How Many Pixels Do You Need?

A lot of people are focused on maximizing pixel count with mega pixels and are not clear what they need for their photo applications (one million pixels = a mega pixel). What you want to do with your photos should be an important factor in choosing your camera and the resolution for final output like web or print.

Each time I make an internet post, construct a PowerPoint presentation or make a print I choose a file size or the number of pixels that are needed to produce a good image. The following information may help you make your choices.

The decision depends on what you need for your specific application or output.  The smaller the application the fewer the pixels needed and vice versa. You don’t need anywhere near as many pixels for a small size Facebook image compared to a large high quality display print.

Pixel is short for picture element, the individual sensing units on a camera sensor and if you don’t have enough for a particular product then it will look “pixelated”: blocky, fuzzy etc. You will actually be starting to see the square or rectangular pixel structure. The total pixel count comes from multiplying the number of pixels along the width of the sensor by the number along the height. So, a 12 MP sensor will have 4288 x 2798 pixels. The height and width values can be changed for different aspect ratios but multiplying the H x W must equal 12 MP.

Several really important qualifications to the pixel discussion are the size of the camera sensor, the size of the pixel on a given sensor, and lens quality.

Sensors come in various sizes from those in small smart phones to large medium format sensors. Related to sensor size is the pixel size or light gathering area of its surface. A small sensor with small pixels will not produce anywhere near the image quality of a full frame DSLR sensor with a much larger surface area and much larger pixels.

And then there is lens quality. All lenses are not created equal even within a given camera manufacturer’s camera format size. Before buying a camera lens check their reviews on sources like Digital Photography Review or the B & H website.

So, 20 MP camera that takes the image through a lens smaller than the tip of your little finger is not going to get anywhere near the image quality of a 20 MP DSLR camera.

Therefore it’s impossible to give an absolute hard and fast answer to this post’s question: how many pixels do you need? The answer is relative to a number of factors but to give some useful guidance, I offer the following examples.

Print Output   With a full frame DSLR camera (full frames all have a 24 x 36 mm sensor but may contain different numbers of pixels,  for example 36 or 50 MP) the quality of print output is determined by how many pixels you put in each inch (PPI) of the output product. Pixels per inch (PPI) is the resolution (R). 

One thing to keep in mind is that the further away you will be viewing a subject like a print, the lower you can go with the resolution. Viewing distance is a factor. Big display prints can be made with 200 PPI and possibly lower and be perfectly acceptable because you are viewing them from a greater distance.

For display prints an R of 300 PPI is often quoted in print manufacturer’s manuals for high quality. I generally print at 240 PPI and produce high quality prints. I challenge anyone to tell the difference between a 240 and 300 PPI print of the same size. I have also printed at 100 PPI and if the image has been carefully taken with low ISO to minimize noise and a sharp lens, it prints well.

Slide Presentations   A PowerPoint presentation with slides formatted to 1024 x 768 and projected on a screen 1.7 meters x 1.0 meter looks great. I use an inexpensive projector for general slide presentations with a resolution of 1024 x 768 and the images are fine. If I was showing clients proofs of a commercial shoot I’d go to a higher resolution projector but for general slide shows the lower R is fine.

Web Images   Computer screens (say in the 22 inch diagonal range) displaying web images might use a 1024 x 768 pixel file to pretty much fill the screen. My website gallery page uses 1600 x 2500 pixel files with a R of 500 KB or less which present well on significantly bigger monitors.

Published recommendations for optimum R: images for social media posts = 72 PPI; average magazine photo = 200 PPI; high end magazine = 250 PPI; fine art book = 300 PPI.

So, before you get obsessed with MP’s give some thought to final output. Do you really need a 50 MP camera that costs big bucks and is quite heavy to carry around when your primary products are web and modestly sized prints rather than big display prints?  The flip side is the person using an 8 MP smart phone and hoping to make nice display prints.