DIGITAL CAMERA SENSORS

A digital camera uses an array of millions of tiny light cavities or "photosites" to record an image. When you press your camera's shutter button and the exposure begins, each of these is uncovered to collect and store photons. Once the exposure finishes, the camera closes each of these photosites, and then tries to assess how many photons fell into each. The relative quantity of photons in each cavity are then sorted into various intensity levels, whose precision is determined by bit depth (0 - 255 for an 8-bit image).

Cavity Array

Light Cavities

However, the above illustration would only create grayscale images, since these cavities are unable to distinguish how much they have of each color. To capture color images, a filter has to be placed over each cavity that permits only particular colors of light. Virtually all current digital cameras can only capture one of three primary colors in each cavity, and so they discard roughly 2/3 of the incoming light. As a result, the camera has to approximate the other two primary colors in order to have full color at every pixel. The most common type of color filter array is called a "Bayer array," shown below.

Color Filter Array

Photosites with Color Filters

A Bayer array consists of alternating rows of red-green and green-blue filters. Notice how the Bayer array contains twice as many green as red or blue sensors. Each primary color does not receive an equal fraction of the total area because the human eye is more sensitive to green light than both red and blue light. Redundancy with green pixels produces an image which appears less noisy and has finer detail than could be accomplished if each color were treated equally. This also explains why noise in the green channel is much less than for the other two primary colors (see "Understanding Image Noise" for an example).

Original Scene
(shown at 200%)

What Your Camera Sees
(through a Bayer array)

Note: Not all digital cameras use a Bayer array, however this is by far the most common setup. The Foveon sensor used in Sigma's SD9 and SD10 captures all three colors at each pixel location. Sony cameras capture four colors in a similar array: red, green, blue and emerald green.

BAYER DEMOSAICING

Bayer "demosaicing" is the process of translating this Bayer array of primary colors into a final image which contains full color information at each pixel. How is this possible if the camera is unable to directly measure full color? One way of understanding this is to instead think of each 2x2 array of red, green and blue as a single full color cavity.

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This would work fine, however most cameras take additional steps to extract even more image information from this color array. If the camera treated all of the colors in each 2x2 array as having landed in the same place, then it would only be able achieve half the resolution in both the horizontal and vertical directions. On the other hand, if a camera computed the color using several overlapping 2x2 arrays, then it could achieve a higher resolution than would be possible with a single set of 2x2 arrays. The following combination of overlapping 2x2 arrays could be used to extract more image information.

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Note how we did not calculate image information at the very edges of the array, since we assumed the image continued on in each direction. If these were actually the edges of the cavity array, then calculations here would be less accurate, since there are no longer pixels on all sides. This is no problem, since information at the very edges of an image can easily be cropped out for cameras with millions of pixels.
Other demosaicing algorithms exist which can extract slightly more resolution, produce images which are less noisy, or adapt to best approximate the image at each location.

DEMOSAICING ARTIFACTS

Images with small-scale detail near the resolution limit of the digital sensor can sometimes trick the demosaicing algorithm—producing an unrealistic looking result. The most common artifact is moiré (pronounced "more-ay"), which may appear as repeating patterns, color artifacts or pixels arranges in an unrealistic maze-like pattern:

Second Photo at ↓ 65% of Above Size

Two separate photos are shown above—each at a different magnification. Note the appearance of moiré in all four bottom squares, in addition to the third square of the first photo (subtle). Both maze-like and color artifacts can be seen in the third square of the downsized version. These artifacts depend on both the type of texture and software used to develop the digital camera's RAW file.

MICROLENS ARRAYS

You might wonder why the first diagram in this tutorial did not place each cavity directly next to each other. Real-world camera sensors do not actually have photosites which cover the entire surface of the sensor. In fact, they often cover just half the total area in order to accommodate other electronics. Each cavity is shown with little peaks between them to direct the photons to one cavity or the other. Digital cameras contain "microlenses" above each photosite to enhance their light-gathering ability. These lenses are analogous to funnels which direct photons into the photosite where the photons would have otherwise been unused.

Well-designed microlenses can improve the photon signal at each photosite, and subsequently create images which have less noise for the same exposure time. Camera manufacturers have been able to use improvements in microlens design to reduce or maintain noise in the latest high-resolution cameras, despite having smaller photosites due to squeezing more megapixels into the same sensor area.

