Posts Tagged ‘resolution’

Wednesday, April 28th 2010

The Myth of the Megapixel Myth

When Canon released the 50D I thought it had too many pixels. And I thought I was smart in thinking so.

Canon, please understand that SLR buyers aren’t as gullible as compact buyers when it comes to megapixels.

But in reality I had fallen for the myth of the megapixel myth.

(This post grew to almost a 1,000 words. If you haven’t got all day, skip to the conclusion.)

Lets start from the beginning.

The megapixel myth refers to the notion that a higher number of megapixels equals a better camera. And in calling it a myth, we are implying that camera manufacturers are increasing the number of megapixels on camera sensors only to trick everyone into constantly upgrading their cameras. Indirectly, we are implying that there is some intermediate number of megapixels that should be considered optimum for a given sensor size.

Ironically, the optimum number of megapixels always seems to be equal to the number in the camera generation one step back from the very latest release. Giving thousands and thousands of spoilt (and misguided) photographer brats an excuse to pour out their disgust in a million forum posts. A bit like I did.

One day it won’t make sense to add more pixels, but we have a long way to go until we reach those numbers. As it is now, we’re still gaining a lot of detail in our photos when the resolution increases.

On photography discussion forums you often hear the claim that a high resolution sensor needs really good glass, or even that it outresolves available lenses.

But those claims simply aren’t true. They are based on an incorrect mental model of how resolution works.

Even if you’re using a really cheap or soft lens, you’ll still get more detail out of it with a higher resolution sensor.

Have a look at these two tests at Photozone.de:

(If you own the 18-55mm IS, don’t scream at me – I’m not claiming it is a soft lens. Read on.)

Scroll down to the section titled MTF. The diagrams show how much detail the lenses can produce on the two cameras. Specifically, they show how many horizontal black and white lines you can fit into the image height before they blend together into a grey mush.

Note that the Extreme Corners which are the softest areas of the lens, produce a higher level of detail on the 15 megapixel camera – just like the centre of the lens. So just because they’re soft on the 8 megapixel camera doesn’t mean they won’t produce more detail on a 15 megapixel camera. This is because the lens and the sensor both combine to produce the details in the final image.

A sharper lens will always give you finer image detail, no matter what the sensor resolution. And a higher resolution sensor will always give you finer image detail, no matter what the lens in front of it!

Some maths

Mathematically, this is an approximation of how it works:

1/I2 = 1/L2 + 1/S2

or

I2 = 1 / (1/L2 + 1/S2)

I is image detail, L is lens resolution and S is sensor resolution. These are linear resolutions, just as in the MTF-charts I referred to above. (Megapixels are two linear resolutions multiplied together, width x height.)

Let’s say we have a camera with S = 2,300 and a lens with L = 3,000. That would give us an image with 1,825 lines per picture height:

I2 = 1 / (1/3,0002 + 1/2,3002) = 1,8252

I = 1,825

If we now buy a better camera with, say, S = 3,200 we’ll get more and finer details in our images:

I2 = 1 / (1/3,0002 + 1/3,2002) = 2,1872

I = 2,187

As you can see, the lens is still able to resolve a lot more detail than we get in the final image.

To get anywhere near the maximum performance out of a lens, the sensor needs to resolve at least three times as much as the lens:

I2 = 1 / (1/3,0002 + 1/9,0002) = 2,8462

I = 2,846

2,846 lines is basically 95% of what the lens in this example can resolve.

As you may have noticed, the numbers in my examples above are not just taken out of the blue. The values for the sensor resolutions correspond to the image heights in pixels of the Canon EOS 350D and Canon EOS 50D.

I chose a lens resolution value that would make the image resolution values (1,825 and 2,187) correspond fairly closely to the average measured resolution in Photozone’s tests that I linked to. In other words, 3,000 line widths (per image height) is probably roughly what the 18-55mm IS can resolve.

Conclusion

So, what does this all mean? Well, since the numbers in the examples above correspond roughly to reality, we can make a simple calculation.

To get 95% of the resolution out of the Canon EF-S 18-55mm IS, or any other half decent lens, we need a sensor ~9,000 pixels high. Which means the width would be 13,500 pixels.

That equals 121.5 megapixels!

Even if we settle for 90% of the lens’ resolution, we need 60 megapixels to get there! Currently, Canon’s cameras are getting something like 60-75% out of the EF-S 18-55mm IS.

These figures obviously sound insane. But it is no more insane than having 12-14 megapixels in a compact camera. (A digital SLR has more than ten times the sensor area of a compact.) Flash memory and hard drives are getting cheaper all the time, so one day it will happen.

In other words, the megapixel myth is a myth in itself. Camera makers are not being tricksy when they add more megapixels. In fact, if we want to get the most out of our lenses we need lots and lots of megapixels!

Of course, there are many other aspects of a camera that are at least as important as the sensor resolution. And when it comes to compact cameras, with sensors no more than 5 or 6 mm wide, we’re probably reaching the upper limits of what makes sense. By now, I’m guessing that compacts are getting practically all the resolution out of their lenses.