Samsung’s new “enthusiast” compact, the TL500 (or EX1 outside the US) was announced at the PMA show in February; but as of this writing, it’s not yet available from the usual mainstream sources. However, reviews are starting to filter out: Both Luminous Landscape and now Photography Blog have given it very positive ratings. (DP Review has a sample gallery posted, which suggests they’ll be posting their own full rundown soon.)

Samsung TL500 (EX1)

Stout Little Fellow

As with any small-sensor compact, there’s still some image-quality compromises. The active area of the TL500′s sensor measures about 7.5 x 5.6 mm, so ISO 800 still shows obvious noise.

However this new Samsung is beginning to look like one of the better options in the “serious compact” segment. (Street prices will start out about $400, presumably to drift downwards from there—that’s higher than a Canon S90, but well below Ricoh and Leica levels.)

Read the rest of this entry »

Well, the much-anticipated “EVIL” cameras from Sony have finally arrived, the NEX-3 and NEX-5.

Sony NEX-3 Body

Big Sensor, Small Camera

The fullest NEX-5 review available so far is at Imaging Resource (while DP Review has vented annoyance that they were given pre-production cameras, and so were forbidden to publish test images).

The clearest win for the NEX system is in body size—which (for an interchangeable-lens, APS-C camera) is quite impressive. I hope all the other EVIL brands are paying very close attention.

The tiny NEXes also make it apparent that zoom lenses become even more ridiculous in the EVIL segment: Compact primes are the obvious way to capitalize on the mirrorless size advantage.

So it’s a mystery that the sole pancake prime offered at launch is a very wide 16mm f/2.8 (24e). Sony clearly needs to fill out its lens lineup still—at least adding a fast “normal” lens (approximately 30mm for the APS-C sensor format).

But perhaps they needed more time to complete something competitive with Panasonic and Samsung’s well-regarded pancakes. Sony’s tiny ultrawide stands alone here, and will admittedly be intriguing for many shooters.

Sony could not resist stuffing in a few more superfluous megapixels: These models use a 14 Mp sensor. But the high-ISO results seem very decent. Not quite class-leading (even compared to Sony’s own 12 Mp sensors used in the Nikon D5000 or the Pentax K-x); but it’s clear that Samsung’s 14 Mp sensor from the NX10 is being left in the dust.

I can tell from Petavoxel’s site traffic that the Samsung NX-10 generated much curiosity wondering if it could accept an adapter for Leica-mount lenses, in M bayonet or 39mm thread. (The answer seems to be no, unless someone can show me otherwise.) But the story for Sony’s new “E” lens mount is much more interesting:

Sony 'E' Flangeback Distance

Shallowest Lens Mount EVAR?

Thanks to Sony for providing that handy sensor-plane mark on the NEX-3. Scaling from the published body dimensions, this is one incredibly shallow lens mount. (The throat diameter is pretty generous too, from what I can estimate). Even for the shallow lens register of Leica lenses, there’s about a centimeter of extra depth to allow a lens adapter.

So the good news is that mechanically, it would be possible to adapt just about any other lens to the E mount. What we don’t know yet is whether Sony will intentionally cripple this function, either by requiring a Sony-chipped lens to be attached or by having poor support for manual focusing.

The user interface of these new NEX cameras follows in the footsteps of the Olympus E-PL1, in being very “point & shoot” oriented. There is no tactile function wheel for manual control, a major downer for any serious user. It remains to be seen whether Sony expands the NEX lineup into “enthusiast” models with better manual controls.

Also mystifying is the lack of an optional eye-level electronic viewfinder. Sony has created a new proprietary accessory port, but currently it is only used for the (included) companion flash, an optical viewfinder for the superwide lens, and a video microphone. It seems impossible that Sony would deliberately handicap themselves by not including connectors for an EVF as well; so I guess we should assume that will come later.

I do respect Sony’s decision to keep the flash separate, given that ISO 1600 shooting seems quite acceptable with this NEX sensor. But again, it does make one wonder whether Sony has a brighter-than-f/2.0 lens option waiting in the wings somewhere.

David Pogue, who writes about technology and gadgets for the New York Times, has spent years mocking the delusion that megapixels define the quality of a camera.

