The megapixel myth

Learning goal

You will be able to tell what megapixels measure and what they don't: that they tell you how many points an image is made of, but not how clean, how sharp or how accurate in color those points are. You will be able to work out roughly, from a photo's pixel dimensions, how large it can be printed, to show that doubling the number of pixels doesn't double the detail, and to recognize the situations where high megapixels really help.

The question

In phone advertising the number grows every year: forty-eight, fifty, a hundred, two hundred megapixels. Some of the cameras shown next to those same ads still say twenty-four megapixels. On paper the decision looks simple: the bigger number wins. But anyone who tries the two tools side by side notices the same thing: especially in difficult light, the camera's photo is cleaner, sharper and richer.

Why can't a 48-megapixel phone beat a 24-megapixel camera? The answer lies in understanding correctly what a megapixel is. Megapixels are a count of detail, not a measure of quality. Think of a book's page count: a thicker book carries more pages, but that doesn't mean it is better written.

Context

Megapixels are the easiest number for selling cameras, because they are a single figure and look more impressive the bigger they get. Sensor size, the quality of the lens or the success of image processing can't be told with a single number. That is why the pixel count is always what stands out in advertising. In the previous lesson you compared two cameras with equal pixel counts and different sensors. This lesson reads that relationship the other way round: what really changes as the pixel count goes up?

Answering this question correctly saves both money and disappointment. Most people see their photos on a phone or computer screen; some get small prints now and then. For these uses, the number of pixels needed is at a level that almost every tool sold today more than meets. Instead of getting caught up in the megapixel race, it is wiser to know what job the pixel count really helps with.

Simple explanation

What do megapixels measure?

A digital photo is made of small colored squares arranged like a grid. Each of these squares is called a pixel. A megapixel means one million pixels. A twenty-four-megapixel photo is usually a grid 6000 pixels wide and 4000 pixels high; multiply the two numbers and you get twenty-four million. The total size of this grid is called the resolution.

Resolution decides how much you can enlarge a photo. A measure often used for good sharpness in print is to have 300 pixels fall in every inch. By this measure a side of 6000 pixels comes to 20 inches, about 51 centimeters. A twenty-four-megapixel photo can be printed at roughly 51 × 34 centimeters so that it looks sharp even up close. For large prints seen from a distance on a wall, fewer pixels are enough. Screens need far fewer pixels: the entire screen of a four-thousand-resolution, or 4K, television is only about 8.3 megapixels.

Why do more pixels not mean a better photo?

Increasing the number of pixels doesn't increase detail as much as people think. What decides the detail in a photo is the number of pixels along one side; the area grows with the square of that number. So to double the detail along a side, you have to quadruple the number of pixels. Forty-eight megapixels is twice twenty-four megapixels, but it carries only about 1.4 times more detail per side. The doubling on paper is much smaller in the difference the eye will see.

More importantly, the pixels have to fit onto the same sensor area. When you put forty-eight million pixels onto a small phone sensor, the pixel size becomes very small and each pixel gathers less light. Less light, as you saw in the previous lesson, means noise. That is why many high-megapixel phones, at their default setting, combine neighboring pixels in groups of four and record a twelve-megapixel photo. In other words, you will often find that forty-eight megapixels are recorded as twelve. On top of that, for every detail to reach a pixel, the lens has to be just as sharp; a small, cheap lens throws a blurry image onto the sensor, however many pixels there are.

These small pixels make a difference when you photograph people too. In low light, small pixels turn noisy in the shadows and the phone hides this by smoothing. Especially on darker skin in shadow, details such as skin texture, eyelashes and individual strands of hair are lost in this smoothing. However high the pixel count, what really protects the detail in a face is the light falling on it.

When do high megapixels help?

High megapixels have a real benefit: cropping headroom. Cropping headroom is the ability to cut away part of a photo and still use the remaining part with enough detail. If you want to photograph a nest at the top of a tree, a fan shouting in the stands or the tile pattern in a mosque dome and zoom in afterward, extra pixels help. In the same way, very large prints, billboards and catalog shots that have to show the finest texture of a product benefit from high resolution.

