why do they call fstop fast?

[BLD_007]

Why is low fstop called fast? the lens doesn't move, the shutter does.

or is there a part in the lens that moves during capture. other than zooming and your normal lens movement

 

[PhotoXopher]

It lets the light in fast; you can get a faster exposure at f/1.4 than you can at f/11.

 

[usayit]

well... technically light doesn't actually move faster... its constant.

Larger aperture (lower number, Av) requires less time (shutter, Tv) to achieve proper exposure.

 

[Dwig]

well... technically light doesn't actually move faster... its constant.

Larger aperture (lower number, Av) ...

equals larger f/stop.

The terms "fast" and "slow", when discussing a lens or a lens' maximum f/stop, date from times long ago (140+ years) when most photographs were taken at the lens' maximum aperture. The plates (this is before flexible plastic film) were so slow that exposures were very long (multiple seconds). The only practical exposure control was exposure time; you generally couldn't afford to stop a lens down as wide open still resulted in painfully long exposures. As wider aperture lenses were developed they were thought of as "fast lenses" because they produced a good exposure in lens time.

 

[BKMOOD]

Simply put...

Let's say your lens is a bucket. This bucket needs to be filled with water (light) before a picture is made. Water comes through the hole in your lens (the aperture).

If the opening of your bucket is small (say, f22 -- a coke bottle sized opening) it will take longer for the bucket to fill with water than if the whole was larger (say, f1.8 -- a jelly jar sized opening). So an f1.8 aperture is faster than a f22 aperture.

The wider the aperture, the bigger the hole, the faster water (and light) comes in.

 

[Dao]

When we take photo, we want a correct exposure.

To have the correct exposure, we need to control the aperture (the amount of light entering the camera at a given time), and the shutter speed (the time allow for the light to enter).

Let's keep the ISO constant.


To have a correct exposure of a full moon, we need aperture of F/11 and shutter speed of 1/250. However, 1/250 shutter speed may not be fast enough for take a photo with a long telephoto lens such as 300mm (assuming no IS/VR/VC/OS) without camera shake issue. In order to have a faster shutter speed, I can increase the size of the aperture (by decreasing the number) so => aperture F/8 and shutter speed 1/500

As you can see, we allow double the amount of light entering the camera (1 stop of light) at a given time. But at the same time, we cut the time that allow the light to enter in half. So technically, we have the same total amount of light hitting the film/sensor at the end.

So for the same exposure, the larger the aperture, the faster the shutter speed.

Let compare these 2 lenses.

Canon EF 50mm F/2.8 macro
Canon EF 50mm F/1.4

Example: Typical office area -> EV 8

EF 50mm F/2.8 macro
Max aperture is F/2.8, with EV8, I need shutter speed of 1/30 at F/2.8

EF 50mm F/1.4
Max aperture is F/1.4, with EV8, I need shutter speed of 1/125 at F/1.4

So which one has a faster shutter speed? Of course, it is the one with larger aperture (F/1.4) and you can say EF 50mm F/1.4 lens is faster than EF 50mm F/2.8 macro lens.



Edit: I found that the 50mm macro lens is a F/2.5, not F/2.8 .

 

[BLD_007]

so will we ever see a lens at like .5 or even .1?

 

[Dao]

Even if they can make a lens like that ... but it is not going to be practical because the DoF is so shadow that you cannot really able capture much.

 

[djacobox372]

so will we ever see a lens at like .5 or even .1?

likely not, the fastest lens ever made was a zeiss f0.7. Special production for nasa, and only a few of them were made. Later Stanley Kubrick used them in his movie "Barry Lyndon" to film under candle light. The DOF is so small in these shots that typically only one actors face is in focus at any time.

 

[LBPhotog]

The front piece of glass on that Zeiss f/0.7 had to have been HUGE, any idea what the focal length was?

 

[usayit]

Remember... it wasn't stills it was for motion picture

Untitled Document

 

[KmH]

Here is a f/0.95 lens:

Leica | 50mm f/0.95 Noctilux-M Aspherical Manual Focus | 11-602

 

[Derrel]

I believe this usage dates back to the latter part of the 19th centrury when the "Waterhouse stop" system was developed. Lenses did not contain iris diaphragms, but were regulated by metal plates, each of which had a round aperture drilled in it. The photographer would use a set of Waterhouse stops as a means to control depth of field and exposure,and exposure at the time was measured in seconds, not fractions of a second. A large Waterhouse stop would be a "fast stop", meaning the exposure time would be "fast", relatively speaking. If a 1.5 second or 2 second exposure was "fast", a more typical exposure would be along the lines of 5 to 30 seconds using a smaller stop.

Here is a web page showing a modern set of Waterhouse stops

Waterhouse Stops