# A few words on Equivalence and comparing systems

Started Apr 2, 2013 | Discussions thread
Re: No you did not, or ...
1

69chevy wrote:

Great Bustard wrote:

Sorry, but that's all incorrect.  Again:

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

In short, 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.

Also, the airy disc size only stays proportional when the relative size of the fstop is also in proportion.

Yes, or when a photo is cropped.

In your scenario, you give the smaller sensors the same size entrance pupil...

For example, 50mm f/2 on mFT and 100mm f/4 on FF (50mm / 2 = 100mm / 4 = 25mm).

...which means the same size airy disc, which means more apparent diffraction for a smaller sensor with higher pixel density.

The airy disc blurs more pixels in your equivalent scenaio, so diffraction is very different (not equivalent).

How many pixels the Airy Disk blurs is irrelevant.  More pixels means more resolution, the larger the Airy Disk, the smaller the resolution increase.

I'm not wrong on this, I promise.

You're confusing the forest for the trees.  There are many sources of blur in a photo, diffraction softening being just one.  If we have two sensors that are otherwise identical, but one with twice the pixel count as the other, mount the same lens in front of both sensors, take a pic of a scene with the same settings, then the photo from the sensor will have greater detail, but this detail advantage will decrease as the aperture narrows (Airy Disk grows).

In short, we have blur caused by fewer pixels, and blur caused by diffraction softening, and the blur caused by diffraction softening is independent from the other sources of blur (pixel count, AA filter, lens aberrations, motion blur, etc.).

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