Number of Megapixels is one of the biggest myth being pushed by salesmen, shopkeepers and even consumers of photography goods. Do they really matter? What is the ideal number of pixels a good camera should have? What are the advantages and disadvantages of having too many or too little of these?
(Lemon Butterfly – Nikon D60 with Nikkor 18-55mm lens, f/8, 1/30 sec with VR. Image was cropped during post-processing to about 5 MP and printed on an A3 paper with excellent results. -Visit Photographing butterflies for more information on photographing butterflies)
First we have to understand what exactly is resolution when it comes to photography.
Digital photographs are composed of smalls dots called pixels. The density of these pixels defines the resolution and the total number of pixels defines the size of the image. The density of pixels is written as PPI or Pixels Per Inch (sometimes wrongly quoted as DPI, which I will talk about later in this article).
Screen Resolution, as the name says it is the resolution of the screen. An image of 4 x 6 inch on the computer will appear to be of different sizes depending on the resolution of the monitor. Most inexpensive displays have 75 – 100 PPI resolution. High end screens have a higher PPI or pixel density. Now do you understand why high end monitors are so hard on the eyes? At the default settings in these monitors, everything looks small due to the high density of pixels. A picture of 200 pixels x 200 pixels on a 140 PPI monitor will appear to be of about 2 Inch by 2 Inch in size. The same picture on a 280 PPI monitor will appear to be of about 1 square inch in size. Mobile phone screens have a higher pixel density than most desktop monitors. An interesting twist to this is that if the image size is defined by the file format then the image will appear the same size on display at their native resolutions and higher than native resolutions. This is a good enough reason why it makes sense to keep the PPI low for images meant to be shown only on digital displays of average to low quality.
Printers, print the photos as small dots. The density is represented by these dots per inch or DPI. For photographs meant to be place in albums and for close viewing, 300 DPI is good. In other words, for printing a A4 size print which has to be seen at really close range, required resolution should be 2400 x 3600 pixels or 8.6 MP. However since an A4 size print is usually seen from a little further distance, the resolution can be reduced a little without any noticeable impact on the print. The resolution for photographs on hoardings is sometimes 30 DPI or even less.
For calculating the size of the image, divide the pixel count by resolution. Keep in mind however that most image sizes are given as horizontal and vertical pixel numbers and the resolution is usually given as diagonal density. For actual calculation one has to go into mathematics….. and you thought that Pythagoras theorem was useless! Thankfully the printer and computer manufacturers have lately started quoting the resolution as horizontal and vertical resolution, making things easier.
When the digital cameras were launched, the resolution was very low. This was the limiting factor when it came to printing images. With advancement in technology the pixel density on the sensors kept increasing and also the sensor sizes. This led to a perception that a camera with higher pixels (or Megapixels) is better. A camera with a sensor that was larger than the predecessor model without any other changes had a higher resolution. The perception was therefore justified to some extent. There were however limitations to the largest size within a specific budget and physical camera size. So the way out was to increase the number of pixels per inch or the density of pixels. This has its own challenges. If there are two sensors of the same physical size but of different pixel densities, the one with higher pixel density will be prone for more electronic noise, if everything else is kept same.
Megapixels and sharpness
These are two different things. Megapixels tell the resolution of the image but the sharpness is determined by the kind of lens used, photography technique, post-processing, contrast in the image and a lot many other factors.
Megapixels and size of the image
This is true to an extent. A high megapixel count means a large image. Sometimes unscrupulous manufactures had interpolation built into their firmware and the image captured by the sensor was presented to the users as a larger size than it actually was. As explained earlier sometimes packing in excess pixels increases the noise so effectively the larger size may not be usable. High number of megapixels do provide an advantage when it comes to cropping. Even after discarding a large part of an image, the photograph may still have enough megapixels to be useful. High MP count images have large file sizes when it comes to megabytes. Post-processing such files can be painfully slow on old computers.
The MP requirements are very low for most uses. An image meant for sharing on facebook will be more than enough at about 1 MP size. Professional stock image companies accept images of 4 MP and above. A 4K display is about 8MP in size.
So if you are planning to buy a new camera, forget the amount of MP and look at the overall image results and features it provides. All the present day cameras from reputed manufacturers have more than required number of pixels. If you already own a good camera, do not upgrade just for the sake of increasing MP.