This is not meant to be a scientific discussion, because I am not a scientist and haven't yet bent my mind around the theories of optics and digital resolution.

However, as each new generation of digital camera comes out, sensor pixel density increases (the number of megapixels jammed into a sensor of a given size), and the discussion of "How many megapixels is enough (or too many)" raises its head.

This is of interest for various reasons. For one thing, lens optics can only resolve to a finite limit, which makes increased megapixels at some point redundant. For another thing, many photographers never intend to print an image at a large enough size to make use of the highest resolutions. Also, denser, smaller pixels introduce increasing amounts of noise, especially at higher ISOs, so that viewing these images at large sizes will make the noise more visible and so cancel out the added detail that one would hope to find. Along a similar line, the higher resolutions will magnify the effect of things like lens softness, camera shake, and "diffraction", the softness that is caused when, at narrow apertures (lens openings), light rays will spill over a pixel "well" into adjacent wells because the narrow aperture forces a sharp angle of projection, and discreet detail will be spread over those pixels, resulting in less actual resolution than the lens and sensor are capable of. Finally, the large resolution files are also large in size. The Canon 1Ds Mk III and the not-yet-released Canon 5D Mk II, for example, produce 21 MP images that create RAW files of about 24 MegaBytes, meaning that photographers will need very large memory cards and computer systems that are quite up-to-date in order to get the best use of these cameras.

Now, all that being said, there are good reasons to welcome more megapixels, up to a point of total uselessnes. Many people do print at large sizes (think of framing a photo and hanging it on your wall -- would an 8" x 10" print really do it justice?). In fact, if I go to, say, a photography booth or a gallery, I'm looking at large prints that "Wow" me, not 4x6 or 8x10 prints. For my own home, I routinely print at sizes of 12x16 and 12x18 for framing and displaying. If I'm capturing an image of, say, a gorgeous landscape, an intricate macro, or a beautiful bird, I want a lens and a camera that is capable of squeezing the most detail possible out of that image. I want to be able to print that image to poster size if I have a reason, so I don't want to be resolution-limited any more than I want to be using a soft lens.

Also, I and many others who are active wildlife shooters (scan my Nature/Birds galleries if you wonder) have the need to crop many shots closely just to get a useable shot of, say, a distant bird. Even with my longest telephoto lenses, sometimes I just don't have enough "reach" to properly frame that bird, but you take the shot because it's there. I know that I can crop that shot for use, whereas with a resolution-limited image I would not have as much freedom to crop and, therefore, would have less useful images.

Another view, referring to the problem of digital resolution out-resolving optical (lens) resolution, has a couple of different angles. First, even if that is the case for a particular camera (such as modern point-and-shoot cameras) it doesn't actually "hurt" anything as long as quality does not suffer at a "normal" viewing size -- in other words, the over-resolved image with either pixel redundancy or diffraction softness will "compress" to be as good or better than an image from a lower resolution image viewed at the same time. Second, for "serious" photographers who actually print large and/or crop close, the "tool" of choice is not the tiny P&S camera but the larger "SLR" bodies that, in the digital world, have much larger sensors than the P&S cameras, and also have much lower pixel densities. In fact, there is only one DSLR today that comes close to out-resolving our better lenses, and that's the Canon 50D with 15MPs. But even that camera has not surpassed the point of fully optimizing lens resolution of our better lenses, so at this point no DSLR camera "wastes" pixels in the strictly theoretical sense (I'm not discussing P&S cameras here per se).

A fundamental concern when it comes to pixel density is digital noise, which becomes evident when using high ISOs (more amplification being given to a "signal" to allow for a higher shutter speed or a narrower aperture) and, especially, when boosting the exposure of a darker area in an image in an image editor. Digital noise is present in any image taken by any digital camera, but it increases as pixels get smaller (pixel density gets greater) for a given generation of cameras. Because of this, Point and Shoot cameras are virtually useless at high ISOs. Smaller DSLRs are greatly improved, and "Full Frame" DSLRs, such as the Canon 1Ds Mk III, the 5D Mk II, the Nikon D3, and the Nikon D700, because of their larger pixels/lower pixel density, have high ISO/low noise performance that allows them to deliver great, clean images in very demanding lighting conditions (but, of course, at a cost). The newest Canon FF bodies pack ~21MPs, but in an area much larger than the smaller "prosumer" DSLRs, so that the pixel density in these sensors is more conservative than, say the Canon 40D or the new Canon 50D, meaning that the 5D Mk II, for instance, will have 21 megapixels that are "cleaner" than the 50Ds 15 MP when shot at the same ISO settings.

