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Tips and Tricks: (Sensor) Size Matters
Here we are again, the close of yet another month...and in this case, the close of another year. It is also time to call the class to order to learn something else about photography. Remember back in the good old days when you used to load a roll of film into the back of the camera and snap away? The image was stored on a frame of film to be developed later by a lab, or your own darkroom. In many ways that has been the way of photography since the dawn of the camera. So, how in the world does digital photography happen...there is no film, its only data. Here is all you ever wanted to know about digital capture but were afraid to ask.
To really understand digital capture we must take a very quick look at film and the different formats. The major players are medium format which has a frame that measures 6 x 4.5 cm up to a panoramic 6 x 17 cm. When you think about it, that is quite large being roughly twice the size of a frame of 35mm film. That frame measures in at 36 x 24mm. The 35mm format is what most of us are familiar with, and is the most popular film format. However, medium format, as well as large format (can be larger than a sheet of copy paper) provides much better quality when it comes to enlargements.
The reason for the difference in quality is very apparent. To obtain a 16 x 20 in print one would have to enlarge a large format negative two times, while the medium and 35mm formats would require exponentially more enlarging to achieve that 16 x 20 in print. So, why does 35mm reign supreme for so many forms of photography....cost, weight, and portability. The larger the format, the larger the camera.
Hey, we started off talking about digital sensors, why do we need to know so much about film? Simple, today's digital cameras are based on film cameras in how they are designed. The sensor in even the most modern digital camera is very closely related to film in both capture, and in reproduction.
The graphic above shows the various sizes of consumer digital sensors. The largest (represented by the white scales) is the same size as a 35mm frame of film. This is why this particular sensor is called a "full frame" sensor. There are several benefits to this sensor format that we will get into later on. The next size down actually represents two different sensors, but they are collectively referred to as the APS-C Sensor (red and blue scales). This is what is found on most of your DSLR (Digital Single Lens Reflex) cameras. The sensor is physically smaller than a full frame sensor or 35mm film. That is not necessarily a bad thing, but we will talk more about that later.
Another popular size in DSLR's is the 4/3 system (green) which was developed from the ground up by several camera manufacturers in an attempt to create smaller cameras and lenses while making a universal mounting system. Below the 4/3 size you will find several increasingly smaller boxes representing the sensor size in the vast majority of point and shoot cameras.
Who cares about the sensor size, my camera is a 12.3MP (Megapixel) monster that fits in my pocket! If you are saying that, you have fallen for the megapixel myth, which is a great advertising gimmick. When it comes to marketing, consumers love numbers that are easy to compare. The sales people love being able to say this 12MP camera is much better than this 8MP model over here. I'm here to tell you that this is only part of the story.
Lets look at how a sensor works shall we? An images sensor is nothing more than a silicone chip which has an array of tiny light-sensing diodes. These diodes are referred to as "picture elements" or pixels. The typical sensor houses filters placed above these pixels that only allow red, blue, and green to enter the pixel and be recorded. Fun fact, there are usually twice as many green sensors as red or blue. My Sony F828 had a four color sensor that divided the greens up to green and emerald, but this approach never did catch on. The number of individual pixels on a sensor is represented by the designation of XXMP or 10MP (10 million pixels) in the advertising material.
For a time, the charge-coupled device (CCD) sensor was the industry standard and was found on cameras from the smallest point and shoot, to the most advanced DSLR. As it turns out the CCD took a lot of power to run and was expensive to produce. The CMOS (complementary metal oxide semiconductor) sensor was more frugal in using energy as well as being cheaper to produce. However, until just recently, the CMOS sensor was prone to higher noise levels than the CCD which made it much less desirable. That has since changed, and you will find the CMOS sensor in a growing number of cameras, both point and shoot and DSLR.
An interesting version of the CCD has been developed by Sigma, and is called the Foveon X3. This sensor deserves mentioning if for no other reason than to explain how it affects the actual megapixel count. The X3 sensor is actually three separate layers of pixels instead of just one plane. Each level is defined by how many pixels it contains, just like that of a normal CCD or CMOS sensor. The difference is, each level might only be 3.4MP but will be listed as a 10.2MP camera. Because of this design, that 10.2MP camera will actually resolve image data more closely to a contemporary 6MP CCD or CMOS. It is up to personal taste as to whether or not the final image quality is on par or not with the other sensors.
