By Remy Levine, Bigstock contributor Before I get into the nitty gritty I need to mention that pixels are only part of the story when it comes to digital photographs. You also need to consider optical resolution, essentially the ability of your camera and lens to accurately direct the light onto the sensor and for that sensor to record it. More pixels does not automatically mean more details or crisper edges. For the purpose of this article we are only discussing digital resolution, pixels and more pixels and even more pixels.
Modern digital cameras are rated in megapixels, scanners and printers are rated in DPI and storage is measured in megabytes or gigabytes, so what do all these numbers mean? They ALL mean the same thing, how much data is being recorded or stored. Megapixels translates to millions of pixels, so one megapixel is one million pixels and six megapixels is six million pixels. DPI, or dots per inch, is similar, but adds another element, physical size. Your one megapixel image might be one inch across or one hundred inches but still have the exact same number of pixels. DPI is what happens when you break it down to a specific physical size and is useful when outputting the image to print or projecting it. For all intents and purposes DPI is not important until the image leaves the computer.
DPI is important when scanning though, since the physical size of the object you are scanning must be considered when choosing a DPI setting for the scan. Scanning a 35mm frame at 2400dpi results in far fewer pixels than scanning a 5x7 print at the same dpi. A one inch square image at 1000dpi has 1000 pixels across and 1000 pixels down, so it is 1000x1000, or one megapixel. A 2000x500 pixel image is also one megapixel, while a 2000x1000 pixel image is two megapixels. I used nice even numbers for simplicity, but essentially you just multiple the two dimensions and divide by one million to get the number of megapixels.
More pixels means more data, generally larger file sizes, and hopefully more detail, though that last bit assumes you haven’t already maxed out your camera’s optical abilities. To use an extreme example, a poor quality lens on a 12MP camera might result in twelve million pixels, but it will still look fuzzy, and probably no better than if it was shot at one or two megapixels.
Anyway, back to digital. Your resolution is a one-time shot, no pun intended. It is limited by the highest starting point, if you shoot the image at 6MP with your camera that is all you get, six million pixels worth of information/resolution. You can add pixels later, but you’re not going to add any resolution, they’re just filler pixels that add no detail.
If you crop an image you are losing pixels, so make sure to do as much of your cropping as possible before taking the picture. If you zoom in on part of your image after taking it you cannot gain detail, the pixels simply don’t hold any more information. This is why you generally want to shoot at the highest resolution you can, then create a smaller file later if needed.
For example, I often shoot with a 4MP camera set to max. I save that file just in case I want to start fresh, then I might crop a bit, adjust the physical size for a printout, and tweak color and contrast. By the time I’m done the image only has 3MP. If I had shot at 3MP I might only have 2.25MP, or ¾ of what I started with. Too low a resolution and pictures start to look grainy and often noisy (like static on the picture) when viewed at normal sizes. This is just as true for film as it is for digital, both are made up of many tiny dots, it’s just how you get those dots that changes.
So what does all this mean in the real world? That is simpler. Your computer screen shows anywhere from 60 to 90 DPI, depending on size and your settings. That means that for it to show an image 1200x800 pixels requires (assuming a generic 75dpi screen) 16”x11” of screen space. That’s a pretty big picture, but not at a very high dpi. If you printed that image at 16x11 it would look grainy and probably kind of fuzzy. Instead, you’d want to print it at a higher DPI, maybe 300dpi, which is a standard print resolution. At 300dpi that same image is only 4”x2.6”. Not very big, huh? This is something to keep in mind when working with stock photography images. What size is your likely customer going to use them at? A great shot is worthless if they can’t print it largely enough to matter. BigStockPhoto.com has an upper limit, don’t be afraid to push that upper limit, it’s far better to be near the top than near the bottom of the resolution range. To get a 7x5 at 300dp requires the picture be 2100x1500 pixels, which is just over three megapixels. In practical terms that means if you shot at three megapixels and didn’t crop at all, that is what you get, but if you cropped much off the image you’re limiting how large the buyer can use your image, which might make them choose something else. This is why the higher megapixel cameras are always big news; it increases the size an image can be printed at without losing perceived quality.
While opinions vary, I feel that roughly six megapixels is a good match for high quality 35mm film for resolution. Of course, the exact numbers would vary with the film type, processing, and an endless number of variables, which is one reason digital is so popular, you don’t have to be a chemist and a mad scientist to get the best results from your camera.
I have one last comment about resolution. In scientific terms, resolution means how well something can resolve an image. Think of it as being able to see more clearly. It is often measured by closely spaced lines on a contrasting background, the more closely spaced lines that can be seen as separate distinct lines the higher the resolution of that image. This is what photographers refer to as the sharpness of a lens, and it is a big variable. While many digital cameras do not offer interchangeable lenses it is still something to check for when purchasing a camera, just having a higher megapixel count doesn’t mean a particular camera has a better resolution. With the digital SLR’s with interchangeable lenses it is important when picking lenses to consider their quality and sharpness. A cheap lens might save some money but result in fuzzy images regardless of how many pixels are involved. This is not just a matter of buying premium brands, as I own several Nikon lenses that are too fuzzy for my taste, and know of equivalent lenses from nearly every brand.
Do your research carefully. Remember, what you start with is the best you get, and that applies to image sharpness and resolution of the optics just as much as the number of pixels. There are many good books on this topic, and it is regularly discussed in photography magazines. Digital photography just changed the way images are recorded, not the principles of light and optics, so all those old books still apply.