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 58-60mm image circle (illustrated left) allowing 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 – unexamined by unshifted captures – is often poor.
Film and sensor size comparison
This actual size comparison shows several image formats and their corresponding image circles.
Red: 36mm x 24mm sensor. Image circle: 43mm diameter
Blue: 48mm x 36mm medium format sensor. Image circle: 60mm diameter
Green: 645 film format. Image circle: 70mm diameter
Black: 6×6 format. Image circle: 80mm diameter
Black: 67 format. Image circle: 90mm diameter
A useful guide to comparing videography and photography sensor formats can be found here >.
| Shift capability with lenses of a given image circle | ||||||
| ⌀= (APS-C) |
⌀=43mm (ie, 35mm) |
⌀=57mm (35mm Shift) |
⌀=65mm (35mm Shift) |
⌀=70mm (ie, 645) |
⌀=80mm (ie, 6×7) |
|
| Micro FourThirds (18×13.5mm) |
H: 10mm* V: 10mm* |
H: 10mm* V: 10mm* |
H: 10mm* V: 10mm* |
H: 10mm* V: 10mm* |
H: 10mm* V: 10mm* |
|
| APS-C (Canon) (22.2×14.8mm) |
Covers, but no shift |
H: 9.5mm V: 11.5mm |
H:12mm* V:12mm* |
H:12mm* V:12mm* |
H:12mm* V:12mm* |
H:12mm* V:12mm* |
| APS-C (DX, Sony) (23.6×15.7mm) |
H: 8.5mm V: 10.5mm |
H:12mm* V:12mm* |
H:12mm* V:12mm* |
H:12mm* V:12mm* |
H:12mm* V:12mm* |
|
| 35mm ‘Full-frame’ (36x24mm) |
Covers, but no shift ** |
H: 8mm V: 10mm |
H: 12mm* V: 12mm |
H: 15mm* V: 15mm* |
H: 15mm* V: 15mm* |
|
| Fujifilm GFX / Hasseblad X2D (44x33mm) |
Some lenses cover ** |
H: 1.5mm V: 1.5mm |
H: 5mm V: 7mm |
H: 9mm V: 10mm |
H: 15mm V: 15mm |
|
| Phase One XT (53.4x40mm) |
Covers, but no shift |
|||||
** Limited by mount or adaptor mechanics.
** Many 35mm lenses have oversized (ie, greater than 43mm diameter) image circles. Where the register permits (ie when adapting any 35mm DSLR lens to any 35mm mirrorless camera), these lenses can be used as tilt/shift optics on 35mm, and/or lenses without movements on sub-medium format digital bodies. For a guide to which lenses are suitable, please see here >
Elephants in the Room
Elephant 1: What about tilt movements?
Tilt movements are much less demanding than shift movements. As a very general (and quite inaccurate) guide, reckon on obtaining 1.25° of tilt for every 1mm of shift movement available.
Elephant 2: Infinity Focus v Macro
All figures quoted refer to infinity focus only. With good cause you may ask “Why? I could live infinitely long and never need to take a picture at infinity focus. Infinity is too far away, and I’m not sure there’s anything photogenic there.” And I would respond: “You’re missing out: go make some bokeh-only masterpieces.”
As well as being the only sane method of standardising the measurement of registration, focal-flange distances and image circles, quoting them at the point of infinity focus reckons with a worst-case scenario. At infinity focus the lens is at the nearest (viable) point to the image plane, projecting the smallest image circle. Therefore coverage is always slightly better than advertised – especially if you shoot in the macro range. At short working distances (ie, sub-30cm) a microfiche lens throws an image circle big enough to cover GFX.
Elephant 3: Medium Format Lies
One might naively assume that the baby of the medium format world – 645 – was so called because it measured 6cm x 4.5cm, logically following from 6×6 and 6×7. It wasn’t necessarily so, and it clearly ain’t now. The ’35mm’ format nobly undersells the reality of its 36mm width, whereas medium format short-changes us with an imaging area of 56x42mm. Things got worse when digital sensors arrived, clinging to the ‘medium format’ label, despite being clipped down to 53.4 x 40mm: surely that’s a ‘540’ sensor? In the post-2020 world, Fujifilm’s GFX flies under the radar to identify as a medium format camera, despite measuring 44mm in width: barely 8mm wider than whatever passes for ‘full frame’ these days – and 16mm smaller than we had a right to expect from all the sixes in 635, 6×6, 67 and 6×9 – actually, 67 is forgiven: it really was 70mm deep. No wonder so many 35mm lenses work with GFX: it’s less medium format than ‘slightly supersized 35mm’.
