Pixel count of Panasonic G9
Posted in News
21/04/2022

What Are Megapixels and How Many Do You Need?

It’s impossible to read about cameras and not encounter the terms “pixels” and “megapixels.” Nowadays most cameras have 20 megapixels (MP) or more, and there are some monstrosities with 100MP. I still have memories of walking into camera shops as a teenager and seeing the higher end models having 2 megapixels, and probably in fifty years everyone will have 600MP. So what is a pixel, and how many do you need?

What are Pixels (and Megapixels)?

A pixel is simply the smallest visual unit making up a digital image. In other words, a digital image is made up of millions of tiny coloured squares, and each of those squares is one pixel.

Pixels_From_Small_area_of_image
The pixels from a tiny portion of the eye. Unfortunately, they are still not large enough for pixel-peeping.

A camera sensor is also said to have pixels. In this context, a pixel refers to the number of photosites on a sensor. Photosites are the individual sensing areas that capture light, which is then translated into pixels through software.

Pixels are also the units used to describe a camera’s resolution. For example, the Fuji X-T4 produces a 6240 x 4160 image, which means an image 6240 pixels long by 4160 pixels wide. This gives a total of 25,958,400 pixels. Since this is such an unwieldy number, it’s better to use the unit of megapixels. One megapixel is simply a million pixels. So, the X-T4 has a resolution of about 26 megapixels.

Total vs. Effective Pixels

Let’s say your friends just cancelled on coming to your birthday party. What’s the remedy? Reading camera specs on B&H, of course! Just knowing that the Canon R5’s DCI 8K records at up to 1300MB/S is sure to make you feel like you don’t even need a birthday.

But if you look too closely, there’s something that may disturb your peaceful evening. Almost every camera has two different values its megapixel count: actual (or total) megapixels, and effective megapixels. What’s the difference between these two values?

Pixel count of Panasonic G9
The “camera effective pixels” and “total pixel number” are two different values! What gives?

The important value for photographers is the number of effective megapixels. This is the number of megapixels that will be in your full-size image when you open up your Raw developer or export a JPEG at maximum size.

For example, the Panasonic G9 is listed as having 20.3 effective megapixels. But what about the G9’s “actual megapixel” value of 21.8 megapixels? Can you unlock these hidden pixels for the low price of $329.95 to get even more precious resolution? Sadly not. Instead, these are pixels on the edge of the sensor outside the imaging area. Why are there extra pixels on the edge of the sensor? There are two main reasons.

1. First Reason: The Way Color Sensors Work

The first reason is the nature of color sensors. For example, consider the Bayer sensor present in almost all color digital cameras. It uses separate photosites for red, green, and blue light:

Layout_Of_Bayer_Sensor
The sensor of cameras with a Bayer sensor have photosites that separately read in green, red, and blue light. These are assembled to produce the pixels you see in the final image

If you shoot Raw (and you should), these are combined by your Raw editor through a process called demosaicing to produce what you see when you open a Raw file. If you shoot JPEG, then the camera does the demosaicing.

However, if there were only as many photosites as the final number of desired pixels, then the edges would not have enough photosites for accurate color information. For example, this is what happens when you try and compute color values only from the edge pixels:

Bayer_Diagram_Lack_Of_Pixels
Using a 5×5 Bayer layout allows you to create a 4×4 (16) pixel image. But a 4×4 Bayer layout will result in incomplete edges

In the example on the right, there are 4×4 or 16 color photosites. Usually, the value of each pixel is computed using four photosites. But when you get to the end of the row, there are only two photosites for the fourth pixel of that row. Therefore, to get a 4×4 grid of pixels in the final image, you actually need a 5×5 grid of photosites so each of the 4×4 pixels has full color information. (I simplified this process a little bit. In reality, the demosaicing stage typically uses a better algorithm than I just outlined.)

2. Second Reason: Black Level and Unwanted Dark Signal

However, these additional edge pixels are not enough to account for all the extra pixels. In fact, most cameras have pixels that are completely obscured from light! You can think of them as pixels with black paint on them. These are the so-called optically black pixels. Why should there be pixels on the sensor that cannot even sense light?

Three_Types_Of_Camera_Sensor_Pixels
Camera sensors have at least two additional types of pixels beyond the ‘effective megapixel’ count. Not to scale

Unfortunately, even in total darkness, a sensor will still generate a signal (the dark signal) that will be translated into something other than pure black. This is undesirable, because obviously you want black to register as black.

This can be partially compensated for by using these optically-black pixels. By reading in the signal generated by these pixels, the camera can apply a correction to the whole image.

This correction is typically derived from a model that depends on temperature, which in turn is estimated from the optically black pixels. In practical terms, the hotter your sensor, the more unwanted signal (noise) comes through, and the camera estimates this via these extra pixels to account for it.

Graph_Of_Dark_Current_Versus_Temperature
The unwanted signal (dark current) is dependent on the temperature, which is best estimated using the optically black pixels. This is a simplified model

A similar technique is used in long-exposure noise reduction, where a dark frame is taken either manually or by the camera to reduce noise. Unfortunately, not all noise can be predicted from the optically black pixels (nor can hot pixels), which is why dark frame subtraction is still useful for long exposures.

How Many Megapixels Do You Need?

Now that I have covered the nitty gritty details of pixels, it’s time for the fun question: how many megapixels do you need? The answer is at least 100MP, and less may cause the universe to explode.

Okay, joking aside, how many megapixels is really enough? Should you get a 45MP camera over a 24MP one? These questions can be answered by considering two things: what is your final output medium, and how much do you need to crop? Let’s go over these two in a little detail.

