Careful choice of focus can help or hurt image sharpness.
This is a companion article to Zeiss Touit Lenses for Sony NEX or Fujifilm X (PT Sept/Oct 2013), but the discussion here applies to lenses in general on any camera. See also Field Curvature in PT Sept 2009.
Most lenses exhibit field curvature, which can result in less sharpness than expected across the frame, often in the mid zones or corners. A typical complaint is “soft corners”, but with many lenses the lack of sharpness is entirely due to focus, not inherent lens unsharpness. Typically, a focus position can be chosen that balances the sharpness across the frame for any particular subject “shape”.
Even mild field curvature is a factor for photographers looking to achieve optimal results for a particular scene, increasingly important on high-resolution digital cameras. Conversely, knowledge of field curvature behavior can also help the photographer avoid a choice of focus that “piles on” and unnecessarily degrades sharpness for a particular scene. Stopping down for depth of field always helps, but with ideal choice of focus, the benefits compound.
The Zeiss Touit 12mm f/2.8 Distagon is a state of the art high performance ultra wide angle for APS-C sensors (18mm equivalent in full frame terms). Field curvature and astigmatism are extremely well corrected for an ultra-wide in the context of the very best 18mm full-frame lenses. Yet even so there are benefits to be had with careful choice of focus.
We first examine the MTF charts for behavioral hints, and then look at a field image. At ƒ/2.8, the edges and corners are less sharp than the central areas, particularly wide open (but the performance is at a very high level even so). The gradual drop-off from center is a typical combination of trace optical aberrations and some very mild field curvature.
Stopping down to ƒ/5.6 yields depth of field sufficient to provide essentially uniform center to edge performance (80% vs 70% has little significance). Between the edges (11.7mm offset) and corners (15mm offset) sharpness drops off somewhat more on the MTF chart, but the total performance is about as high as one will find for this class of lens.
But are the corners actually unsharp? Or perhaps just as sharp, but at a different distance, e.g., is there field curvature involved? One cannot say from the MTF chart. For that, field shots must be used.
In the field. Well, in the creek actually.
Let’s see if the field curvature theory pans out, and what if anything we might do about it.
Focus is at medium range on the dark central boulder, a balancing point between the immediate foreground and the heap of wood in the background. The use of aperture ƒ/4 is well suited to our analysis: ƒ/4 largely eliminates minor optical aberrations, but does not add so much depth of field as to obscure what is really going on with the zone of sharpness. Overall rendition here at ƒ/4 is already exceptional for an ultra wide-angle lens (24-megapixel Sony NEX-7 from raw format).
Observations using the original full-res image:
• Sharpness is high across the frame in the expected plane of focus, but with hints of a rearward sharpness bias towards the edges, more so to the corners.
• The foreground corners are visibly blurred, the most unsharp area of the frame.
• Sharpness in the top corners (distance) is at a surprisingly high level, even better than sharpness in the distance at center (top of frame).
The unduly sharp top corners (well behind the nominal plane of focus) correspond inversely to the unduly unsharp bottom corners. While the camera was tilted down slightly, it is field curvature that is responsible for the corner sharpness behavior: the zone of sharpness curves rearward slightly as the corners are approached: this makes the foreground corners unduly blurred and the background corners unduly sharp. This can be seen in the full resolution image by inspection.
Improving / balancing the overall sharpness in this example
To optimize the image sharpness in this particular case, the field curvature and the “shape” of the scene must both be taken into account: call it “fitting the curve”. Stopping down to ƒ/5.6 or ƒ/8 or ƒ/11 would improve total sharpness via depth of field, but when handholding as here that option might not be viable in shutter speed terms. And regardless of aperture (and its depth of field), carefully focus placement can create a strikingly sharp image even wide open at ƒ/2.8: it is often where sharpness is achieved that creates the visual impact of being sharp, even if much of the scene is less sharp (consider razor sharp eyes in a portrait, same idea).
Here, focusing can help: assuming a desire for the foreground to be uniformly sharp edge to edge, a modestly closer focus (perhaps 1-2 feet) would pull the lower foreground area into sharper focus, with the foreground corners improving notably, even if not fully sharp. This would push the background a bit more out of focus, but the tradeoff might be worthwhile. As a possible bonus, the increased sharp/unsharp difference between foreground and background might actually improve the 3D feel of the image by juxtaposing a sharper foreground against a somewhat more blurred background. The focus adjustment slightly forward plus stopping down to ƒ/5.6 would doubly help the foreground, and reclaim the background sharpness versus ƒ/4. Had the photographer instead focused in the distance, even ƒ/11 would not be enough to reclaim the foreground sharpness.
In the field, one can never be entirely sure of the optimal focus point, but at least one can avoid focusing badly for the scene for any particular lens. In this case, focusing beyond the black boulder would be trouble for foreground sharpness, even at ƒ/11. On a tripod where any aperture can be chosen without concern for shutter speed, careful choice of focus at ƒ/8 would likely deliver highly satisfactory results without incurring the negative effects of diffraction: high sharpness and high brilliance together.
Every lens has its own flavor field curvature behavior, a flat-field lens being uncommon. With knowledge of the actual lens behavior, careful selection of the point of focus can yield benefits to total image sharpness that can be surprisingly worthwhile. Conversely, the wrong-direction focus bias can accentuate the unsharpness in some areas of the image.
The Zeiss Touit 12mm f/2.8 Distagon exhibits very mild rearward field curvature in the corner areas that is easy to visualize (as compared with a pronounced wave-type field curvature as with some designs). A small focus compensation for scenes in which the foreground is prominent could provide satisfying for the demanding photographer.