What is Resolution?
Resolution is a term that is tossed around rather loosely in the camera world. Most often, it is thought of as synonymous with “megapixels,” and camera companies are generally fine with you believing this. The term “resolution,” however, existed long before photography went digital. In reality, the resolution of any camera is determined by many factors. Sensors, lenses, settings, and technique all play a role in determining the final clarity of your images.
The general thinking of “more is better” when it comes to megapixels is not necessarily wrong, but it doesn’t mean that by doubling your megapixels you will double your apparent image quality. To understand camera resolution, I like to compare it to another electronics industry: televisions. Currently, TV manufacturers are making much ado about 4K, or “Ultra High Definition,” sets. On paper, 4K TV looks great, promising four times the resolution of full 1080p HD. You can look at a graph like this one and think, “wow!”
Unfortunately, such illustrations serve for very little in the real world. It turns out, most people who own HD televisions today can’t even tell the difference between the smaller 720p and full 1080p resolutions, even on very large televisions. The reasons for this vary, but the most obvious one is viewing distance. It is uncomfortable to sit much closer than 10 feet to a 50” TV, but our eyes are simply not capable of discerning sufficient detail at that distance to truly appreciate 1080p. Perceived sharpness depends on the ratio between screen size and viewing distance, so to truly appreciate the added resolution of 4K, one would need to sit unbearably close to a very large screen (akin to getting stuck in the front row of a movie theater).
Why am I taking up time talking about television? Because a 50” 1080p display is roughly two (2) megapixels. That’s it. And if a measly two megapixels makes for a very attractive 50” image from a comfortable viewing distance, then you should be able to sleep easy tonight knowing that your camera has a sufficient number of pixels. Of course, still photography is inherently different from the moving images that generally grace your television, and people tend to look at photographs much closer than they do video. Additionally, we all like to be able to crop, so having more pixels to start with doesn’t hurt. Still, the fact remains: any modern digital camera has more than enough pixels to suit the vast majority of photographers’ needs, and for those of you who need the best possible resolution you can get, it’s time to look at other factors.
If the marketing hype is to be believed, then today’s point-and-shoot cameras offer the same resolution as many professional level DSLRs (Digital Single Lens Reflex). Yet, anyone who has compared their point-and-shoot images to those from a DSLR knows that they are not as sharp, crisp, clear, or whatever adjective you choose to describe it. This is usually true even when the point-and-shoot has more megapixels than the DSLR. The biggest factor limiting resolution on the point-and-shoot is not its pixel count, but the physical size of its imaging sensor.
Small sensors are what makes it possible to have such small, portable cameras; but that portability comes at a cost. Subjective differences aside, small-sensor cameras simply cannot resolve as much detail as larger-sensor cameras (like DSLRs). There are two main reasons for this. First, a smaller sensor sees less light, meaning the processor has to work harder to generate a proper exposure. This leads to increased noise (think “grain”). This is where the term signal-to-noise ratio (SNR) comes in, and refers to the number of pixels that contain true image data (signal) compared to the pixels that carry noise. If you have 16 megapixels but half of them are noise, you're not getting 16MP worth of resolution. No camera on the market today has a SNR that is anywhere near that bad (2:1), but the point is that noise affects resolution. Larger sensors have inherently better SNR because they capture more light, and thus have increased resolution over point-and-shoot sensors, even when they don't have as many megapixels.
The second issue is diffraction. Diffraction is the result of light waves creating interference as they spread out after passing through the aperture of a lens. There is a lot of techo-mumbo-jumbo going on here and you can read all about it on Wikipedia if you so desire, but for the sake of this article, just know this: the smaller the aperture, the worse the diffraction. The effect of diffraction is essentially a smudging of detail, which will impact cameras with smaller pixels more so than cameras with larger pixels. It is not limited to point-and-shoot cameras, but because those cameras typically have the smallest pixels, the effect is more pronounced. Because diffraction is related to the size of an aperture, you will sometimes see a “diffraction limit” mentioned as a particular aperture for a particular camera. For example, the diffraction limit for a 36MP Nikon D800 is around f/8. Shooting at smaller apertures (f/16, f/22 ...) will result in a loss clarity as the actual light passing through the lens does not contain enough detail to realize the full potential of that sensor. Small sensor cameras will hit their diffraction limit much sooner, perhaps even as low as f/2.8 depending on the camera. If your point-and-shoot contains a 16MP sensor, chances are that very rarely is your camera actually resolving that much detail.
Resolution is also affected by the quality of the lens. On interchangeable-lens cameras, you can usually tell the good ones from the not-as-good ones just by looking at the price tags. Lenses affect resolution mainly through contrast. So-called “high frequency” areas of an image—areas of fine detail with stark contrast—often prove challenging for any lens. Chromatic aberration, which appears as colored fringing around edges, is common here. Lenses typically show better contrast and sharpness when stopped down to the middle of their aperture range, but sometimes this point is beyond the diffraction limit of a given camera. Knowing the details of your sensor and lens will help you set everything correctly to maximize resolution.
Which brings us to the final point: technique matters. Never has a tripod been more important than with today’s ultra-high megapixel cameras. Whether that’s a 36MP DSLR or a 16MP point-and-shoot, putting your camera on a tripod is the first step toward maximizing image quality. If your camera has the option to use a remote shutter release, this will also help improve resolution as it removes your (shaking) hand from the camera. Even the steadiest hand will cause motion blur at the microscopic scale of pixels. For DSLR’s, using a shutter delay or mirror lock-up function will also remove vibration caused by the slapping of the mirror as it raises out of the way. Now, these techniques may not matter to most people in average, everyday shooting situations, but for those of you looking to truly maximize detail, especially for making large prints, they can be very important.
As for whether or not to get a 4K TV, well, I’ll leave that up to you.