For example, he once ran a demonstration showing that random viewers couldn’t see much difference in a row of enormous, 16 x 24″ prints, even when the pixel counts varied wildly.

But Pogue made an odd aside last week, at the conclusion of his compact-camera buyer’s guide:

“As the ridiculous megapixel race winds down at last, …”

…a comment which left me scratching my head in confusion.

Perhaps he’s been busy—avalanched under press releases for all those new tablet e-readers. Or maybe he’s aggravated that the megapixel race didn’t stop at 7 Mp, as he hoped in 2006?

Believe me, I understand the frustration; the desire to throw up your hands, declare victory, and retreat.

But in reality, the megapixel war still rages—most obviously among point & shoot cameras. (And it’s the buyers of these mass-market models who are most likely to take advice from newspaper articles, rather than from some specialist geek website.)

Imaging Resource 12-14.9 Mp Digicams

Imaging Resource Breaks it Down

Among current point & shoots, I see a sickening 40 different compacts trumpeting 14 megapixel sensors. And new models are being introduced every day.

With the teensy sensors used in these cameras, diffraction, lens aberrations and noise make such ridiculous pixel counts meaningless—fraudulent, in fact.

Now, Pogue begins his compact-model roundup by noting some limitations inherent to all small cameras: shutter lag, grain, and blown highlights. But he hasn’t much followed his own oft-stated advice: Choose a camera based on its sensor size, not pixel count.

Seven of Pogue’s nine selections have pixels smaller than 1.54 microns. (The Nikon’s are a ludicrous 1.43 microns.) His Panasonic pick does a smidge better, at 1.56 µm.

But compare these to the 2.0 microns of the (still fairly compact) Canon S90—each of its pixels can collect about 70% more light.

His one choice I might grudgingly accept is the 10 Mp Fujifilm F70EXR. Besides having 1.77 micron pixels, this model offers a special low-light mode. Ironically, it works by pairing up pixels, turning it into a 5 megapixel camera! Hey, it’s a Pyrrhic victory, but I’ll take it.

But Pogue’s other picks simply have pixels that are too small, by any reasonable criterion.

I do admit that anyone forced to buy a compact digicam today—lets say your old one just died, and you’re leaving on a trip tomorrow—faces very limited choices.

If need be, you might hunt for a model using one of the new generation of 10 Mp back-illuminated CMOS sensors. For example, Sony’s “Exmor R” chip (versus the regular, non-R kind) works some special tricks to wring the most out of its 1.7 micron pixels.

Meanwhile, even among serious DSLRs, megapixel marketing creeps onwards too. Canon just introduced its Rebel T2i (550D outside the US), with an 18 megapixel sensor.

Canon Rebel T2i

The Amazing Shrinking Pixel Goes DSLR

This is actually rather worrying. Aren’t “enthusiast” photographers supposed to know better? That smaller pixels compromise other aspects of performance, like dynamic range and noise?

Stuffing 18 million pixels into the same 22.3 x 14.9mm sensor area makes each pixel 4.3 microns wide. This is the same pixel pitch that causes Micro Four Thirds cameras to struggle with noise when pushed up to ISO 800.

Consider the 12 Mp Pentax K-x, praised for its high-ISO performance. It uses 5.5 micron pixels instead. This gives each pixel 63% more light-gathering area.

Also remember that on the T2i’s sensor, each millimeter of sensor width contains 232 pixels.

But it is very rare for a real-world lens to resolve detail at that scale with reasonable contrast. If one can do so, it will only be at a single, optimum, middle f/stop. That’s not especially practical.

(Aberrations limit sharpness at wide f/stops; diffraction creates blur at smaller ones—in APS-C cameras, typically f/8 or smaller. For a more technical discussion, start here.)

I wish we could say that megapixel marketing madness had finally ended.

But I’m not seeing any evidence this is true.

Sensor Size, Part II

February 7, 2010

As I noted in an earlier post, camera makers quote sensor sizes in mystifying “fractional inch” designations. They’re much less forthcoming in giving us the actual, active dimensions of the chip.

Is this because they’re embarrassed? Even a throwaway Kodak Fun Saver uses the generous dimensions of 35mm film; while today’s $300 digital compacts might use a chip with only 3% of that area.

