At its core each camera has a device, similar to a computer chip, called the sensor. The sensor is broken into a regular grid of small elements, called pixels, which are all individual light sensors. The number of pixels in a sensor is a useful number in describing a camera because it tells you how finely the image can be resolved. For modern cameras, this number is generally in the millions of pixels, and so the unit megapixel (1 megapixel = 1,000,000 pixels) is used.
As the number of pixels on the sensor increases, so does its ability to resolve details, or resolution. This is due to the decrease in the size of each grid element, and is illustrated by the figure below. The increase in resolution by decreasing the pixel size does not come for free though. Smaller pixels are struck by much less light from the scene and thus are more highly susceptible to noise. We'll talk about this more in the section on ISO, but for now just remember that a very small sensor with 8 MP resolution is not the same as a large sensor with 8 MP resolution.
The pixels are arranged on a grid, and the number of pixels along the length and width of the sensor tells us the sensor's aspect ratio, or the ratio of its width to its height. Most consumer cameras tend to be in the aspect ratio of 4:3 (the same as on a non-widescreen monitor). SLR cameras tend to be in the aspect ratio of 3:2 (think a 4"x6" print), and some cameras are starting to appear in 16:9 (high-definition television). Although images can always be cropped to whatever aspect ratio you like at a later date, it is sometimes useful to pick a camera that matches where you intend to use the images the most.
Personally, I prefer the 3:2 aspect ratio. This is probably because it is close to the golden ratio, but who knows.
One common concern people have is if they have enough image resolution for printing. If you print with too low a resolution the image will appear unsharp like in the 50x50 image above. So, it becomes important to figure out what the required image resolution is for a particular print size. Two concepts related to this are pixels-per-inch (ppi) and dots-per-inch (dpi). There is a difference between the two, and it is subtle. The former corresponds to how an image is represented, and the latter corresponds to how and image is printed. If you care to learn more in depth, just click the ppi and dpi links.
Traditional wisdom has held that 300 pixels-per-inch (ppi) are needed for good quality prints, but people also argue that 150 ppi is enough. When possible, I prefer to have things at 300 ppi, but I think 150 is quite enough. It really depends on where the viewer is when looking at the image. Beyond 300 ppi, you don't really gain much because your eye cannot distinguish a difference at a short distance. A billboard you see hanging on a building is not printed at 300 ppi because it doesn't need to be. If you're printing something very large and don't mind if it looks a bit unsharp if people are less than a step away from it, 100 ppi works just fine. I have a 1 m x 4 m panorama hanging on my wall that was printed it at 100 ppi, and I think it looks great! The table below can be used as a handy reference for judging the maximum size of an image.
Almost all digital cameras on the market are greater than 8 MP these days meaning that most cameras you buy can make quite nice prints. At higher resolutions though, you have more flexibility after the photo is taken for cropping and other adjustments. So, the pixel war is dead. Long live the feature war!
When the images are saved there are a couple of different formats they can be saved in. The most common is with JPEG compression. This compression method is very good at reducing the size of the image file, but comes at the price of loss of details in the image. If no compression were used on the images below, the original 8-bit image would be 180 kilobytes (kb). Even JPEG at 100% quality JPEG reduces that amount, and the difference to the highest compression is a factor of almost 50%.
Most cameras will allow you to select between different image resolutions (the number of pixels stored) as well as the quality of the JPEG compression. This tends to be with either S, M, or L for the former and the little curve for the latter.
When using JPEGs, I would recommend shooting with the largest image possible and high image quality. This is because you can always reduce the image quality and size in post-processing, but you can never increase it. Memory card space is inexpensive these days, and unless you are on a very long trip, you're unlikely to run out of space.
The advantage of JPEG is that the files are relatively small (even a 20 MP image is only 3-4 megabytes (MB)), and that many image handling programs can work with them. There are a few disadvantages though, and they're not solely related to compression artifacts.
You may have heard the terms 8-bit and 16-bit before. These refer to the digital bits which are used to store a particular value. In actuality, a pixel isn't a single element, but rather made up of three different color component values: red, green, and blue. Each of these values is represented to the computer as a number signifying their intensity. An 8-bit number can store 256 different values. A 16-bit one can store 65,536 different values. Since a pixel is made of 3 elements, the total number of different colors an 8-bit pixel can represent is 256*256*256 = 16 million. Not bad, right? Well if we use 16-bit pixels, we can store 281 trillion. Now that's quite a bit more than people can actually see, but in these small changes we have an incredible amount of flexibility during post-processing.
Some sensors are only 8-bit, so there's not much need for more there. In many modern cameras, sensors can read up to 14-bit pixel values. So what happens to the extra information? Well, the camera automatically analyzes the scene and tries to do its best to get the right information crammed into that 8-bit image. Using other file formats, though, it is possible to save the files with this information and a bit more.
You may have noticed in the image above that there is a mode called RAW. This is a vendor-, and often camera-specific format that allows the camera to save the extra information. Many, but not all, image processing tools support RAW image processing. All this extra information doesn't come for free though. An image with my 21 MP camera is stored at around 20-30 MB. This means that every 500 images take up roughly 16 GB of space. Luckily though, disk space is really inexpensive these days, so you shouldn't have too much trouble storing the files. Depending on your computer speed though, they may be a bit slow.
The RAW vs. JPG debate is one that also rages on the web, and I'm a staunch supporter of the RAW format for everything. Some people say that you should shoot RAW only when you want high quality images and then for "normal" use, just use JPEG. For me the slight inconvenience of always having massive files is much better than having to switch between formats (what if you forget to switch?) and having disparate types of files to handle. If you really don't want to deal with the extra disk space then simply shoot RAW. Do your edits, and then export the files to JPEG. You're then free to remove the original RAW images or do with them as you like.
In my opinion, if you want to bring your photography to the next level, you really should shoot in RAW. The post-processing options you have with it are simply amazing and you can always save the final image to JPEG to save space. I'll touch on the advantages of RAW in other sections, but for now you'll just have to trust me.
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