Understanding Megapixels

Having a 6MP (megapixel) camera may be better than you think. Megapixels, or native resolution, plays a small role in the value of an image as would any other feature. What we need to do is differentiate resolution from quality, or in a sense actual resolution, from spectral resolution.

Most buyers will either be using a stock image for print, advertisement or some other form of commercial publication. In a perfect world, the higher the resolution, the better.

Let's take my camera for example, a Canon Digital Rebel (the old one); the camera features a 6.3MP sensor, giving you roughly an image with 3,072 pixels across, and 2,048 pixels vertically. Most stock agencies and advertising firms request their images to be printable at 300 DPI (dots per inch); this is considered by most to be the professional standard. So let us figure out how large of a print can be achieved from this camera.

It's really quite simple: divide the number of pixels by the number of dots desired per inch.

3,072 pixels / 300 dots per inch = 10.24 inches
2,048 pixels / 300 dots per inch = 6.8 inches

Now we know that a 6.3MP image will give you roughly a 10.24" x 6.8" image output, barely enough to fill the whole page of a magazine at letter size. You will find that some clients and every-day art admirers might have no problem with an image printed at 150DPI (which would give you roughly 20.48" x 13.65" print resolution).

One important thing to keep in mind is that the actual resolution of an image is the resolution captured by the camera. Do not think that by resizing an image in Photoshop to come out with a larger image, that you will, in effect, increase the image's printable resolution. Scaling up an image to print at 20.48" x 13.65" @ 300 DPI will yield the same quality, if not worse, than keeping the image at its native resolution and printing at 150 DPI, since the information to fill in for those other 150 dots per inch has to be guessed by the computer when you are resizing.

Although a 6MP camera can produce images capable of roughly 10"x6" of printable resolution, a camera yielding 8 or even 12 megapixels will not necessarily produce a better image at the same dots per inch.

To understand why a higher megapixel camera often doesn't produce any better results than a lower megapixel camera, we need to briefly understand more about sensor technology, and even film.

Let's go back to basics for a moment.

A standard 35mm film negative consists of an area 36 x 24 mm. That is roughly the dimension that light covers when used in most 35mm film cameras (or of some full frame digital SLR cameras). Film is a little funny when compared to digital sensors, since film relies on chemical reactions to light. With the size of a 35mm negative, something like high-end Velvia 50 film theoretically has enough detail per area, that if scanned-in with the proper equipment can produce a 16MP image. Most off-the-shelf film will produce around 3 to 6 depending on the brand, speed and type, the type of scanner used, and how correctly the image was captured.

Now let us get back to the digital portion of this answer. We already understand that a film negative is roughly 36x24 in size, called 'Full Frame'. Then there are standard consumer digital cameras, which are usually called 'Point and Shoot'. You can often identify a consumer camera by the fact that it has a fixed lens system, and the sensor is always exposed, (one way to know is if you can preview an image on the LCD prior to actually shooting the subject). The size of a consumer digital's sensor is roughly 2/3" which is approximately 8.8 x 6.6 mm; that is nearly four times smaller the size of a standard 35mm negative.

Digital SLRs usually have a larger sensor;

the Nikon D70 has a sensor size of 23.7 x 15.6 mm. Because most Digital SLR cameras use the same 35mm lens as their film counterparts, as opposed to having a fixed lens specifically for that sensor size, digital SLR has a crop factor attached to them. Crop factor is a ratio to give consumers a better idea of how the focal length or results will be affected by the camera. In this case the Nikon D70 has a 1.5x crop factor. You can use crop factor as a quick way to judge how close a sensor is to full frame size. Most Canon DSLRs are 1.6x until you get into the professional series (1.3x and full frame). There are even some that are closer to your standard consumer digitals such as the Olympus E-300 that features a 2x crop factor.

Why is understanding the crop factor and sensor size useful?

Let us use an analogy to demonstrate. Say we want to use buckets to capture rain, the buckets representing the individual photosites or pixels and the rain representing passes of light. Say we have an area of land roughly 4 feet on either side. I want to setup 16 buckets in this area and 64 buckets next to it. In order to fit 64 buckets into the same area I must shrink the size of each bucket. When the rain starts coming down, the smaller buckets will start to overflow sooner. The overflow can be represented by noise & grain, as well as hot pixels commonly seen on digital cameras. To eliminate the problem you either have to increase the area for the bucket, to allow for more water to be contained before overflow, or to decrease the amount of buckets used.

Keep in mind that a consumer 8MP camera has a smaller sensor size compared to a Digital SLR with 6MP. This is why most consumer cameras generate much more noise and hot pixels in their images. In many cases, a consumer digital camera with lower resolution may be a better purchase than a 12 megapixel model.

Regarding salability, suppose you took a picture from a digital camera at 6 megapixels and then uploaded it, hoping it's size would sell, then you get another from an individual using a 12MP camera with a similar or smaller sensor. It is true that the 12MP image will print at a higher resolution, but the quality at full detail will be inferior to that of the 6MP image.

A Disclaimer:

In general practice most of the information here would hold true. One thing to keep in mind is that as time goes by, the technology used in digital cameras become increasingly complex, correcting issues such as noise in certain areas and quality in others. The answers here are to help you get a general understanding that while resolution does play some role in an image's appearance, it is better to have overall quality than resolution.

By Karl Blessing

Karl is a code junkie and has been in web development for the past five years and is currently a C.T.O. at Takecharge Productions.

Source : Article from iStockPhoto.com