Next silly question

OK now on to lens questions .

I keep hearing folk talking about 'fast glass' .

Sooooooo - just what is 'fast glass' ?

Now remember - for me you have to make it simple and non-geeky

The next idiot question will come once this bit of nomenclature is in my head

Space Shuttle Windows.

John

Hmmm. I think its lenses that have large apertures, F1.7, F1.4, F1.2 etc. Allowing you to collect more light and shoot with a faster shutter speed for the conditions.

Small apertures leave you with a slow shutter speed and more likelihood of shake/blue etc.

(Most lenses on the earth's surface are fast at aprox 1000 mph.)

No no no, is not about speedy travelling at all, it about drinkning! A 'fast glass' is the European equivalent of a quick pint

Seriously now, it's about how large the aperture of a lens is. The bigger the aperture (i.e. smaller F number) the 'faster' the lens. Hence F1.2 is faster than F2.8. The term might be due to the fact a larger aperture allows smaller exposure time, so the lens is it's quicker / faster.

Lucian

If you want to change systems, Leica Noctilux 50mm f/0.95, Voigtlander 50mm f1.1 Nokton.

The fastest pentax is the F1.2 50mm (I'm sure somebody will correct if wrong).

OK, now I need a lesson. Just what is the largest aperture out there?
I don't really understand the aperture number system, and was wondering if it's possible to better a Leica f0.95, as stated above.
Regards

Stanley Kubrick famously adapted a Zeiss f0.7 lens to use on a film camera when filming the candlelit scenes in Barry Lydon. The lens was apparently one of 3 specially made for NASA.

There's a list here.

Well.. in absolute terms the largest apertures are on big long telephotos. The f number is a relative aperture (which is much more useful to know when calculating exposure) and is the ratio of the focal length to the actual aperture as seen when looking through the front of the lens. Thus a 50 mm f2.0 lens will appear to have an aperture of 25mm when looked at from the front.

[geek mode on] Strictly speaking the aperture is f1:2.0, but the 1: is usually ommited. However this sometimes creates confusion when a 50mm f1:2 is listed on ebay as a 50mm f1.2 - the former been common as muck and cheap as chips, the latter being very 'fast glass' and not quite a hens tooth but still quite expensive. [geek mode off]

So for example the Pentax 500mm f4.5 has an apparent aperture from the front of about 110mm. This is, of course bigger than the actual lens mount, but the lens 'concentrates' the light down so it can get through that space.

According to Wikepedia, the fastest lens made was a Carl Zeiss 50mm f0.7

[edit - beaten to it by Tim]

hmm what do you mean by looked at from the front? im always wondering whats the ratio 1:2 is all about..is it the ratio of the front and rear glass? (that was my assumption)

When you actualy look at the lens from the front, with the aperture wide open, the actual aperture is the diameter of the 'hole' you can see. With standard and telephoto prime lenses this is often the same as the diameter of the front element. The f number of the lens is the focal length of the lens divided by this diameter.

Thus my nice litle SMC M 135 3.5 has a front element about 38mm in diameter 135/38 = more or less 3.5, My slightly scruffy SMC M 28 2.8 is harder for me to measure, because the hole is smaller than the diameter of the front element, but it sure looks to be about 10mm. (28/10=2.8 ). My diminutive SMC M 40 2.8 is easy to measure at 14mm - 40/14 = about 2.8. The fastest lens I have with me at present is my SMC M 50/1.7 which has maximum aperture of 33mm. 50/33=1.7 (more or less). I would quote my SMC 17 f4 fisheye but that would only invite envy...

PS to see the aperture wide open on a lens without an aperture ring, just slide the sprung lever on the rear of the lens clockwise.

ah, thank you for the information Tony.
so a 50mm 1:0.7 have a diameter bigger than the focal length = 71mm ? wow

I thought that fast glass was a specific type of glass that had photon acceleration properties. That allowed shooting to happen a shorter shutter speed - for example with normal glass a speed of 1/125 may be required but the same scene, light level and aperture using a lens with Fast PA Glass would only need 1/200 second. It helps reduce blur and shake and improves low light performance.

OK - since this has now got rather further than I think I'm ready for - let me back track and summarise using my lenses

18-55 DA L fstop 3.5 to 5.6

50-200 D-Xenon fstop 4 to 5.6

40mm Ltd fstop 2.8

So the fastest lens I possess is the 40mm Ltd [ which since it's a different class of lens doesn't really surprise me I have to say] .

Next job is to try and take the rest in and make sense of all that information from Amilner .

You only need to know that f8 or any other aperture is the same on every lens, regardless of the size of lens elements it needs to achieve that.

Zoom lenses are often variable aperture because the focal length chjanges, but the size of the glass doesn't. A 100mm lens has to be a lot bigger than a 50mm lens to achieve an f2 aperture, for example.

John - you said


Yes that makes sense since it's the 'hole' made as the 'blades' open that is the actual aperture through which light enters .

I'm still chewing over the next bit