February 19, 2010
The stats show that my post, “the Great Megapixel Swindle,” continues to get quite a lot of traffic. That’s a little unnerving, given that it was written as a quick, off-the-cuff tantrum. If I’d known how many folks would read it, I would have said several things more precisely.
Around the internet, the “Swindle” spawned many, many discussion threads—I can’t keep track of them all. I did try to respond to many of the questions and misunderstandings that I was seeing, in a followup post here. And I’ve expanded on the same issues in many other posts as well.
Today I’ve noticed a thread over at Rangefinder Forum which raises the question, “isn’t it unfair to use a crop from the background? Naturally that looks bad, since it’s out of focus.”
First, the real point this example makes is this: Cameras with tiny pixels must use aggressive post-processing to reduce noise; and this can cause strange, unnatural-looking artifacts. (There’s more on the subject here.)
But on the question of depth of field, I should clarify a bit.
The EXIF data for this shot shows the Olympus FE-26 was set at the wide end of its zoom range—namely 6.3mm. Such a short focal length implies extreme depth of field. The f/stop used was f/4.6.
The H. Lee White is a 700-foot-long Great Lakes freighter. I’m not exactly sure how far the camera was from the subject; but it’s doubtful that it was closer than 15 feet.
Now, we can’t blindly apply standard depth-of-field tables here. The standard calculation (e.g. if you scroll down to Olympus FE-26 here) uses a circle of confusion of 0.005 mm for this sensor format.
But when you are looking at such an extreme enlargement, the assumptions behind that break down (CoC is generally referenced to viewing an 8×10 print at a moderate distance).
But consider that this camera has a sensor size of about 6 x 4.5mm, and so each individual pixel is 1.53 microns wide. In other words, 0.00153 mm.
Clearly, the meaningful circle of confusion can’t be smaller than one pixel. Given the resolution loss that happens with Bayer interpolation, 0.002 mm seems a realistic CoC.
So the out-of-focus blur is actually negligible compared to the pixel size in this case.
You can use an alternate depth of field calculator which lets you input arbitrary values if you’d like to explore this further yourself.