Film Speed/ShutterSpeed/F stop

[paragon]

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Please excuse this question, but

 

Is 2.8 twice 5.6 and if so - Is there any reason to believe that 2.8 @1/125 with a 200ASA film is NOT the same as 5.6 @ 1/125 with a 400ASA film - if you see what i mean - or is it all a lot more complicated?

 

(apart from DOF and grain - that is)

[andybarton]

Those two scenario will give you the same exposure.

 

The halving of the light hitting the film due to the smaller aperture is compensated for by the doubling of the film speed.

[paragon]

Andy

 

That's what I thought but taking the first point

 

The aperture at 1.4 on my Nikor S 5cm/1.4 looks a lot bigger than twice 2.8

 

The same looking at the f2.8 verus f5.6

[stunsworth]

Please excuse this question, but

 

Is 2.8 twice 5.6 and if so - Is there any reason to believe that 2.8 @1/125 with a 200ASA film is NOT the same as 5.6 @ 1/125 with a 400ASA film)

 

No they're not the same, Andy's wrong I'm afraid. If you shoot at f2.8 with a 200 film the aperture will be f4 when you shoot a 400 film with the same shutter speed.

 

Doubling the aperture number lets in a quarter of the light, not half.

[jfgilbert]

Aperture (f stop) is the ratio of the focal length to the diameter of the diaphragm. The amount of light captured is proportional to the surface of the opening, so it is proportional to the square of the diameter of the opening. That's why f stops are graduated in multiples of the square root of 2, i.e. 1.4. Each f-stop divides the diameter by 1.4, thus the surface by 2, and the amount of light by 2.

So, 2 to 2.8 = half the light. 2.8 to 4 = half the light again, etc.

Equivalent exposures for the same speed would be f2 at ISO 100 or f2.8 at ISO 200 or f4 at ISO 400, etc.

Or, for the same ISO, f2.8 at 1/500s = f4 at 1/250s = f5.6 at 1/125th, etc.

Hope that answers your question.

Jacques

[budrichard]

To make it simple, you need to memorise the f stops, 1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, 22, 32. 64 if I remember correctly. Each change is either 1/2 the light or 2 twice the light depending on whether you open or close the diaphragm. Shutter speeds are usually marked to give either 1/2 or twice the exposure depending on whether you set a slower or faster speed. Once you know one correct exposure, the rest is easy.

 

Your example of 2.8 at 1/125 with ASA 200, would translate to 4.0 at 1/125 for ASA 400 because the film is twice as sensitive, therefore you need half the light or 1 stop down, i.e. f4.0. -Dick

[paragon]

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To make it simple, you need to memorise the f stops, 1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, 22, 32. 64 if I remember correctly. Each change is either 1/2 the light or 2 twice the light depending on whether you open or close the diaphragm. Shutter speeds are usually marked to give either 1/2 or twice the exposure depending on whether you set a slower or faster speed. Once you know one correct exposure, the rest is easy.

 

Your example of 2.8 at 1/125 with ASA 200, would translate to 4.0 at 1/125 for ASA 400 because the film is twice as sensitive, therefore you need half the light or 1 stop down, i.e. f4.0. -Dick

 

Thanks Dick - that's easier

 

BUT thanks Jacques for the more technical stuff

 

"throwing the meter out" is taking me back to the 70s