A word about zones and image circles

Our 'Zone' system: not to be confused with Ansel Adams (and Fred Archer's) . . .

You may have noticed and/or been confused by the convention used here
and on our sibling site in which lenses' resolving ability are considered in 'zones'
rather than the more commonplace 'centre, edge and corner' designation.

There are several reasons why we felt a more explicit system would be helpful.

1. Correspondence to MTF charts.

MTF charts plot performance on the y-axis against distance from the centre of the image circle on the x-axis, portraying how the lens' resolving ability is distributed across the frame. In a perfectly centered world, all points 15mm distant from the centre would be identical. Naturally this doesn't happen: sharpness is also affected by distance to the subject: most lenses are slightly (sometimes distinctly) optimised for either near- or far-field performance, and nowadays manufacturers don't tell you whether the chart is an infinity or 1m. But I digress . . . the point of our tests is to illustrate data with real world examples: hence the Zones – Zone A is the bit of the chart on the left, Zone B the middle, and Zone C represents the last 6mm or so the image circle.

2. Clarity and accuracy

Canon alone deploys three different sensor sizes, which greatly diminishes the usefulness of a term such as 'corner'. At the risk of stating the obvious, the long edge of the frame is much closer to the middle of the frame than the short one, as this diagram demonstrates.

Many people writing about lenses seem to forgot this. Here you see that both 'edges' of a full frame capture are well outside the part of the image circle required by an APS-H sensor camera like the 40D, but they are also at quite different radii from the image centre and will probably be rendered dissimilarly on a 36mm sensor. Clearly it is more accurate to say that the centre of the long edge lies mainly in Zone B, and therefore is likely to be better rendered than the short edge, which lies entirely in Zone C, heading into the badlands of a 35mm lens' image circle.

3. Eunoia

Sometimes badly translated as 'beautiful thinking'; actually it refers to healthy thinking. Those weaned on – or still using – view cameras are conscious of the fact that photography is the delicate act of snatching a slice of a cone of light thrown into the air. It's good to remember that, despite advances in digital capture technology, at its heart nothing has changed. When shopping for a large format lens, the size of the image circle it projects at the film plane is a vital statistic: a bigger image circle means not only greater movements, but often sharper rendition at the edge of the film, using only the 'sweet spot' of the lens. And nothing has changed for DSLR users.

Large format photography quickly shows you that an image 'circle' has no boundary: instead, a broad fuzzy region of penumbra shades gradually out of blackness into light. The zone system is a reminder that, despite the rectangular window we hold up to the world, in reality, we're working with circular puddles of light. Remembering this makes shift lenses and stitching techniques much easier to understand.

It's why Hasselblad lenses have sharper corners: they're bigger, heavier and throw bigger image circles. It's why the most reliable way to get sharp corners at wide apertures is to use a shift lens designed for an area bigger than the sensor you're using.

For the purpose of evaluating 35mm-format lenses, we have divided a 58mm diameter image circle into four zones.
A 58mm diameter image circle is the maximum typically projected by a 35mm-designed shift lenses (11mm + 36mm + 11mm).
The diagram also demonstrates why tests performed on an APS or DX 'crop-sensor' camera are pretty useless for 'full frame'
sensor users like most Canon pro-shooters and of course photographers shooting with now Nikon's 'FX' cameras like the D3.

Because almost all our tests have been conducted from the beginning on 36mm or larger sensors, users of cameras with smaller sensors
can just ignore Zone C results. You'll also notice that a 35mm shift lens like the Zuiko f2.8 will cover a 48mm x 36mm medium format sensor perfectly . . . .

Here we have an actual size comparison of several image formats and their corresponding image circles. You may be surprised to learn that 645, 6x6 and 6x7 don't measure up to their titles.

Red lines: 36mm x 24mm sensor
Minimum image circle: 43mm diameter

Blue lines: 48mm x 36mm medium format sensor
Minimum image circle: 60mm diameter

Green lines: 645 format (56mm x 42mm)
Minimum image circle: 70mm diameter

Black lines: 6x6 format (56mm x 56mm)
Minimum image circle: 80mm diameter

Black lines: 67 format (56mm x 70mm)
Minimum image circle: 90mm diameter

All medium format lenses, have the capacity to be used as shift lenses on 36mm sensors with scope for at least 17mm movements. Their 70mm image circles offer possibilities for results superior to 35mm shift lenses which typically have 58-60mm image circles. On the flipside, these lenses will often work well with MF digital sensors that only need a 60mm image circle.

Using a Nikon D3 or Canon 5D or 1 Series full frame camera and a shift lens such as the Nikon 85PC or Canon 90mm TS typically permits horizontal movements of up 10mm, which is fine to explore a modest 58mm image circle. You have to think of these as medium format lenses: they throw a similarly sized image circle. Some of the smarter shift designs, like the Olympus 35mm f2.8 exploit the fact that greater vertical movements are possible when the camera is in landscape orientation. If your camera has a smaller sensor, you won't be probing its outer reaches.

I've overlaid in blue a 48x36mm medium format sensor such as the Phase P25, Leaf Aptus 22 or Mamiya ZD. Once you've grasped that these are really lenses for a larger format, you might get quite excited by the possibility of using an Olympus 24mm PC, or Schneider 28mm on your Mamiya 645/ZD, instead of the new Mamiya 28mm f4.5. No problem.

Deploying the larger 80mm image circle of a Hasselblad lens designed to cover 6x6, using a Zörk or Mirex adaptor, offers far greater scope for movements: with the camera in landscape orientation, the limiting factor in the vertical range is the shift mechanism rather than the image circle. At present, no-one offers an adaptor that lets us shift 24mm vertically . . . .

The 'Blad Zeiss lenses are all very sharp in the corners, so you can expect excellent shifted performance all the way to 20mm+ with a 36mm sensor. As the diagram shows, 20mm + 36mm + 20mm takes us 'off-piste' into areas beyond that which would be recorded on 6x6 film. However, the shift adaptor enables us to efficiently 'map' those perfectly useable areas to the side of the frame (think circular Zones!), and we're still inboard of the corners.