f-stop

PaulieC

I was pondering this last night...how much light in an f-stop?

magicbastarder

how do you mean? actual light energy?

Zillah

First of all you'll have to define what unit of measurement you intend on using.

Roen

I was taught that it was an arbitrary measurement in college. Bearing in mind each stop is a halving and a doubling of the light it would be kind of hard to quantify. For instance, half the amount of light at f 22 would be significantly smaller figure than half the amount of light at f 2.8

I'm not that maths crafty though so don't take me up on it.

Valentia

I think you are better talking in EV's rather than f-stops. Have a look at this.

PaulieC

I suppose I meant volume of light, if there is such a thing. I know about halving and doubling using shutterspeeds, aperture and ISO, but what is the amount you are actually doubling and halving? If it's an arbitrary measurement, how do you know if it is actually doubled or halved?

Wish I had never thought of this - my head is sore now.

magicbastarder

'volume of light' is just as dependent on shutter speed too; you'd have to qualify the volume with related time.

also, volume of light is also highly dependent on ambient light levels.

pete4130

An aperture of 2.8 lets in twice the amount of light compared to an aperture of 4.0 at a constant light intensity. It achieves this by reducing the area of the aperture in half (using the simple pi by the radius squared). Obviously if its really bright out and you switched from 2.8 to 4.0 and the the sun came from behind a cloud the intensity of the light going through the 4.0 aperture could be greater than when it was dull going through an aperture of 2.8.

The shutter speed merely determines how much time the light hits the film/sensor which is dependant on the light intensity and the aperture used.
to get correct exposure you need the same amount of light all the time for example and you use both the aperture and shutter speed to obtain this.

A simple (although quirky) analogy of this is to imagine you film/sensor as being a kitchen sink. The tap is the aperture. The water is the light.
To get a good exposure the sink needs to be full up. You have several options.

1) You can turn the tap on full (having a big aperture like 2.8) and the sink fills up very quickly (fast shutter speed like 1/1000th second for example).

2) You turn the tap on a little so it trickles (a small aperture like f22 for example) but it would take a longer time for the sink to fill up (1/15th second)

As the amount of light coming through the aperture gets halved, the amount of time that the shutter needs to stay open for will double in time....

- f2.8 - 1/1000th second

- f4.0 - 1/500th second

- f5.6 - 1/250th second

- f8.0 - 125th second

- f11 - 1/60th second

- f16 - 1/30th second

- f22 - 1/15th second

I hope this makes some sense and hasn't cause more confusion for anyone?

Pete.

Zillah

cambo_gueno wrote:

I suppose I meant volume of light, if there is such a thing.

Basically you'd need to measure it in terms of its energy. The net result has nothing at all to do with photography though

If theres any physicists around I'm sure they could explain all sorts of objective conventions for measuring intensity or "volume" of light.

mathias

And the light meter in your camera is the last variable in all this , that actually measures the light reflecting off your subject ,

So for a given aperture , you focus your camera ( take a light reading ) and adjust your speed until the meter in your camera indicates a correct exposure ,

Likewise for a given speed , you focus , and adjust your aperature until you have a correct exposure , in all cases the camera is going to take a light reading from the subject and adjust accordingly.

If theres any physicists around I'm sure they could explain all sorts of objective conventions for measuring intensity or "volume" of light.

There are all sorts of measurements for light , lumens , candlepower , intensity , brightness , and none of them belong here really , it would be a very long post with no relevant information to photography,

The Best book Ive ever read on how a camera works is this one , its cheap and answers all the above questions , I cannot recommend this book enough ,

http://www.amazon.com/Understanding-Exposure-Photographs-Digital-Updated/dp/0817463003/ref=pd_bbs_sr_1/104-7687309-2581545?ie=UTF8&s=books&qid=1177509169&sr=8-1

Also for the basics of light metering in cameras only , check here ,

http://www.cambridgeincolour.com/tutorials/camera-metering.htm ,

Valentia

I suppose that for every combination of f-stop, shutter speed and ISO there must be the same "volume" of light that exposes properly at that combination. That's where I think EV values come in. Each EV value is the same "volume" of light regardless.

I don't know of any other name for it but EV works for me, I think :rolleyes:

magicbastarder

Valentia wrote:

Each EV value is the same "volume" of light regardless.

for a given level of ambient light, to be pedantic.

PaulieC

Valentia wrote:

I suppose that for every combination of f-stop, shutter speed and ISO there must be the same "volume" of light that exposes properly at that combination.

Yes, this is what I meant, but was so unable to express adequately, thanks.

I guess Borg would be the only person able to tell us.

mathias

For the physics basics ,

http://www.aelgroup.com/how%20to%20measure%20light.pdf


Camera light meters are usually set up for something like 10 -18% of middle grey reflectance , your exposure ( depending on whether your camera is set to evaluative , spot , or partial etc ) will set the exposure on whether its more or less than this figure.

Naturally this will depend on your lens , lighting conditions etc.

Valentia

magicbastarder wrote:

for a given level of ambient light, to be pedantic.

The light doesn't need to be ambient, to be even more pedantic. :eek:

Zillah

Valentia wrote:

I suppose that for every combination of f-stop, shutter speed and ISO there must be the same "volume" of light that exposes properly at that combination. That's where I think EV values come in. Each EV value is the same "volume" of light regardless.

Continuing the trend of being pedantic, ISO has nothing to do with how much light enters the camera, it determines how the sensor/film reacts to the amount of light that enters the camera

Valentia

Zillah wrote:

Continuing the trend of being pedantic, ISO has nothing to do with how much light enters the camera, it determines how the sensor/film reacts to the amount of light that enters the camera

I know that But you can't have an EV value without taking the ISO value as part of the equation. The EV value is based on 100 ISO. Again, I think.

mathias

I suppose that for every combination of f-stop, shutter speed and ISO there must be the same "volume" of light that exposes properly at that combination. That's where I think EV values come in. Each EV value is the same "volume" of light regardless.

Just to point out also ,
that although there may be 6-8 combinations of settings that give you the same EV value or "correct exposure " , that is far from saying that they will all produce the same picture , they most definitely will not.

For any given EV value , the widest aperture will have the shallowest depth of field , great for macro , and the smallest aperture will have the deepest depth of field , great for landscapes.

Likewise , for the same EV value , the fastest shutter speed will freeze action , good for sports , whereas the slowest will blur , great for waterfalls etc.

Zillah

mathias wrote:

and the smallest aperture will have the deepest depth of field , great for landscapes.

Does it matter really? I'd have thought most of the shot would be at infinity anyway.

mathias

Depends on the shot your taking , most eye catching landscapes have some foreground interest , and if you want that in focus as well as everything else then a nice small aperture helps !!

magicbastarder

if everything is essentially at infinity, f8 or thereabouts is usually your best bet, quality wise.

minikin

not true, the max resolving power (measured in lpmm) varies from lens to lens depending on focal length, quality and amount of elements etc

to answer the original question:
one stop = two heaped tablespoons of light.

Roen

And no lens will ever perform at it's best when focused to infinity.