The common 1/2.33″ or 1/2.5″ sensors used in current point & shoots measure roughly 6mm across. That’s, you know…  not big:

US Penny Versus Tiny Sensor

A 6 x 4.5 mm Sensor vs. Honest Abe

Now, even when you don’t have any “official” specs about the chip used in a camera, it’s usually possible to work out the sensor dimensions indirectly.

All you need is the actual focal length(s) of the camera lens; plus the manufacturer’s stated “35mm equivalents.”

Here’s a camera marked with its true, optical focal lengths. (When the smaller number is under 10mm, you’re seeing true, not “equivalent” focal lengths.)

Compact Camera Lens Markings

Focal Lengths Marked as 5.4 to 10.8mm

The first thing we need to know is that “equivalency” is usually based on the diagonal angle of view of the lens. The next point is that (true) focal lengths scale directly in proportion to the dimensions of the image format.

A frame of 35mm film has these dimensions:

35mm Film Dimensions

A Film Frame Has a Diagonal of 43.3mm

Notice that film’s 43.3mm diagonal is a smaller number than the 70mm “equivalent” f.l. that was quoted for the long end of the zoom range. Telephoto focal lengths will always be longer than the image diagonal.

So, the digital sensor’s diagonal must also be smaller than the lens’s true focal length when zoomed in: 10.8mm.

Divide 43.3 by 70 and you get 0.62; multiply 10.8mm by that and you get 6.7mm as the diagonal of the sensor chip.

Likewise: 43.3 divided by 35 = 1.24; multiply 5.4mm by that and you also get 6.7mm for the diagonal.

But wait, that’s not so useful—didn’t we want to know the chip’s width and height?

Well, compact cameras almost always use 4:3 image proportions (the old “television” aspect ratio). And so, conveniently, the diagonal has a nice easy-to-remember relationship to the sides.

3:4:5 Ratio in Sensor Dimensions

Rembember the 3-4-5 Triangle?

In other words, the chip is 60% as tall as the diagonal; and it’s 80% as wide.

So for the sensor we’re talking about, a 6.7mm diagonal means it’s about 5.3mm wide and 4.0mm tall. This is what the industry calls a 1/2.7″ chip size.

And that’s a lot smaller than Lincoln’s head.

Know Your Noise

February 5, 2010

I’ve just begun messing around with a Nikon Coolpix P60. This was one of Nikon’s upper-level point & shoots from back in 2008.

It’s not the greatest camera in its class, and it’s not the worst. For a pocketable 8 megapixel model, the P60 is probably about average.

Nikon Coolpix P60

Nikon Coolpix P60

Its pixels are about 1.9 microns across; today, 1.5 or 1.4 microns has become the norm. In other words, the P60′s pixels have 60-80% more light-gathering area than the ones used in a typical 2010 compact.

So let’s take a quick look at how well it handles noise.

We’ll start with an image where the camera is set to ISO 80, the lowest available. And I’ve zoomed to 135e, for a close view of my (charming) model:

P60 ISO 80 Close-Up

Click for Larger Version

This is the high-quality version, to show us what the textures and details ought to look like. (Although notice that the P60, like any small-pixel camera, is struggling to keep the highlights from blowing out.)

Now we zoom out the lens, and look at some detail crops to see how well the image quality holds up. Here’s the same view of the subject, at ISO 80, ISO 200, and ISO 800 (these are now crops using about 40% of the frame width).

It’s not a huge surprise that ISO 800 looks very grainy:

ISO 800 Crop 1, at 100%

100% view, ISO 800

The top of the camera no longer shows its original texture; any apparent detail is just the noise itself.

P60 Crop 2 at ISO 800

Subtle Details Getting Lost in Noise

While at ISO 80 you could still read “München Germany” below the lens, that detail is gone now.

But let’s give Nikon some credit: Chroma noise is very well controlled here, so the speckles do not have distracting “rainbow confetti” colors. Aesthetically, this noise is fairly inoffensive.

However, what may be more worrying is that even at a moderate ISO 200, we still see some anomalies:

P60 Crop 1 at ISO 200

100% View, ISO 200

Instead of obvious noise, the issue is more subtle here: Rather than looking entirely photographic, the image almost begins to look painted. Noise-reduction processing has kicked in, even at a fairly low ISO—and it’s adding some of its own odd artifacts.

