Indy's Camera Review - Digital SLR's
Sensors
©2009 James Melatis - webmaster@indigotide.com
It is not hard to see that more pixels are an important part of better
quality print images, and the sensors in todays digital cameras have plenty.
Camera manufacturers understand that average consumers don't look far past
the number of pixels, so they are always touting that their camera is better
because it has higher megapixel resolution. 
Actually, the image height in pixels is what determines the best possible resolution on an 8x10 landscape image straight from the camera. While the number of megapixels you really need depends on the height to width ratios available on the camera, the bottom line is that in landscape orientation the height needs to be at least 2400 pixels no matter what the megapixels or height to width ratio is. An 8" x 10" photo needs to be at least 2400H x 3000W to get 300 DPI (Dots Per Inch) right out of the camera. Just divide the image pixel height by 8 to calculate the best possible DPI resolution available for an 8x10.
Why 300 DPI? Simply because 300 DPI is the minimum quality for an image destined to reach publication in the print world. But that is just the absolute minimum for an 8 x 10. We would also like to have extra room to crop out unwanted picture elements and still stay at 8 x 10 and 300 DPI. A good estimate might be at least 3600 x 4500 pixels to give us that extra room (16.2 megapixels, 4:5 ratio), which can also get us a 400 DPI image with a slight crop. This is definitely workable for great 8 x 10's. Who needs film now?
How many megapixels do we want? 500 DPI, 600 DPI? Consider that your typical inkjet photo printer can do 720 DPI. Wouldn't it be great to have a digital camera that has the same resolution as the printer? Is it even theoretically possible that sensor and camera technology could reach this level? Everyone probably has their own ideas, but the higher the DPI out of the camera, the better the quality in our prints. An 8" x 10" at 720 DPI would need to be at least 5760 x 7200 pixels right out of the camera. That would be 41.4 megapixels in a full frame 4:5 ratio sensor. Plenty of extra room to crop out a high quality 8" x 10" at a minimum 600 DPI without having to re-sample the image and lower the original quality, and up to 720 DPI with no crop. This would be perfect! It really boils down to if or when camera manufacturers will be able to make D-SLR's like this with the power and speed to move that many pixels fast enough, and that most people can afford. The following table may help you to better understand the relationship between megapixels, image size, and image quality.
| 4 x 5 DPI | 5 x 7 DPI | 8 x 10 DPI | Image Size | 4:5 Ratio Mega Pixels |
|
| 375 | 300 | --- | 1500 | 2100 | 3.2 |
| 600 | 428 | 300 | 2400 | 3000 | 7.2 |
| 800 | 571 | 400 | 3200 | 4000 | 12.8 |
| 900 | 640 | 450 | 3600 | 4500 | 16.2 |
| 1000 | 714 | 500 | 4000 | 5000 | 20.0 |
| 1200 | 857 | 600 | 4800 | 6000 | 28.8 |
| 1440 | 1028 | 720 | 5760 | 7200 | 41.4 |
For example, a typical 4-megapixel sensor used in some pocket consumer cameras has 3-micron square photo sites, or 9 square microns per photo site. Higher end 4-megapixel sensors have 7 to 11 micron square photo sites (49 square microns to 121 square microns). These sensors all have the same number of pixels, but there is a big difference in sensor size, cost, and quality. This is because the physically larger the area of the individual photoreceptors, the less noise, and better color accuracy is obtained; and the bigger they are, the more they cost.
Like dot pitch on a TV, you might think that smaller and closer together photoreceptors would give you higher quality, but you have to remember that the lens has to present a smaller optical frame to the smaller sensor. With the physically larger sensor and standard 35mm lenses, the optical frame size is huge by comparison, increasing the accuracy. The larger more accurate photoreceptors are actually just as close together in relation to the total size of the optical frame.
It's not just the total number of pixels; each individual photoreceptor on some sensors has 13 times better resolution than your basic pocket camera!
Another thing to understand about consumer sensors is the color depth. Manufacturers claim 48-bit color on the box (16 bits per channel), but be careful because they usually mean FILE FORMATS, not sensors. Cameras may be able to save files in 24 or 48 bit sRGB color format (16-bit TIF), but the lower cost sensors may only "see" in 24 bit color (16.7 Million Colors) with an 8-bit per color channel analog to digital converter. Usually these cameras only offer saving files in 24-bit compressed JPEG format.
A higher quality sensor "sees" in 36-bit color (68.7 BILLION Colors) with a 12-bit per color channel analog to digital converter, and the camera saves in 12-bit RAW, 24 or 48-bit sRGB, 24 or 48-bit modified RGB, or 48-bit Adobe RGB Color profile (16-bits per color channel). This is a difference of over 4,000 TIMES as many colors as the lower cost sensor.
While Adobe RGB color space may seem a little flat out of the camera, the colors are more natural and consistent with what your eye actually sees. This is what a camera should do; capture exactly and only what is really there. If you want, you can adjust the colors later in Photoshop. If you use modified RGB and let the camera adjust the colors like most point and shoot cameras do, it is harder to adjust later if you don't like what the camera does. This is why "seeing" more colors is important. Cameras that "see" fewer colors have to substitute something close, and that something is sometimes unnatural.
In spite of what camera manufacturers would have you believe, more megapixels are not the only requirement for a better image. If it was, then professional photographers would be using low cost pocket cameras these days.
- D-SLR's see more colors
- D-SLR's use professional quality lenses.
- D-SLR's present a larger optical frame to the sensor.
- D-SLR's have larger, faster, more accurate, and lower noise sensors.
- D-SLR's have options for the best formats for saving images without losing any color data.
