The recent announcement of the 36-megapixel Nikon D800 has brought forth a certain number of predictable reactions, a few lauding it as the Second Coming and many more dismissing the utility of more megapixels.
Most such opinions are based on mythical (or at least mistaken) beliefs about what pixel counts really mean. Let's see how many of them I can shoot down in one column.
I've written on this subject many times before, so I'm not going to present detailed arguments. I am simply going to assert facts. To keep the number of comments I have to answer to a minimum, please remember to check to see if your doubts have already been addressed in the cited columns and my comments before you take issue with my myth-busting.
The sharpness of your photographs will increase in proportion to the number of pixels.
Reality: Well, technically, the square root of the number of pixels, but that's not really the issue. Quadrupling the number of pixels (doubling the resolution of the sensor) is no more guaranteed to double the sharpness of your photographs than doubling the sharpness of the film you were using did. Image sharpness is a convolution of multiple factors. Improving sensor resolution improves image sharpness, but it won't make a proportional difference unless it is by far the fuzziest link in the chain. For more on this, see my article "Diffraction In Perspective."
Getting more megapixels is just about making bigger prints.
Reality: For a minority, yes, but it's more commonly about making sharper prints at whatever size you're choosing to print. Good printers can render more than 500 PPI of fine detail and human eyes can perceive considerably more than that. Digital cameras are only about 50% efficient in terms of resolution (e.g., a 24-megapixel camera delivers about 12 megapixels of resolvable detail), so you don't have to be printing very large to see a perceivable difference from more megapixels. See "How Sharp Is Your Printer? How Sharp Are Your Eyes?"
Whether you care about that difference, of course, depends upon your needs. But the difference is there.
More megapixels inevitably means noisier photographs and/or poorer high ISO performance.
Reality: That would be true if the camera manufacturer took exactly the same chip technology and exactly the same support electronics and simply crammed more pixels into the same size chip. That never happens! Those pixel-count changes are always accompanied by other improvements in the chips and the support circuitry. If we had reached the physical limits of image quality, those improvements wouldn't be able to compensate for the additional pixels. We haven't; we're several stops away from that. New cameras frequently have higher pixel counts, lower noise, and better ISO performance, all at the same time. See "Photography at the Speed of Light" and "Something Old and Something New."
It's pointless to add more pixels because lenses aren't good enough.
Reality: I have no idea where this one came from, because it's contradicted by decades of lens and camera test data. I'm going to cut through the morass of minutia-based arguments about pixel dimensions, filter geometries, and Airy disks and lay it out in very simple terms. A 16-megapixel 35mm-sized Bayer array sensor is going to resolve around 50 line pairs per millimeter (lp/mm). A 36-megapixel sensor will resolve around 75 lp/mm. Even mediocre 35mm lenses will hit 75 lp/mm at some aperture over some portion of their field of view. This is true of both fixed focal length and zoom lenses.
Decent (not at all exceptional) 35mm-format lenses can hit 75 lp/mm at just about all apertures and will do so over most of the field of view for at least one aperture. They'll show peak resolutions more like twice that. Really good lenses (not necessarily expensive ones) will be able to exceed 75 line pair per millimeter without even trying hard over most/all of the field of view and will have peak resolutions three or more times that.
Furthermore, until the lens resolution drops to only half that of the sensor, improving sensor resolution will produce an observable improvement in image resolution.
This is just a useless horsepower race by manufacturers; people don't need all those megapixels.
Reality: Sure, it's a horsepower race, but saying nobody needs it would be like saying nobody needed 120 format over 35mm or sheet film formats over 120 roll film. Most people don't, to be sure. Even more won't be willing to pay the premium. Count me in that group; I may still very well lay out a grand for the 24-megapixel Sony NEX-7, but there's no way I'm laying out $3K for the boat anchor Nikon D800. But that's just me (and, similarly, your priorities are just you).
Let's also remember that not everybody likes to compose uncropped, full frame. Actually, most photographers don't (although many digital photographers have been forced into it). If 50% more pixels affords them the opportunity to do some reasonable cropping without unreasonable sacrifices in sharpness, they will be happy campers.
In other words, it's not about you. It's not even about me.
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It's likely I have missed some myths, but I'm sure I've done enough busting to enrage some of the myth-perpetuators. Can't wait to see what shows up in my e-mail inbox.
Physicist, astronomer, photographer and fine printer Ctein has written hundreds of articles for dozens of magazines about the science and technique of photography over four decades. It's no myth that his weekly TOP column appears on Wednesdays.
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Original contents copyright 2012 by Michael C. Johnston and/or the bylined author. All Rights Reserved.