About FF lens coverage

[ProfHankD]

With a fair number of 44x33mm cameras now getting into the consumer price range, and the extremely cheap option of capturing 48x36mm three-image-stitches using the Budgie Leica-M-to-Sony-FE shift adapter, coverage circle is becoming relevant even for designed-for-FF lenses. Most lenses cover somewhat larger diameter circles than the diagonal of the format for which they were designed, and using FF lenses for larger formats offers both cost advantages and a much wider range of lens options.

Some relevant frame diagonals (rounded). Formats a FF lens might cover are shown in bold:

• 43.3mm for 36x24mm (i.e., 135 film or FF sensor)
• 46.7mm for a 33x33mm square crop of a 44x33mm sensor
• 50.5mm for a 44x24.8mm 16:9 aspect crop of a 44x33mm sensor
• 50.9mm for a 36x36mm square crop of 48x36mm (i.e., Budgie)
• 52.9mm for a 44.29.3mm 3:2 aspect crop of a 44x33mm sensor
• 55mm for 44x33mm sensor
• 57.7mm for a 48x32mm 3:2 aspect crop of 48x36mm (i.e., Budgie)
• 60mm for 48x36mm (i.e., Budgie)
• 69.4mm for 56x41mm (i.e., 645 film)
• 79.2mm for 56x56mm (i.e., 6x6cm or 2 1/4" film)

Measuring coverage is difficult because it depends on focus distance and aperture, but also because shedding light on an area isn't the same thing as saying there is high-quality image detail painted there. There is also the fact that tiny amounts of decentering could cause a change in the usable coverage circle, so even apparently identical lens copies might vary significantly.

I am currently using Budgie on a Sony A7RII to measure coverage circle, which might also introduce some imprecision. I shoot a frame with the lens maximally offset on Budgie, which means one edge is in the center, and then I simply measure the pixel distance from the center of that edge to where the image circle intersects the frame edges -- which should average out to the radius (correcting for minor decentering). Since the A7RII resolves approximately 7952 pixels in 36mm, I simply multiply the sum of the two radius measures by 36/7952 to get the coverage circle diameter in mm, and I round that to the nearest integer value. I can't measure coverages greater than 60mm this way, but almost no FF lenses quite make it to 60mm (at non-close-up focus distances).

So, I've started adding coverage estimates to the summaries on my lenses page. Here are the lenses I've made entries for thus far, all of which happen to have Minolta SR/MC/MD mount (because that's the M adapter I happened to have on Budgie):

• (M44,H47) Vivitar 19mm f/3.8
• (M46,H48) Vivitar 24mm f/2
• (S48,H52) Vivitar 28mm f/2.0 Close Focus Wide
• (M44,H45) Auto Promura MC 28mm f/2
• (S49,M52) Minolta MC W Rokkor Si 28mm f/2.5
• (S51,M57) Minolta MD 28mm f/2.8
• (S52,M54) Starblitz Auto Macro 28mm f/2.8
• (S55,H57) Minolta MD Rokkor-X 45mm f/2
• (H48,H49) Minolta MC Rokkor-X PG 50mm f/1.4
• (M54,H56) Minolta MC Rokkor-X PF 50mm f/1.7
• (M53,H55) Minolta MD 50mm f/1.7
• (S55,H57) Minolta MD 50mm f/2
• (S54,H55) Minolta MC Macro Rokkor-X QF f/3.5
• (H49,H47) Minolta MC Rokkor-PG 58mm f/1.2
• (H50,H51) Vivitar Macro 100mm f/3.5
• (H48,H49) Minolta MD Macro Rokkor-X 100mm f/4
• (H51,H49) Minolta MD Tele Rokkor-X 135mm f/2.8
• (H51,H49) Minolta MD 135mm f/3.5

Each estimate is in the form of a pair of numbers each prefixed by an H, M, or S. For example, (S46,M48) means that wide open this lens very Softly tapers off to a 46mm diameter coverage and stopped down to the smallest aperture (largest f/number) it has a Moderately sharp approach to vignetting at 48mm diameter coverage. A lens that very suddenly and sharply vignettes, showing a clear line at the coverage edge, will be designated as Hard vignetting. Of course, these numbers are approximate and should be taken only as "ballpark" estimates; use them at your own risk.

It's also worth noting that 1.2X teleconverters were very commonly marketed as "glass adapters" to allow infinity focus when adapting a FF lens in a longer mount to a shorter mount. A 1.2X teleconverter that does not itself vignette should extend the coverage circle of a FF lens by about 1.2X. That means a nominal 43.3mm coverage would turn into 52mm, and any FF lens that can natively cover 45.8mm would be able to cover 44x33mm.

 

[JimKasson]

Thanks for that, Hank.

 

[dbateman]

Interesting.

I have the Minolta 45mm f2 and it does have broad coverage on the GFX camera.

