DPI is dots per inch, and is often used to describe the (max) resolution a printer can offer, but it is also applicable for scanners. Contrary to PPI, the DPI for a printer or a scanner is not fixed but can vary subject to the print mode. So you can print and scan at different DPI levels, subject to your needs. The higher the resolution required, the longer the scan or print will take, so when you don’t nee the high resolution, it is nice to be able to prioritise speed over resolution.
When printing, the DPI needs to be significantly higher than the PPI for a screen, and this is because the printer builds a colour combining different base colours, and hence more base colour dots in combination are needed to make up for a similar dot on the screen. So when you read the spec sheet for a good colour printer, the DPI is often much higher than the PPI for a screen.
PPI is pixels-per-inch or points-per-inch. It is used to express the resolution for a screen, say a monitor, a television, a tablet or a smartphone, just to mention a few.
If you think of a game of chess measuring 1 inch by 1 inch, the PPI is 8. You have 8 pixels across and 8 pixels down, giving a total of 64 pixels. So the PPI does not count the total number of pixels per inch, but tells you how many pixels you will find on both sides of the 1 inch by 1 inch square.
The PPI cannot change. The resolution of your monitor or your television has the PPI set from the factory and it remains fixed – no parameter of configuration can change that.
The PPI is a grid into which the pixels in your image can fit. If your image file has a lot more resolution than the PPI, the screen simply cannot do justice to the quality of the file and will have to scale down the resolution of the file to match the PPI of the screen. You can counter this when you edit by zooming in on the image in which case the constant PPI would be applied to a small(er) part of the image and hence would yield a relatively higher resolution.
Typically the PPI increases the smaller the device is. My iPhone 13 has a PPI of 440 approx, whereas the 32 inch AOC monitor I use to write this has a PPI of 140. You would think that the iPhone is a much better screen than the AOC, but here you have to factor in the viewing distance: The greater the viewing distance, the less PPI you will need. I find this to be a bit counterintuitive, but have accepted that it is the way it is. If a billboard was made using a monitor, you would not need 140 PPI’s!
Many want their camera to have a lot of megapixels, and primarily use the files produced to present on a UHD monitor (typically 32″ in size). The resolution is 3840 x 2160 equal to 8.3 megapixels which is much less than say the 24mp you would find in a Nikon Z6ii or a Nikon D750.
Banding is when the gradual transition from light to dark is not represented in a smooth and gradual way in the image, but rather abrupt changes from one level to another. It often happens in bands, just like you can see a height curve on a map. So that beautiful setting sun is not so beautiful, as the sky above it is shown as bands of red, orange and yellow!
Left: Exported with a image quality of 10%, right: ditto at 100%. The banding is visible in the image left as lines tilting to the left.
On this homepage you will find a lot of banding going on, and that is because I have to export the images highly compressed to support fast load time.
Banding is not for colors only. Here the same image as before, but zoomed in a bit more ad converted to B&W. Banding is just as visible in the shades of grey as it is in colors.
So banding is very often caused by compression, i.e. that a JPG image is throwing away too much information as it compresses the file to save space. Being a bit of a photo nerd, I often notice banding then I watch a movie on my 48″ LG TV, and I think the banding also here is caused by compression.
Banding is easy to see when a color is transitioning slowly from dark to light, but it is just as noticeable in black and white (see above).
Resolution
Banding is not because the image has too little resolution, but because the information stored to reproduce each pixel is too little. So a fix can be to make sure the images are stored with 16-bits of information per channel rather than 8-bits, i.e. to shoot in RAW or TIFF formats and make sure the information per channel is 16-bits. However, the issue with banding is often not at the source, but when exporting it to JPGs and my best advice here is to go as light as possible with the compression.
Another fix is to introduce a bit of noise when editing the image, so the noise acts as details added to the image and hence softens the transition along the banding. This can be a good strategy if your image is created with too little information per pixel.
You can get into some very technical discussion around banding and why it happens and some photographers are very frustrated to find banding in their prints that is not on screen etc. I am no expert here and do not have all the answers. My ambition here was just to give some insights to what banding is and a few high level fixes should it come your way.