Image Circles v Image Quality
As noted, it’s recently become common practice to over-engineer lenses (especially wide-angles) with larger than necessary image circles. This makes sense. Your 14mm lens with ‘magically sharp’ corners wide open isn’t doing the impossible (ie, rendering the petering edge of its light puddle without aberrations) – it’s simply projecting a bigger image and allowing it to be mechanically cropped.
Terry Pratchett once memorably said: “Light thinks it’s the fastest thing in the universe – but it’s wrong. Because wherever it goes, it finds the darkness got there first.” Every lens vignettes. The question is: when? Within the sensor’s view? Just outside it? Or far beyond? Not all lenses vignette with the same gradient, but at the edge of their image circle the gradient increases steeply as it fades to black.
Hand in hand with this deteroration comes an unavoidable increase in field curvature and spherical and chromatic aberration. Postponing this decline further outboard reaps obvious benefits. That’s why, earlier in this article, ‘cover’ was set in quotes. The economically-motivated desire to use 35mm lenses – even shift lenses – on GFX must be offset against the fact that we’re doing the photographic equivalent of driving along the hard shoulder: we’re ploughing through the crud flung outward into places motorists don’t usually go: there’s nothing to stop you going there, but it will be mucky.
As well as an increase in aberrations and a loss of contrast, defocused areas suffer, too. The ‘swirly bokeh’ look may be having its moment, but like any fad based on an integrity shortcoming, it won’t last. Or, at least, it won’t ever replace the ‘classiness’ of a timelessly correct rendition. It’s a nice look, but niche. For me, as it happens, much more niche than nice. Pertinent here, though, is that thinking about image circle size is fundamental to a creative decision: lenses that ‘only just’ cover your format will tend to have strongly swirly bokeh; lenses with image circles over-engineered for your format, will tend to render bokeh more consistently across the frame, and have less field curvature.
Similarly, when it comes to defocused rendition of specular highlights (‘bokeh balls’ and ‘cat-eyes’) it’s well known that you can control their appearance by stopping down. Wide open, every lens distorts these highlights at the edge of its image circle. Stopping down alters the light path in such a way that mechanical clipping no longer manifests – though the shape of the diaphragm blades will. This is one of the advantages of a faster lens: f1.2 primes tend to render circular highlights by around f2; whereas f2 primes often still suffer from ‘cat’s eyes’ at f2.8. Somewhat analogously, though not for exactly the same reason, over-sizing the image circle smooths irregularities in bokeh rendition. Shooting with a 36×24 sensor at f2, circular bokeh balls can be obtained equally well by stopping down at f1.2 prime, or using an f2 lens in the sweet spot of its 80mm image circle.
On the subject of sweet spots, you often read that cropping into the sweet spot of a bigger lens gives a sharper image. Up to a point this is true, and it can be a useful way of leveling performance across a frame. However, within the context of using a big old lens on a new little camera – which is often the backdrop to such chatter – modern lenses aren’t monstrously lossy into the corners, and old lenses (especially those intended for lazier, larger formats) often didn’t resolve to the highest levels required by today’s picky little pixels. Particularly when used wide open, or at close range, the choice of a simple vintage lens often results in consistent overall softness, which may not be the sweetness desired.
Switching back to the positive – and if we set aside the reality that most photographs are now delivered to eyeballs with their optical imperfections dialed out by lens profiling – image circles impact geometry. There are notable examples (for instance the Canon 16-35mm f2.8 L Mark III) of 35mm lenses that appear to have well-corrected geometry for their intended use, and have enough illumination to cover GFX, but hide strong waveform distortion outside the 35mm frame. Conversely, there are lenses like the Pentax 645 FA 35mm that suffer from similar waveform distortion in Zone D that become well behaved when cropped to 35mm – until shifted, when the resultant asymmetry is tricky to correct. There are also medium format lenses with quite liveable barreling that look more heavily contorted than you recall when cropped to 35mm, where the distortion’s peak occurs.
