It depends. Don’t you hate those types of answers? Unfortunately, there is no simple answer, because you have input resolution and output resolutions, and sub-types inside those two high levels.
Cut to the chase
If all you do is share your pictures online or make 4×6, 5×7, or 8×10 prints, then pretty much any camera made within the last 5-10 years will give you more image resolution than you need and you don’t have anything to worry about and shouldn’t even need to think about it. Carry on snapping away. If you want to learn about why, then read on.
Keeping it simple
In the interest of distilling information down to useful bite size chunks, let’s start with output resolution.
The vast majority of us don’t actually print our pictures, however, print resolutions are typically a lot higher than screen resolutions for a given amount of surface area, so we’ll talk in terms of print resolution.
For this discussion, we’ll equate dots and pixels as the same thing, so terms like dpi (dots per inch) and ppi (pixels per inch) mean the same thing. Likewise, we’ll say a pixel and a dot are a single discrete full color entity that we can see. It may be made up of one or more smaller sub-parts, like a red element, a green element, and a blue element in a computer display, or multiple ink droplets on a piece of paper, but for this discussion it’s one discrete visible full color unit.
It turns out that the human eye actually does have a finite amount resolution in terms of how we would describe it if it where a digital sensor. If we where to take resolution and print it onto a piece of paper, and look at it up close and personal, the magic number where we stop being able to discern fine detail sits right around 300 dots per inch, or 150 line pairs per inch on the paper (or screen), meaning that if we take 150 black lines that are 1 pixel wide and 300 pixels tall, 150 white lines that are also 1 pixel wide and 300 pixels tall, and alternate between them, when printed on the paper so that all 300 lines fit into onto a 1×1 inch square, it would actually look like a grey square to most of us instead of look like alternating black and white lines. This is why most magazines print at 300 dpi, and your iPhone’s Retina display is also about 300 dpi. Spatially speaking, our vision starts to poop out and adding more image resolution than that generally does not make the picture have more detail or look sharper to our eyes.
So, with this 300 dpi number, it’s pretty easy to do some simple math and extrapolate out how much output resolution we need for the various ways we look at our pictures: for a 4×6 inch print, 300 dpi times 4 inches is 1200 pixels on the short edge, and 300 dpi times 6 inches is 1800 pixels on the long edge, or an image that is 1800×1200 pixels. That’s a measly 2.1 megapixels.
The same math for an 8×12 (or 8×10) print comes out to 3600×2400 pixels, or a very modest 8.6 megapixels. A 16×24 (or 16×20) print is 7200×4800 pixels, or 34.5 megapixels. Now we’re starting to get into some serious resolution.
The 16×24 print size not withstanding, if we take display sizes into account, we soon discover some correlations. The average computer display or HD TV can comfortably display the 1800×1200 resolution image with little to no scaling and look quite good. A newer 4K display can display the 3600×2400 resolution image with little to no scaling and look quite good. It should be noted that the aspect ratios between print and screen aren’t the same, so if you never intend to print, you can crop your images to 16:9 aspect to match your display and simply size your output to either HD (1920×1080) or UHD (3840×2160) resolution and call it a day.
What I do
I’ve standardized my “house format” if you will to that of a 16×24 (or 16×20) print size, meaning all of my keeper images, regardless of their input resolution get scaled up or down to 7200×4800 pixels. That’s my working/master resolution (as of 2016).
For paying clients, for standard uses, they get 3600×2400 (or 3840×2160) pixels as the deliverable (scaled down from the 7200×4800 master) unless they’re going to print larger than 8×10, and in that case, the conversation shifts to commissioning me to get the image and do the print for them, since I’ll typically want to capture more resolution than normal and work with a print service that specializes in larger print sizes, and that involves renting gear that is appropriate for what the client wants in terms of output size. Depending on the size or the aspect ratio of the print, I may break from the 7200×4800 and go larger, but typically, that is starting to get into higher end output, and very low volume. Keep in mind, a 16×20 or 16×24 print while not really huge, is not small. It’s 4 times the size of an 8×10. It’s big enough, you have a frame built for it and hang it on a wall.
This is where it starts to get a little techie and can be a bit confusing if you’re not a tech head, so lets take it a little at a time. I saved input resolution for last since what resolution you want/need to output tends to drive what resolution you need to acquire to use as your input.