Benefits of size
As I mentioned before, having a full frame sensor (compared to a 35mm frame of film) has its benefits. For one, the sensor is completely compatible with your collection of SLR (film) lenses. This is great news for those that have a collection of Nikon F mount lenses that they used with their F-bodies. They will work just as they did with that film body. The smaller APS-C or 4/3 sized sensors will require a conversion factor to determine actual focal length...more on that in a minute.
The other major benefit is noise suppression. Take two 10MP sensors, with one being a full frame, and one being an APS-C sized sensor. There are 10 Million pixels on both sensors, however, the pixels are larger on the bigger sensor. This makes them more sensitive to light (more surface area), and makes it easier for the sensor to capture more detail. It also allows the camera to operate at higher ISO's with less noise than the smaller sensor counterparts. This is the main reason that there is so much push to get full framed sensor cameras on the market. This argument is becoming less and less compelling with each new generation of DSLR, however.
The noise issue is becoming less and less of a problem with the APS-C sized sensors. With advances in the CMOS technology, the smaller sensored cameras can shoot well up into the ISO 3200 range, and even more before noise becomes an issue. The quality of the sensors is has come extremely close to matching the detail captured by the full frame sensor as well. So, it is looking like the APS-C sensors and the full framed sensors are on an even playing field. I would agree with that. However, there is one other thing that I would like to put into the mix, and that is the crop factor.
Remember when I said that full framed sensors are completely compatible with film lenses? Well, so are the APS-C sensors, with one caveat. They each come with a crop factor to take into account the smaller sensor. The image that is produced by the lens, and shown to the sensor is still the same regardless of the sensor size, but since the sensor is smaller, there is a portion of the image that is lost around the edges. This is what is knows as the crop factor.
The two APS-C sized sensors which are used by Canon and Nikon require you to multiply a conversion of 1.6x and 1.5x (respectively) to the focal length as displayed on the lens. The 4/3 system requires a conversion of 2x. For instance, on a Canon APS-C body, a lens of 10mm will actually be equivalent to 16mm in 35mm (full frame) format. A 100mm lens will be equivalent to a 160mm lens in the 35mm format. As you can see, as the lens gets longer (on the telephoto end), the difference is compounded. While this cropping factor is detrimental to the wide angle (short end) it is a great boon to the telephoto end. While you only have a 300mm lens in your bag 'o goodies, if you put it on a cropped frame camera then you will effectively be shooting with a 480mm lens with no loss of light. The full frame camera would have to have a much more expensive telephoto lens to match that focal length.
What about the wide angle you ask? Well, there are digital only lenses that are extreme wide angle, going to around 10mm to duplicate the old standby of 17mm wide angle lenses from the film era. On the subject of digital only lenses, they usually cannot be used satisfactorily on a film orfull framed camera. They will introduce significant vignetting since they are optically designed to present a smaller image to the smaller (than full framed) sensor.
Where does that leave the point and shoot?
We have spent most of our time here talking about DSLR's because in a point and shoot, there is no correction to be made. However, with the inherently smaller sensor, the point and shoots are very prone to digital noise, and with all of those millions of pixels crammed into a very small space, detail can suffer. You might find that you get better quality pictures from a sensor with less pixels when you are looking at such a small sensor size. Normally speaking 7MP, or even 5MP cameras will provide file sizes large enough to satisfy even moderate enlargements. But the salesman will have a hard time telling you that the 5MP camera is better than the 10MP camera won't he?
Written by Greg A. Kiser
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Oy vay...I'm going to have to read this one again. ;) C.
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When you say noise level, are you talking about clarity? more noise equals less detail? You've mentioned noise level before, and I thought I understood it, but could you explain it again for me? The point and shoot cameras have made me lazy and I've forgotten what I used to know about taking pictures with an SLR! bea
http://journals.aol.com/bgilmore725/Wanderer/ -
They look intimidating. The pictures probably look great.'
Julie -
My camera is a 3.2MP. I've found it's enough mega pixels for my use. I have another new camera (still in the box) that's a 7.1MP~but I like the old one...I like the feel of it in my hand and the ease to use it. Sometimes it's all very complicated, I wonder if the little Brownie I had and the 127 film wasn't easier? Linda in Washington state http://journals.aol.com/lsfp1
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1/28/08 1:12 AM