Blog 11: What are megapixels and why should I care?

The past decade, all camera manufacturers have been waging what we photographers call a “megapixel war”. One week, a brand would release a 10 megapixel camera, the next week a competitor would release a camera with 15 megapixels. The term “megapixel”, also revered to as MP, has become somewhat of an sales pitch in itself. But what is a megapixel and why should you care? Is more megapixels always better? What are the pros and cons of having more megapixels? In this blog, I will try to answer these questions.

What are megapixels?

A pixel is a light sensitive cell that resides on a sensor (figure 1). The amount of MP is a term for the amount of pixels on a sensor. If a sensor contains one MP, it holds 1.000.000 pixels! That means that a sensor with 10 MP holds 10 million pixels. 

Figure 1: coloured pixels on a sensor

The amount of pixels on a digital sensor is comparable to the resolution of a computer screen. Nowadays, a resolution of 1280x1024  or higher is most common. This means that the computer screen has 1280 pixels aligned horizontally and 1024 pixels vertically. This amounts to approximately 1.3 megapixels. Digital sensors now have a much higher resolution than that. For instance, a 20 MP sensor has a resolution of 5184x3456. This means that it contains 5184 pixels horizontally and 3456 pixels vertically and a grand total of 20.000.000 pixels! But what are the advantages of having that many pixels on a sensor? Don’t “they” always say that the amount of MP on the sensor don’t matter? Let’s see if that is true.

Advantages of high MP camera’s

There are two major advantages when it comes to having a lot of MP. The first, and foremost, is the fact that a high amount of  megapixels means more data being collected. This results in more details in the photograph. Figure 2 depicts this. Keep in mind that every block is one megapixel, thus contains 1.000.000 pixels.

Figure 2: amount of pixels on a sensor. More pixels means more details captured.

This effect is most obvious when taking photographs of a scene with a lot of small details. For instance, grass in a landscape or cloth during a portrait shoot. When you look at figure 3, which is the same as figure 2 but with added grass, you will see what I mean by this. The 8 MP sensor has just 2 vertical MP to capture the entire height of the grass, while the 48 MP sensor has 6. This simply means that the 48 MP sensor will capture 3 times as much details in the vertical axis than the 8 MP. In photography, this is often called  “resolving power”. In other words: will you see very blade of grass as an individual object, or does it become a green blur?

Figuur 3: More datapoints (pixels) means more details

The second great advantage has to do with printing. More megapixels will allow you to print your work on bigger formats without the loss of quality. Of course, you don’t need 20 MP to print photographs of 10x15cm. However, if you want to print a poster of let’s say 160x240 cm, you will need all the pixels you can get. This all has to do with something we call PPI, or Pixels Per Inch. The higher the PPI, the sharper the prints will be.

Now, a rule of thumb, depending on who you ask, is that the minimum PPI must always be 150. Using this fact we can set up a formula to calculate the maximum enlargement of the digital photograph.

The formula reads:

This means, for a photograph with a 10 MP resolution (3648x2736 pixels), that the maximum enlargement is 3648/150 = 24,32 inch, approximately 60 cm in length. The maximum width then is 2736/150 = 18 inch, approximately 45cm. So the maximum magnification of a 10MP photograph is approximately 60x45cm. If you want to print larger than that, you will have to lower the PPI and thus get a less sharp image. It is kind of hard to find a digital camera with less than 18 megapixels nowadays so, if your are not planning on making prints bigger than 60x45 cm, you don’t really need to worry about high megapixel count. However, if you are planning on making billboards, wallpaper, life size posters of yourself (or a foxy model), you might want to consider getting a camera with more megapixels. For comparison, here is the same size photograph, the left one is 300 PPI, the right one 50 PPI.

Figure 4: PPI comparison

Disadvantages to high megapixel count

Of course, as with everything, there are some disadvantages when it comes to having a lot of megapixels on your sensor. Perhaps the biggest con is that of image noise. Image noise is created by pixels that start “leaking” light on the neighbouring pixels because they are full. This is comparable to an ice cube tray. If you have 8 (or 12 as in this example) slots in this tray, you can fit in a decent amount of water in them before the slots start leaking into others.

Figure 5: more pixels on the same surface means smaller pixels

However, change the amount of slots to 48, as in figure 2, but keep the sensor size the same, and the pixels will have to be much smaller. Smaller pixels leak light faster, thus creating more image noise. If you raise the ISO on you camera, the pixels will be more sensitive to light and thus be “full” quicker.

Figure 6: image noise at high ISO settings

Of course this is not a problem when shooting landscapes. That is, if you brought a tripod as all landscape photographers should, or if you shoot in a studio where you can cater the light to your needs. It will become a problem for a street- or wedding photographer that have to up their ISO values indoors or during dusk, to keep a fast shutter speed. So the amount of megapixels that are useful to you, depends on what you want to do. High megapixel cameras are more suited for landscape and studio photographers, while lower MP cameras (creating less noise) are more suited for street-, wildlife- and wedding photographers.

A second disadvantage of a high megapixel count is that lens flaws become more obvious. All lenses contain minor flaws that cause aberration( figure 7), deforming, vignetting etc.

Figure 7: lens flaw: chromatic aberration

Lenses that use fluorite glass have less problems with lens flaws but are more expensive. It is safe to say that all flaws in a lens subtract from overall image quality. Therefore, if you want to use a camera with a high megapixel count, you should also buy adequate lenses (read: the more expensive ones).

That being said, I sometimes see people buy a very expensive camera body. Because the body is so expensive, they don’t have enough money for proper glass, so they buy a cheap lens with it. This is a very common mistake. The cheap lens will not make the most out of the resolution your expensive body will offer. Instead, do the opposite. Buy a more expensive lens with a less expensive body and your photographs will contain more detail, more contrast and more colour! Of course, if you want the best, go for a high megapixel camera (if you shoot studio or landscape) AND get an expensive lens.

One last disadvantage of a high MP count is the file size. A RAW file made with a 20 MP camera is approximately 25 MB. If you have a 40 MP camera, it will quickly shoot up to about 50MB per photograph. This means you need bigger memory cards, a faster PC and more hard drive space. Also, because of the higher resolution, you have to spent more time processing the images because you will see every little flaw in that models skin, every crooked hair, every speck of dust..

So, In summary:

-         - The amount of megapixels that is USEFUL for you, depends on what you want to shoot

-         -  High MP count means more details In your photographs

-         - High MP count means bigger enlargement without loss of quality

-         - Less MP means less image noise at higher ISO values

-         - Less MP means less bothersome lens flaws

-         - More expensive lens on a cheaper body is better than the other way around

So much for his blog! I hope that you have once again learned something useful. Check out  http://photojitsu.nl/blogpost.html for more blogposts and useful stuff to read about photography. In addition, if you want to follow me and my work, or just want to drop in to say hello, check out my facebook page www.facebook.com/photojitsu or twitter @photojitsu_nl.

Until next time!