This stuff can be very confusing if you’re just starting out in photography, not helped by the fact that Resolution and Megapixels are often used as “headline” features when selling a camera. You’re supposed to be impressed by how much resolution a camera has got, but as we’ll see in this section, bigger isn’t always better.
The number of megapixels you’ll see quoted simple means the number of light sensitive picture sites (pixels) that make up the image. The term “pixels” is an abbreviation of “picture elements”, and strictly speaking isn’t accurate when talking about camera sensors, but since the term is in such common use, we’ll stick to it, as it will only cause confusion otherwise.
So a 6 megapixel sensor in a camera would be made up of something like 2000 x 3000 pixels. The true number is often slightly lower than this, but only by a tiny amount, and really nothing to worry about. The exact number will of course vary depending on your camera, as well as the dimensions of your sensor. My Nikon D4, for example, with 16 megapixels, has an image sensor that contains 4928 x 3280 pixels, and my D800 with 36 megapixels has 7360 x 4912. Do the maths, and you’ll see these numbers add up to 16.16 million, and 36.15 million, but no-one tends to bother with numbers smaller than a million.
Resolution – Size and Cropping
More pixels means more resolution – therefore you’re able to make bigger images without messing about in software. The size of image you start with – the “native” image, if you will – is larger with a 16MP sensor than a 10MP sensor, and so on. If you were to take an image straight out of the 2 cameras in the video, and just print it at full size – the 16MP one (D4) would be 41 cm x 27 cm, and the 10MP one (Lumix) would be 33cm x 18cm.
You can change image size easily in software like Photoshop, but the important principle to understand is that if you want to make the image smaller, you’re discarding information, so you’re not losing quality. If, on the other hand, you want to make the image larger than it is, the software needs to do something called interpolation, whereby it imagines what pixels need to be inserted between the existing ones in order to make the image bigger. Up to a certain point there’s not much loss of quality when you do this, but obviously you can’t keep doing it for long before image quality starts to degrade quite rapidly.
So, we know that starting with a larger resolution image allows you to enlarge more without any loss of quality. Image size gets a bit more complicated though.
The actual size in physical terms of an image will also be affected by how many pixels per inch (ppi) or dots per inch (dpi) your image is. This can easily get confusing, as it’s not truly fixed. If we take one of the example images above from the Nikon D4 at 16 MP, the size is listed as 41 cm x 27 cm. However, this is at 300 pixels per inch, which is a fairly standard resolution for printing purposes. This simply means that there are 300 pixels to each inch of image. This may sound like a lot, but is quite a common setting to print at for smaller prints. If you have very few pixels per inch, then each pixel becomes more visible – take it to extremes and you’d have just one pixel per inch, and your image would be very blocky indeed!
So, more pixels per inch means you’ll have a higher image quality, but it’s not quite as simple as that. Never is, is it? For starters, with larger images you can actually get away with a lower PPI count, because you generally don’t view large images too close. Secondly, whilst the resolution of your camera will be fixed, the pixels per inch can be changed with software like photoshop. For example, the same image that’s 41 cm x 27 cm at 300 PPI, can be changed to 173 cm x 115 cm at 72 PPI. Doing this hasn’t actually altered the resolution of the image – there are still 16 million pixels – they’re just less dense. This will allow you to display your image larger, but the image won’t stand up to close scrutiny, as now there will be less pixels to each inch of the image.
This is where things can get confusing – the same image in different PPIs can give you wildly different sizes. It’s why if you want to be accurate when talking about resolution you should either list how many pixels there are, or be specific about what PPI the physical meaasurements represents. So, either say “2000 x 3000 pixels” or something like “41 cm x 27 cm at 300 PPI”.
You’re also able to crop into an image to make a different picture and still get a usable shot. You can see here how I’ve taken a 36 MP image, and cropped into it to create a new image. The new one is still 10MP, and so still OK to use in a lot of situations. You may be able to select different resolutions in your “image quality” settings, but we’ll come onto that in more detail later when we talk about file types. Getting ahead of ourselves slightly on that score – unless you’ve got a very good reason not to, I’d always suggest using the maximum resolution you can – that way you’ve got more options to crop later on if you so choose.
Bigger is not always better
Bigger is not always better. In my experience, quality images start around 12MP, and go up from there. More resolution means much bigger files sizes, so your camera may have a smaller buffer (that’s how many shots you can take before it has to stop and think) plus, you’ll obviously need bigger or more memory cards. You’ll also slow things down as you progress through your digital workflow on a computer. A 16MP image may take 2 seconds to process in Photoshop or similar, whilst a 36MP one might take as much as 12 – multiply that by hundreds of images over the course of a shoot, and that’s quite a difference.