1. What is Your Final Output?

If you’re mostly displaying your photos on the internet, you do not need that many megapixels at all. For example, a 4K monitor can be covered by 8.3 megapixels. (An 8K monitor, on the other hand, needs 33.2 megapixels. Very few people have such high resolution monitors, but they’re becoming at least a bit more widespread, so keep the 33 MP mark in mind if you wish to create 8K desktop backgrounds.)

Printing is another great way to display your work, and at least at larger print sizes, it tends to demand a larger number of megapixels. For close viewing distances, most people recommend printing at 300 pixels per inch. What does that mean? I means for every inch on your print, you’ll want that inch to be spanned by 300 pixels. It’s not a hard rule, meaning that if you have 270 pixels per inch, your print will look pretty good. So how many megapixels do you need for printing? Just take a look at this chart for some common large print sizes:

Print size (inches) Resolution for 300ppi Megapixels for 300ppi Resolution for 250ppi Megapixels for 250ppi
8 x 10 2400 x 3000 7.2 MP 2000 x 2500 5.0 MP
12 x 18 3600 x 5400 19.4 MP 3000 x 4500 13.5 MP
16 x 24 4800 x 7200 34.6 MP 4000 x 6000 24.0 MP
24 x 36 7200 x 10,800 77.8 6000 x 9000 54 MP
32 x 48 9600 x 14,400 138.2 MP 8000 x 12000 96 MP

As you can see, the number of megapixels you need becomes insane as the print size goes up. I guess everybody who wants to make large prints should go out now and buy the Fuji GFX 100S, right? Well, not exactly. The number of pixels you need is also dependent on the typical viewing distance!

Gull_Spreading_Water_With_Wings
The full-sized version of this image came out to 20MP, or just enough for a 12×18 print at 300PPI

People’s vision is such that a low-resolution print can look perfectly sharp if viewed from further away. To be specific, if you need about 300 PPI at one distance, doubling that distance will reduce your requirement to 150 PPI. And moving twice the distance will divide the number of megapixels required by four.

Since larger prints won’t tend to be viewed as closely, I have come up with a more realistic chart for pixel requirements:

Print size (inches) PPI Resolution Megapixels
8 x 10 300 2400 x 3000 7.2 MP
12 x 18 260 3120 x 4680 14.6 MP
16 x 24 220 3520 x 5280 18.6 MP
24 x 36 200 4800 x 7200 34.6 MP
32 x 48 180 5760 x 8640 50 MP

This is based on my own personal preferences and thinking about where I’d put different-sized prints in my house. In other words, it’s highly scientific and not up for dispute. (Well, at least it’s a good starting point.)

How much resolution you need also depends on the print medium and the subject. A photo with a lot of fine details like feathers will appear worse if those details are obliterated compared to a photo of a person’s face at the same resolution.

Groundhog_Up_close_portrait
Nikon Z6 + Nikon Z 500mm f/5.6 PF @ 500mm, ISO 2200, 1/320, f/5.6

From these considerations, I recommend the following: If you are happy keeping your prints at most 16×24 inches, almost any modern sensor will be fine (since the entry point on cameras today is usually at least 20MP). So, this means any recent micro four thirds, APS-C camera, or low-resolution full-frame camera will suffice. Even if you lack a bit of resolution for a print at your desired PPI, you can use software methods that can do advanced upscaling. Some cameras like the Panasonic GH6 also have pixel-shift or high resolution modes that are suitable for some subjects and provide more resolution.

On the other hand, if you want to print 24×36 or higher, you will have more freedom with a high-resolution full-frame sensor like that in the Nikon Z7, Canon R5, or Sony A1. An even higher resolution full-frame camera, like the Sony a7R IVA, which has 61MP, is an excellent choice for those who need to make massive prints.

And if none of these is satisfactory, then the Fuji GFX 100S is an amazing camera.

2. How Much Do You Need to Crop?

The second consideration is cropping, which in some cases is unavoidable. As a wildlife photographer for instance, I am often cropping because not all species are easy to get close to. Cropping is also common in macro photography, because the size of the subject in the photo is often limited by the maximum magnification of the macro lens.

So for shooters who use need to crop substantially, I would recommend the higher megapixel bodies like the Canon R5 over lower megapixel ones like the Canon R6. Looking at the print chart above, the 45MP of the Canon R5 will give many more print options. Even after a 1.5x crop, the 45MP of the Canon R5 will still leave you 20MP, whereas the 20MP of the Canon R6 will become 8.9MP.

Alternatively, if you use a long enough lens, you can “crop” by using a crop-sensor camera, i.e., micro four thirds or aps-c. Most of these cameras are about 16 or 24 megapixels at the most, but thanks to their crop factor, you could end up putting more total pixels on a distant subject than a typical full frame camera could manage.

Black_capped_Chickadee_In_Colourful_Vegetation
Despite being taken on a D500 at 500mm, I still cropped this image 1.5x and ended up with 8.8 MP

As a final reason to buy a camera with more megapixels, there’s no denying that pixel peeping is a very relaxing activity. More pixels can be an option for their therapeutic properties.

Conclusion

A pixel is the fundamental building block of an image, and generally, the more pixels, the better. However, photographers are very lucky with modern cameras because most of them have more than enough pixels for almost any situation. For very large printing and cropping, it is definitely worthwhile to have more pixels, and so cameras in the 40-60MP range can be very useful. However, even a 20MP camera can make a very nice large print, and fewer pixels should not hold you back. I look forward to hearing how 100MP is the ultimate level of photography in the comments!

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