New Insight into Megapixels vs. Sensitivity Debate

Discussion in 'Digital Photography' started by anubis, Jun 26, 2008.

  1. anubis macrumors 6502a

    Feb 7, 2003
    Marketers would have us believe that camera performance is tied only to the number of megapixels the sensor has (especially in the case of point-and-shoot cameras). We are lead to believe that there is a direct correlation between megapixels and performance. However, this is not the case.

    In order to keep costs low and keep modularity high (as is especially the case with DSLR), overall sensor size rarely changes. For example, ALL ASP-C cameras have 23.6x15.7mm sensor, regardless of what model it is. (I realize that this dimension varies slightly manufacturer to manufacturer but for the most part, each company will stick with one dimension that works for it.) The sensor size doesn't change with an increase in megapixels. (In fact, today's point-and-shoot cameras are getting ever small sensor sizes. The reason is because 'thin is in' and in order for all of your various physical laws to line up [hyperfocal distance, aberation balancing, airy disk, etc.] the thinner the camera, the smaller the sensor has to be. Another advantage to smaller sensor size is lower cost.)

    Example: The Canon 40D, 400D, 30D, 450D, 20D, 350D, 300D, 10D, D60 and D30 all have the same sensor SIZE although they have different numbers of PIXELS.

    The only way for camera manufacturers to increase megapixels while keeping the sensor size the same (or smaller) is to decrease the physical size of each pixel on the sensor. For example, consider a pixel size of 5.6 micron on an APS-C sensor. That would give you about 11.8 megapixel camera. Let's compare that to a pixel size of 5.1 micron also on an APS-C sensor. That would give a 14.2 megapixel camera. Are those extra megapixels worth it?

    Some quick calculations reveal the following.

    Effects of shrinking pixels from 5.6 micron to 5.1 micron:

    Light sensitivity -17%
    Quantum Efficiency -2%
    Saturation Level -17%
    Dynamic range -1.6dB
    Signal-to-noise ratio -1.8dB
    Pixel Nonuniformity -19%

    As you can see, if you have the same sensor size (and again, digital camera makers usually stick with the same standard sensor size for many, many years) and go for the higher megapixel, you will see a substantial decrease in sensor performance.

    Of course, at the same time megapixels are increasing, the technology used to build silicon sensors is also improving. So while pixels are getting smaller, in general manufacturers are able to use newer techniques to reduce noise, increase quantum efficiency, etc. However, given two sensors of the same size and made at around the same time, the lower megapixel camera will work better as long as you don't need to make poster size prints.

    Conclusion: Carefully consider the number of megapixels you NEED and don't shell out extra dough ONLY to get more megapixels because you think your camera performance will increase substantially. Hint: You only NEED a huge number of megapixels if you're in the business of making poster size prints.
  2. Chappers macrumors 68020


    Aug 12, 2003
    At home
  3. seenew macrumors 68000


    Dec 1, 2005
  4. 66217 Guest

    Jan 30, 2006
    I'm no expert in this technical issues, but I would guess that there is more to picture quality than just the sensor size. There is a sweet point of pixels in a sensor, so less pixels on same year cameras won't always mean better.

    Nonetheless, I found your post interesting, tho, as seenew, I would like seeing some sources to the info you presented.
  5. rogersmj macrumors 68020


    Sep 10, 2006
    Indianapolis, IN
    Good info, but hardly new. It's pretty well-known that, all other things being equal, more MP in the same size sensor is going to reduce your sensitivity *slightly*. The D40x and the D40 are great examples. With more pixel divisions in the same sensor space, the D40x's sensor contains smaller pixel sensors and more areas that can't receive light (the "walls" between each pixel). Ken Rockwell has an article about it on his site too.
  6. OreoCookie macrumors 68030

    Apr 14, 2001
    Sendai, Japan
    Well, yes, there are ways to improve image quality while keeping other parameters constant. But the overall gist is correct: unless you improve your chip structures at the same time, smaller pixel sizes (to increase resolution) will worsen most other parameters.

    There is a reason why resolution hasn't increased significantly from 12 MP (with the exception of the Ds series Canons and very expensive medium format digibacks): it's just the sweet spot and for most applications, an increase in resolution does not lead to better results (because the lenses are the limiting factor) and most people quite frankly don't need more than 12 MP.