But that benefit has a price. High-megapixel files take up much more space, slow down the computer and show shake more clearly: as pixels get smaller, even the slightest tremor of the hand spreads across several pixels. So the decision is not to make a number bigger but to choose the number that fits your needs. For social media and the family album, twelve megapixels is more than enough; for large prints and heavy cropping, more can make sense.

Visual evidence

Imagine two photos of a child playing in a park on the same evening, after sunset. One was taken with a forty-eight-megapixel phone, the other with a twenty-four-megapixel full-frame camera. When you see both side by side on a phone screen, the difference is barely there: the phone frame is a little flatter and brighter, the camera frame a little softer in its light.

Now imagine printing both at 30 × 45 centimeters and hanging them on the wall. In the phone frame, the child's face and hair have softened like a watercolor; the eyelashes can't be made out, and on the cheek in shadow there is a fine noise and patches flattened here and there. The grass has turned into a green carpet, and the individual leaves are lost. In the camera frame, though, the freckles on the face, the individual strands of hair and the streetlight reflected in the pupil are sharp. Individual leaves can be made out in the grass. The phone frame has twice as many pixels, but the one showing more detail on the wall is the camera frame. The difference lies not in the number of pixels but in how much light, and how sharp an image, each pixel receives.

Common mistakes

Summary

A megapixel is a count, not a quality grade; a 48-megapixel phone can't beat a 24-megapixel camera, because pixels multiplied on a small sensor get smaller and gather less light, and detail per side grows not with the pixel count but with its square root. High megapixels help most with cropping headroom and large prints; for screens and albums, almost all of today's tools carry enough pixels.

Mini assignment

In this assignment you calculate the size of your own photo.

1. Open the details of a photo from your phone or camera and note its pixel dimensions, for example 4000 × 3000.
2. Divide the long side by 300 and multiply by 2.54; this is, in centimeters, how long a print can be while still looking sharp up close.
3. Crop a corner of the same photo down to a quarter of its size and look at how the remaining image looks on screen.
4. If your phone has a high-megapixel mode, shoot the same scene at both settings and compare the size of the two files and the noise in the shadows.

Check yourself

• Could you correctly calculate the print size from your photo's pixel dimensions?
• Could you see where the detail starts to break up in the image you cropped?
• Could you observe the difference in file size and noise between the two settings?

Glossary

Mini quiz

1. What do megapixels measure?

• A) How many points the image is made of
• B) How clean the image is
• C) The sharpness of the lens
• D) The accuracy of the colors

2. By how many times must the pixel count increase to double the detail along a side?

• A) Two times
• B) Three times
• C) Four times
• D) Eight times

3. What is the main drawback of putting a large number of pixels onto a small phone sensor?

• A) The colors come out reversed
• B) Each pixel gets smaller and gathers less light, and noise increases
• C) The photo is recorded in black and white
• D) The screen breaks

4. In which situation does the real benefit of high megapixels show up most?

• A) When sharing the photo only on a phone screen
• B) When photographing fast-moving subjects in low light
• C) When you want to keep files small
• D) When cropping the photo heavily afterward or printing it very large

Answers

1. A — Megapixels tell you only the number of pixels; cleanness, sharpness and color accuracy depend on sensor size, the lens and the light.
2. C — Detail grows with the number of pixels along a side, and area with its square; doubling the side quadruples the area, that is, the number of pixels.
3. B — Fitting more pixels into the same small area makes each pixel smaller; a small pixel gathers little light, and in low light noise increases.
4. D — Extra pixels leave enough detail even when cropped and keep sharpness in very large prints; there is no need for them when sharing on a screen.

Summary card

• Megapixels = the number of points, not quality.
• 24 MP ≈ 6000 × 4000 pixels ≈ a sharp print of 51 × 34 cm (300 pixels/inch).
• To get 2 times the detail along a side you need 4 times the pixels; 48 MP is only about 1.4 times sharper than 24 MP.
• Many pixels on a small sensor = small pixels = more noise.
• High megapixels: for cropping headroom and large prints.

Bridge to the next lesson

Time to leave the numbers and return to the body of the camera. Everyone who picks up a camera for the first time pauses for a moment in front of the crowd of buttons, dials and menus on it. At the next stop the question is this: what are all these buttons for? There you will see why getting to know a few frequently used controls is enough to start with, and how the rest falls into place over time.