Megapixels — how many is enough?

When digital cameras first started appearing in the shops, a megapixel was considered a big deal and it was definitely worth stretching your budget to get a 1-megapixel camera. But now the megapixel counts are reaching 20 and more. Is this really necessary?

Do you need a 20-megapixel camera?

You probably do if you are working on extreme high-quality print jobs or for some of the serious high-end photo agencies. Professional photographers have some reasons for big megapixel counts, which I discuss in this article. But most people would rarely need to go beyond about 10-15 megapixels, even if they want to print nice big photos to a high standard on their home ink-jet printer.

What’s a megapixel?

A megapixel is one million pixels. So if your camera takes a 1-megapixel photo then the resulting image would measure something like 1,200 pixels wide by 900 pixels deep.

1,200 x 900 = 1,080,000. There’s your one million.

It might sound like a lot, but how many pixels do you need to print that photo? The answer depends on how tightly you pack those pixels together when you print it.

Your typical newspaper photo gets between 150 and 200 rows of dots (pixels) per inch — measured horizontally and also vertically — into its photos.

Glossy magazines tend to work at higher resolutions to get that really nice quality, printing their pics at between 240 and 360 dots per inch.

From now on, I’ll refer to dots per inch as ‘dpi’.

What’s the best-quality print resolution for glossy magazines?

Many experts say that once you go beyond 300 dpi, the dots start becoming so small that the human eye can’t see any difference. However we already know that some printers work at 360 dpi. I’m pretty sure no one has eyes good enough to see any difference beyond 360 dpi.

How big would your 1-megapixel photo print?

Let’s imagine we print your 1 megapixel photo in a newspaper.

At 200 dpi your 1,200 x 900 pixel photo would be:

(1,200 divided by 200) inches wide and
(900 divided by 200) inches deep.

Which comes to 6 inches wide by 4 and a half inches deep.

Okay for a smallish pic, but if you wanted to stretch that photo across the whole front page you’d quickly see its limitations. It would look fuzzy and, well, just plain awful unless you stood a long, long way away from it.

And if it was printed in a glossy magazine?

Let’s take the high-end example of 360 dpi.

Your 1,200 x 900 pixel photo would be:

(1,200 divided by 360) inches wide and
(900 divided by 360) inches deep.

Which comes to 3 and a bit inches wide by 2 and a half inches deep.

In other words, a typical small magazine photo.

Printing your photos at home

Modern inkjet printers boast about printing your stuff at resolutions of thousands of dots per inch. Wow, that sure does sound good, and true enough, the output quality from those printers is very good indeed. But not as outrageously, freakishly good as the numbers might make you think.

You see, they use a different sort of technology which results in a way of measuring dots per inch that creates very big numbers. So an inkjet printer working at 4,800 x 2,400 dpi resolution does NOT require a 4,800 x 2,400 pixels-per-inch photo to print properly. In fact, if you tried to push a photo at that resolution through your home printer I have a hunch it would either take forever to print or crash your computer (and I won’t even start explaining the bit about having a different resolution vertically and horizontally). Instead, you could save your pic at 360 dpi and print it at the highest settings. Assuming you used the right inks and paper and so on, it would come out looking superb.

That’s enough talk about 1 megapixel. What about your typical camera today?

Your average digital SLR camera now will probably exceed 10 megapixels in its image resolution. Let’s look at the numbers to see how big that kind of photo would print. And don’t worry: I’ll be doing all the maths for you.

A typical 10 megapixel photo might be 3,888 x 2,592 pixels.

3,888 multiplied by 2,592 equals 10,077,696. There’s your ten megapixels.

Using the same formula as above we can see it’s more than enough pixels to print your photos:

In a newspaper, it would print about 19 and a half inches wide by 13 inches deep. Which is wider than a full broadsheet newspaper page.

In a glossy magazine at the high end of the scale (360 dpi) the same photo would print at about 11 inches by 7 inches. Almost enough to fill an A4 page. And don’t forget this was at the high end of resolution settings. At 300 dpi your image will print slighter bigger than an A4 page.

And printing at home, you could save your 10 megapixel file at a very reasonable 200 dpi and it would easily be big enough to fill an A3 page.

Okay, but what if you want to chop off half the photo and still print it big?

Hah! That’s the key point! Most pro photographers don’t need to print photos much bigger than about 10 megapixels unless they’re doing high-resolution glossy posters or something fancy like that. But sometimes a portion of their photo isn’t needed. They want to chop off parts of it (crop the photo) and still have enough pixels left over to print it nice and big. So they might start with a 12 megapixel photo, for example. Then discard a sixth of those pixels when they crop it. And they’re still left with a 10 megapixel photo.

That’s the main reason why you’d want to go higher than about 10 megapixels.

So more megapixels is good then, is it?

Well yes, and no.

The ‘yes’ reasons

More pixels means that you have the luxury of having more to throw away if you need to crop the photo, while still having enough pixels left over for your printing needs. And of course, for the pros working towards large-format high-quality printing, they will need big pixel counts.

The ‘no’ reasons.

