High-Mp Point & Shoots: Take Two
January 29, 2010
My Petavoxel post about the “Great Megapixel Swindle,” from January 19th, has now been viewed more than 60,000 times. (Which I find kind of scary.)
And there were lots of noisy comments elsewhere, complaining that I had stacked the deck unfairly. I made a teensy crop from a cheap camera—and no surprise, it looked bad!
Fair enough. So let’s start over, stacking the deck as much as possible in favor of the photo looking good. Please look at a couple of new images:
(I am grateful to both gentlemen for posting these images under Creative Commons licensing. Paulo was also kind enough to email me his straight-from-camera JPEG; it’s the source of the crops below.)
I choose these because the camera used to take both is the Panasonic Lumix TZ5. This was among the final high-end point & shoot models to receive a full profile from DP Review; so you can have an independent opinion of its quality.
A 9 megapixel model from 2008, the TZ5 has now been replaced by a zoomier 12 Mp version. But the earlier TZ5 remains the second most popular Lumix on Flickr. Its pixels measure about 1.7 microns wide—that’s actually on the large side, compared to today’s typical compacts.
DP Review were rather enthusiastic about this camera, praising the lens in particular. It’s a Leica-branded 10x zoom, with 4 aspheric surfaces and 11 elements, including one of ED glass. This is some rather high-end stuff.
I’d be the first to admit our sample photos look crisp and vibrant at any normal viewing size. And both Paulo and Simon tell me they’re quite pleased with their TZ5’s.
Note that these well-lit shots show off the TZ5 under the best possible circumstances. The sensitivity is at ISO 100 (the lowest setting), which gives the least noise. The shutter speeds are easily high enough to freeze any hand shake at the focal lengths used. But the apertures have been held to the widest possible, which minimizes diffraction.
Even with all these advantages, we begin to see some artifacts that any compact-sensor camera is prone to.
Within the same photo, a white surface in sunlight can be 500 times brighter than a dark surface in shade. That’s about 9 f/stops of difference.
And smaller pixels inherently have lower dynamic range. So even in well-exposed photos like these two, the highlight areas can blow out to a textureless white:
Examining a highlight area in Photoshop, we see that the brightness levels in the selection have completely “hit the ceiling,” so that no detail can be recovered:
So small-sensor cameras will always struggle with high-contrast scenes (unless some tricky HDR processing is used). Yes, shooting in RAW could rescue a bit more from the highlights; but I don’t know of any camera under $350 which offers this option.
What about graininess in the image? (Digital-camera fans use the word “noise,” since we’re discussing electronic signals.)
Even under uniform illumination, photon counts can vary randomly between neighboring pixels. The larger the pixels, the more this averages out. Small pixels inherently have more random brightness variations—that is, higher noise.
So any compact camera must devote some fraction of its processor power to noise reduction, attempting to smooth out those speckles.
Selecting higher ISO sensitivity settings means the pixel noise is amplified even more; and so, noise-reduction must get even more aggressive. Ultimately, this causes a loss of detail, as DP Review’s TZ5 test clearly showed (scroll down to the hair sample, especially).
But our ISO 100 samples should be best-case scenario for noise. Yet even so, the NR processing is leaving some “false detail” in areas that should look smooth:
The effect is stronger in the darker parts of an image. And as you raise ISO, it will just become more obvious.
Might these simply be JPEG artifacts?
These samples are not highly compressed files—the JPEGs are about 4 MB each. Compression with JPEG can add visible “sparklies” around high-contrast edges; but flat areas of color should compress very cleanly. I say this texture comes from noise (or, noise reduction).
A camera of 9 megapixels would seem to promise rather high resolution. But this camera’s sensor is about 1/3rd the area of an aspirin tablet. So is there truly any detail for all those pixels to resolve?
Lets look at some 100% crops near the center of the photos, where lens performance ought to be at its best.
The high-contrast edges here show a lot of “snap.” But the camera’s own sharpening algorithm may take some of the credit for that.
Looking at the lower-contrast details in the shadows (like the hanging tags), the impression is noticeably softer. I feel that the optical resolution is starting to fall apart before we reach individual pixels.
The TZ5 lens gains a whole f/stop when zoomed to the wide end. At this setting, detail is a bit softer still—notice the grille in the archway. At a 4.7mm focal length, depth of field is enormous; so I don’t believe we’re seeing focus error here.
Note that the Airy disk at f/3.3 is about 4.4 microns (for green light). As I discussed before, diffraction means that no lens, no matter how flawless, can focus a pinpoint of light any smaller than this.
But a 4.4 micron-wide blur covers almost four of the TZ5’s pixels. And at any smaller aperture, the Airy disk expands even further. Thus, I remain skeptical that packing in pixels more densely than in the TZ5 would extract more true, optical detail over what we can see here.
Now, let me be clear.
Paulo tells me, “I love my TZ5. It’s my everyday use camera (being much lighter and more compact than the 500D).”
There’s a need for cameras like this. Among pocketable models, it’s probably among the nicest available.
But even under these best-case circumstances, we start to see some limits imposed by its 1.7 micron pixels. And since the TZ5’s release in 2008, pixels in point & shoots have only shrunken more.
It’s not an accident that DP Review made the final winner of their “enthusiast” roundup Panasonic’s own LX3—a small camera, but with a larger sensor. Its pixel density of 24 Mp per sq. cm is half of today’s worst-case models. The result is that the LX3’s “high ISO performance puts most competitors to shame.”
I’m not against small cameras. I’m not even against high megapixels, if you have a genuine need for them (assuming the sensor is large enough).
But today, pixels have shrunk too far. It’s time to stop.