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Photography

Understanding the Fundamentals of Camera Sensors

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Light travels through a lens, the shutter opens, and a moment is preserved by capturing it on the camera's sensor. This chip is an absolute essential in creating digital images. However, you may not have a good idea of how it all works. If you're wanting to demystify the magic of how your digital SLR works, look no further than today's Basix article all about camera sensors.

Megapixels & Resolution

If there is one thing that the average camera user knows about a camera's sensor, it is the megapixel count. Loved by the beginner, the amount of pixels on a camera's sensor determines the amount of data that can be captured.

What do megapixels actually mean? Each "megapixel" a million pixels capable of captures bits of color that will result in an image. Let's take a file from my Nikon D300 for example. The maximum resolution of a file produced by the D300 is 4288 x 2848. On the long side of an image are 4,288 pixels, while the shorter side has 2,848 pixels. If we multiply 4288x2848, the resulting number is 12.2 million. Want to know the D300's megapixel count? You guessed it, 12.2 megapixels. (Nikon quotes it as 12.3.)

While megapixels are a valuable measurement of a camera sensor's capabilities, more megapixels is not always better. One reason that camera companies have somewhat capped the number of megapixels that they will put into a sensor is that more megapixels usually means higher levels of noise.

There is also a law of diminishing returns. Digital cameras have been able to produce large print sizes for years with 6 or fewer megapixels. This isn't going to change - any camera that you're going to buy today is now capable of producing large prints. However, before upgrading to the 18 megapixel camera, ask yourself what you will use that massive resolution for. While professionals may need huge amounts of resolution for their purposes, if you're just getting started with photography, don't buy into the megapixel myth.

Don't get me wrong, having the extra resolution for cropping is great. Just don't purchase one camera over another for megapixels alone. In conclusion, megapixels are a measure of only one of a camera's capabilities.

Noise & Sensor Sensitivity

A camera's "ISO" settings adjust the sensitivity of the sensor to light. In the film camera days, the ISO was associated with the film you loaded in the camera and could not be changed. Digital sensors have the advantage of being adjustable from shot to shot.

You may know that when you are in low light, you should consider bumping up the ISO to accommodate a usable shutter speed. A friend once asked me that since a higher ISO allowed for greater light capture, why shouldn't we always set the ISO as high as it could go? Wasn't this the same as a super fast lens or a slower shutter speed?

He was right in thinking that this was the case - in fact, raising the ISO does allow for more shutter speed or aperture flexibility. However, this comes at a cost. A camera's sensor works best at the lowest ISO. At this setting, you're going to see the best colors, lowest amount of noise, and overall highest image quality.

Noise is basically the digital era equivalent of grain. It's all of those fine little dots you'll see, especially in dark shots. I performed some tests below with my Nikon D300 so you can see the differences in ISO.

As the ISO setting increases, the image may increase in noise and decrease in overall quality.

From sensor to sensor, ISO performance varies. One of the biggest strides in recent camera technology are the high ISO capabilities of modern cameras. Yesterday's ISO 400 is now matched in quality by today's ISO 800. The boundaries of low light performance continue to be pushed to levels that were never before possible.

Sensor Size

Not all camera sensors are built equally. Each company uses their own technologies and specifications in building the latest sensors for the latest cameras. The specifications used have a great impact on the overall quality of the sensor, and as a result, the images made with it.

One of the major factors that determines image quality is the physical size of this sensor. This is why a DSLR is going to produce better images than almost any point and shoot. The size of the sensor in a pocket sized camera is merely a fraction of its SLR counterpart. Typically, larger sensors will also perform better in high ISO situations - an effect that can certainly be witnessed when comparing point and shoots to even entry level DSLR's.

By now, you may have heard of an effect called a "crop factor". This term helps us describe the size of a camera sensor in relation to a "standard" size. What is this standard size? The reference point is a "full frame" sensor, meaning a sensor that is the same size as a 35mm frame of film. Any sensor that is smaller than a full frame sensor is going to experience the crop factor.

