This article is intended to clarify all the questions that one might have in regards to the megapixels, it’s relationship with printing, why do manufacturer’s make cameras with higher megapixels and also why people buy such cameras.
Well, what is a Megapixel anyway?
A pixel is a miniature of a square in the image sensor, which contains the digital information. Megapixel is nothing but the total no. of pixels in an image file. It’s more like an area of a rectangle/square (L ✕ B).
When it comes to image, the length is defined as width and the breadth is defined as height, so the horizontal measurement in pixels is multiplied by vertical measurement in pixels. The result is in pixels, when you divide it by a million it gives you the result in megapixels. For instance Canon 5D MKIII camera has 22.30 MP(as in canon’s website), the width and height of the image file is 5760 ✕ 3840 pixels = 22118400 pixels, divide by a million you get 22.12 Mega pixels.The resolution of digital cameras are defined in megapixels, as the least area of a digital image can be measured in pixels.
What can more megapixels practically do?
More megapixels helps us to make bigger prints, that’s it. Smaller megapixels can make big prints at the cost of reducing the ppi, ie., reducing the quality, more about ppi later. There is a cost you have to pay for using more megapixels, powerful computer, high capacity memory cards, high capacity hard disk drives all at a higher cost than usual requirement. If you are not going to make prints more than 12 inches on the longer side, than you don’t need camera more than 10 megapixels.
The below table should clarify the relationship between ppi and the print sizes we can make, with reference to the size of the given image.
You might have noticed in the table (click on the table to see it fully), as the ppi decreases one can make bigger prints, the tradeoff is quality and alternatively you can make much bigger prints with more megapixels, here the trade off is cost.
This table is self-explanatory and is the reason why medium format camera is being used in high-end advertising shoots so far. Now Canon is attempting to give a tough to the medium format camera manufacturers.
Let’s see what is ppi, straight answer-pixel per inch on the print. The photo quality print can be achieved by maintaining 300 ppi (300 pixels printed on one inch of the paper). But the quality is in reference with viewing distance. The less the ppi the less the quality, but this can be made unnoticed by increasing the distance. Remember the billboards, it is actually printed at around 40 ppi, but it is not obvious to our eyes because we are looking at 50 mtrs away from it, at least. For art galleries we definitely need to print at 300 ppi, since the viewing distance is less than one meter or so.
I am not touching much maths behind the PPI and DPI (DPI is dots per inch), all the PPI information is converted in to DPI when it goes to print, because printers have to use an entirely different way of reproducing digital image on the paper by measuring dpi. Since this article is intended to clarify all questions about the megapixels and it’s limitations, let us not enter in to the printing medium in detail. Let’s keep it for another day. If you are looking for a print related article to read, I suggest you to click here.
Does more megapixels means, more quality to your image?
To be more precise “Does more megapixels on a same sensor mean that there is more quality to the image?” The straight answer is NO. If the question is changed “Does more megapixels on a bigger sensor add more quality to the image?” Now, the answer is YES.
Note : I am mentioning quality in reference with the size of the print, an image can be converted. And kindly do remember that an image quality depends on many things from camera mounted on a tripod, subject focus nailed precisely, exposure made accurately, appropriate aperture, good quality optics, processed well to maintain the tonal range, appropriate sharpening for print etc., All the image sizes mentioned above are the full size images directly from the camera, if you have a habit of cropping heavily, you need to buy a more megapixel camera only.
Coming back to our question – when the manufacturer pushes in more pixels in the same area of the sensor, image quality degrades. Since more pixels will let in less signal-to-noise ratio, hence more noise and more diffraction. But the prevailing manufacturer’s trend is to make people believe that more megapixels results in a good camera. That is the reason they make ‘Point and Shoot’ cameras which practically have less than an inch of sensor area with higher megapixels in it.
Now Canon have introduced this in their full frame sector and as well for the first time in full frame cameras, with whopping 50.6 megapixels. As of now Canon have released two images from the camera,the original can be seen here, the another one can be seen here.
This article will become much more technical hereafter and this topic itself is a complex one, I have spent a lot of time reading many articles to understand this and write it in a simpler way. If you are a beginner in photography, I recommend you come back here at a later point in time, once you remember this post.
Before we go further, we need to know one important thing about sensor, it is pixel pitch. It is the centre to centre distance between two pixels in microns. If the pixels are big enough the distance will be short, conversely if the pixels are tiny and more in number in the same area, even then the distance between two pixels will be less. If a manufacturer makes sensor with less megapixels(big pixels) they are unable to market the camera(at-least it appears so). In this way the pixels are perform well in regards to image quality and less noise, since the signal to noise ratio is good.
But the other way, with more pixels pumped into smaller sensor area the signal to noise ratio lowers and it lets in more noise to the image, degrading the quality. However the manufacturers advertise only the megapixels.
In my earlier post regarding the specs of two cameras, I have mentioned about the low ISO performance of the cameras, now you must be able to conclude why it is that way. Because the manufacturer pumped in lot of pixels in to the sensor, the sensor will have less signal to noise ratio and will be creating noises. Hence Canon might have thought, stretching beyond ISO 6400 would produce furthermore noise and they put a stop there. If you wonder what is signal to noise ratio, head to this wiki page.
The above two canon cameras comes at pixel pitch of 4.14 micron pixels. Whereas canon 5D MK III have a pixel pitch of 6.25 micron pixels which appears to be possible optimum based on the article in Luminous landscape. And Canon 1D MK II have a pixel pitch of 8.2 micron, a side by side comparison of two images taken by 1D MK II and Canon S70 which have 2.30 micron as pixel pitch can be seen here at Clarkvision. Or could Canon have achieved the optimum pixel pitch as 4.14 micron? We will have to wait and see the lab reports for ISO performance at 6400 and in general image quality even at base ISO. Base ISO is nothing but the default ISO setting of the sensor, canon full frame camera sensor comes at 100 as their base ISO, some says as native ISO.
Conversely, in the name of high quality, if one expects to have one megapixel in a full frame camera which becomes impractical, because it results in a print size of 4.20” ✕ 2.80” at 300 ppi !!! So, there must be an “optimum pixel pitch” which delivers the highest quality of print possible in the present technologies for any size of sensor. I was unable to find any useful article,except a paper presented from the stanford university. If you come across about optimum pixel pitch do let me know in comments or drop me a mail, would be highly interested in knowing. Or is this not been revealed by the manufacturers, so that people will start looking out for these details in the cameras before they buy?
Note: In this article I mean sensor as CMOS sensor. CCD sensor is expensive than CMOS and presently almost all the camera manufacturers are using CMOS sensors with Bayer filters in their digital cameras except Sigma, where they have started using Foveon sensors, more about that later.
Hope this article has put some light behind the sensor and megapixels. Has the intended purpose been served as per the title of the article? Did this article help you? or do you disagree with me somewhere? Do let me know in comments or drop me an email.