Diffraction Limit

Started 11 months ago | Discussions thread
Great Bustard
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Speaking of clues...
In reply to LTZ470, 11 months ago

LTZ470 wrote:

Hallalujah!..Someone with a clue!

Steen Bay wrote:

"Also depends on where you look. The lens aberrations are usually stronger in the corners than in the center, so the point at which the resolution starts to decrease will most often be at a smaller aperture if looking at the border/corner resolution."

Yes, Steen did write that, didn't he? He wrote it in this post:

http://www.dpreview.com/forums/post/52061974

where he is responding to me, where I quote from the Equivalence Essay (I'm the author, by the way) on diffraction:

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

Diffraction softening is unavoidable at any aperture, and worsens as the lens is stopped down. However, other factors mask the effects of the increasing diffraction softening: the increasing DOF and the lessening lens aberrations.

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In terms of cross-format comparisons, all systems suffer the same from diffraction softening at the same DOF. This does not mean that all systems resolve the same detail at the same DOF, as diffraction softening is but one of many sources of blur (lens aberrations, motion blur, large pixels, etc.). However, the more we stop down (the deeper the DOF), diffraction increasingly becomes the dominant source of blur. By the time we reach the equivalent of f/32 on FF (f/22 on APS-C, f/16 on mFT and 4/3), the differences in resolution between systems is trivial.

Let me now fill in the "..." from the same link (please excuse the repetition, and take care to note the portion I highlighted in bold which was omitted from the first quote, as it was not particularly relevant at the time -- in fact, there's a lot more about diffraction in that link that I also did not quote, which, in fact, is why I gave the link):

Diffraction softening is unavoidable at any aperture, and worsens as the lens is stopped down. However, other factors mask the effects of the increasing diffraction softening: the increasing DOF and the lessening lens aberrations. As the DOF increases, more and more of the photo is rendered "in focus", making the photo appear sharper. In addition, as the aperture narrows, the aberrations in the lens lessen. For wide apertures, the increasing DOF and lessening lens aberrations far outweigh the effects of diffraction softening. At small apertures, the reverse is true. In the interim (usually around a two stop interval), the two effects roughly cancel each other out, and the balance point for the edges typically lags the balance point for the center by around a stop (the edges usually suffer greater aberrations than the center). In fact, it is not uncommon for diffraction softening to be dominant right from wide open for lenses slower than f/5.6 equivalent on FF, and thus these lenses are sharpest wide open (for the portions of the scene within the DOF, of course).

The optimum DOF is often more a matter of artistic intent than resolved detail. Clearly, more shallow DOFs have less of the scene within critical focus, but this is by design. What is not by design is that, at very wider apertures, lens aberrations reduce the detail even for the portions of the scene within the DOF, so even if the photographer prefers the more shallow DOF, they may choose to stop down simply to render more detail where detail is important. Likewise, while a photographer may stop down with the intent to get as much of the scene as possible within the DOF so as to have a more detailed photo, portions of the scene that were within the DOF at wider apertures will ne softer due to the effects of diffraction, so the photographer must balance the increase in detail gained by bringing more of the scene within the DOF against loosing detail for portions of the scene that were within the DOF at wider apertures. In addition, deeper DOFs require smaller apertures, which means either longer shutter speeds (increasing the risk/amount of motion blur and/or camera shake) or greater noise since less light will fall on the sensor at more narrow apertures for a given shutter speed.

However, the relationship between diffraction softening and pixel density is largely misunderstood. For a given sensor size and lens, more pixels always result in more detail. 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.

In terms of cross-format comparisons, all systems suffer the same from diffraction softening at the same DOF. This does not mean that all systems resolve the same detail at the same DOF, as diffraction softening is but one of many sources of blur (lens aberrations, motion blur, large pixels, etc.). However, the more we stop down (the deeper the DOF), diffraction increasingly becomes the dominant source of blur. By the time we reach the equivalent of f/32 on FF (f/22 on APS-C, f/16 on mFT and 4/3), the differences in resolution between systems is trivial.

Imagine that. I found it terribly curious that Steen felt it necessary to add that tidbit in, as it didn't really pertain to the situation at hand, except as an aside, when it was contained within the link I gave.

Furthermore, let me quote more from my post:

In short, it is entirely possible for the FZ200 to be sharper at f/4 than it is at f/2.8, even though at f/2.8 it is already well within the realm of strong diffraction softening, and the lesser lens aberrations at f/4 may outweigh the increased diffraction softening.

Regardless, the effects of diffraction softening at f/2.8 on an FZ200 are identical to the effects of diffraction softening at f/8 on mFT and f/16 on FF, it's just that diffraction softening is one of many forms of blur.

So, where does that leave us? Oh yes -- it leaves us back where we started: you do not understand diffraction in the least.

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