Examples
Example 1: Using 35mm Lenses on Micro Four-Thirds Bodies
A 35mm lens has an image circle of at least 43mm diameter – although many lenses have undocumented ‘bonus coverage’ (they were over-designed for the format) and at certain apertures and working distances throw image circles as large as 50mm or more in diameter. More on this in a moment. However, assuming a worst-case minimum, the image circle of any 35mm lens permits a vertical shift of 13mm and horizontal shift of 12mm. In practice, the 38mm throat diameter of the Micro FourThirds mount is a hard limit, and the free internal diameter of the adaptor’s shift mechanism may impose further limitations. These factors tend to limit practical shift-ability to 10mm in each direction. Lenses with larger image circles (for whatever format they were intended) offer no greater functionality here: the limit is the diameter of the bayonet mount and adaptor providing movements.
Shrinking the image circle of a 35mm lens with a Speedbooster or other focal length reducer still amply covers a M43 sensor – in principle even still permitting vertical shifts of almost 6mm and horizontal shifts of 5mm. No adaptor presently exists to allow this functionality.
The ‘squarer’ M43 format permits greater movement within a circular field of view. As we will see, when using sensors based on a 4:3 format (APS-C and 35mm) there’s a greater difference between maximum horizontal and vertical shifts.
Example 2: Using 35mm Lenses on APS-C Bodies
For everyone but Canon, APS-C is a standardised size of 23.6mm x 15.7mm (illustrated below). Standard 35mm lenses permit 8.5mm of horizontal shift and just over 10mm of vertical shift. The smaller Canon APS-C sensor (22.2mm x 14.8mm) allows slight greater freedom of movement: 9.5mm horizontally and almost 11.5mm vertically.
Example 3: Using DSLR-Era 35mm Tilt/Shift Lenses on Fujifilm GFX
Because each manufacturer had differently dimensioned mounts and register distances, the image circles of 35mm tilt/shift lenses are not standardised – hence the varation in ‘advertised functionality’ (below). Not for the first time, Nikon paid the penalty for its decision to use a smaller diameter mount than its main rival, Canon: the Canon TS/E range consistently offered slightly more in the way of shift, and consistently performed better in Zone D, than either Nikon’s PCs or the Olympus and Pentax Shift lenses. Lessons learnt, though: the relatively huge Nikon Z mount (and it’s ultra-short register) paves the way for a competitive advantage over its rivals that will last a generation.
However, advertised functionality was often different to actual potential: On the first-generation 24mm f3.5 TS-E, Canon honestly marked the last 2mm of movement in red, and in the manual advised against deploying horizontal shifts in excess of 7mm. Some manufacturers offered similar advice, but the difference between the demands of horizontal and vertical shifts weren’t always made this clear. The diagram below may help.
To put this into ‘perspective’, a lens permitting a 10mm vertical shift on 35mm is required to have an image circle of 57mm diameter. For a 10mm horizontal shift to be possible, the image circle must be at least 61mm in diameter. To shift 12mm in each direction, the image circle will need to approach 65mm.
But here throat diameters hove into view: albeit not quite as constricted as M42’s 42mm diameter, Nikon’s F mount was throttled to 44mm in diameter; Pentax K was 45mm; Olympus OM was 46mm; and Canon’s EF mount was 47mm. Hence . . .
