Aperture Below 1.0 fstop?

[Severin Barenbold]

Hello all,

 

So a buddy and I have been trying to figure out how an aperture of 0.7 is possible, as we thought that an aperture of 1.0 is theoretically 100% of the possible light...

 

We think it has something to do with reflectors channeling additional light, but we dont really know.

 

We did a little look through the forum to see if the question hasn't already been answered but didn't find anything, so apologies if this has been tackled before.

 

Thanks.

sevi

[John Sprung]

The "f/" number is the focal length of the lens divided by the diameter of the aperture as it appears at the front element. The lower limits for various cameras are more practical and mechanical than theoretical. If you had a shutter very close to the film plane, and a really huge piece of glass, you might be able to reach f/0.5 or f/0.3. Maybe an old condenser lens from an enlarger could be mounted to do that. But, of course, the image would look like crap.

 

Kubrick had an f/0.7 lens made for "Barry Lyndon", but IIRC, they couldn't use it with a mirror shutter camera.

 

Because of the glass optical block in three chip video cameras, it's impossible to make a lens faster than f/1.45 for them. Again, that's a purely mechanical issue, the lens would have to be where the prism is to get any faster.

 

 

 

 

-- J.S.

[Brian Drysdale]

The lens with a f0.7 has been made, it was made by Zeiss for NASA. Stanley Kubrick used a couple to shoot the candle scenes in Barry Lyndon. http://www.visual-memory.co.uk/sk/ac/len/page1.htm

 

Some lenses have an aperture of fO.95.

 

The f number is a mathematical relationship between the entrance pupil and the focal length of the lens, rather than how much of the available light a lens can collect.

 

http://en.wikipedia.org/wiki/F-number

[K Borowski]

An F/stop is just the mathematical ratio between lens length and diameter of the aperture. (rounded metal opening in the lens).

 

A T-stop represents the actual transmission of light that enters the lens in comparison to the light that reaches the film plane.

Edited December 2, 2010 by K Borowski

[Vincent Sweeney]

Unless you are staying wider or doing city/landscapes, shooting actors that move with lenses which open up that far can be near impossible. Lets say you have a 50mm lens, which is common, and it opened up to .7 and your actor was 6 feet from you. This will give you something like 1.8" of acceptable focus range, depending on CoC (I'm averaging) but that's close generally. Ever tried doing that? A 35mm lens would give you about 3.5" but at a 2 it gives you about 10", the 50mm jumps to about 5" at a 2, at 6 feet.

 

It's hard enough just to keep someone in a sit-down interview sharp at 2.8 on a 50mm.

[Chris Keth]

There's a 25mm f0.7 switar for bolexes that are lovely lenses.

 

I think the fastest lens possible would be a solid hemisphere of glass with the flat side in contact with the film emulsion. The focal length would be quite short and the entrance pupil would be larger than the diameter of the glass itself because of the very wide angle of view. In fact, the angle of view would be so wide that normal means of measuring the entrance pupil don't really work any more.

[Chris Keth]

An F/stop is just the mathematical ratio between lens length and diameter of the aperture. (rounded metal opening in the lens).

 

A T-stop represents the actual transmission of light that enters the lens in comparison to the light that reaches the film plane.

 

I know you're a stickler for accuracy. An f-stop is not the ratio between focal length and aperture. It's the ratio between focal length and entrance pupil.

[John Sprung]

It's the ratio between focal length and entrance pupil.

 

Yes, and the entrance pupil is the diameter of the aperture as it appears at the front element.

 

 

 

 

-- J.S.

[Brian Drysdale]

Unless you are staying wider or doing city/landscapes, shooting actors that move with lenses which open up that far can be near impossible. Lets say you have a 50mm lens, which is common, and it opened up to .7 and your actor was 6 feet from you.

 

At f1.4 on a FF35 DSLR a lens could have a pretty similar DOF to f0.7 on 35mm motion picture, but this doesn't stop some people attempting it! How well it works is another matter.

 

On Barry Lyndon they used a video camera at 90 degrees to the film camera so that the distances could be marked out on a monitor screen.

Edited December 2, 2010 by Brian Drysdale

[Colum O Dwyer]

This is a great topic and something I've been wondering for quite a while! Thanks lads

[K Borowski]

I know you're a stickler for accuracy. An f-stop is not the ratio between focal length and aperture. It's the ratio between focal length and entrance pupil.

 

 

Woah, Chris, I am a stickler on accuracy only as it pertains to *other people.* :P I always forget if the formula is for the radius or diameter of the aperture; notice that I left that out. I used to always think that the focal length of the lens was the actual length of the physical lens barrel, too.

 

 

The root for the word "aperture," PERT, also is seen in words such as "portal." It's a fancy word for a hole from Latin. What value this has, is dubious at best, but you can either impress or annoy your friends with this knowledge.

[Chris Millar]

There's a 25mm f0.7 switar for bolexes that are lovely lenses.

 

Not that I know of ...

 

I just sold a f0.95 1" C-mount lens but the Switar is a f1.1 26mm

 

But in the grand scheme of things I do wonder what use it is of me pointing this out :rolleyes:

[Simon Wyss]

There's a 25mm f0.7 switar for bolexes that are lovely lenses.

That’d be new to me, too. There was the 10-element D-mount Switar 13-0.9 for 8 Millimeter cameras, five groups of two cemented elements each. Brass barrel.

[Leo Anthony Vale]

There's a 25mm f0.7 switar for bolexes that are lovely lenses.

 

There was a Century Precision 25mm f/0.78 C-mt.

It was a Japanese CCTV lens. They also had a 25mm f/0.95, which was also a japanese CCTV lens.

 

I have no idea about the picture quality, nor whether it could clear the prisms/mirrors of all

reflex C-mt. cameras.

[Mei Lewis]

...

On Barry Lyndon they used a video camera at 90 degrees to the film camera so that the distances could be marked out on a monitor screen.

 

I can;t figure out what that means. The video camera was poiting in a different direction?

[David Mullen ASC]

I can;t figure out what that means. The video camera was poiting in a different direction?

 

The video camera was watching the action from a right angle to the film camera so it could see the subject on one side of the video frame and the film camera lens on the other side of frame, with the video monitor screen marked up as to the distance between the two.

[Paul Bartok]

So technically you can go lower than F 1 but then does that mean T 1 would be 100% light? if the T relation is how much light is actually passing through the lens

[Simon Wyss]

You seem to be stuck with an absolute thinking. Who’d define what 100 % light is?

 

Please read all posts. It’s been said that f and T numbers are mathematical relations between two values. F 1.0 says that the effective aperture diameter is equal to the lens’ focal length such as 40 mm vs. 40 mm. The diameter of the iris diaphragm inside the lens measures 40 mm full open. From there you can stop down to higher ratios according to an internationally accepted series of values. The increments are the square root of 1, sq. r. of 2, sq. r. of 4, sq. r. of 8, and so on. Below 1 you have the inversion of these: sq. r. of ½, sq. r. of ¼, sq. r. of ⅛, and so forth.

 

Why is this so? Because when you alter the iris opening in linear fashion, the surface of the opening changes in the square function. Remember surface calculation?

[Chris Millar]

'T relation' is how much light passes through compared to the ideal amount as determined by an f-stop.

[Chris Millar]

I didn't see Simons reply - but yes, you're thinking in absolute terms where t-stop is more relative in nature