Is 18 Mpixels Enough?

Started May 16, 2012 | Discussions thread
Jeff Peterman
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Re: Mega pixels - myths and reality
In reply to Pimpleportrait, May 16, 2012

Having just discussed this in another forum ...

Pixel count alone isn't worth much. Here's some math to help:

To avoid seeing the individual pixels, the general consensus is that you need 200 pixels per inch, with 300 being better. (Some claim they can see improvement up to 600 pixels per inch.) For an 8x10 print, that's 1600x2000, or 3.2 MP, so a 3.2

MP sensor is enough for a standard 8x10 print (or a Web page). In practice, if the subject has a noisy image, like a shot of a pile of leaves, or a crowd of people from a distance, you may not see the pixels, but they can become apparent with a smoother subject.

People rarely look closely at images bigger that about 16x20 - with bigger images, the viewing distance increases and that can make resolution lower than 200 ppi work well. But for 200 ppi on a 16x20 inch print, you need 3200x4000, roughly 12 MP. So it could be argued that 12 MP is enough, BUT, people often want to crop, and if you crop to the center 50% of an image, that cuts the pixel count by 4, making an argument for a 48 MP sensor with a 16x20 print! The argument only holds if you want to look closely at such as large print, AND the print has lots of critical detail.

But there's another issue. Unless the pixel count is increased through increasing the sensor size, more pixels means higher pixel density, which means a lower signal to noise ratio, and removal of the noise in processing can decrease effective pixels so that a noise-reduced 20 MP image can have less detail than than a 10 MP image.

If you increase the number of pixels without changing the sensor size, you're increasing the pixel density. As you increase the pixel density, you're reducing the light capturing region per pixel, which tends to reduce the signal per pixel, while the electronic noise remains the same. This electronic noise can reduce the "quality" of the data from the pixel. As a result, if you look at a 100% crop from images of the same size sensor but different pixel densities, the image from the higher density sensor will look worse.

However, this gets complicated quickly!

For example, if I compare a 100% crop from my 10 MP Canon 40D and a 100% crop from my 18 MP Canon 7D, both with the same size sensor, the image from the 7D clearly looks worse. BUT, Canon put a lot of effort into reducing sensor noise when developing the 7D sensor, so the increase in noise is less than the increase in pixel density. As a result, when the images from the two bodies are viewed AT THE SAME SIZE (which means more pixels per inch for the 7D than for the 40D), the image from the 7D looks cleaner. This means that you can crop images from the 7D more than you can from the 40D for the same image quality, but not as much as you might predict from pixel count alone.

Note that in some cases, especially with P&S models, sensor density is increased without improvements in sensor noise, and the net result can actually be a worse
image at the same final size!
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Jeff Peterman

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