The Minolta MC 58mm f1.4 also has a much larger image circle than the 58mm f1.2 lens.

The Minolta 55mm f1.7 lens also has a large image circle, similar or larger than the 45mm f2.

 

[ProfHankD]

The earlier versions of Budgie had a thicker wall on the FE-mount part. However, there's now a 20230505 version which thins the thickness of the FE-mount walls to the bare minimum that still appears to be structurally sound -- making the clear central region 4mm larger and chamfering the edges for 1mm as well. This was done because the width of the FE-mount itself is a potential cause for vignetting, so a thinner wall might vignette less. Apparently, for some lenses it does make a difference...

• (S51,M57) Minolta MD 28mm f/2.8

Here's a (heavily compressed) shot at f/11 using the new 20230505 version of Budgie:

View attachment d8d5c8d84eff4042ae947a704911bbad.jpg

This is showing M59 vignetting rather than M57, so the FE-mount itself was causing a little bit of the vignetting for that lens. I suspect a Nikon Z body version would allow this lens to reach the full M60 needed to cover 48x36mm... but I've not yet designed a Z version (and probably will not unless somebody gives me a Z-mount body). It is not possible to make a version hitting infinity focus for Canon's RF-mount or L-mount; there isn't enough clearance to make an M-mount shiftable with the same range of movement.

Here's what the new Budgie looks like:


Budgie 20230505

Notice that the top edge of Budgie is now also changed so that there is more clearance with the body grip and it is much easier to press the FE-mount release button with Budgie mounted. Earlier versions were particularly difficult to remove once mounted because Budgie partly extended over the release button. This change was made possible by replacing the relatively heavy follower pin at the top with a smaller sliding track. In theory, the new design is weaker, but it doesn't yet seem to be a problem. The new design does require a bit of more precise welding in assembly.

 

[ProfHankD]

• (M44,H47) Vivitar 19mm f/3.8
• (M46,H48) Vivitar 24mm f/2
• (S48,H52) Vivitar 28mm f/2.0 Close Focus Wide
• (M44,H45) Auto Promura MC 28mm f/2
• (S49,M52) Minolta MC W Rokkor Si 28mm f/2.5
• (S51,M57) Minolta MD 28mm f/2.8
• (S52,M54) Starblitz Auto Macro 28mm f/2.8
• (S55,H57) Minolta MD Rokkor-X 45mm f/2
• (H48,H49) Minolta MC Rokkor-X PG 50mm f/1.4
• (M54,H56) Minolta MC Rokkor-X PF 50mm f/1.7
• (M53,H55) Minolta MD 50mm f/1.7
• (S55,H57) Minolta MD 50mm f/2
• (S54,H55) Minolta MC Macro Rokkor-X QF f/3.5
• (H49,H47) Minolta MC Rokkor-PG 58mm f/1.2
• (H50,H51) Vivitar Macro 100mm f/3.5
• (H48,H49) Minolta MD Macro Rokkor-X 100mm f/4
• (H51,H49) Minolta MD Tele Rokkor-X 135mm f/2.8
• (H51,H49) Minolta MD 135mm f/3.5

One more Minolta I forgot:

• @35(H44,H44) @70(M57,H50) Minolta MD 35-70mm f/3.5

Those were all Minolta SR/MC/MD; now some Canon FL/FD/FDn on the 20230505 version of Budgie:

• (H44,H45) Vivitar 20mm f/3.8
• (M54,H54) Canon FD 24mm f/2.8
• @24(H41,H43) @40(H56,H54) Sun 24-40mm f/3.5
• (M52,M57) Canon FDn 28mm f/2.8
• (H49,H52) SIgma Mini Wide II 28mm f/2.8
• (M51,H56) Canon FD 28mm f/3.5
• (H51,H53) Vivitar 35mm f/1.9
• (M52,H56) Canon FDn 35mm f/2
• (M53,H58) Canon FDn 35mm f/2.8
• @35(H44,H47) @70(M60,H46) Canon FDn 35-70mm f/3.5-4.5
• (H48,H48) Canon FD 50mm f/1.4 "chrome nose"
• (H52,H54) Canon FD 50mm f/1.8
• (H52,H54) Canon FDn 50mm f/1.8
• (M56,H56) Macro Canon FL 50mm f/3.5
• (H47,H47) Canon FL 55mm f/1.2
• (H51,H48) Tamron SP 52B 90mm f/2.5
• (H49,H48) Canon FDn 135mm f/3.5

The above includes the first one to not clear the standard full-frame format: the Sun 24-40mm really does give dark corners wide open at 24mm on FF.

Note that I have not tested all my lenses in these mounts, just many of the ones that would be most desirable if they did cover a larger format. Longer lenses are not as interesting because there are plenty of those natively covering more than 60mm circles.

 

[RhonaldJR]

Thanks for this list. I am on the fence about the FF lens on MF. This gives a good starting point in the near future.

Regards,

Rhonald