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

Started 3 months ago | Discussions thread
jrtrent Veteran Member • Posts: 6,183
Re: Lens testing

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."

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.

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?

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."

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