P60 Crop 2 at ISO 200

A "Watercolors" Look in the Lens Reflections

Remember—ideally, it’s supposed to look like this:

Lens Detail, Good Original

Back to the Original

Of course, this “painterly” impression is much less noticeable at any reasonable viewing size. We’re pixel-peeping here.

Yet there’s something troubling about a camera that re-draws your photographs—even if it does so very tastefully.

Questions About Pen Jr.

February 3, 2010

Hints and rumors had been swirling that Olympus was about to add another “digital pen” to its micro Four Thirds lineup. And today Olympus finally took the wraps off the new E-PL1.

DP Review has a preview which runs down all the features, controls, and how it compares to other µ4/3 models.

Econo-µ4/3 from Olympus, the E-PL1

Econo-µ4/3 from Olympus, the E-PL1

(DP Review seems to be conspicuously boycotting the term “EVIL”—electronic viewfinder, interchangeable lens—which the rest of the camera world has jokingly embraced.)

With Olympus already fielding an E-P1 and an E-P2, you might overlook that letter “L” in the new model’s name. But it’s a significant letter. It means “less expensive.”

The E-PL1 comes in at a couple hundred dollars cheaper than the E-P1. So for a street price not that much higher than “serious” compacts like the Canon G11 or the Panasonic LX3, now a shopper can buy a true interchangeable-lens camera, with a much larger sensor chip.

The pixel size in the E-PL1 is dramatically larger than other compact cameras: about 4.3 microns wide. Each pixel covers more than 4 times the area of those in “enthusiast” compacts. And compared to today’s silliest point & shoots, they’re 9 times larger. Theoretically at least, this should reduce image noise, and boost high-ISO range.

And Petavoxel says, that’s A Good Thing.

The E-PL1 apparently shares the same sensor as the E-P1 and E-P2 (even their anti-shake feature). What remains unknown is whether any of Olympus’s cost-cutting has compromised signal processing, or read-out noise. Remember that Panasonic’s most compact µ4/3 model, the GF1, has some trouble matching the noise performance of its larger-bodied siblings.

Since the whole justification for micro Four Thirds is the promise of better image quality in a small camera, it would be rather lame if Olympus blew this one. I’ll be waiting to see.

The controls of the E-PL1 are very much in the style of a point & shoot—not a DSLR. Despite the “PASM” options on the mode dial, there are no control wheels to adjust settings. I suspect it will be a rather aggravating experience to burrow into the menu system and make several button presses, just to change the f/stop.

That’s a disappointment, especially compared to the nifty control ring of the Canon S90. Or let’s not even mention the breathtaking clarity of the Leica X1.

But here’s hoping Olympus sells a billion E-PL1′s. It does offer frustrated point & shooters a real promise of much-improved picture quality.

And frankly, micro Four Thirds cameras have been overpriced so far. So if price competition has finally arrived in µ4/3 cameras, that’s good for everyone.

Resolving Pixels

January 27, 2010

Gather ’round the campfire, children; Grampa has a story to tell, about a wild and primative time long ago.

In about 1996, a techie friend of mine went out and bought one of the very first digital cameras a normal civilian could find. It cost something insane, like $600; and you held it like a weird pair of binoculars:

Kodak DC40 Paleo-Digital Camera

Kodak DC40, circa 1996

It had no removable memory (it could store 48 photos internally), and no zoom lens, either. In fact its lens was “focus free” —meaning anything closer than 4 feet was a blur. It was an electronic Brownie.

Most remarkable is that the images were only 756 x 504 pixels. That means about 380,000 total. (Check the manual if you don’t believe me.)

Yes, children, this was a dark age before megapixels. Today, we’d round that off and call it 0.4 Mp.

I would be the first to admit this was inadequate. You could maybe get away with 756 x 504 on the web, unmagnified; but as for cropping and enlarging those images, there was no hope. Even a 4-inch-tall print could look a little raspy and lacking in detail.

In 1998, people really took notice when the film-photography powerhouse Canon brought out their first “serious” digital camera, the PowerShot Pro 70. It had autofocus, an F/2.0 zoom, and Compact Flash slots. And—it could shoot 1.6 megapixel images.

