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File Size and Resolutions in Photography

by Steph Doran (follow)
Blog (393)      Articles (166)     
What is DPI and PPI?
Before we begin, let’s have a look at the difference between DPI and PPI. These two terms are sometimes (wrongly) used interchangeably, but they do not mean the same thing. DPI means “Dots Per Inch,” and is a term that we use when referring to the resolution of a printed image.
When a printer lays ink onto the paper, it will space out the dots of ink according to the DPI. A higher DPI means that there are more dots of ink in one square inch of the print, whereas a lower DPI means that there are fewer dots per inch.

Obviously, the higher the DPI, the more fine detail the image will appear to have. PPI, on the other hand, means “Pixels Per Inch,” and we use this term for digital files. Because digital files are made up of square pixels, rather than dots of pigment, a different terminology is used.

What is Resolution?
When we talk about resolution in terms of digital images, we are talking about the number of pixels in an image. This number is usually written in terms of width by height (for example, 1000 x 200px). A high resolution photograph has more pixels than a low resolution photograph, which means it is larger. You may have heard the terms “low res” and “high res” before, and these are just abbreviations of the aforementioned terms.

The highest possible resolution that you can capture will depend on your camera’s sensor. For example, a 50 megapixel camera will be able to capture images of a higher resolution than a 12 megapixel camera.

Resolution also relates to the size of a monitor or screen, and this will affect how your image is displayed. A higher resolution monitor will display an image smaller than a lower resolution monitor, because it has more PPI.

For example, let’s pretend we have an image which is 1024 x 800 pixels. We have two monitors- one set to 1024 x 768, and another set to 1920 x 1080. That same image will appear larger on the 1024 x 768 screen, because its pixel dimensions are almost the same. However, on the monitor that is set to 1920 x 1080, the image will appear smaller because there are ‘leftover’ pixels in the dimensions.

What is Aspect Ratio?
The term aspect ratio refers to the ‘shape’ of your image, or the ratio of width to height. Traditional cameras were of the 5:4 format, but the default ratio for modern 35mm DSLRs is 3:2. In simple terms, a 3:2 aspect ratio means that the image is 1.5 times wide as it is high. For a square image, both width and height are equal, so the aspect ratio is 1:1.

Different Sizes for Different Purposes

Making Prints
Naturally, we want to create the best quality prints that we can, and this means working with the highest resolution available. In most circumstances, 300PPI is standard, but 150PPI will still yield decent results. The exact dimensions of the image will depend on the size of your desired print.

Emailing To Friends
We generally use smaller images in emails because large images would take far too long to send and receive. Using low-res images is the norm with email, so keep your dimensions under 1000px, and your PPI low.

Displaying on Your Website
Choosing a size for the images in your online portfolio is sometimes a tradeoff- too large, and they will take far too long to load, but too small and they might look low quality. Finding a balance is key. A lower resolution is fine for web, but you may find you also have to make the dimensions smaller too, depending on your camera’s sensor, this may even need to be half the size of the original image.

Instagram Snaps
1080x1080px is the current standard for Instagram pictures.


Don’t confuse image size with file size. Image size refers to the height and width of the image (for example, 800 x 800 px), whereas file size refers to the amount of space the image will take up on your computer (for example, 800MB.)

Always start with the biggest image that you have. It’s easy to make an image smaller, and still obtain a high-quality result, but trying to make small images larger will result in blur, pixellation, and potential tantrums.

When making an image smaller, always save it separately to your large masterfile. Once you make that file smaller, you can’t return to the larger original without losing quality.
Give yourself the best chance of retaining quality by shooting RAW, or at least the best quality JPEG that your camera can capture.


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