – you CAN NOT see your images at their NATIVE 300 PPI resolution when you are working on them. Typically you’ll work on your images whilst viewing them at about 1/3rd native pixel resolution.
Yes, you can see 2/3rds native on a 15″ MacBook Pro Retina – but who the hell wants to do this – the display area is minuscule and its display gamut is pathetically small. 😉
Getting back to the two Adder images, you’ll notice that the one thing that does change with pixel resolution is the linear dimensions.
Whilst the 300 PPI version is a tiny 3″ x 2″ image, the 72 PPI version is a whopping 12″ x 8″ by comparison – now you can perhaps understand why I said earlier that the implications of pixel resolution for print are fundamental.
Just FYI – when I decide I’m going to create a small jpeg to post on my website, blog, a forum, Flickr or whatever – I NEVER ‘down sample’ to the usual 72 PPI that get’s touted around by idiots and no-nothing fools as “the essential thing to do”.
What a waste of time and effort!
Exporting a small jpeg at ‘full pixel resolution’ misses out the unnecessary step of down sampling and has an added bonus – anyone trying to send the image direct from browser to a printer ends up with a print the size of a matchbox, not a full sheet of A4.
It won’t stop image theft – but it does confuse ’em!
I’ve got a lot more to say on the topic of resolution and I’ll continue in a later post, but there is one thing related to PPI that is my biggest ‘pet peeve’:
PPI and DPI – They Are NOT The Same Thing
Nothing makes my blood boil more than the persistent ‘mix up’ between pixels per inch and dots per inch.
Pixels per inch is EXACTLY what we’ve looked at here – PIXEL RESOLUTION; and it has got absolutely NOTHING to do with dots per inch, which is a measure of printer OUTPUT resolution.
Take a look inside your printer driver; here we are inside the driver for an Epson 3000 printer:
The Printer Driver for the Epson 3000 printer. Inside the print settings we can see the output resolutions in DPI – Dots Per Inch.
Images would be really tiny if those resolutions were anything to do with pixel density.
It surprises a lot of people when they come to the realisation that pixels are huge in comparison to printer dots – yes, it can take nearly 400 printer dots (20 dots square) to print 1 square pixel in an image at 300 PPI native.
See you in my next post!
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Excellent explanation of PPI and thanks for taking the time to post it….:-)
Great article , there is a lot of information advising downsizing good to know I don’t have to bother . Regards
Another excellent article Andy.
Andy, A very clear explanation of something that is badly written in many magazines where the writers clearly do not have a clue. In fact, the whole matter of resolution and the confusion between what the lens does and what a sensor achieves makes me groan. There is a great myth about sensor sizes, pixel numbers and resolution perpetuated for purely commercial reasons. I am working on some stuff for Imaging Resource of what sharpness means…a sucker for punishment.
best
Paul