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Old Feb 7, 2008, 1:26 PM   #1
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I am trying to educate myself on the relevance of lens resolution vs MegaPixels. I have a lens which tests at about 900 LP/PH (a pretty good lens). I know if I were to put this on a digital camera with a 6 PIXEL sensor (3 by 2 pixels), the sensor would be the limiting factor. My question is:
How many MP on an APS-C sensor are required to make the above lens the limiting resolution factor? (or both of equal limiting factor)
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Old Feb 7, 2008, 4:45 PM   #2
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A specification of "lp/ph" means "line pairs per picture height". The "line pairs" refers to alternating black and white horizontal and vertical lines on a test image. 900 lp/ph is quite good.

Doing some quick math might lead someone to believe that any sensor with a resolution of at least 1800 pixels should be able to match 900 lp/ph, but this ignores the impact of the Bayer filter (see http://en.wikipedia.org/wiki/Bayer_filter ), which effectively halves the actual resolution of a digital camera. That would imply that an image sensor with a resolution of 3600 pixels is needed to surpass a lens witha resolution of 900 lp/ph.

But the measurement does not require that the alternating black and white lines be rendered accurately, but merely that they be distinguishable as alternating black and white lines, not a gray box. In order to do that, an image sensor will probably need a resolution of somewhere between 1800 and 2700 pixels.

Anybody else?
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Old Feb 7, 2008, 6:11 PM   #3
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Thanks TCav

Based on your explanation, my KM 7D with 2000 pixels in the PH is pretty much compatible with this lens. The 2880 pixels in the PH of the alpha 700 would probably represent the maximum number of PH pixels that this lens could make use of. You have answered my question. There is no burning need to buy the A700 right now.
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Old Feb 7, 2008, 8:26 PM   #4
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hingarfi wrote:
Quote:
Thanks TCav

Based on your explanation, my KM 7D with 2000 pixels in the PH is pretty much compatible with this lens. The 2880 pixels in the PH of the alpha 700 would probably represent the maximum number of PH pixels that this lens could make use of. You have answered my question. There is no burning need to buy the A700 right now.
It's not an issue of compatibility; it's an issue of whether the lens or the image sensor is the limiting factor for image sharpness.

Yes, the vertical resolution of 2000 pixels for the image sensor in your 7D is the limiting factor when paired with your 900 lp/ph lens. But when paired with the 2856 pixel vertical resolution of the image sensor in the A700, the lens becomes the limiting factor.
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Old Feb 7, 2008, 9:10 PM   #5
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Yes, the vertical resolution of 2000 pixels for the image sensor in your 7D is the limiting factor when paired with your 900 lp/ph lens. But when paired with the 2856 pixel vertical resolution of the image sensor in the A700, the lens becomes the limiting factor.
Thanks TCav. You have explained this well and I undestand the math quite a bit better now. Looks like a rule of thumb is: You need about 3 pixels for each LP that the lens will resolve. (I'm guessing that most of the cheap PS 10 MP cameras have way too many pixels for the quality of lens they have)
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