The red box represents the area that would be captured by a full frame sensor, while the blue area is respresentative of a crop factor camera's field of view.

You probably know that to crop an image is to use a selected portion of the image, or to basically select a smaller region of it. On a crop factor camera, you are getting a field of view that is tighter than that of a full frame sensor.

Believe it or not, there are sensors larger than 35mm "full frame" sizes. Digital medium format is a growing field that is favored by product and studio photographers for the massive resolution that can be offered. Phase One is now offering a medium format 80 megapixel camera and competitors such as Mamiya and Hasselblad are sure to follow with similar offerings.

How does a sensor work?

Today, our sensors are digital. Years ago, the "sensor" was film. Both of these are essentially the medium on which images are recorded. A lens plus some type of sensor is the basic equation for creating an image. There are a lot of other pieces in the machine, but these are the two keys to creating an image.

As mentioned earlier, there are a few different technologies that drive camera sensors. Two of the most popular types of sensors are "CCD" (charge-coupled device) and "CMOS" (complementary metal oxide semiconductor). CCD sensors work by transporting electrical charge and converting it to a digital signal. CMOS sensors use red, green, and blue color filters and pass data through metal wiring and onto photo diodes. Most modern sensors are of the CMOS variety. The CCD sensors had somewhat reached their technological limits and less common in digital cameras.

In addition to the common CCD and CMOS, Sigma has developed their own type of sensor called "Foveon" that has caused quite the stir. Using a proprietary technology, Sigma claims that their new SD1 camera will be able to achieve 46 megapixel images using an APS-C sized sensor. This is done by using a three-layer sensor, with each layer being responsible for 15.3 megapixels. Some have disputed the total validity of this lofty claim, and with the camera yet to be released, the jury is still out. But the "Foveon" sensor has been around for a few years now, and other (lower resolution) cameras have been made using it. You can do some research and see if you prefer the results from this sensor.

Sigma's Foveon sensor technology claims ultra high resolutions by use of a unique layered sensor.

Sensor Care

Have you noticed any dark spots on your photos? Perhaps while taking a landscape photo, you notice some small dark regions on the bright blue sky. Although you can clone them out easily in Photoshop, what you're viewing are dust spots on the sensor. More accurately, the dust spots are on the filter on top of the sensor.

While not a huge problem, they are an annoyance that you may want to deal with. There are a couple of steps that you can take to rid your sensor of the dust bunnies. The first thing that I would recommend is to use a "Rocket Blower" style tool to try and blow loose dust off of the sensor. These are a great tools to have on hand for all types of camera gear cleanup.

To use the Rocket Blower, first put your camera in bulb shutter speed mode. In this mode, holding the shutter release button opens the shutter, exposing the sensor, until releasing it. Doing so allows access to the usually protected sensor. After the mirror is up, use the rocket blower to blow a few puffs of air into the sensor area. Holding the camera upside down ensures that gravity will do its part in pushing dust out.

A Rocket Blower is a great tool for sensor cleaning.

The alternate method is to use "contact" cleaning, or a method of cleaning in which the sensor is touched in order to remove dust and particles. This type of cleaning is usually employed when the dust is more serious. There are a number of methods, some using sensor brushes and others using liquid solutions.

Keep in mind serious gearheads may scream at you for using this method. Using the bulb shutter mode means that the sensor is on, and a charged sensor (especially a CCD sensor) will actually attract dust with a static-like draw. To "properly" clean your camera, consult to owner's manual. There is usually a cleaning mode that allows access to the sensor while it is off, although you may need to buy a special plug. With this is mind, I've cleaned my sensors using the methods above for years with no noticeable negative effects.

When my camera needs a more serious cleaning, I send it off, simple as that. It isn't worth risking your camera trying an at-home hackjob.

Conclusion

The digital camera sensor has revolutioned photography. With technology seeming to improve everyday, who knows what could be possible in the next few years? The last 10 years have seen the digital sensor become a part of everyday life, and the next 10 could be just as exciting as high ISO performance and image quality improve.

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