Pixel Density

Discussion in 'Beyond the Basics' started by DjBeau, Sep 3, 2009.

  1. DjBeau

    DjBeau TPF Noob!

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    Hi

    I know lots of people are saying that you can't tell the difference between the quality of a full frame image and a crop sensor image. Well, those people should obviously buy some glasses.

    One of the things that really stands out on fullframe is the increased sharpness (in the center, anyway). The reason for this is - as far as I understand - that the pixel density is lower on a full frame which means the pixels are farther apart and do not "bleed" into each other.

    Now, here's the question:

    Is a 12 mega pixel full frame sensor sharper than one of 24 mega pixels? I mean, since you're cramming more pixels into a sensor of the same size wouldn't that mean bigger density and therefore more bleeding?

    Or do that extra amount of pixels to define the image simply balance out the bleeding problem?

    Regards
    Beau
     
  2. Plato

    Plato TPF Noob!

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    I've never seen pixel blood.

    A lower density does not mean that the pixels are further apart. It means that the pixels are larger. The issue is electrical noise (the simplest example is static), especially at high ISO values. The noise is always present but it impacts small pixels greater than it does large pixels.

    However, this is a bit of over-analysis. Just as with large frame sizes with film, large sensors produce better results than small sensors. The decision that each photographer must make is whether or not the difference affects his/her own pictures. If all of your shots are in bright sunlight at ISO-100 and you'll never print anything larger than a 4X6, even the DX sensor is major overkill.
     
  3. Moglex

    Moglex TPF Noob!

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    The reason that FF is sharper with any given lens is that the lens can produce a certain number of LPPM (Line Pairs Per Millimetre).

    Obviously there are less millimetres for it to produce line pairs over with a crop frame sensor than one that has the full monty.

    It really is that simple.

    As Plato says, though, there are tradeoffs to consider - it isn't all about the ultimate sharpnesss.
     
  4. Village Idiot

    Village Idiot No longer a newbie, moving up!

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    Crop sensors should be sharper over all since they're using less of the edges, which aren't as sharp as the center of a piece of glass.

    Maybe it's the shallower DOF you're noticing?
     
  5. Moglex

    Moglex TPF Noob!

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    He did specifically say: 'In the center, anyway'.
     
  6. Gaerek

    Gaerek TPF Noob!

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    Really? I'd be willing to bet that even most pros would be hard pressed to tell the difference between a photo shot with a crop frame and full frame at lower ISOs. This is especially true at web resolutions and anything the size of an 8x10 or smaller. Is there a difference, most certainly. But the only people who will really be able to tell the difference are pixel peepers.

    I guess I need glasses for my 20/20 vision. :p
     
  7. DjBeau

    DjBeau TPF Noob!

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    I am of course aware that there are other things to consider and that I might be starting a **** storm. On certain pictures, though, especially close-up portraits, I think the sharpeness does matter quite a lot. I've been working with crop as well as full frame and it's really not that hard for me to see the difference.

    @ Plato: But does that mean that the pixels on a 24mpx are smaller than those on a 12mpx and will thereby be more influenced by electrical noise?

    @ Moglex: Can you elaborate a bit on your post? I don't think I quite fully understand it :)
     
  8. JamesMason

    JamesMason TPF Noob!

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    yes, taking into account sensor size i.e. full frame, 1.6x, ect.
     
  9. Garbz

    Garbz No longer a newbie, moving up!

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    Noise yes, Noise does not necessarily reduce sharpness though. Noise reduction reduces sharpness.

    What you will see is the difference in the lens. Diffraction amongst other things determine a finite size that any given point will be projected by the lens. If your sensor has more megapixels in the same area, and the point is larger than any single megapixel what you end up with is reduced visible sharpness.

    But is it less sharp? One could consider it like this. You have a printed photo that looks perfectly sharp to the naked eye. Now you look at it under a magnifying glass and find that it is not as sharp, But is the photo itself now less sharp, or are you just showing it's flaws?

    The easiest way around the problem... Downsample. take your 24mpx image and convert it to 12mpx, and it will look just as sharp as it was previously.
     
  10. musicaleCA

    musicaleCA TPF Noob!

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    Woohoo! My favourite "trick". :lol: (That is, downsampling an image can definitely save it. There are plenty of times when I look at an image and think "aw nuts, they moved a few millimetres, it's not sharp" and just downsample.)
     
  11. Moglex

    Moglex TPF Noob!

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    The resolution of a lens is measured in LPPM or 'Line Pairs Per Millimetre'.

    This is exactly what it says: the number of pairs of lines (one black, one white) that can be differentiated per millimetre at the focal plane.

    The resolution of the image is determined by the resolution of the sensor (or film) in conjunction with the resolution of the lens.

    Let:
    Rs = resolution of the sensor
    Rl = resolution of the lens
    Ri = resolution of the image

    Then Ri = 1 / ( 1/Rs + 1/Rl )

    Caveat: This is simplifying considerably by ignoring, for example, edge effects and the Bayer filters on digital sensors. The numerical answer should not be taken as an accurate figure but the working shows the general way the figures will tend to pan out.

    If your sensor has a horizontal resolution of 5000 pixels this is equivalent to 2500 lp (since it takes two pixels to represent a pair of lines).

    Thus a full frame sensor has a resolution of 2500/36 = 69 lppm
    And a Canon crop frame a resolution of 2500/22.7 = 110 lppm

    Say your lens has a resolution of 100 lppm

    Then the effective lppm for the full frame lens = 1/(1/69+1/100) = 40 lppm
    and the crop frame = 1/(1/110+1/100) = 52 lppm

    And the effective lp's are 40*36 = 1440 and 52 *22.7 = 1271 for the full and crop frame respectively.

    Thus the FF will be approximately 13% sharper.
     
  12. DjBeau

    DjBeau TPF Noob!

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    Thank you so much for all your answers, especially you Moglex.

    I'm of course not trying to start another tedious debate on FF vs crop - just searching for the scientific explanation to what I see with my eyes.

    This raises a few more questions.

    I tried to use your formula and double the amount of pixels to 10.000 = 5000 lp. On a FF this adds up to about 2093 effective lp's which is 45% more than on a FF sensor with only 5000 pixels. Does this mean that a 24mpx FF sensor is 45% sharper than a 12mpx FF sensor or are there other factors to take into consideration? It doesn't really seem likely to me.
     
    Last edited: Sep 4, 2009

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