Just because your phone has a 20 megapixel camera doesn't mean you'll actually take good pictures. Here's what you really need to know.
Buy any smartphone, and one of the headline features will be the megapixel count of the cameras that come on the device. Typically the front "selfie" camera will have a lower megapixel count than the camera at the rear of the phone, because you're usually holding the camera a lot closer to your face at the front than the subject you're shooting at the rear.
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What are megapixels?
To understand what a megapixel is, you need to understand what a pixel is. A pixel is, in visual/IT terms, a single dot in a visual image.
If you think back to the bad old days of CRT TVs, if you stared really closely at the screen, you could make out the individual little red, blue and green onscreen dots. Each of those was a pixel. Stare closely at your LCD or OLED TV and you probably won't see the same thing, because today's TVs (and any other digital display) tend to pack in millions of pixels into the same space, leading to crisper images that appear more natural.
Each pixel's job in a digital camera of any type is to capture the image data of whatever you're pointing the camera at, relative to the reflected light from that object. A megapixel (MP) is a count of the number of pixels in an image, where the mega- prefix denotes a million. So an 8MP smartphone camera contains a sensor with 8 million pixels, a 16MP camera has 16 million pixels on its sensor and so on.
How many megapixels do I actually need?
There's no absolute hard and fast answer to this question, because it depends on how you define "need". In the context of images, the most important consideration how and where you intend to display them.
It's feasible to print lower-megapixel prints at larger sizes, but typically a higher megapixel count will allow you to more comfortably print A4 or poster-sized prints simply because you're expanding on a greater pixel count without making individual pixels more evident.
What about if you're only ever showing your photos on a digital screen? There, you can use some simple maths to determine a minimum megapixel count. A Full HD TV, for example, has a pixel count that equates to only 2MP, so anything better than that is in essence being downscaled. If you're viewing images on a 4K display, that figure bumps up to 8MP.
The reality for just about anything that isn't a pure budget phone is that it's likely to shoot at 8MP or better anyway right now, and that's not a trend that's going to reverse suddenly. If you never print, a higher megapixel count isn't going to be quite as critical a factor, although there are other areas where it can be surprisingly handy.
Using a higher megapixel rating to create a zoom is one of those areas. The last few years have seen a few camera/smartphone hybrids, such as Samsung's Galaxy Zoom or Panasonic's Lumix CM1, but for the most part, true optical zoom isn't something you get on a smartphone. What you can do however is the digital equivalent of zooming in by cropping a higher megapixel photo down to a lower megapixel equivalent.
That sounds confusing, so here's a quick example. If you took a photo of a bird that was flying far away, you might only get one shot with a 16MP camera. That gives you a shot with 4,920 by 3,264 pixels in it, but the bird might be the actual point of interest in the shot, with a lot of blue sky that you don't actually want. With a traditional camera with optical zoom, you'd simply zoom in to make the bird more of a focal point.
With a digital photo, if you cut down the sides to say, 3008 by 2000 pixels, you'd have the bird as much more of the focus of the shot, and you'd end up with what was, numerically, a 6MP photo from your 16MP original. Crop in closer to, say, 2048 by 1536, and you'd have an even closer looking photo at 3MP resolution. It's entirely feasible that even with that level of cropping, you could end up with a reasonably pleasant image, although obviously not one in quite as much detail as if you were able to get up close with your avian friend.
If all you've got is a 3MP camera, however, cropping in would bring the resolution much further down, leaving you with a much blockier bird shot, suitable perhaps only as an online avatar pic, but not useful for print purposes.
The one notable downside to a larger megapixel photo is that you've got to store all those pixels somewhere. If you're always shooting at the maximum capability of your smartphone, you'll use up a lot more space than if you shoot at lower resolutions. (If you set your phone to automatically upload pictures to a cloud service like iCloud, Google Drive or Dropbox, you'll also chew through a lot of data.)
Most smartphone camera apps allow you to set the megapixel count of each shot, so if you're using a phone with fixed storage, like an iPhone of any generation or the newer Samsung Galaxy S6 or Galaxy Note 5, turning the resolution down on your shots can be a great way to save storage space.
