When first getting into digital photography; whether taking pictures with a fancy camera or just snapping pics with your cellphone, one of the first words to get thrown out and about is the word megapixel. So what is a megapixel or mp? In short it’s a short way of saying ‘a million pixels’ or a million little dots that will be used to make your image. So a 5mp camera creates an image consisting of 5 million pixels. A 24mp camera produces an image consisting of 24 million pixels.
Right off the bat as consumers we all tend to jump right in and say “Give me more megapixels!” Because more is better right?
Not always. In part a lot of it depends on what you’re going to do with all of those millions of little pixels that make up your image. Nowadays most digital pictures end up getting uploaded online or shared digitally via email or text messages and those images are viewed on computer screens or smaller cell phone screens. Display screens are marketed in terms of their resolution – which just means how many dots they will display. Here’s a few examples of common sizes and how many megapixels they are showing you at a time.
Apple i6 = 1,334 x 750 = 1,00,050 pixels or 1 mp.
Apple i6+ = 1,920 x 1,080 = 2,073,600 for roughly 2 mp.
Samsung Galaxy S6, Note 4, Note 5 = 1,440 x 2,560 = 3,686,400 pixels or 3.7 mp.
1080p TV … same as the Apple i6+ above at 2 mp.
720p TV = 1,280 x 720 = 921,600 or less than 1 mp.
Generic widescreen monitor = 1,920 x 1,200 = 2,304,000 or 2.3 mp.
4K TV = 3,840 x 2860 = 8,294,400 or 8.3mp
Now that we have a rough idea of how many pixels we’re actually looking at on a screen let’s look at would would happen if we were to take a picture using a 16mp camera like the one found in my phone. It’s a Galaxy Note 4, so when I take a picture I get an image with a resolution of 2,988 x 5,312 pixels. The rough equivalent to 15.8mp. The camera stores that image at that size, but can only display it at 1,440 x 2,560. That means that when I look at the image on my phone the processor in it is throwing out more than 75% of the actual pixels that were captured to make the photo.
The math for that is: 3.78mp divided by the 15.8mp that the phone can display = 23.4. That means that only 23.4 percent of the pixels are being used.
If you were to show that same image on the i6+ the math is worse and you’re only really seeing one eighth or roughly only 12% of the original pixels. And at 720p you’re seeing about 1/6th of the original pixels. Even if you were to display the picture on a 4K monitor you would still only be seeing a little over 1/2 of the pixels of the original picture.
Why Capture all the Extra Pixels ?
If all of those pixels are being ignored, why capture them in the first place? After all if a 4K monitor can’t display more than 8.3mp why should there be a need for a camera to shoot more than 8.3mp? There are two reasons for this. The first is that a lot of times there are parts of an image that you just don’t need to keep that can be edited or cropped off.
Take the following pictures. The first shows what the camera captured, but I didn’t really want all the extra space around the emblem so I cropped some of it off. The original was a 24mp image, after cropping it was 13mp. That means I cut down the pixels by about half but I’m still able to display the image in a 4K monitor without losing any detail.
But let’s look at a more extreme cropping scenario.
Here’s the original picture shot at 24mp. There’s a lot of dead space in this picture. All I wanted from this shot was just the bird. But the lens I had on the camera right then wouldn’t zoom in any closer. So all I could do was take the shot and then crop out the parts I didn’t want later.
After cropping I had this image…
The second image has been cut down from 6,000 x 4,000 pixels down to only 1,821 x 1,216. So a 24mp image got cropped down to a 2.2mp image. Even with that extreme amount of cropping the picture is still bigger than adequate if being displayed full size on a computer or even on a lot of phones. Had I started with an 8.3mp image and tried to crop it down to this image the cropped size would have ended up being 1,165 x 868 pixels. That means that anything beyond 720p or an Apple iPhone screen and the picture quality would suffer when displayed because the image would have to be stretched to fill the screen, or would just only display on a portion of the screen.
So cropping is one reason why all those extra pixels are needed.
The second reason goes back to the root of photography when photos were taken on film and then the negatives had to be processed and the image transferred over to some other medium like photo paper if you wanted to be able to pass them around and see them. Yep, I’m talking about good old fashioned printing. While display screens have fixed resolutions of x many pixels across and down; printers use a different system.
Printers will print in terms of Dots Per Inch or dpi. It works like this. Imagine a square that is 1 inch by 1 inch across. If you had a really giant marker like one of those bingo daubers and you smacked it down in the middle of that box you would basically fill it up. If you built a printer using bingo daubers as you’re equipment then you’d end up with a 1dpi resolution. One dot for every inch. So a picture printed on a piece of 8.5″ x 11″ paper would be 8×11 or 88 dots. So the highest resolution picture you could accurately display would be 8×11 pixels. That’s a .000088mp image for those that just want to know. Anything more and you’d need a bigger paper; or you’d have to start picking which pixels to leave out.
Just because the math is so absurd; at 1 dot per inch a 24mp image at 6,000 x 4,000 pixels would need a piece of paper 6,000 inches by 4,000 inches; that translates down to 500ft x 333 ft. That would be a picture that roughly three football fields side by side. Definitely not something to stick in your wallet. Plus you’d have to be in an airplane for the picture to look normal.
Modern printers do a far better job of printing than at 1 dot per inch mercifully. As a very broad rule of thumb a lot of people will print out photographs at 300dpi. This is a very generalized statement. There are a ton of other factors but for the sake of understanding why megapixels matter let’s just work with a number of 300dpi. If you want your head to explode you can go do an internet search for PPI vs DPI.
Using the 300dpi number as our guide the previous 6,000 x 4,000 pixel image would now only display on a true one dot per pixel piece of paper that is 20″ x 13.3″. An 8.3mp image would only display print at 300dpi out to about 13″ across. My severely cropped bird photo would only come in at 6″ x 4″. These are just examples. This absolutely doesn’t mean that you can’t print bigger than this. Depending on the software, the printer, how far away you’re going to view the image from and the quality that you’re willing to accept you can go bigger.
The main point is that all of those megapixels really aren’t there for you to see onscreen. They’re there for you to print.
What can you do with this information?
First of all you need to think about what do you want to do with your images. If you’re never going to print them out and all you’re doing is emailing friends or posting them online to site like facebook, maybe you should resize them. Storing an 8mb full resolution picture that will display the same as a resized 300k picture is a waste of storage. Since most people are taking and keeping pictures on their phones; that space adds up fast. Also sending all those full size images eats up your data plan on your phone. One 8mb photo or two dozen resized images for the same bandwidth? It’s your choice.
If I were a conspiracy theorist I would propose that data carriers want more megapixels in cell phone cameras specifically to eat up data from your limit.
Second, if you’re a photographer and you wan’t to keep people from making large high quality prints of your images but you still want them visible online- only upload images that have been resized down. The other upside to only uploading smaller files is that it will allow your website to load faster; rather than transferring a huge image file only to have the computer at the other end scale the image down. As long as you keep the full size original you can always make larger prints for yourself or if someone asks for one.
That’s it for now. I know I threw a lot of numbers out this time around, but image resolution is all about the numbers game. The trick is don’t fall for the hype that more megapixels means better picture. Most people are really never seeing 75% of those extra pixels the paid for. It doesn’t take a 16, 20, 24, or 32mp camera to have a good usable online image. Anything over 8mp will work perfectly for almost all digital displays without any fear of losing pixels. As long as you’re not having to crop out a lot; it’s only if you’re going to make really large size prints that you should start to worry about megapixels.