Velocity Reviews > f value and shutter speed : what is the relation as a mathematical expression?

# f value and shutter speed : what is the relation as a mathematical expression?

n
Guest
Posts: n/a

 11-17-2003
I would like to understand how much faster my shutter speed could be
letting in the same amount of light with a faster lens.

For example, for a particular shot I used an f 2.8 lens and a shutter
speed of 1/50.

If I had had a different lens with an f value of 1.2 how long would
the shutter have needed to stay open to get the same light?

Am I right in guessing that an f 1.4 lens would need to stay open half
as long as an f 2.8?

What sort of lens would have an f1.0 value? Or an f0.5 value?

jam
Guest
Posts: n/a

 11-17-2003
You'll find the concepts and math you need at

f-number of a lens with the f-number of a given exposure. The former
describes the minimum f-number (maximum opening) the lens can achieve.
(Greater openings require larger lenses and higher costs for reasons
--
Jeremy McCreary
Denver, CO
-------------------------------------------

"n" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) om...
| I would like to understand how much faster my shutter speed could be
| letting in the same amount of light with a faster lens.
|
| For example, for a particular shot I used an f 2.8 lens and a
shutter
| speed of 1/50.
|
| If I had had a different lens with an f value of 1.2 how long would
| the shutter have needed to stay open to get the same light?
|
| Am I right in guessing that an f 1.4 lens would need to stay open
half
| as long as an f 2.8?
|
| What sort of lens would have an f1.0 value? Or an f0.5 value?
|

Tom Thackrey
Guest
Posts: n/a

 11-17-2003

On 17-Nov-2003, http://www.velocityreviews.com/forums/(E-Mail Removed) (n) wrote:

> I would like to understand how much faster my shutter speed could be
> letting in the same amount of light with a faster lens.
>
> For example, for a particular shot I used an f 2.8 lens and a shutter
> speed of 1/50.
>
> If I had had a different lens with an f value of 1.2 how long would
> the shutter have needed to stay open to get the same light?
>
> Am I right in guessing that an f 1.4 lens would need to stay open half
> as long as an f 2.8?
>
> What sort of lens would have an f1.0 value? Or an f0.5 value?
>

The f/stops represent a doubling of the amount of light for each full stop,
the stops are 1.0,1.4,2.0,2.8,4.0,5.6,8.0 ...
If your exposure is correct at 1/50sec and f/2.8, then going to f1.4 would
be doubling the exposure twice so, you would have to increase the shutter
speed to 1/200th (4x) to get the same total exposure.

f number is the ratio of the focal length of the lens to the size of the
aperture opening. That's why it's hard to get large apertures on long
lenses. So, an f/0.5 lens would have an opening twice it's focal length. A
50mm lens would have a 100mm aperture opening.

http://www.wildthingsphoto.com/tips/tip9807.htm

--
Tom Thackrey
www.creative-light.com
tom (at) creative (dash) light (dot) com
do NOT send email to (E-Mail Removed) (it's reserved for spammers)

MarkH
Guest
Posts: n/a

 11-17-2003
(E-Mail Removed) (n) wrote in
news:(E-Mail Removed) om:

> Am I right in guessing that an f 1.4 lens would need to stay open half
> as long as an f 2.8?

Quarter! The f number is a ratio of diameter of aperture to the length of
the lens, double the aperture gives 4x the light. Light gathering is
related to the area of the opening whereas the f number is the width of the
opening.

When you look at the f-stop numbers, the square of those numbers show why
each one lets in half as much light (not exact, there is some rounding):
f-stop Square of f-stop
1 1
1.4 2
2 4
2.8 8
4 16
5.6 32
8 64
11 128
16 256
22 512

I found that when I was trying to understand the f numbers the squaring of
those numbers helped me to make sense of it.

--
Mark Heyes (New Zealand)
See my pics at http://homepages.ihug.co.nz/~markh/
"There are 10 types of people, those that
understand binary and those that don't"

Alan Browne
Guest
Posts: n/a

 11-17-2003

n wrote:

> I would like to understand how much faster my shutter speed could be
> letting in the same amount of light with a faster lens.
>
> For example, for a particular shot I used an f 2.8 lens and a shutter
> speed of 1/50.
>
> If I had had a different lens with an f value of 1.2 how long would
> the shutter have needed to stay open to get the same light?

Look up reciprocity. Basically a stop on the lens is a doubling or
halving of light. A 'stop' of speed is a doubling or halving of time.
a 'stop' of sensitivity is a doubling or halving of ISO number.

So, for a given exposure (let's assume it is correct), a 2 stop change
in aperture that allows more light needs to be compensated reciprically
in speed or sensitivity or both by a total of 2 stops.

> Am I right in guessing that an f 1.4 lens would need to stay open half
> as long as an f 2.8?

f/2.8 to f/1.4 is two stops. So 1/4 the time on the shutter.

