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I've seen on Flickr that the photographers specify values for four settings about their photos (where exif data is available) -

  • Shutter speed
  • F number
  • ISO
  • Focal Length

An example can be found here.

I would like to know the aperture size (diameter), but they don't specify it. Knowing that the F/number is the ratio of the focal length on the aperture, why is it so important?

It appears that F/number is less precise than having both aperture and focal length (because it's impossible to know about the aperture and focal length if only F/number is provided). Plus, as it is a ratio, it's more complicated to understand than the two numbers it's based on.

So which information is more relevant, and why?

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4 Answers 4

up vote 14 down vote accepted

The f-stop is more directly relevant to photographers because it normalizes out the focal length. It then gives you a measure of how bright the image will be on the sensor relative to the scene brightness.

For example, if a scene is well exposed with a 50 mm lens at f/8 and 1/200 second, then it will be well exposed with any other lens at f/8 and 1/200 second. Therefore (ignoring secondary effects like light loss in the glass of the lens), f-stop, ISO, and shutter speed tell you the exposure. Focal length then tells you the image width angle, but exposure differences due to focal length have already been factored out by the f-stop value. That is why f-stop is used instead of raw aperture diameter.

Raw aperture diameter does matter for things like diffraction effects, but that's another secondary issue in most cases.

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Thanks ! That makes it clearer to me –  user1717735 Apr 15 at 13:06

Like a lot of things that seem odd today, the f stop is a historical tradition.

Early photographers, were manually setting their camera exposure, and in the very early days without the help of a light meter.

They needed a way of expressing the light passing capacity of a lens in a way that was portable (you didn't need to learn a new set of numbers for each lens), easy to remember/use (didn't want to have to do math every time they took a picture) and proportional to shutter speeds (so that you could easily trade off aperture for exposure time). The scheme that was ultimately adopted was what we know as the f stop.

So these f stops weren't metadata, they were (and for some cameras, still are today) part of the physical controls on the lens barrel used to control how much light struck the negative.

For a more complete overview of the history, see wikipedia:

http://en.wikipedia.org/wiki/F-number#Origins_of_relative_aperture

Looking at the article, it also seems that there is a difference between the effective aperture and the size of the physical aperture. This would be another reason to use a dimensionless ratio instead of a physical size.

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they were part of the physical controls on the lens barrel used to control how much light struck the negative. "Were"? :) Some people still use manual film cameras. –  dodgethesteamroller Apr 15 at 21:40
    
Certainly -- I still have a Nikon FM2 and used to be a huge darkroom snob. I'm sure as long as Leica is in business and view cameras are sold there will always be mechanical aperture rings. –  David Rouse Apr 16 at 12:50

Actually, when the aperture is stated as "f/#", that is the aperture diameter measurement, where f is the focal length and # is the f-number.

f-number = focal_length/aperture_diameter

So, a 50mm lens at f/4 has an aperture opening diameter of 12.5mm.

And a 100mm lens at f/4 has an aperture opening diameter of 25mm.

This is why we prefer using the f-number notation: f/4 gives you the same amount of light regardless of the focal length of the lens, while you'd have to remember that one stop down from 12.5mm diameter on a 50mm is 8.92mm, but the same change on a 100mm lens is stopping down from 25mm to 17.85mm with the same math for every possible focal length, rather than just going from f/4 to f/5.6.

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The original question is a bit unclear because the OP indicates wanting more information than provided, but not really why.

For the EXIF details that are provided:

  • F stop is important because it directly affects depth of field.

  • Focal length is important because it indicates how much magnification the lens imparts compared to actual (human eye) view.

  • Shutter speed indicates how much the motion was stopped, or allowed to blur.

  • ISO indicates the overall light sensitivity of the film or sensor, which constrains all other exposure factors.

Maximum aperture is interesting only as a measure of how fast (and expensive!) a lens is. Actual aperture is meaningless by itself (as in 1.25 inch aperture, without a focal length).

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1  
Welcome to Photography Stack Exchange! Could you elaborate on your last paragraph? I think that’s the kind of information that the OP was looking for. –  bdesham Apr 15 at 18:28

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