How Many Pixels Do I Need?

Megapixel versus Resolution
How big a print can I get?

Mega Pixel versus Resolution

The common perception that "higher the mega pixel, better will be the resolution". Interestingly this idea is not accurate. The word resolution means the ability to capture small details. It depends on many variables, like sensor technology, dark count, sensor size, optics and so on!

It is true that starting with 640X400 pixel web cam, the resolution steadily increases with the increase of pixel count, till it reaches a threashold. According to my opinion, once you cross the level of 2 Mega pixel, higher

	High density	Medium density	Low Density

mega pixel does not directly translates into better resolution. There after, the other variables like lens quality, Sensor size and sensor technology play the vital role. A sensor manufacturer can put same number of sensor pixels on a small sensor or a bigger sensor. So the smaller sensor will have higher sensor density than the bigger sensor, but specification-wise they are both same mega pixel sensor! Now take a look into these three photographs of a birds eye. They are reproduced with three different pixel densities. How the resolution of the lowest density looks against that of a higher density? Same is true for a image sensor. But there is a snag with high sensor density also. Higher the density, higher the noise, lower the light sensitivity and magnification of lens aberrations! All these factors depend on the manufacturing technology of the image sensor. So a lower megapixel sensor can produce much high fidelity in color and resolution than a higher megapixel sensor.

Dark Count and Dark Noise
All optical sensor suffers from this malady. What happens here is the sensor produces electric charge on its own with time and elevated temperature. This charge has nothing to do with the optical image. So even in total darkness, the sensor produces signal, which appears as noise. It increases with exposure time and ambient temperature. Professional Digital cameras employ special circuit and firmware to cancel out some of this Dark Count / noise. That is why all the consumer level digital cameras are so noisy in time exposure and low light situation. Unfortunately, no camera manufacturer ever provides (at least to my knowledge) a Dark Count specification of their sensor. So it is best to take few sample time exposure and low light shot to check it yourself.

Lastly, but not least, look for the quality of the optics. You may not believe your eyes what a lens quality can do to your photograph without changing the megapixel resolution of your camera. This test can only be performed in a Digital SLR like D30. Try to shoot the same subject using same camera setting using two similar focal length lens

	Quataray 70 210mm 140$	Sigma 70 200mm 750$	Canon 70 200mm 1350$

manufactured by different manufacturer, or even from the same manufacturer! Though very unfair comparison, but to prove the point, see the table below, same focal length lenses from different manufacturer targeted for different consumer levels at the same camera store cost from 140$ to 1350$. Why almost 10 times price difference? Mainly because of superior optical quality. The image made with 140$ lens on same D30 will look very soft and unacceptable when compared with picture taken with 1350$ lens. In this case image sensor and rest of the electronics of the camera is common. For a fixed lens camera, you are in no luck of ever replacing the lens. So if the lens of a higher megapixel camera is of inferior quality, then you are much better off with a lower megapixel camera with a better lens.

So the moral is, do not overemphasize on megapixel for cameras above 2.1 megapixel rating. Check for quality of image and optics, which will pay back far more than the raw megapixel number.

How big a print I can make?

The photoprinter does not understand megapixel. It only cares about Dot Per Inch or DPI. Whereas our computer monitor are generally set for displaying 72 DPI resolution. This resolution is too small for any photo printer, as a photo realistic print requires at least 300 DPI resolution. If you lower the DPI, you may get a bigger print, but the print quality will degrade. It will show pixilation and jaggies at the edge, especially on the curved or diagonal lines. There are, however, ways to make bigger quality prints from a lower megapixel source by using special software tools like Genuine Fractal. We will discuss it later in the FAQ.

A simple unitary method calculation gives you how big photo-realistic prints you can get from your images. The key is considering each pixel as a dot. Now let us do a simple sum, what will be a photoquality print size from a 640x480 size image?:

Using same calculation, I have computed the maximum photoquality prints achievable with various popular image sizes.