Chuck,
Thanks to you and to Dean for asking the question. I was curious as well,
but hated to display my ignorance. You explained it very nicely.
Jim Nichols
Tullahoma, TN USA
----- Original Message -----
From: "Chuck Norcutt" <chucknorcutt@xxxxxxxxxxxxxxxx>
To: "Olympus mail list" <olympus@xxxxxxxxxxxxxxxxx>
Cc: "Dean Hansen" <hanse112@xxxxxxx>
Sent: Wednesday, August 01, 2012 6:14 PM
Subject: Re: [OM] 3MP image on 10MP sensor?
> Dean Hansen wrote to me today saying:
>
>> Hi Chuck, You recently posted on the OM list: "That's OK but you
>> should realize that, at f/13, your 10MP camera is producing only
>> about 3MP images." Guess I know more about insects than I do about
>> digital cameras. Could you explain this, or direct me to a web page
>> that explains this? Many thanks! It will be interesting what Moose
>> thinks of his OM-D. Best wishes, Dean
>
> So I wrote a response explaining it as best I could to which Dean
> replied that he thought it was a well written response and that I should
> post that response to the OM list. He thought perhaps he wasn't the
> only one who didn't fully understand what I said. So here's what I told
> him:
> -------------------------------------------------------------------
>
> It's actually fairly simple. First consider a point source of light
> such as a star. Despite their actual size, stars (except for the sun)
> are so far away that they are essentially a mathematical point. No
> telescope in the world is powerful enough to image a star as anything
> but a point.
>
> However, when an optical system attempts to image that point source of
> light various abberations will conspire to make that point source into
> something larger and measurable. One of those aberrations is
> diffraction which is the bending of a light beam as it passes by an
> edge. Diffraction is present in ALL optical systems as they all have
> edges... the edges of lens cells, the edges of aperture blades, etc. The
> bending of the beam distorts it and causes the image of the star to grow
> larger.
>
> Finally, the control factor for diffraction is focal ratio. The smaller
> the aperture (higher the f-number) the worse the diffraction. In the
> case of a film camera, once the diffraction controlled size of a point
> source of light exceeds the diameter of a film grain it takes more than
> one grain to capture that point. In the case of a digital camera, once
> diffraction has swelled the size of that mathematical point to something
> larger than a pixel it takes more than one pixel to record that point.
>
> The smaller the size of the pixels the more prone they are to the
> effects of small aperture. In the case of Jim's E-510 with its 10 MP
> sensor (note: very tiny pixels on a 10MP 4/3 sensor) when the camera is
> set to f/13 diffraction has swelled the size of those point sources to
> where they require more than 3 pixels to record a single point source.
> Therefore, although the camera has 10 MP, only 3 MP of data can actually
> be resolved. On the other hand, my Canon 5D with 12.7 MP does not run
> into diffraction limitations until it's past f/11 because the individual
> pixels on that large sensor are much larger diameter. The same effect
> is seen on 35mm film vs. medium and large format film. Large format
> especially often uses very small apertures like f/32 or f/64. But the
> film is very large and the final image does not have to be magnified
> very much.
>
> Now, having said all that, something out of focus is blurring the image
> in a fashion somewhat like diffraction. Although diffraction might be
> limiting the E-510 to a 3MP image at f/13, f/13 might be producing
> greater depth of field and still make a pleasing image. Even so, the
> total resolution of the final image can't exceed 3 MP.
>
> If you'd like to dig deeper see this Luminous Landscape article. In
> particular pay attention to Table 3 which lists the maximum resolution
> obtainable at various apertures on different sized sensors. Note that
> each sensor size has 3 columns of data: Since diffraction is dependent
> not only on focal ratio but also on wave length the table gives
> different resolution values for red, green and blue light (with
> wavelengths specified). Blue light, having the highest frequency and
> energy is the least affected by diffraction. Red the most.
>
> The actual subject of the article is lens resolution but as limited by
> diffraction and pixel dimensions. Note that the diffraction values are
> theoretical for a perfect lens. Real world resolution will be somewhat
> less since no lens is perfect.
> <http://www.luminous-landscape.com/tutorials/resolution.shtml>
>
> Chuck
> --
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