So, how many megapixels can we use? How many do we need? How many are too much? You see, there is no "right" answer to this question. Most photographers don't need the latest and greatest, and would not want to spend thousands of dollars on something they don't need. But that doesn't mean that the latest, such as the Canon 50D or 5D Mk II, are "useless", and for many photographers they will in fact give better images than anything they've had before.

For those who in the future will need a new camera but don't want or need more megapixels, the newer bodies have what I think is a very nifty feature: "sRAW". This is a smaller RAW file that you can set your camera to produce, so that, say, instead of a 21 MP RAW file from a 5D Mk II you can, with the sRAW setting, get a 10 MP RAW file, which means you still have the high-quality tonal data in RAW, but also get a very manageable 10 MP, which is good enough for almost anything. Of course, if you are one who really wants the largest prints or the closest crops you really do want the 21 MP RAW, but then you knew that, right?

One last point: Yes, the higher resolution pics will show more flaws, of both your optics and your technique although, as I pointed out earlier, they won't "hurt" the image at normal viewing distance. However, there is something to stress here: if you want to get the best out of you camera, you need to always use the best techniques of both lens use, of steady shooting, and of composition (to avoid unnecessary cropping).

I'm challenged by this all the time in my wildlife photography. I'm lugging a heavy body with a heavy long lens and often shooting hand-held or with a monopod, and I'm shooting at, say, birds in flight or small critters where I'm in motion and sharpness is critical. I get home and review my shots at "full size"/100% and find that some are just too soft. They look OK when viewed at half-size, though. What does that say? It says that if my technique is not just right, then instead of getting the most of my 10 Megapixel camera, I am in essence shooting 5 Megapixel images. Now, if I were to shoot with a 15 MP camera, what would I end up with? Still, a 5 MP "useable" image.

Another example is landscape/scenic shooting. If you're "serious" about landscape photography, getting the sharpest detailed shot is extremely important. Imagine shooting a once-in-a-lifetime spectacular, beautiful scene, then getting home and finding that it was useless when viewed at anything larger than a 4x6 snapshot. I don't know about you, but that to me would be very disappointing. So, as far as I'm concerned, there is no excuse for a serious landscape photographer to not have and use a very sturdy tripod and to have a habit of good techniques for such shooting, such as dealing with wind, Mirror Lockup, release cables, all of that. The same thing goes with macro shooting and other "serious" closeups -- again, who wants to try for a fantastic image that could be a glorious 21 MP image that could hang in a gallery, but end up with just something to file away as a "nice" snapshot?

Another note before posting some images -- Some will argue, but the points I've made above illustrate what is to me an important point: if you're wondering about image resolution, especially the ability to print large and/or crop close, and if you're wondering about lens sharpness and the best technique for image sharpness, the "acid test" is viewing your image at "full size"/100%.

Let's say, for example, your favorite picture looks nifty at a "normal" viewing size. What this means is that it will print up to, say, an 8x10 and look just as nifty. However, when you look at it at a 100% size, you may see the effects of, say camera movement or lens softness. This means that, first, you won't be able to get a close crop that is useable, and that you can't print at a larger size without showing those effects. You have, in essence, lowered the useable resolution of your image. So, if you want to get the best out of your 15 MP 50D or your 21 MP 5D, you can differentiate the good from the better from the best, and you can then judge between a good snapshot, a good 8x10, and an image that could hang with pride in a gallery.

I've been browsing my archives to see images that could be useful in this discussion. It'll take a while to find and post a good number of them that might illustrate various points. But at this point I'm going to post two shots and 100% crops as a frame of reference and a bit of a teaser. The "teaser" part is that they're taken almost two years ago with a Canon S3 IS -- and "advanced" point-and-shoot 6MP camera. This is a low resolution camera compared to recent ones, but still it packs 6 Megapixels into a sensor that is a small fraction of the size of, say, a full frame DSLR: the S3 has a sensor size of 5.75x4.31mm, the 5D has a size of 35.8x23.9mm, so that's a "crop factor of about 6, meaning the 5D sensor is about 36 times the area of the S3 sensor.

As you will see, even a point and shoot is capable of some pretty nice resolution of detail, meaning that it is still up to the photographer to get the most of his/her gear, even a P&S!