Those specs seemed impressive enough to outweigh its $1100 introductory price, and its cartoonishly odd appearance:

Canon PowerShot Pro 70

Canon Weighs In At 1.6 Mp

Brrrr! But hey—at least 1.6 megapixels is four times as good as that sad old Kodak, right?

Well actually… We need to stop for a minute here, and talk about what “resolution” really means.

Doubling resolution means two finely-spaced details can still be distinguished, even when they’re half as far apart. (We’re assuming here that the lens is perfectly sharp, and only the sensor resolution matters.)

To get this doubling, the spacing between the pixels must be scrunched down, so there are twice as many of them per inch.

3x4 pixels

Original, Low Resolution

But remember—a sensor chip has both width and height. To get a doubling of resolution, you must quadruple the number of pixels:

6x8 pixels

New, Doubled Resolution; 4x the Pixels

And each time you want to double the resolution… The pixel count quadruples again:

12x16 Pixels

Doubling Resolution Makes the Pixel Count Mushroom

Now we fast-forward to 2006. The explosion of mass-market digital cameras is in full swing. Cameras have shrunk to the size of soap bars, even including a 3x zoom.

And despite this, linear resolution has doubled again, compared to the 1.6 Mp Canon. Now, sensors were up to a crazy 6, even 7 Mp. Six million pixels!

One notable example was the FujiFilm F30, a 6 Mp model:

FujiFilm FinePix F30

Doubling Resolution Again, FujiFilm's F30

With some clever sensor design and noise-reduction techniques, its images stayed quite usable even at high ISO sensitivities—startlingly so, compared to its contemporaries.

But what of today? Surely technology should be marching onwards! Don’t we deserve another upwards ratchet in the resolution race?

Well, it’s not that simple. First off, 6 Mp is a plateau which satisfies almost all of the real-world uses we put our photographs to.

We can make excellent 8×10″ prints, view them onscreen (even zooming in considerably), and spare our hard disks the strain of bloated file sizes.

Nikon’s D40 DLSR was one of the runaway success-stories among recent digital cameras; it remains the 3rd most popular Nikon among Flickr users. It was built to be affordable, and is hardly as stout as Nikon’s pro models—but you don’t hear many owners whining about inadequate pixels. Click on a few sample shots and check for yourself.

And you guessed it: Six megapixels.

But, on the remote chance that you really do need more resolution… keep in mind our quadrupling math above. If for some reason, 6 Mp doesn’t satisfy you, the next step up isn’t 10, or 12.

It’s twenty-four megapixels.

The good news is, those cameras do exist! Including a basic lens, I think we can fix you up for about $2400.

So enjoy that extra resolution. I’m sure you have some perfectly good reason to want it.

How Big Are My Pixels?

January 21, 2010

In the firestorm of comments about the Great Megapixel Swindle,  a couple of questions kept coming up: “Instead of megapixels, what should I be looking at? And how do I even know what chip size a camera has?”

Well, cameramakers designate chip sizes using an almost incomprehensible naming system (inherited from video tubes, if you really must know) using numbers like 1/2.3″. In fact, if we don our conspiracy tinfoil hats, it’s almost as if they’re deliberately making it hard to understand the true size.

Thankfully there’s a table decoding various sensor formats here. (Though that page has grown a little dated: There are no longer any 2/3″-sensor models on the market, sadly.)

But the number I really care about is, what is the size of the pixels? Yes, new technology might still come up with a few sensitivity-enhancing tweaks. But loosely speaking, the bigger each pixel is, the better.

Pixel width is quoted in microns—sometimes abbreviated µm or um. But it’s not often listed directly in camera specs.

But many in-depth review sites like DPRreview will give the “pixel density” in their model listings. Note the ridiculous jump from the consumer point & shoots (between 35 and 50 megapixels per square centimeter) versus the serious DSLRs (1.4 to 3.3).

Might this tell us something?