Full stops (rounded) are: f/1 f/1.4 f/2 f/2.8 f/4 f/5.6
f/8 f11 f/16 f/22 f/32 ...

>
> What sort of lens would have an f1.0 value? Or an f0.5 value?

There used to be a Canon f/1.0, but it has been discontinued, presumably
for very low benefit/price. The theoretical limit is (I believe) f/0.7
and at that the lens rear element would be in contact with the film/CCD
so no room for the mirror or shutter. The shutter would need to be in
the lens or in front of it.

Alan

--
e-meil: there's no such thing as a FreeLunch.

Alan Browne
Guest
Posts: n/a

 11-17-2003
Correction below ((CORR)).

Alan Browne wrote:

>
>
> n wrote:
>
>> I would like to understand how much faster my shutter speed could be
>> letting in the same amount of light with a faster lens.
>>
>> For example, for a particular shot I used an f 2.8 lens and a shutter
>> speed of 1/50.
>>
>> If I had had a different lens with an f value of 1.2 how long would
>> the shutter have needed to stay open to get the same light?

>
>
> Look up reciprocity. Basically a stop on the lens is a doubling or
> halving of light. A 'stop' of speed is a doubling or halving of time. a
> 'stop' of sensitivity is a doubling or halving of ISO number.
>
> So, for a given exposure (let's assume it is correct), a 2 stop change
> in aperture that allows more light needs to be compensated reciprically
> in speed or sensitivity or both by a total of 2 stops.
>
>> Am I right in guessing that an f 1.4 lens would need to stay open half
>> as long as an f 2.8?

>
>
> f/2.8 to f/1.4 is two stops. So 1/4 the time on the shutter.
>
> Full stops (rounded) are: f/1 f/1.4 f/2 f/2.8 f/4 f/5.6
> f/8 f11 f/16 f/22 f/32 ...
>
>>
>> What sort of lens would have an f1.0 value? Or an f0.5 value?

>
>
> There used to be a ((CORR)) Canon f/1.2 ((CORR)), but it has been discontinued, presumably
> for very low benefit/price. The theoretical limit is (I believe) f/0.7
> and at that the lens rear element would be in contact with the film/CCD
> so no room for the mirror or shutter. The shutter would need to be in
> the lens or in front of it.
>
> Alan
>

--
e-meil: there's no such thing as a FreeLunch.

Flux
Guest
Posts: n/a

 11-17-2003
MarkH wrote:

> (E-Mail Removed) (n) wrote in
> news:(E-Mail Removed) om:
>
>
>>Am I right in guessing that an f 1.4 lens would need to stay open half
>>as long as an f 2.8?

>
>
> Quarter! The f number is a ratio of diameter of aperture to the length of
> the lens, double the aperture gives 4x the light. Light gathering is
> related to the area of the opening whereas the f number is the width of the
> opening.
>
> When you look at the f-stop numbers, the square of those numbers show why
> each one lets in half as much light (not exact, there is some rounding):
> f-stop Square of f-stop
> 1 1
> 1.4 2
> 2 4
> 2.8 8
> 4 16
> 5.6 32
> 8 64
> 11 128
> 16 256
> 22 512
>
> I found that when I was trying to understand the f numbers the squaring of
> those numbers helped me to make sense of it.

It follows from this that each stop number is equal to the previous stop
multiplied by the square root of 2. This means that if you are at an
arbitrary aperture number (say, 4.5), to get double the exposure you'd have
to open up to 4.5/SQRT(2) = 3.2 (rounded).

This happens because the light intensity is directly proportional to the
area of the opening, and that is proportional to the diameter squared (and
the f number is the ratio between the focal length and the diameter).

Flux

Argon3
Guest
Posts: n/a

 11-17-2003
I seem to remember that Stanley Kubrick used lenses faster than f1.0 to film
"Barry Lindon". The scenes were lit by candlelight...at the time it was quite
a technical achievement.

argon

Ken'
Guest
Posts: n/a

 11-17-2003
I don't think there is such a thing.
Leica have a 50 MM F 1.0 but it is the only one as far as I know.
Ken'

"Argon3" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> I seem to remember that Stanley Kubrick used lenses faster than f1.0 to

film
> "Barry Lindon". The scenes were lit by candlelight...at the time it was

quite
> a technical achievement.
>
> argon

Phil Stripling
Guest
Posts: n/a

 11-17-2003
(E-Mail Removed) (Argon3) writes:

> I seem to remember that Stanley Kubrick used lenses faster than f1.0 to
> film "Barry Lindon". The scenes were lit by candlelight...at the time it
> was quite a technical achievement.

"Fortunately, I found just such a lens, one of a group of ten which Zeiss
speed of fO.7, and it was 100% faster than the fastest movie lens."
http://www.visual-memory.co.uk/amk/d...erview.bl.html

I recall an interview where he made a different statement about the source
of his cameras, but I can't find it. My recollection is that the cameras
were in existence at a studio, and he conned them out of them in some way.

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