Impossibly fast 50mm?

[Ko.Fe.]

And as Reddesert pointed out, even if the theoretical DOF is sufficient for the purpose, still it must be at all possible to achieve it by correct focusing.

Yep, portrait where only one eyelash is in focus and rest is in bokeh is very much possible.

 

[Dan Fromm]

But there is this lens
https://petapixel.com/2013/08/06/carl-zeiss-super-q-gigantar-40mm-f0-33-the-fastest-lens-ever-made/

Chan, that was my late friend and neighbor Charlie Barringer's lens. It was a joke. f/0.33 is impossible in air. The largest maximum aperture possible in air is f/0.50, as was mentioned earlier in this discussion.

If you want faster than f/0.50, you'll need an immersion objective as used in high magnification microscopes.

 

[AgX]

Well, there are good and bad jokes. And expensive jokes...

 

[miha]

Chan, that was my late friend and neighbor Charlie Barringer's lens. It was a joke. f/0.33 is impossible in air. The largest maximum aperture possible in air is f/0.50, as was mentioned earlier in this discussion.

If you want faster than f/0.50, you'll need an immersion objective as used in high magnification microscopes.

I dont think immersion oil (with 100x microscope objectives or similar) has anything to do with the lens f-stop.

 

[_T_]

Your thread title says it all. A 50mm lens with a 100mm aperture provides a paraxial f/# of 0.5, but the working f/# is significantly slower due to cos^4 law roll-off even on-axis, where f/# is usually specified. At f/0.5 the on-axis marginal rays intercept the image plane at 45 degrees, at which angle relative illumination is down 25% for the marginal rays due to cos^4 law. Quick and dirty estimation tells me that, on-axis, the effective speed is thus actually around f/0.9-f/1.0, but with exponentially worse image quality compared to an actual f/0.9 lens due to aberration dependency on f/#. The same rough calculation says Kubrick’s f/0.7 lens was effectively f/0.85 or so (marginal ray illumination is down to about 47% on-axis)

Off-axis it’s even worse, as at some point the ray angles exceed Brewster’s angle (about 54 degrees for digital camera cover plates, 56.5 degrees for gelatin) and simply reflect off the imaging media, where they then bounce around inside the camera and then contribute to veiling glare and loss of contrast. Effective f/# off-axis would drop to f/4 or less even without vignetting.

An *effective* f/0.5 cannot be achieved, as Brewster angle and cos^4 limits effective f/# to 0.7 or so..it’s an asymptotic approach iirc.

So there you go. Note this limit has nothing to do with technology to fabricate / assemble, practicality of using a lens, nor even image quality. Technology and practically do not limit design work. Laws of physics, however, cannot be broken and create a true limit.

Once you can follow what I wrote, you will no longer have to say you don’t understand why they don’t make a 50mm f/0.5 lens.

-Jason

So there's really no reason to bother trying to make a lens with an aperture that large. At some point making the aperture wider begins to decrease the brightness and contrast of the image it produces and until it becomes less effective than making a similar lens with a smaller aperture.

 

[Dan Fromm]

I dont think immersion oil (with 100x microscope objectives or similar) has anything to do with the lens f-stop.

miha, the medium's refractive index limits the maximum aperture possible. That's why immersion objectives, with oil between the lens' front surface and the subject, can have apertures greater than f/0.5 and why lenses with air between lens and subject can't.

 

[Nodda Duma]

So there's really no reason to bother trying to make a lens with an aperture that large. At some point making the aperture wider begins to decrease the brightness and contrast of the image it produces and until it becomes less effective than making a similar lens with a smaller aperture.

f/#, to be precise, but yes aside from special cases such as the example Dan mentioned above (oil immersion objectives).

Large apertures — “light buckets” in astronomy parlance — naturally have a role to play... note that aperture and f/# are two related but different parameters.

 

[miha]

miha, the medium's refractive index limits the maximum aperture possible. That's why immersion objectives, with oil between the lens' front surface and the subject, can have apertures greater than f/0.5 and why lenses with air between lens and subject can't.