So as a handy conversion reference, here’s how to translate some of those density numbers into actual pixel sizes*

  • 50 Mp/sq. cm —> 1.4 micron pixels

[e.g. 14-megapixel compacts]

  • 35 Mp/sq. cm —> 1.7 micron pixels

[10-megapixel compacts]

  • 24 Mp/sq. cm —> 2.0 micron pixels

["enthusiast" compacts, e.g Panasonic LX3]

  • 16 Mp/sq. cm —> 2.5 micron pixels

[Fujifilm F31fd, circa 2007]

  • 5 Mp/sq. cm —> 4.3 micron pixels

[New micro Four Thirds models]

  • 3.3 Mp/sq. cm —> 5.5 micron pixels

[typical APS-C sensor DSLR]

  • 1.4 Mp/sq. cm —> 8.5 micron pixels

[professional Nikon DSLR]

Now, for the reasons I’ve vented about before, the smallest pixel that makes any sense to me is about 2 microns across. Making cameras pocketable dictates smaller sensor sizes; but unless the chip is under 24 Mp/sq. cm, you’ll definitely compromise low-light capability.

But the real leap in quality comes when you drop to “single digits” in pixel density. Even compared to enthusiast compacts, those DSLR-style pixels have 5 to 8 times the light-gathering surface. That really makes a difference.

What this world needs badly is more little cameras with big pixels. I hope we get some soon.

*Note I’m really quoting the “pixel pitch.” Each actual pixel loses a bit of light-gathering area to its wiring traces. But microlenses overtop give nearly 100% coverage; and so they’re really the relevant width in terms of light-gathering area.

Megapixel Recap

January 20, 2010

I woke this morning to discover that my post about megapixel madness was up to 344 comments on the Reddit “technology” page, as well as generating a lot of talk at The Consumerist. It seems to have struck a nerve.

There’s nothing like having 20,000 people suddenly read your words to make you panic, “could I have explained that a little better?” Let me say a couple of things more clearly:

  • Yes, some people DO need more megapixels

Anyone who makes unusually large prints, or who routinely crops out small areas of the frame, does benefit from higher megapixel counts. However, those pixels are only useful if they can add sharp, noise-free information.

The typical point & shoot CCD would fit on top of a pencil eraser. There are fundamental limits on how much detail you can wring out of them. So, the giant-print-making, ruthlessly-cropping photographer really needs to shop for an “enthusiast” camera model—one with a larger sensor chip.

  • Diffraction sets theoretical limits on image detail

Many more people viewed the “Swindle” post than read my explanation of diffraction. The key point is that even if you have a lens that is well-designed and flawless, light waves will not focus to a perfect point. The small, blurred “Airy disks” set a theoretical limit on how much actual detail a lens can resolve.

Up to a point, “oversampling” a blurry image with denser pixel spacing can be useful. But today’s point & shoots have clearly crossed the line where the pixels are MUCH smaller than the Airy disk, and squeezing in more pixels accomplishes nothing.

Plus, making pixels tinier actually worsens image quality in other respects. Marketing compact cameras by boasting higher megapixel counts is simply dishonest.

  • Higher-quality lenses can’t fix this

Better lenses are preferable to bad ones; but diffraction puts a ceiling on what even the best lens can do (yes, even one with a German brand-name on it).

To get greater true image detail, the entire camera must scale up in size. This makes the Airy disks a smaller fraction of the sensor dimensions.

  • Tiny pixels are low-quality pixels

A pixel that intercepts less light is handicapped from the start. Its weaker signal is closer to the noise floor of the read-out electronics. There’s more random brightness variations between adjacent pixels. Each pixel reaches saturation more quickly—blowing out the highlights to a featureless white.

I’m aware of the theory that higher-resolving but noisier pixels are okay, because in any real-world output, several pixels get blended together. But I’ve seen enough photos with weird “grit” in what ought to be clean blue skies to be suspicious of this.

First, random pixel fluctuations interact in strange ways with the color demosaicing algorithm. Distracting color speckles and rainbowing seem apparent at scales much larger than the individual pixels.

Second, the camera’s noise-reduction algorithm can add its own unnatural artifacts—obscuring true detail with weird daubs of waxy color. (This was the problem highlighted in my example photo.) It’s better to have less noise from the start.

  • Many compacts perform much better than this one

That’s true. But isn’t reading an exaggerated polemic much more fun?