| Lens | Horizontal Shift | Vertical Shift | Indicated Image Circle Diameter |
| Arax 35mm f2.8 Tilt/Shift | 11mm | 11mm | 63mm |
| Canon 17mm f4 | 12mm | 12mm | 65mm |
| Canon 24mm f3.5 TS-E | 12mm (not advised beyond 7mm) | 12mm | 57mm |
| Canon 24mm f3.5 TS-E II | 12mm | 12mm | 65mm |
| Canon 45mm f2.8 TS-E | 12mm | 12mm | 65mm |
| Canon 50mm f2.8 TS-E Macro | 12mm | 12mm | 65mm |
| Canon 90mm f2.8 TS-E | 12mm | 12mm | 65mm |
| Canon 90mm f2.8 TS-E Macro | 12mm | 12mm | 65mm |
| Canon 135mm f4 TS-E Macro | 12mm | 12mm | 65mm |
| Contax 35mm f2.8 PC-Distagon | 10mm | 10mm | 61mm |
| Laowa 15mm f4.5 | 11mm | 11mm | 63mm |
| Laowa 20mm f4 | 11mm | 11mm | 63mm |
| Nikon 19mm f4 PC-Nikkor | 12mm | 12mm | 65mm |
| Nikon 24mm PC-E | 11.5mm | 11.5mm | 63mm |
| Nikon 28mm f3.5 PC | 11mm (not advised beyond 8mm) | 11mm | 57mm |
| Nikon 28mm f4 PC | 11mm (not advised beyond 8mm) | 11mm | 57mm |
| Nikon 85mm f2.8 PC-Nikkor | 12mm | 12mm | 65mm |
| Olympus 24mm f3.5 Shift | 8mm | 10mm | 57mm |
| Olympus 35mm f2.8 Shift | 10mm | 10mm | 61mm |
| Pentax K 28mm f3.5 Shift | 11mm | 11mm | 63mm |
| Samyang 24mm TS | 12mm | 12mm | 65mm |
| Schneider Super-Angulon 28mm | 11mm (not advised beyond 9mm) | 11mm | 59mm |
Having established the probable sizes of actual image circles, we can predict their behaviour on Fujifilm GFX series bodies. Lenses like the later Canon TS-E models perform well on 35mm partly by virtue of improved correction, but also because of significantly larger image circles. A shift lens capable of a vignette-free 12mm horizontal shift on a 36x24mm sensor is capable of 15mm vertical shift and is only constrained from doing so by the limitations of the camera body. Liberated into the 65mm throat of a Fujifilm GFX body, the full potential of its 65mm dimater image circle can now be seen.
The 43.8mm x 32.9mm sensor of the GFX requires an image circle of approximately 55mm. Therefore, even 35mm tilt/shift lenses contraindicated for use above 7-8mm of horizontal shift will ‘cover’ a GFX sensor – whatever that means. ‘Coverage’ doesn’t guarantee freedom from vignetting, or chromatic and spherical aberration. In fact few tilt/shift lenses made before 2000 were serviceable with respect to Zone D performance, especially at wide apertures.
However, recent designs – notably the Mark II Canon 24TS/E, all the final Canon EF Macro tilt/shift lenses and (with some caveats) the Samyang 24mm T/S – threw 65mm image circles, and are useable across a GFX-recorded frame: often very sharp in Zones A-C. Furthermore, the compact proportions of the GFX’ 3:2 sensor (theoretically) permits such lenses vertical shift movements of up to 7mm and horizontal movements of almost 5mm – though, again, even a heavily stopped-down 35mm tilt/shift lens will not perform optimally under the scrutiny of a 100MP sensor this far into Zone D.
Also note that even the best of the 35mm tilt/shift lenses do not project image circles large enough to cover medium format (645) film bodies. Conversely, lenses designed to cover 645 offer greater potential for movements when adapted to 35mm and Fujifilm GFX.
Example 4: Using Medium Format (645) Lenses on 35mm Camera Bodies
See above. Even the smallest medium format lenses have image circles of at least 70mm in diameter. Depending on the throat diameter of the camera, horizontal and vertical shift movements of at least 15mm are possible. In this application, the mechanical light path typically becomes an obstruction before the limits of the lens are tested.
Example 5: Using Medium Format (645) Lenses on Fujifilm GFX
The 70mm image circle of a medium format lens covering 645 film (56x42mm) is sufficient to provide just over 10mm of vertical shift and 9mm of horizontal shift.
Example 6: Using a Lens with a 80mm Image Circle on Fujifilm GFX
Fujifilm’s 30mm Tilt/Shift lens has an image circle that may approach 80mm in diameter, given the current trend for in-built image circle redundancy to ensure the lens renders acceptably across its useable projection. But many other lenses also throw image circles of this size, large enough to provide 14mm of horizontal shift and 17mm of vertical shift – including enlarger lenses of c.90mm focal length, 35mm cine projection lenses longer than 90mm designed for 35mm film, most 70mm cine projection lenses, some lenses designed for medium format slide projection, the Mamiya 645 50mm Shift lens, and all 6×7 and 6×9 medium format lenses such as the Pentax 67 and Mamiya RZ. The longer Hasseblad 6×6 lenses reach and sometimes exceed the 90mm requirement for movements on GFX, and all large format lenses qualify.
Choking on an image circle much above 80mm, the Fujifilm’s 65mm throat diameter makes application of larger and longer lenses unnecessary (and unworkable), when larger movements are needed. Indeed, many adaptors impose their own limitations before the Fujifilm body becomes an issue.