I understand that oil has a similar refractive index to the glass lens it touches however this has to with NA (numerical aperture), not f-stop. NA is an indicator of the resolution.

 

[tomkatf]

If memory still serves me, the Canon 0.9 lens did not have a very good reputation and was not a market success because pics not that good and lens just too large to be practical. I haven’t seen one in person since about 1970. High prices most likely due to collectors’ interest rather than photographers’.
However, I found this thread fascinating. Kudos to all contributors!

No kidding... these guys are just too damned smart!

 

[Dan Fromm]

I understand that oil has a similar refractive index to the glass lens it touches however this has to with NA (numerical aperture), not f-stop. NA is an indicator of the resolution.

miha, numerical aperture = 1/(2*f/number)

And the oil's refractive index doesn't have to match the glass' RI.

 

[Nodda Duma]

To expand on what Dan states above, Numerical aperture and f/# both describe the same thing: marginal ray angle into the image plane. Use of one or the other depends on convention.

 

[miha]

miha, numerical aperture = 1/(2*f/number)

I see. Thanks for the explanation Dan.

 

[k.hendrik]

what were you hoping to shoot with it, portraits? still lives? landscapes ? there are ways of achieving selective focus and bokeh&c without a super fast lens like that, that don't cost 12grand+...
have fun !
John

like this one: Rolleiflex sl66se. With lens up very selective focus
https://www.flickr.com/photos/77846450@N00/49426791272/in/dateposted-public/

for sale for half a 12 grand

 

[reddesert]

Here's an example of what can be done if you have a large budget, an optical shop, and two mad-genius optical designers: https://ui.adsabs.harvard.edu/abs/1996AAS...188.5406E/abstract

We have produced optical designs for two new spectrographs currently under construction at UCO/Lick Observatory which will be used on the Keck II telescope. The first is DEIMOS, a dual-field areal spectrograph with a single on-axis reflecting collimator and two huge 15.0-inch f/1.29 wide-field broad-passband refracting cameras. The second is ESI, a single-beam prism cross-dispersed echellette spectrograph using an off-axis reflecting collimator with an even faster 12.1-inch f/1.07 broad-passband camera. ...

[The 15 and 12.1 inch numbers are the focal lengths.] These were designed for digital detectors and have a flat field, of diameter ~ 6" and 2.75" respectively, so about equal to covering a 4x5 film or 6x4.5 respectively. These use rather exotic optics, such as aspheres and enormous calcium fluoride lenses. The lens cells and mounting are of course both very large and very precise; these cameras are the size of beer kegs and weigh hundreds of pounds (not even counting the detector).

Even so, neither of these is faster than f/1. I don't know anything about the bokeh.

There are likely more exotic designs in the classified world but that's out of my league.

 

[jjphoto]

One person's uselessly shallow dof is another's holy grail. I'd love to have access to an F0.5 50mm lens but I know i could never afford it.

A lenses aberrations are what give it it's look or character so complaining about its optical issues is kind of missing the point. It might render very special images for portraits and fashion.

 

[neeksgeek]

I borrowed a friend’s 25mm f/0.95 Voigtlander (for digital micro four-thirds, a “normal” lens). I don’t believe shallower depth-of-field than that would be any use, it’s really quite shallow. But it was nice being able to shoot hand-held, at dusk, on a cloudy night, using ISO 100!

 

[Wallendo]

This would be the type of lens which would be hand assembled on demand and sold primarily to Sheikhs and other billionaires.

With the widespread availability of ultra high ISO sensors on digital cameras (I know this is an analog thread), I don't think there would be much of a market for this lens.

 

[jjphoto]

This would be the type of lens which would be hand assembled on demand and sold primarily to Sheikhs and other billionaires.

With the widespread availability of ultra high ISO sensors on digital cameras (I know this is an analog thread), I don't think there would be much of a market for this lens.

