The X100S produces excellent photos, albeit with a few caveats.
Lori Grunin
I've been reviewing hardware and software, devising testing methodology and handed out buying advice for what seems like forever; I'm currently absorbed by computers and gaming hardware, but previously spent many years concentrating on cameras. I've also volunteered with a cat rescue for over 15 years doing adoptions, designing marketing materials, managing volunteers and, of course, photographing cats.
The sharpness of the lens, high resolvability of the sensor and smart noise and JPEG processing all contribute to the X100S' excellent JPEG quality as high as ISO 1600 in good light. The extra-thin optical low-pass filter on the sensor does result in some rendering oddities, though; notice how the lack of antialiasing distorts the text.
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Sharpness
The camera resolves detail very well, but to my eyes it looks a hair oversharpened.
(1/125 sec, f10, ISO 200, AWB, multi metering)
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Bokeh
The 9-bladed aperture and low-noise sensor results in lovely soft bokeh.
(1/125 sec, f4, ISO 200, AWB, multi metering)
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Lens at f2
While the lens generally has extremely good center sharpness and reasonably good edge sharpness, f2 is close to unusable. You can see the huge difference simply between f2 and f2.8 in the center; the outer edges become decent at f4.
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Lens at f2 redux
This is what f2 looks like in practice.
(1/125 sec, f4=2, ISO 200, AWB, spot metering)
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Highlight recovery
The camera rates fairly well on highlight and shadow recovery (top is JPEG, bottom is raw).
(1/125 sec, f3.6, ISO 200, AWB, multi metering)
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ISO 400 JPEG
The camera produces extremely clean, crisp JPEGs at middle ISO sensitivities.
(1/60 sec, f3.6, ISO 400, AWB, spot metering)
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ISO 800 JPEG
There's no softening even at ISO 800.
(1/110 sec, f4, ISO 800, AWB, spot metering)
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ISO 1600, no noise reduction
What little color noise appears at ISO 1600 is extremly fine-grained compared to other APS-C sensors. You can see some yellow blotchiness start to appear, though.
(1/60 sec, f2, ISO 1600, AWB, multi metering)
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ISO 3200 JPEG
This is pretty sharp for ISO 3200.
(1/30 sec, f4, ISO 3200, AWB, spot metering)
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ISO 6400 raw vs. JPEG
ISO 6400 is the first setting where there was a clear advantage to shooting raw over JPEG for getting better image detail.
(1/125 sec, f4, ISO 6400, AWB, spot metering)
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Color
The X100S' default color settings (called Film Simulations) produce pleasing images, with somewhat higher contrast than I like and pumped up saturation.
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Saturated reds and pinks
The X100S' handling of bright, saturated reds and pinks is better than the X100's, but there are still some issues with hue shifts in the Standard default color setting (Provia film simulation). Switching to anothing setting prevents the orange areas you see here, but there are still some areas which render clipped. Additionally, these aren't quite the right color in any of the JPEG settings.