The 'no' reasons can be broken up into a few categories:

FILE SIZE
More pixels means bigger files sizes, and that means:

• slowing down your camera
• clogging up your camera’s buffer more quickly during fast-burst shooting
• filling up your camera’s storage card more quickly
• slowing down image transfers to your computer
• filling up your computer’s hard drive more quickly
• bigger archiving backup-facilities
• Bigger files take more memory and are more cumbersome to work on in your image-editing software.

LIMITS IN WHAT YOUR LENS CAN DO
Even under ideal conditions, most lenses struggle to capture enough detail to exceed what 10-15 megapixels can hold. This is one reason why the professionals with their high-end cameras spend several thousands of dollars more on high-end lenses.

LIMITS IN WHAT YOUR SENSOR CAN DO
I’ll use some rough numbers here. The geeks and purists might take offense, but they’ll probably agree with my overall message.

A 10-megapixel camera will require close enough to 10 million pixels on its sensor. Now, since the sensor in your typical digital SLR is only about a square inch in size, that means each pixel on its surface must be about a ten-millionth of a square inch in size! In fact, if you take into account the way they build them, there has to be a tiny gap between each pixel. So the pixels are even smaller.

Try to imagine how small a ten-millionth of a square inch is. No, don’t bother. It’s crazy-small. In fact, using current limits of technology, it’s barely enough area for capturing enough photons to get a useful sample of light.

So when a pixel in your sensor gets too small, it might get things wrong. Instead of registering a green pixel for an area of grass, it might not capture enough green light, and so it bungles things and records a red pixel instead. Or a white pixel in area that was supposed to be in shadow. Your photo starts getting speckled with pixels of the wrong tone. That speckling is called noise and too much noise will greatly reduce the image quality in your photos.

Noise happens when the sensor pixels are unable to get a sufficient sampling of light. I was able to get this much noise (seen at 100% resolution) by maxing out my camera's ISO settings, which forced the camera to take the shot with a much faster exposure than it would have normally used

Can you see now, how too high a megapixel count can result in lower image quality?

This is why compact cameras, which have very small sensors combined with unrealistically high megapixel counts, tend to produce noisy photos, especially in the shadow areas and weak-light situations where there just aren’t enough photons landing on those miniscule sensor pixels.

Now, the clever boffins who create the cameras’ sensors are getting better at their job all the time, and the sizes they shrink those pixels down to are getting smaller and smaller. Also, I think it is fair to say that some brands of camera appear to perform better than others. But some of those marketing types all too often push for megapixel counts in excess of what even those smart boffins can handle. And for what benefit? In my opinion it is just for some sort of shallow advertising claims. You can imagine the kind of conversations you’d get in their R&D departments:

I find this aspect of cameras bizarre, because in all other areas the manufacturers do seem to listen very carefully to the top photographers.

And a bit of yes and no

When you reduce your image size in your image-editing software, some of the noise tends to go away. Shrink your images down enough and noise pretty much disappears altogether. So, a large photo with a bit of noise can be shrunk, and sometimes end up with the same image quality as a less-noisy photo taken at lower resolution.

For this reason, some people argue that more pixels are better, because in good light you get great pics. And in low light you just sacrifice some image size later on. The question is, how much you have to reduce your images to make them usable.

How much noise is okay?

If you print your photo out and it looks great, then obviously the amount of noise in your photo is acceptable. Too much noise though, and you will notice colours starting to look wrong and detail being lost.

But the good news: the camera manufacturers are well aware of these issues and have some great people working on sensor design, which means cameras should keep getting better.

So is it yes, or no?

Perhaps the maths at the beginning of this article was a bit boring. But it doesn’t take much number-crunching to see there are more ‘no’ reasons than ‘yes’. In other words, the problems with giant megapixel counts are more than the benefits for most of us.

So how many megapixels do you need?

Professional photographers do sometimes need to capture very high resolution images. But for everyone else, my personal advice is not to worry too much about giant numbers of megapixels. Chances are your camera will have more than enough. Unless you plan on doing high-end large-format printing or heavy cropping of your images, you probably won’t need much more than about 10 or perhaps 15 megapixels. More than that and your lens might not capture enough detail to make any difference anyway.

On those occasions when you need super-high resolution, consider the option of taking a few photos in a panorama and stitching them together in your image-editing software.

Instead of looking for super-high megapixel counts, in my opinion most people would be better off looking into camera performance. That doesn’t mean examining every pixel of your photos on screen. It means looking at how well the camera works in all sorts of conditions and all sorts of light, including low light. If you’re a wildlife photographer you’ll really appreciate a camera that works well in low light, in those times close to sunset and sunrise when many animals come out of hiding. Low light (high ISO) photography is becoming more of an issue with digital camera manufacturers and in the coming years I hope to see them putting more emphasis on that, rather than on stacking in ever more millions of pixels.

Beginners’ series on
digital SLR photography

• A beginners’ guide — basics for Digital SLRs
• Learning from examples — picking up where the first article left off
• Understanding histograms — making sense of this handy tool
• Preventing washed-out photos —Why it happens, and how to prevent it
• Understanding Exposure Compensation — Why your photo subject can look so badly exposed, and what to do about it
• Megapixels — how many is enough?

Photo Sales

• Photo Library — Pics of Australian critters

An unhelpful guide

• Wildlife Photography — this guide will not make you into a better photographer
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