This kind of lens has nothing to do with providing a fast working aperture for low light, although this is an obvious use too, but its purpose would be its very shallow DOF. Anyone who doesn't appreciate the aesthetic possibilities of such things obviously wouldn't be in the market for one either. It's a bit like me being ignorant of the latest high end super telephoto lenses, which I am, because I am not their market either.

The dof of the 0.5/50 on a 35mm frame would be about the same as a 4.5/360 on an 8x10 when focused at 5 metres.

My calculations might be wrong so can some one please cross check, but the DOF of a 50mm lens at F0.5 and at 5 metres (a distance I chose because it's a typical distance for a full length portrait with such a focal length) would be approx 30cm (I had to extrapolate from other measurements so I appreciate that this might be wrong, but I doubt it). If you look at the DOF for an 8x10 camera using a 360mm F4.5 lens (a lens which is readily available and costs as little as approx USD250) the dof for the same distance is almost exactly the same at 0.32m (using https://www.dofmaster.com/dofjs.html ).

This type of dof, lens and camera (the 8x10) are quite desirable for many so I see no reason that a 50mm F0.5 lens wouldn't be as desirable and useful to its intended (potential) users.

Re price, well that depends on many things. If made by a Chinese maker it might be quite affordable but if made by Leica or Zeiss then only oligarchs need apply.

 

[reddesert]

The field of view of a 35mm camera with a 50mm lens at 5 meters (magnification = 1/100) is 2.4 x 3.6 meters, which is a bit large for a full length portrait IMO. One might want to use a fast 100mm and step back a little. Or step back a little more: Nikon actually makes a 200mm f/2 lens, B&H price US$5700.

To first order, the DOF is the same for the same entrance pupil and camera position, so a 360mm f/4.5 lens (entrance pupil 80mm diameter) on 8x10 would make a similar image to a 50mm f/0.63 lens on 35mm. However, people use 8x10 for reasons other than simply small DOF. Just getting a hypothetical super-speed lens won't instantly duplicate the 8x10 experience.

The thing that several people in this thread are trying to explain is that f/0.5 is a physical limit, not just a practical limit. In order to form an image that isn't simply a comatic blur, a lens has to have numerical aperture less than 1, and that means f-number greater than 0.5. Nodda Duma is an actual optical designer (unlike me - I'm a physicist with some lens design books), and he pointed out that even the hypothetical f/0.5 lens would have unusably great light falloff.

If people want to engage in speculation about making and using a 100mm f/1 lens, that's great. It would be more challenging than the 200mm f/2 lens so that gives a starting point for what it would take and cost.

 

[jjphoto]

The field of view of a 35mm camera with a 50mm lens at 5 meters (magnification = 1/100) is 2.4 x 3.6 meters, which is a bit large for a full length portrait IMO. One might want to use a fast 100mm and step back a little. Or step back a little more: Nikon actually makes a 200mm f/2 lens, B&H price US$5700
....
If people want to engage in speculation about making and using a 100mm f/1 lens, that's great. It would be more challenging than the 200mm f/2 lens so that gives a starting point for what it would take and cost.

As it happens, I've had a Leica Summicron-R 2/180 since the 1990s, and by the way, you often can't step back hence the practical use of fast 50's.

I do regularly shoot full length portraits with (about 6 different) F1.2 50-58mm lenses and find the approx 5m distance is a reasonable example based on personal experience. The exact distance doesn't matter anyway, as long as the same distance is used comparing 35mm and 8x10.

 

[Nodda Duma]

@jjphoto check back through the charts here.

https://www.photrio.com/forum/threads/charts-of-depth-of-field-vs-focal-length-scaling-f.169727/

I think your estimate is pretty close, and you might find the info useful.

cheers,
Jason

 

[jjphoto]

@jjphoto check back through the charts here.

https://www.photrio.com/forum/threads/charts-of-depth-of-field-vs-focal-length-scaling-f.169727/

I think your estimate is pretty close, and you might find the info useful.

cheers,
Jason

Thanks, very interesting.