Help a novice: sharp or unsharp images? Should I return my X-T30?

Started 3 months ago | Discussions thread
Gerry Winterbourne Forum Pro • Posts: 18,726
Re: Lens testing

jrtrent wrote:

Gerry Winterbourne wrote:

jrtrent wrote:

"The centre is slightly soft wide open at f/2, with the peak performance achieved in the f/2.8-f/11 range. Diffraction sets in at f/16. The edges are soft from f/2-f/2.8, sharpening up at f/4, with f/5.6-f/11 the optimum settings."

That's a weird way of putting things. Diffraction doesn't "cut in" - it's always present for all lenses. As with all lenses its effects start to be noticeable round about f/4-5.6, and on APSA-C the effects are quite severe by f/11. f/16 is hardly ever useful on APS-C except for macro work.

That's interesting. Here's a quote from Bob Atkins that I've long thought was accurate:

"If you want to keep your images sharp, don't use f32 with an APS-C DSLR. The effects of diffraction are clearly visible at f32 and significantly degrade the image. Use f22 only if you have no choice. Optimal sharpness depends on the lens. For a lens with significant aberrations (e.g. a consumer zoom at maximum focal length and minimum focus distance), stopping down to f16 may give optimum results. For a lens with less aberrations (e.g. a consumer zoom used at infinity focus), optimum performance is around f11, though both f8 and f16 are very similar. For a really good lens like the EF 300/4L, with well corrected aberrations, performance may peak at f5.6, but be good from f4 to f11. f16 is acceptable, but f22 and smaller apertures should be avoided."

All lenses suffer aberrations, which are caused by the curvature of the glass. This is steeper at the outer zone of a lens so aberrations are greater when a lens is wide open. The result is that virtually all lenses are softer wide open than stopped down. If this were the only factor then we could just keep stopping down forever to get sharper sand sharper photos.

However, diffraction is an effect caused by the edge of an aperture: as all lenses have apertures, all lenses suffer from diffraction. Although this is an inexact description, it's easy to conceptualise the idea that small apertures have a higher edge-to-middle ratio so diffraction gets worse as the aperture reduces.

The result is that unless a lens is really well corrected it will start soft wide open, improve on first stopping down as aberrations have less effect but then get softer again as the effect of diffraction steadily increases. The point where the two adverse effects balance, giving best resolution, is the "sweet spot" of the lens.

The better corrected the lower f-number is the sweet spot; a really good prime lens might be at f/4 or (occasionally) lower; decent lenses tend to be around f/5.6, while cheaper "kit" lenses and similar might be f/8 or f/11.

All the above is true but it doesn't matter in the slightest if the picture you get is sharp enough. That, of course, is to a large extent a matter of personal taste. However, as a broad generalisation (which is borne out by questions here asking "why are my pictures soft") the point at which the effects of diffraction starts to be noticeable in making images soft is about f/16 on FF and one f-number smaller for each reduction in sensor size (f/11on APS-C, f/8 on M43 etc). To be safe I usually advise one f-number less if that's possible.

This, however, is an older article now, and he used an 8 mp camera. From the calculator at Cambridge in Colour I see that diffraction limits change with increasing resolution. My 6 mp DSLR is not diffraction limited at f/11, but the OP's 26 mp Fuji is already diffraction limited at f/8.

Diffraction "limit" isn't, in my experience, a helpful term. Relative softening is a gradual effect; there's never a fixed point at which things suddenly dissolve into loss of detail. Things like local contrast, the size of individual things in the frame, even comparison with other parts of the picture affect how we decide what's sharp enough.

How does this translate to comparative results? For example, I just don't get the depth of field I want most of the time at f/5.6, so I'm going to stop down to f/8 and even f/11 on occasion. Does the fact that I'm not diffraction limited at those apertures with my old camera mean its output is going to look better than if I used those same apertures with the OP's kit, or will the newer camera still produce better output at those apertures despite being diffraction limited?

DOF is never san absolute range of distances. All the standard DOF calculators use a set of assumptions about lots of things like size of picture looked at, distance from it, acuity of the viewer's eyes and so on. They ignore diffraction Its effect is relatively unimportant for FF, which is what was the smallest common size when the calculations were developed; but as diffraction softening increases the size of the blur circle - which is effectively the circle of confusion plus other effects - than stop down too far and instead of increasing DOF you can reduce it.

I rarely use a longer FL than 35mm on APS-C. My most used lens has its sweet spot sat f/5.6. When I put 35mm f/5.6 into a DOF calculator36'. for 1.5 crop I get hyperfocal distance = 36'. So if I focus there my DOF runs from 18' to infinity, which covers most situations. I use 43mm on FF, which by pure coincidence gives the same hyperfocal distance. I rarely use hyperfocal focusing in practice but with those lenses (or shorter) I virtually never need to go beyond f/8 for sufficient DOF.

As you see, the loss of resolution at f/8 on my 35mm lens is small - less than the ~15% that is often cited as the "just noticeable difference". Even f/11, though it is heading that way, is mathematically tolerable.

Edit: I'll add that the lens test I quoted used an X-A7 camera body with a 24.2 mp sensor. If we end up getting more government money, an X-T200 with that XC 35mm F2 sounds tempting. PC Mag's review (using an X-T200) also suggested that the smaller apertures are quite usable:

"It's a lens that, on today's cameras, delivers nearly as much resolution wide open as it does when stopped down, notching an excellent 2,735 lines at f/2 and settling in at a slightly better 2,800 at smaller f-stops. It hits 2,900 lines, close to outstanding, at f/8, and drops off just a little bit at f/11 (2,775 lines). The weakest resolution is at f/16, but images are still in the good range (2,440 lines). You needn't fret about distortion—there's none—nor worry about a heavy vignette."

Yes; that's consistent with the OP's results. But remember that many these days view their photos on screens that are a lot bigger than the 8x10" that is assumed in DOF calculations. Other than macro (where - see above - the size of objects is relatively large in the frame) I'd stick to the smaller f-numbers to be safe.

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I'm happy for anyone to edit any of my photos and display the results
First camera 1953, first Pentax 1985, first DSLR 2006

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