10 Top Photography Composition Rules

There are no fixed rules in photography, but there are guidelines which can often help you to enhance the impact of your photos.
It may sound clichéd, but the only rule in photography is that there are no rules. However, there are are number of established composition guidelines which can be applied in almost any situation, to enhance the impact of a scene.
These guidelines will help you take more compelling photographs, lending them a natural balance, drawing attention to the important parts of the scene, or leading the viewer's eye through the image.
Once you are familiar with these composition tips, you'll be surprised at just how universal most of them are. You'll spot them everywhere, and you'll find it easy to see why some photos "work" while others feel like simple snapshots.

Rule of Thirds

Imagine that your image is divided into 9 equal segments by 2 vertical and 2 horizontal lines. The rule of thirds says that you should position the most important elements in your scene along these lines, or at the points where they intersect.
Doing so will add balance and interest to your photo. Some cameras even offer an option to superimpose a rule of thirds grid over the LCD screen, making it even easier to use.

Notice how the building and horizon are aligned along rule-of-thirds lines. Image by Trey Ratcliff.

Balancing Elements

Placing your main subject off-centre, as with the rule of thirds, creates a more interesting photo, but it can leave a void in the scene which can make it feel empty. You should balance the "weight" of your subject by including another object of lesser importance to fill the space.

Here, the visual "weight" of the road sign is balanced by the building on the other side of the shot. Image by Shannon Kokoska.

Leading Lines

When we look at a photo our eye is naturally drawn along lines. By thinking about how you place lines in your composition, you can affect the way we view the image, pulling us into the picture, towards the subject, or on a journey "through" the scene. There are many different types of line - straight, diagonal, curvy, zigzag, radial etc - and each can be used to enhance our photo's composition.

The road in this photo draws your eye through the scene. Image by Pierre Metivier.

Symmetry and Patterns

We are surrounded by symmetry and patterns, both natural and man-made., They can make for very eye-catching compositions, particularly in situations where they are not expected. Another great way to use them is to break the symmetry or pattern in some way, introducing tension and a focal point to the scene.

The symmetry of this chapel is broken by the bucket in the bottom right corner. Image by Fabio Montalto.

Viewpoint

Before photographing your subject, take time to think about where you will shoot it from. Our viewpoint has a massive impact on the composition of our photo, and as a result it can greatly affect the message that the shot conveys. Rather than just shooting from eye level, consider photographing from high above, down at ground level, from the side, from the back, from a long way away, from very close up, and so on.

The unusual viewpoint chosen here creates an intriguing and slightly abstract photo. Image by ronsho.

Background

How many times have you taken what you thought would be a great shot, only to find that the final image lacks impact because the subject blends into a busy background? The human eye is excellent at distinguishing between different elements in a scene, whereas a camera has a tendency to flatten the foreground and background, and this can often ruin an otherwise great photo. Thankfully this problem is usually easy to overcome at the time of shooting - look around for a plain and unobtrusive background and compose your shot so that it doesn't distract or detract from the subject.

The plain background in this composition ensures nothing distracts from the subject. Image by Philipp Naderer.

Depth

Because photography is a two-dimensional medium, we have to choose our composition carefully to conveys the sense of depth that was present in the actual scene. You can create depth in a photo by including objects in the foreground, middle ground and background. Another useful composition technique is overlapping, where you deliberately partially obscure one object with another. The human eye naturally recognises these layers and mentally separates them out, creating an image with more depth.

Emphasise your scene's depth by including interesting subjects at varying distances from the camera. Image by Jule Berlin.

Framing

The world is full of objects which make perfect natural frames, such as trees, archways and holes. By placing these around the edge of the composition you help to isolate the main subject from the outside world. The result is a more focused image which draws your eye naturally to the main point of interest.

Here, the surrounding hills form a natural frame, and the piece of wood provides a focal point. Image by Sally Crossthwaite.

Cropping

Often a photo will lack impact because the main subject is so small it becomes lost among the clutter of its surroundings. By cropping tight around the subject you eliminate the background "noise", ensuring the subject gets the viewer's undivided attention.

Cut out all unnecessary details to keep keep the viewer's attention focused on the subject. Image by Hien Nguyen.

Experimentation

With the dawn of the digital age in photography we no longer have to worry about film processing costs or running out of shots. As a result, experimenting with our photos' composition has become a real possibility; we can fire off tons of shots and delete the unwanted ones later at absolutely no extra cost. Take advantage of this fact and experiment with your composition - you never know whether an idea will work until you try it.

Digital photography allows us to experiment with different compositions until we find the perfect one. Image by Jule Berlin.

Composition in photography is far from a science, and as a result all of the "rules" above should be taken with a pinch of salt. If they don't work in your scene, ignore them; if you find a great composition that contradicts them, then go ahead and shoot it anyway. But they can often prove to be spot on, and are worth at least considering whenever you are out and about with your camera.