by Jeff Park, former Technical Development Manager, fotolibra
Bigger is not always better.
PPI doesn't affect digital image resolution or quality at all. It simply maps a virtual size (the pixel) to a physical one (an inch) in order to produce a physical representation for print or display.
The number of dots per inch is simply used for scaling an image in physical terms. That is to say that — in terms of content — an image can be 72ppi, or 300ppi, still be exactly the same image, and be used in exactly the same way. The difference is only shown when the two images are represented physically. An image displayed at 72 ppi will be larger, while an image displayed at 300ppi will be smaller and finer. All images uploaded to fotolibra should be 300ppi which is generally regarded as the optimum for reproduction in print.
The size of a file, on its own, is not the best indication of its quality. Thanks to improvements in image compression, a high quality file can be reduced to a fraction of its original file size.
JPEG compression is capable of reducing the file size of an image to a fraction of its raw size, making transfers across networks much more efficient. It does this at the cost of image quality; however when compressed properly, the loss is noticeable only under close inspection. More recent types of compression — such as the Portable Network Graphics (PNG) format — are able to achieve respectable levels of compression while maintaining 100% of the quality of the original.
Image files on fotoLibra should be saved as JPGs, but must not be compressed any further.
Megapixels give a good indication of image resolution, but not all pixels are created equal, and neither are the ways they're pieced together.
There's more to an image than the size of it. Content, colour, crispness and size are all factors in an image, and the Megapixel rating only gives you an indication of the result in one of them, size. The individual pixel sensors in a digital camera can vary in level of brightness they produce in a certain colour. Individually, this is acceptable, but when there's a variance among a group of pixels, it quickly turns into distortion.
Many digital cameras use 'Interpolation' — a type of smart pixel averaging — to increase the resolution of an image, and thus the Megapixel rating. On the pixel level, this can create blurring, resulting in decreased quality at this level. Check your camera's manual to see if they use it — they may use some trade name to describe it, but look out for the giveaway words 'enhanced' or 'enhancement'.
In many cases, it is infinitely preferable to have a camera with a fair Megapixel rating and a high quality image sensor to one with a high Megapixel rating and only a fair quality image sensor. Build quality — like image quality — counts.
NB Do not confuse MP (Megapixels) with MB (Megabytes) — they are not the same.