Megapixels, which are the ‘dots’ on the sensor which make up the resolution of a digital file. Why do they matter and to what extent? I am about to argue two sides to this argument because it is not an easy one to solve. Those two sides include on a screen and on a print because they are vastly different!
Screens and viewing your photos.
This is the most cut and dry side of the two. Ok, so you take a picture, edit it, and are looking at it on your monitor. If you do not zoom in to pixel peep at 100% there is going to be almost no difference between an 8 megapixel and 50 megapixel image. The reason is because your screen is maxing out at a certain resolution of most likely around 1080p in today’s world. This means you max out the resolution at the equivalent of a 2 megapixel image or about the same as a camera phone from 2005! Even with a 5k display, which is extremely rare, it maxes out at 14.75 megapixels.
As for online viewing, it is going to be even less important. Facebook compresses photos down to a tiny version of the original (it is truly awful). Even professional photography sites downscale images anyways to increase speed and responsiveness. You probably don’t need more than 5-8 megapixels for anywhere on the web!
Printing images on paper, metal, canvas, etc…
This is where it gets complex. This is where the lens, megapixels, material, and size matter. I will try to keep it as simple as possible, but you might have to read through this a couple times to really understand what is all at play.
Lens and camera
So, first of all there is a camera with a lens in front of it. This is what captures the initial image. The resolution of the camera matters not at all if the lens it captures it through and the technique used are improper.
The lens is an analog device that squeezes and bends light to focus it all onto the sensor. Some lenses do this better than others, and they usually cost more because of it. If you want a good description of what lenses do the best job check out dxomark.com for their perceptual megapixels as well as other geeky camera stuff.
The next thing is the technique. If the captured image blurs for whatever reason you are capturing less megapixels of sharpness because of it. If you miss focus, have too slow a shutter speed, or have a high ISO there will be less detail perceived.
This is why having more megapixels actually makes it harder to make good use of them. The minuet detail captured requires more attention, stability, accuracy, and skill to keep. The more resolution a camera packs, the more things that exist to prevent you from taking full use of them. These are all at play when you look at blowing up an image for printing.
This is the biggest factor, and it is one that will affect your megapixels decision the most. If you want to print large like anything bigger than an 8x12 you will need to consider the resolution of your camera.
Printing larger means you are spreading out the pixels over a larger area causing pixelization to occur if the file size is too small. You cannot print a 20x30 on paper at the highest quality with a 12 megapixel image. You can do things that will help, but there will be a lack of detail up close. This is the type of thing that requires the use of a high-megapixel camera. What I am talking about here is the DPI (Dots Per Square Inch) of a print. Ideal DPI is around 300, and if the resolution that you shoot an image at is below this when you print the quality will deteriorate.
Fine-art prints that are blown up to cover half of a wall need to be shot at a high resolution. This could be one high resolution camera, or it could be a low resolution camera shooting a panorama to get more detail. It just needs more detail to look good! There is one thing to note here though…
If nobody is going to view your print up close enough to pick out the small details you still won’t need all those megapixels. Say you are shooting a billboard. Nobody is coming within 50 feet of that behemoth of a print. You can shoot a full size billboard with a 4 megapixel camera just fine. Viewing distance matters just like the “retina display” does with apple devices because they calculate in the “normal” viewing distance into what the human eye can perceive as far as detail.
Everything prints differently. Paper is the traditional way for people to print, and typically it holds up best when it comes to showing off all the details. This means that as a print size grows the more quickly you may notice pixelization. Metal prints aren’t far behind though, and canvas prints retain the least detail per inch which allows for larger prints from smaller files.
What does this all mean? Well if you have an image that is only 12 megapixels it may only be able to print an 8x12 photo on paper or 12x18 on canvas due to different properties of the materials. Take into account the printer and material printed on to best understand their properties and get the highest quality print
The megapixels DO matter, but only for a limited amount of people in a very specific usage scenario. Most photographers need not care about getting the latest and greatest camera with a bazillion megapixels. You probably only maybe need 8 megapixels for about 99% of what you do with photography!