What other specs are important for the camera?
What's important to realise is that when it comes to image quality, it's not just a raw megapixel numbers game.
A smartphone with an 8 megapixel camera might actually take better pictures than one with a 21 megapixel camera, irrespective of camera operator skill.
That's because there's more to taking images in a variety of situations than just cramming a higher megapixel count into a camera.
Firstly, there's the size of the individual pixels to take into consideration, especially if you're shooting in low-light situations. Some smartphones, such as Apple's iPhone lines have cameras with lower megapixel counts, but larger individual pixel sizes within that overall count.
That matters, because the function of each pixel is to capture the light from whatever you're pointing the camera at. A larger pixel size, measured in nanometers, allows more light to make its way to the sensor, which means it's more sensitive to light in most situations. The larger the pixel size, the larger the overall sensor and the better the general capabilities of the smartphone camera.
Software can also play an important role here. Many smartphones offer camera apps finely tuned to the specifics of the onboard camera optics on that phone, making them better or worse than other smartphones even with the same raw megapixel count.
Then there's the issue of phones with dual lenses, each with their own optical quality. Dual lens camera phones are very big right now, but there's a wide array of approaches when it comes to what you do with that secondary lens. Some phones, such as the Samsung Galaxy Note 8 or iPhone 8 Plus offer a secondary telephoto lens. Others offer wide angle lenses, such as LG's G6.
You may also see secondary lenses that only shoot in monochrome, as is the case with Huawei's P10 Plus and Mate 10 handsets. This is both for the effect and in order to mix colours between both lenses for a more pleasing overall picture. Some dual lens cameras in the mid-range tend to have very low megapixel secondary cameras that are there purely to provide photographic "bokeh", or background focus blur, and on those phones you typically can't access the secondary camera as a separate entity.
Above that, different camera apps give you differing levels of control over your smartphone's functionality, from whether or not it'll shoot in RAW format to shutter timing and even specific modes for selfie shots to "beautify" your face -- although results there can vary quite widely.
It's worth pointing out, however, that it is perfectly feasible to take very good photographs with a lower quality or smaller sensor. There are always the issues of composition skill and timing to take into account. Having more tools in your camera toolbox, even with a smartphone camera, gives you more possibilities, but it won't automatically make you a better photographer.
Some smartphone cameras also feature optical image stabilisation. This packs tiny sensors into the camera lens to detect and correct for the minute vibrations that your hands make when taking photos. The practical end effect of all that technology should be a significant reduction in the amount of blur in your shots, although again the scale of lenses in this case means that they can't apply as much correction as on a "full" lens camera such as a DSLR.
Are some phones better for particular apps?
There really isn't a hard and fast rule that can declare a given camera "best" for, say, Instagram, Facebook or Flickr, because the reality of each given photograph should be judged on its own merits.
If, however, you're very serious about your smartphone photography, you're going to largely want to concentrate on the premium end of the sector, because this is where most phone manufacturers combine the majority of their smartphone camera design smarts.
Again, you can take great shots with an ordinary smartphone camera, but the cheaper models often have very slow startup times, so you miss shots, or slow focus speeds, so you may end up with blurry photos.
Are all smartphone cameras so good it doesn't matter?
No, or at least not yet. You can easily buy a smartphone for a couple of hundred bucks with a megapixel count that's identical to that of a high-end DSLR that could cost several thousand for the camera body alone, let alone the lenses to go with it, but the gulf in terms of photographic possibilities is immense.
What is clear with the current crop of smartphone cameras, especially in the premium space, is that they're certainly good enough for most everyday purposes, especially if you're only sharing shots online or viewing them on a smartphone screen. There's very little case for most compact point and shoot digital cameras because of this fact.
In the more budget-centric space, while you may see a camera touted as being anywhere from 8 to 21 megapixels in pure pixel count, it's not the only consideration to bear in mind when assessing its overall photographic quality.
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