Let me be clear that my complaint is about TINY CHIP point & shoots. The new micro Four Thirds cameras (which I am following closely) were created specifically to address the shortcomings of small-sensor cameras, while remaining pocketable. But they cost a lot, at least so far.

Mainly, my complaint is about honesty. Camera makers are slapping big “14 megapixel” stickers onto cameras with tiny chips.

I just want people to understand that—as The Consumerist headlined it—these are “Marketing Lies.”

There is a great article at cambridgeincolour.com about the role diffraction plays in digital-camera resolution.

The issue is that at microscopic scales, the wavelike nature of light makes it act in a slightly “squishy” way. Points of light brought to focus by a lens are smeared out by a certain irreducible amount—even if all lens aberrations are perfectly corrected.

Instead of a sharp pinpoint, light is actually focused into a fuzzy bulls-eye pattern. Its bright center is named the Airy disk, after the British scientist who first described it.

Interestingly, the diameter of the Airy disk is unaffected by lens focal length, or image size; it depends only on the f/ratio of the lens. As you stop down the aperture, a bigger fraction of the light fans outwards from its intended path, and so the wider the Airy disk blur becomes.

With lens aberrations, the opposite is true: they create the most blur at widest apertures. On stopping down, sharpness improves.

So most film-camera lenses give their sharpest images at the middle of the f/stop range—the “sweet spot” where the combined effects from diffraction and lens aberrations are lowest. That’s one way to interpret the old photography rule, “f/8 and be there.”

Anyway, it’s easy to figure out the Airy disk size. The diameter in microns is about 1.35 times the f/number (using the green wavelength our eyes see most brightly). So, for example, the Airy disk at f/4 is 5.4 microns across.

The shocking thing few camera-buyers realize is that these fuzzy blobs are often larger than the individual pixels in a digital camera sensor.

Airy Disk versus Pixels

Pixels much smaller than the Airy Disk add no detail

The problem is most egregious in the world of point & shoots. Everyone seems to want the highest possible megapixels, in a camera the size of a deck of cards. There’s no way to do this without making each pixel extremely tiny. While the pixels in a good DSLR sensor might be 5 microns wide, the latest megapixel-mad point & shoots shrink each one to 1.5 microns or less. You start to see the problem.

We need to be a little careful about relating Airy disk size to pixel size, though. Sensor pixels have a Bayer pattern of color filters over them; and the final RGB image pixels are the result of a demosaicing algorithm. Also, every digital camera applies some amount of sharpening. This can, to some extent, counteract the diffraction blur.

But you can’t generate detail that was never recorded to begin with.

My assumption is simply that when the Airy disk fully covers four sensor pixels (as shown above), you have reached the point where diffraction makes additional pixels useless—no additional detail can be extracted. (This is a more generous criteria than many other folks’ reckoning.)

Let’s consider a typical point & shoot. Although its lens might open to f/2.8 at the widest zoom setting, at a “normal” focal length the maximum aperture is more like f/3.7. At this f/stop, the Airy disk is 5 microns across; it would fully illuminate four pixels of 1.7 micron width.

So how many megapixels could you get, if a single pixel is 1.7 microns?

Take a typical P&S chip size of 5.9 x 4.4 mm (a size better known by the cryptic designation 1/2.3″). At 3470 x 2603 pixels, you’d have a 9 megapixel camera.

Adding more pixels will not capture more detail. Neither will improved chip technology—we’ve hit a fundamental limit of optics.

Remember, this is all at the lens’s widest aperture (i.e., the one giving the poorest lens performance). As you stop down from there, the diffraction just gets worse.

Yet today’s models continue their mad race to ever-higher megapixel counts. Ten, twelve—now even 14 Mp are being sold.

This is where I start using the word “fraud.” Customers are being sold on these higher numbers with the implication it will make their photos better. This is simply a lie. All the higher megapixels deliver is needlessly bloated file sizes.

People forget that “full” HDTV is only 2 megapixels (1920 x 1080). Or that a 6 Mp camera can make a fine 8″ x 10″ print. A camera with 2 micron pixels is just about the limit, in allowing you to stop down the lens at all. That means staying under 7 Mp, given typical point & shoot chip dimensions.

And the more important point is this: Shoppers shouldn’t give their money to companies who lie to them.

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