You may have noticed and/or been confused by the convention used here in which lens performance is addressed by ‘Zones’ rather than the more commonplace ‘centre, edge and corner’ designation. At the risk of treading on the toes of Ansel Adams of Fred Archer, I feel that this, more empirically explicit, system is helpful. Here’s why . . .
Rectangular image formats are masks: lenses throw fuzzy, circular puddles of light. In assessing lens performance, failure to think radially leads to misjudgment. Consider the MTF chart: it depicts (for a flat target) resolution (on the Y axis) at distances from the image centre. Dividing the chart into concentric rings of fixed radii enables us to compare lenses designed for different formats on a level playing field, and better connect MTF charts to real-world photography.
All lenses perform optimally in the hot centre of their image circle: within the limits of diffraction, resolution naturally peaks in Zone A (a central circle of 16mm diameter). Moving outward from this sweet spot into Zone B (8-16mm from the centre) gradually introduces light fall-off and a decline in fine (> 30 lp/mm) resolution. In the Twilight of Zone C (16-24mm from the centre) aberrations tend to dominate as the penumbral Outer Limits approach.
Reminded of the unvarying physics of light and shade, one wonders why lens designers don’t simply make lenses that throw bigger circles than the 43mm diameter required to cover a 36mm sensor. And of course, they do: the superior Zone C performance of shift lenses such as the Canon 24mm T/S II is largely a function of their 60mm image circles. One of Hasselblad’s trade secrets is that they design their lenses with oversized image circles. In general, adapting down a lens designed for a larger format frequently performs well at the perimeter of a smaller sensor.
To allow for discussion of lenses whose 60mm image circles cater for shift movements, or cover medium format, we have extended the terminology to include Zone D (image circles up to 64mm in diameter).
Corner of what?
Lens reviews struggle with the ambiguity of terms like ‘edge’ – which is meaningless unless qualified; Canon alone deploys three sensor sizes. At the risk of stating the obvious, the long edge of the frame is closer to the middle of the frame than the short one. Some lens reviewers seem to have forgotten.
A sample of the upper middle edge of a 35mm frame is barely out of Zone A, but a sample taken from the middle right is almost in Zone C – or well into the badlands of Zone B: the MTF chart predicts significantly degraded performance there. Lenses don’t throw square images; sensors capture rectangular crops from an image circle.
As you’ll notice from the MTF chart, a step change in performance is typical at around 16mm from the image centre of a full frame-covering lens. When seeking sharp corners with a 35mm sensor, it’s all about Zone C (C for Corner). Note that Zone C isn’t a simple triangular cut off: it’s a pair of concave stripes running along the short side of the frame. Full-frame corners are deep in the badlands.
Wherever possible, I display samples taken from the middle of each zone to depict as accurately as possible the range of performance across the frame. Klaus Schroff’s Optical Limits site (formerly Photozone) similarly provides three sample points (centre, ‘border’? and corner).
This system visibly illustrates the common conundrum of a lens that performs well on APS-C, but fails when transferred to a full frame sensor. For APS-C, the ‘corners’ are in Zone B. Zone C doesn’t get a look-in. Similarly, adapting a lens from 35mm > Micro Four/Thirds, the smaller pixel pitch places great demands on its Zone A acuity which is only required to extend a little into Zone B to deliver wiry corners. Again, other factors will complicate this picture, but relevant MTF-predicted resolution is a good starting point when considering lenses adaptable to smaller formats.
Image circles and lens movements
Photographers who cut their teeth on large format systems grew up appreciating coverage: longer lenses generally have bigger image circles that allow greater capacity for tilt/shift/rise/fall movements, whereas wide-angles – with their smaller circles – don’t permit as much fun in this regard.
A tilt/shift lens designed for use on a 35mm format typically has a minimum 58mm image circle (illustrated left) allowing puny 11mm horizontal movements (marked red) in landscape orientation. These lenses have image circles big enough to be adaptable to cover 645 (marked blue).
Conversely, medium format lenses make excellent shift lenses for smaller formats via suitable adaptors, as do 35mm lenses for M43 cameras – providing they are aberration-free across a wide portion of their image circle. Zone D performance of most 35mm T/S lenses – hiding from sight in unshifted captures – is often dire.
Film and sensor size comparison
This actual size comparison shows several image formats and their corresponding image circles. You may be surprised to learn that 645, 6×6 and 6×7 don’t measure up to their titles.
Red: 36mm x 24mm sensor. Minimum image circle: 43mm diameter
Blue: 48mm x 36mm medium format sensor. Minimum image circle: 60mm diameter
Green: 645 film format (56mm x 42mm). Minimum image circle: 70mm diameter
Black: 6×6 format (56mm x 56mm). Minimum image circle: 80mm diameter
Black: 67 format (56mm x 70mm). Minimum image circle: 90mm diameter
The larger (> 70mm) image circles of medium format lenses offer an expanded range of operation, and the possibility of results superior to 35mm shift lenses which typically have 58-60mm image circles that constrain movements to around 10mm on each axis.