Diffraction Limit Discussion Continuation

Started Feb 21, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 38,500
There we have it!

Jonny Boyd wrote:

I'm not interested in dissecting the previous discussion. I'd rather go forwards than backwards. What I said before and think is most relevant to the present discussion, is that lower resolution sensors have more of a plateau than a peak, so you get the same resolution effectively at lower apertures as you would at the theoretical peak resolution aperture.

I would agree with this, except I would say "nearly the same resolution" instead of "same resolution".

Therefore a low res sensor isn't diffraction limited at the same aperture as a high res camera.

You really need to defined "diffraction limited". For reference:


Notice also there is no defined 'limit' where the resolution suddenly falls due to diffraction, it is a smooth and even drop-off. The 'limit' is just a bogus idea. McHugh has taken a well defined optical term - a 'diffraction limited' system is one so good that diffraction is the only limit on its performance - turned it inside out and made it into something senseless.

More specifically, as I said upthread:

To the uneducated on diffraction, if you say one system is "diffraction limited at f/4" and another system is "diffraction limited at f/5.6", they take it to mean that the system that is "diffraction limited at f/4" will deliver a lower resolution photo at f/5.6 than the system that is "diffraction limited at f/5.6".

Let me illustrate what I mean. Here's a hypothetical lens attached to a number of different hypothetical sensors covering a large range of resolutions. The units for resolution are the same throughout and the table below calculates the systems resolution for each aperture for each sensor.

When we plot the data on a chart, it looks like this:

The highest resolution sensors are virtually indistinguishable from the lens resolution. Below is a table showing what percentage of lens resolution is reached at different apertures by different sensors.

With the low res sensors, resolution is pretty flat across the range. The table below shows the percentage of sensor resolution used at each aperture by each sensor.

If we looked at the numbers with a high enough precision, we would indeed find peak resolution at the same aperture for every lens.

This last sentence, as supported with your charts and graphs, is *exactly* what "we" have been saying.

In practice though, there is no noticeable difference in resolution at any aperture for the lowest resolution sensors.

This would be because the blur from lens aberrations and diffraction are subsumed by the blur of low pixel count.

With high res sensors, you'll gain sharpness by using the peak aperture, but as sensor resolution decreases, you get less and less advantage from the peak aperture and suffer less of a penalty for stopping down.



A common myth is that smaller pixels suffer more from diffraction than larger pixels. On the contrary, for a given sensor size and lens, smaller pixels always result in more detail. That said, 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. Eventually, the additional detail afforded by the extra pixels becomes trivial (most certainly by f/32 on FF). See here for an excellent example of the effect of pixel size on diffraction softening.

Here's one last table that shows the % of peak resolution that you get at each aperture:

If we said that a resolution difference of 0.1% was the limit of perception, then for all practical purposes, peak resolution is indeed a plateau that stretches over several apertures for smaller sensor sizes.


Mathematically of course, there is always a peak aperture independent of the sensor. And in practice, the relationship between lens and sensor resolution for a system may be such that no combination of lens and sensor produces a plateau.

The bottom line is there is no reason to fear stopping down to get the necessary DOF with a sensor that has more pixels.  If you are stopping down past the peak aperture, you will not realize the full potential of the higher pixel count sensor, but you will resolve more detail than you would have with the lower pixel count sensor, although that resolution advantage may be trivial, depending on the difference in pixel count and how far you are stopped down.

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