When it comes to input resolution, it can be greatly affected by lots of different factors, so now is a great time to talk about the concept of “effective resolution”. For example, when you take a picture, how much visible resolution ends up in your image that you can see as fine detail is affected by things such as mirror slap (if you’re shooting an SLR or DSLR), shutter shock, how long your shutter is open (which affects how much movement happened, which shows up as blurring), your hand shaking the camera when you press the shutter button (which shows up as blurring), how deep your depth of field is (which affects how much of your image is in really sharp focus), how much image noise the sensor is introducing into the image, how much diffraction is happening in your lens (depending on your lens f-stop), and how much spatial resolution the lens you’re using is capable of actually putting onto your camera sensor. These things that I just listed are all things that will affect how much resolution you’re effectively putting into your image regardless of whether your shooting film or digital, full frame, medium format, large format, APS-C, Micro 4/3, or smaller. We haven’t even started talking about the raw image sensor resolution yet.
In short, that nice crispy 24 megapixel camera you just picked up? Unless you’re using a really high resolution lens (which is incredibly expensive), and practicing some pretty rigorous shooting process to keep your camera movement and vibration under control, you’re not getting anywhere near 24 megapixels of resolution when you take a picture. Even then, your camera sensor is hiding a dirty little secret.
You see, a 24 megapixel camera outputs an image that is 6000×4000 pixels. It does not actually have 6000 red pixels, 6000 green pixels, and 6000 blue pixels across. The same goes vertically. Nope. What it does have is 6000×4000 light detecting sensors, that then has a color filter array placed over it (usually in a bayer pattern). The color filter array takes that 6000×4000 pixels and divides it up between red, green, and blue. Since human vision is most sensitive to green, a full half of the sensor resolution gets filtered to green, and the remaining half goes to red and blue, which each get a quarter of the resolution. To get to an image that you can actually see, this then goes through a demosaicing process into an image that is 6000 red, green, and blue pixels by 4000 red, green, and blue pixels.
What this means is that for a 24 megapixel image captured with a 24 megapixel camera, you are effectively seeing 12 megapixels of green, and 6 megapixels each of red and blue. Even though, spatially speaking, you have 24 million light sensing elements on your sensor, you are not getting 24 megapixels of full color information. It’s actually more like 6 to 9 megapixels of full color information, which interestingly enough is right there in the ball park of making a really nice 8×10 print.
This is why medium format cameras have been 40+ megapixels for a while now. It’s less about getting the raw spatial resolution, and more about effectively getting more full color spatial resolution. This is why an 8×10 print from a 50 megapixel Canon 5Ds looks so much better than the same picture taken with a ten year old Canon Digital Rebel XTi that’s 10 megapixels. It’s not about the raw spatial resolution, since we can’t really see more than 300 dpi on the page anyway, it’s about getting 300 dpi of full color information.
What about large prints?
But wait a minute, photographers have been making large prints with cameras that don’t have anywhere near that resolution for a while now and they look great. What gives? Well, as it turns out, the larger you print, the less spatial resolution you actually need. It sounds counter intuitive, but once you start getting into 16×20 or larger print sizes, you stop looking at it up close and personal like you would a smaller print, and instead stand back to take it in. The further away from the print you stand, the less dpi your eyes can actually resolve on the print. This is why a 65″ 1080p HDTV which is only 2 megapixels still looks good. You sit further away from it than you would a smaller TV. Combine that with the fact that our brains are very good at filling in missing information, all the photographer has to do is make sure that the image is scaled up in a way that pixelation isn’t obvious if inspected up close. Our brains will do the rest.
With that being said, for larger print sizes more camera resolution will generally result in a better looking output image simply because we’re then putting more raw spatial and full color resolution into the image and have a lot more real estate that we can fill up with that resolution, until we effectively have more than 300 dpi that we’re putting on the print surface.
So how much image resolution do we really need? For the average person sharing online and making 8×10 or smaller prints, a camera that is at least 6 or 7 megapixels will provide totally usable images. The larger you print, the more resolution you’ll want. Digital cameras have just recently gotten to the point where we can actually capture and put all the full color resolution that we can see into an 8×10 print, which makes for super fabulous prints, so this is a great time to be taking pictures.