    Kodak has recognized this and they are putting effort in improving the IQ but maintaining a resolution of about 12 MP. One of the things they are looking into is new color patterns apart from today's Bayer pattern.
  7. Abstract macrumors Penryn


    Dec 27, 2002
    Location Location Location

    This is, of course, absolutely true. However, I think you're preaching to the choir on this issue. ;)

    However, the wording is a bit inaccurate. When you say, "...the lower megapixel camera will work better as long as you don't need to make poster size prints", this isn't technically true. If noise and S/N wasn't an issue, more pixels always results in a better image. If you want to capture more detail, you'll need more pixels. Of course, you need to think about the sensitivity you use, but what you said is only true with regards to noise. "Best" is a relative term. If I want to capture the most detail, and am certain that I'm going to be shooting at ISO 100-200, and a maximum of ISO 400, then I probably won't mind an APS-C sized sensor with 16 MP.
  8. Hmac macrumors 68020

    May 30, 2007
    Midwest USA
    As digital photography migrates from APS-C to 35mm full-frame sensors (at least, in the upper range of cameras), now we can have an increased number of pixels, and larger ones to boot. What's the sweet spot in number of pixels vs size in these larger sensors?
  9. anubis thread starter macrumors 6502a

    Feb 7, 2003
    Thanks to everyone who replied...

    Just want to clarify a few points....

    Given two sensors that are made at the same time using the same wafer fabrication and technologies, the lower megapixel sensors will always have the better sensitivity characteristics. (Of course, if the megapixels start to get too low, all the sensitivity in the world won't matter when you start to suffer from spatial noise). The people talking about a "sweet spot" are a little confused... what you mean to say is "given TODAY'S state of circuit design, wafer fabrication, microlens arrays, AR coatings, etc., ~12MP is the best tradeoff between sensitivity and spatial resolution". That is to say, 2 or 3 years ago, 12MP would definitely NOT have been the sweet spot. In 2 or 3 years from today, when sensor manufacturing has improved even more, 14MP may be the sweet spot. But today, 14MP may represent an unacceptable tradeoff in sensivitiy compared to increased spatial resolution.

    At any rate, I just wanted to provide you guys with a quantitative comparison between two sensors. I know we've all seen qualitative comparisons but I wanted to provide all of you with the benefit of hard data to help you make your camera purchase decisions.

    Source: My brain. A calculator. Vector Calculus. Principles of Optics by Born and Wolf. Classical Electrodynamics by Jackson. Digital Cameras: Smaller Pixels for Higher Resolution by Dalsa Imaging.
  10. OreoCookie macrumors 68030

    Apr 14, 2001
    Sendai, Japan
    Going from 12 to 14 MP is not a huge improvement, it is 14 % more pixel -- even if your lens is good enough, even then, most people won't be able to tell a difference.

    At any rate, I just wanted to provide you guys with a quantitative comparison between two sensors. I know we've all seen qualitative comparisons but I wanted to provide all of you with the benefit of hard data to help you make your camera purchase decisions.
    You've found something concerning digital sensors in Jackson?
    (I'm not trying to argue here, your arguments are sound, but what have you found in Jackson that pertains to digital sensors?)
  11. Grimace macrumors 68040


    Feb 17, 2003
    with Hamburglar.
    This isn't terribly new...

    But, historically speaking, it's not as though Canon, Nikon etc. intentionally left the MP on a sensor as low. Noise reduction technology and manufacturing cost are the biggest hurdles.
  12. Westside guy macrumors 603

    Westside guy

    Oct 15, 2003
    The soggy side of the Pacific NW
    I can see a big glaring problem with this discussion. There's this unstated (and very common) assumption that you're assuming is true a priori - you're assuming that the limiting factor is always (one aspect or another of) the sensor. If you're shooting with pro lenses on a tripod, at the len's aperture sweet spot, using mirror lock-up and a remote shutter release then sure - the quality of your photo is probably sensor limited, one way or the other. But when we're in the range of 8-10 megapixels or more, even with decent technique the average hand-held shot is probably being limited by something other than sensor resolution; e.g. hand movement, mirror slap, or even the optical limits of that consumer-grade lens that's being used. A lower-res image shining on a high-res sensor still results in a lower-res image, no matter how many pieces of it are captured.
  13. Phrasikleia macrumors 601


    Feb 24, 2008
    Over there------->
    I don't think anyone is making the assumption that the "limiting always the sensor". The crux of the matter is--all other factors being equal--whether or not less is more when it comes to megapixels.
  14. 147798 Suspended

    Dec 29, 2007
    I agree with most of the posts here "nothing (new) to see here. Move along"
  15. anubis thread starter macrumors 6502a

    Feb 7, 2003
    True that
  16. xper macrumors 6502


    Dec 15, 2005
    Sweden - Halmstad
    "Effects of shrinking pixels from 5.6 micron to 5.1 micron"

    The result would be pretty correct if there was no advancement in how the lens is produced, the alteration in microlenses minimize the effects of using a sensor with smaller micron technology.

    Edit: Well, i just saw that a section in the original post said something about this so nevermind ;)
  17. keltorsori macrumors regular

    Jul 24, 2002
    It's all about physics, law of optics. This insane need to manufacture a 100 megapixel camera that will fit in your lapel only makes for really crappy pictures, especially in low light. (Retired photographer). I'd rather buy a 3-4 year old 4 or 5 megapixel camera used if I was going for a point and shoot than anything they're making today.

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