A few words on Equivalence and comparing systems

Started Apr 2, 2013 | Discussions thread
Detail Man
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Re: Photosite Density in relation to Diffraction Effects
In reply to Great Bustard, Apr 3, 2013

Great Bustard wrote:

... the effects of diffraction softening are also included, and the pixel size has nothing to do with it:  the proportion of the photo that the Airy Disk takes up when the FF photo is cropped by a factor of two also doubles (as does the DOF when the photo is displayed at the original size).

... diffraction softening is a result of the proportion of the photo that the Airy Disk spans, and that proportion is the same for all systems at the same DOF, regardless of sensor size or pixel count.

For a given sensor size and lens, more pixels always result in more detail -- that's a fact.  As we stop down and the DOF deepens, we reach a point where we begin to lose detail due to diffraction softening.  As a consequence, photos made with more pixels will begin to lose their detail advantage earlier and quicker than images made with fewer pixels, but they will always retain more detail ...

... the effects of diffraction softening affect all systems equally at the same AOV and DOF, not the same f-ratio.  This is because the diameter of the Airy Disk takes up the same proportion of the sensor (and displayed photo) at the same DOF on all systems.  However, that does not mean that all systems record the same detail at the same AOV and DOF.  Even though diffraction softening affects all systems equally at the same AOV and DOF, the system that began with more detail will retain more detail (although, as the DOF deepens, all systems asymptotically approach zero detail).

While the diameter of the Airy Disk is the same for the same color and f-ratio, regardless of the sensor size, the effect of the diffraction softening is not the same across formats.  The reason is that the proportion of the sensor that is covered by the Airy Disk is not the same since the sensors are not the same size ...

... the system that began with more resolution will always retain more resolution, but that resolution will asymptotically vanish as the DOF deepens.  In absolute terms, the earliest we will notice the effects of diffraction softening is when the diameter of the Airy Disk exceeds that of a pixel (two pixels for a Bayer CFA), but, depending on how large the photo is displayed, we may not notice until the diameter of the Airy Disk is much larger.

... for two sensors of a given size, the sensor with a greater pixel density does not suffer more from diffraction softening due to the smaller pixels.  We will simply notice the effects of diffraction softening earlier (at wider apertures) since we had more resolution to begin with as a result of the smaller pixels (presuming, of course, that we display the photo large enough that we can resolve individual pixels) ...

It seems important to note that composite system resolution (as assessed via the composite system MTF response) is not something independent of lens-system diffraction effects in relation to individual photosite size - in a manner that is independent of proportion of image-frame.

When the MTF response of the photosite (combined with any AA filter assembly that may exist on the image-sensor) decreases to a point where the product of its multiplication by the MTF response resulting from diffraction through a circular aperture opening reduces the composite MTF of the system in total, photosite size does in that case limit resolution of the composite system.

Higher photosite density can result in more detail (that the image-sensor can potentially resolve) - but that has little or even no effect in situations where the MTF response due to diffraction through a circular aperture has already reduced the composite MTF response to an extent where loss of resolution due to diffraction dominates relative to the MTF response of the photosite (combined with any AA filter assembly that may exist on the image-sensor).

The above effect is independent of what relative proportion of the recorded image-frame the diameter of the Airy disk or the photosite (combined with any AA filter assembly that may exist on the image-sensor) represents - it relates to the relative proportions of their individual physical dimensions.

"The sensor with a greater pixel density does not suffer more from diffraction" - but the composite system MTF response also does not (necessarily) benefit from greater pixel density.

Resolution that vanishes as a result of lens-system diffraction is not meaningful. Little to be gained from increasing photosite density beyond amounts that correspond to what are dominating effects of diffraction through a circular aperture of the lens-system used at its widest opening.

.

Am interested in your thoughts (related to composite MTF as a function of photosite density), GB.

DM ...

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