For low aspect ratio photos, e.g. 1:1, 6:5, 5:4, and 4:3, the 3:2 ratio of APS-C and 35mm sensors wastes a significant portion of the image circle. A fairly modest 28% increase in sensor area allows:
- All ratios from 3:2 to 1:1 to make native use of the lens, providing 61% more image area in the case of 1:1 (square) photos
- Electronic viewfinder (live-view) overscan for easier subject tracking
- Aspect ratio to be chosen in post-processing without additional cropping, if the full sensor readout is saved.
- Correction of barrel distortion with less or none of the cropping typically incurred
Formats like 5:4 and 1:1 may be less popular than 3:2 but they are hardly rare. The fractional increase in sensor area (and the mechanical and electronic changes needed to support it) shouldn't have a major effect on price; it wouldn't push a camera into a different class entirely (cf. the ~132% sensor area increase from APS-C to 35mm). Even at significant cost this would be a fine differentiating feature for flagship cameras; since it is often stated that lenses are the primary investment in a photographic system getting the most out of them should be a strong selling point.
Given the benefits listed above, illustrated below, I have to wonder why isn't this already commonplace on high-end cameras? Is there something that makes manufacturing a sensor, shutter, or IBIS unit 28% taller really that prohibitive? Are the benefits I believe exist incorrect or unappealing? Are people simply unaware of this technical option and therefore haven't voiced a demand for it?
Preemptively addressing possible concerns:
- File size, buffer speed, etc. should remain for same for a selected 3:2 aspect
- A standard 3:2 EVF could be used by simply displaying other formats a bit smaller
- Vignetting need not be shown unless a full-sensor-readout option is specifically selected
For APS-C lenses my proposal would be a 23.50mm × 19.95mm sensor, shown in figure 1 in light blue along with standard APS-C and 35mm "full frame" sizes (wine and pink respectively) overlaid with the the APS-C minimum image circle. Figure 2 highlights the substantial difference in image area between a 1:1 crop of the APS-C sensor (green), and the native 1:1 frame (dashed).
Referencing the figure 1, if a standard 3:2 image is desired there is unused area of the proposed sensor. This can be put to good use to provide a wider view in the electronic viewfinder which would make it easier to track a subject, e.g. birds in flight. (Appropriate crop guides for the target format would be overlaid.)
Ad hoc crop in post
If an option is provided to save the full sensor readout to RAW (or JPEG for that matter) one could defer the decision of aspect ratio to post without any additional cropping. This could be used to save a poorly composed photograph, or simply for changing artistic desire. It would also improve the ability to meet a client's request for printing a given image in a specific format, e.g. 8×10, 11×14 20×24.
Barrel distortion, common in wide-angle lenses, typically requires additional cropping (yellow frame) after correction—either in-camera or post—because the sensor (green frame) does not capture all of the distorted image. (fig. 3)
Without changing the image circle it is still possible to capture a wider usable angle of view if the sensor is extended. (fig. 4) Obviously the recoverable area depends on the aspect ratio, with only the top and bottom recoverable at 3:2 and only the sides at 1:1.
Wafer die yield
An answer made the claim
A sensor area with more square aspect ratio leads to considerably worse wafer yields.
Attempting an approximation using CALY die yield calculator I see a 35% increase in cost for the proposed sensor over APS-C, compared to a 232% increase for 35mm.
Standard APS-C 23.50 × 15.60 die on a 300mm wafer:
Using the proposed 23.50 × 19.95 die with the same parameters:
As expected a far higher yield than 35.7 × 23.8 "full frame":