Diffraction Limit Discussion Continuation

Started 5 months ago | Discussions thread
s_grins
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Re: Enlargement factor has been ignored
In reply to Great Bustard, 5 months ago

Great Bustard wrote:

s_grins wrote:

Mike Davis wrote:

If two sensors are the same size, but one has more pixels than the other, there's some probability that sooner or later, the guy equipped with the higher pixel count is going to make a larger print than the guy who has the lower pixel count on the same size sensor. Using the higher number of pixels on the same size sensor to produce a larger print causes the Airy disk diameters at the sensor, for any given f-Number, to suffer more magnification in the final print than they would in a smaller print, and thus a greater likelihood of diffraction inhibiting a desired print resolution. The photographer has to shrink the Airy disks at both the sensor and in the final print (after enlargement), by opening up so that his desired print resolution is not compromised by the larger Airy disks that come with the greater enlargement factor.

Beating the dead horse...

Do you really think that airy disk reveals itself on the print (final of course) as disk?

The Airy Disk "reveals itself" in terms of the blur it introduces into the photo, where the blur is a function of the relative size of the Airy Disk to the photo, not the absolute size:

http://www.josephjamesphotography.com/equivalence/index.htm#diffraction

For the same color and f-ratio, the Airy Disk will have the same diameter, but span a smaller portion of a larger sensor than a smaller sensor, thus resulting in less diffraction softening in the final photo. On the other hand, for the same color and DOF, the Airy Disk spans the same proportion of all sensors, and thus the effect of diffraction softening is the same for all systems at the same DOF.

Let's work an example using green light (λ = 530 nm = 0.00053mm). The diameter of the Airy Disk at f/8 is 2.44 · 0.00053mm·8 = 0.0103mm, and the diameter of the Airy Disk at f/4 is half as much -- 0.0052mm. For FF, the diameter of the Airy Disk represents 0.0103mm / 43.3mm = 0.024% of the sensor diagonal at f/8 and 0.005mm / 21.6mm = 0.012% of the diagonal at f/4. For mFT (4/3), the diameter of the Airy Disk represents 0.0103mm / 21.6mm = 0.048% at f/8 and 0.005mm / 21.6mm = 0.024% at f/4.

Thus, at the same f-ratio, we can see that the diameter of the Airy Disk represents half the proportion of a FF sensor as mFT (4/3), but at the same DOF, the diameter of the Airy Disk represents the same proportion of the sensor. In other words, all systems will suffer the same amount of diffraction softening at the same DOF and display dimensions.

However, the system that began with more resolution will always retain more resolution, but that resolution advantage 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.

Typically, the effects of diffraction softening do not even begin to become apparent until f/11 on FF (f/7.1 on APS-C and f/5.6 on mFT -- 4/3), and start to become strong by f/22 on FF (f/14 on APS-C and f/11 on mFT -- 4/3). By f/32 on FF (f/22 on APS-C, f/16 on mFT -- 4/3) the effects of diffraction softening are so strong that there is little difference in resolution between systems, regardless of the lens, sensor size, or pixel count.

We can now summarize the effects of diffraction softening as follows:

  • There is no "diffraction limit" except when resolution falls to zero.
  • There is a point when the effects of diffraction softening will become the dominant source of blur, and this point will vary from lens to lens as well as where in the frame we are looking (e.g. center vs edges, where the edges typically lag a stop behind the center).
  • All else equal, more pixels will always resolve more detail.
  • All systems suffer the same diffraction softening at the same DOF, but do not necessarily resolve the same detail at the same DOF, as diffraction softening is merely one of many forms of blur (e.g. lens aberrations, motion blur, large pixels, etc.).
  • As the DOF deepens, all systems asymptotically lose detail, and by f/32 on FF (f/22 on APS-C, f/16 on mFT -- 4/3), the differences in resolution between systems is trivial, regardless of the lens, sensor size, or pixel count.

Thank you for been educated.

But there is one thing that bothers me. Diffraction reveals itself in my camera as a dull image with excessive CA. I did not see any blur caused by diffraction. Probably, I use wrong lens

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