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F stops and aperture size


Ok, it's silly question time now. I know when someone answers my question below I am going to feel quite silly but at the moment, this is confusing me somewhat.

My understanding of f stop numbers is that they are a ratio of the lens focal length to the aperture diameter. Therefore, if this correct,a 90mm f2 lens @ f2 has an aperture opening of 45mm in diameter, a ratio of 1:2. The same calculations would see a 50mm lens at f2 with an aperture opening of 25mm, also a ratio of 1:2. This does not confuse me except that when alternating between lenses I notice that the shutter speed stays the same. If the aperture size is different, either 45mm or 25mm depending on the lens mounted, why doesn't the shutter speed change in accordance with the differing amount of light entering? My confusion arose after reading a Kodak Workshop Series book which seemed to contradict itself in two different paragraphs. If someone could clear this up for me I would greatly appreciate it. Thanks.



>Why exactly that happens I couldn't answer with any scientific depth, either. But independently from the focal length of the lens you cut the light going through in halve when you close down 1 stop. > Two lenses of different focal lengths theoretically always let through the same quantity of light if it set to the same f/stop, irregardly if you have the 45mm opening or the 25 mm one you mentioned in your ex&le. So you don't need to change aperture. However with longer lenses you must correct exposure a bit as they lose some light (too much glass in there I guess). If you have TTL the camera does it automatically. With hand metering you have to do it yourself.

Hope this brings some light on your question.


Basically in longer focal length lenses you have a wider opening in absolute terms but the effective distance from the film is further. If you double the focal length the light is a quarter strength by the time it reaches the film, so you need four times the aperture area to let the same amount of light in. That's why apertures are like they are (essentially based on the square root of 2 and multiples thereof) which are ratios and therefore independent of focal length, which makes it consistent for all lenses. Other things come into play such as the number of lens elements and the types of coatings but this is the biggest effect.

Hope this helps



Same answer as Alex, only he beat me to it

Basicaly light looses intensity while traveling down a tube. The longer the focal length, the less light reaches the film.

The fact that most people don't think of this is because we are used to think of light as spot source with neat paralel rays. This is what we learn at school and most photography classes. While in fact (day)light is a bit of a physics nightmare , it bounces, waves, ripples and interferes.
Sometimes it's easier to think of light as a schizofrenic entity, wanting to be white noise and a single frequency laser at the same time.


Well-Known Member
> You are partly right that f stop is the ratio of focal length to the diameter of the lens opening. More exactly it is the ratio of the lens opening area (i.e. ability to transmit light) to the distance from the lens to the film. Since the shorter lens is closer to the film plane, the f stop is effectively identical and the shutter speed remains the same. But if you think about it, when you work macro, you must adjust the f stop (or exposure) for the extreme distance the lens is now from the film. You can use a constant f stop for normal work because the distance from the film to the lens between infinity and say, three feet, is inconsequential. But in reality, an 80mm F2.0 lens, when focused at three feet, is, in reality an F2.2 (or something like that) lens. Not enough to affect exposure enough to worry about it.


Well-Known Member
> Light does not lose intensity "travelling through a tube." It loses intensity according to the inverse square law with distance. A light at two feet gives four times the light as at four feet. 2 squared is 4, 4 squared is 16. Therefore the ratio of 16 to 4 or four to one.


Well-Known Member
My confusion arose after reading a Kodak Workshop Series book which seemed to contradict itself in two different paragraphs. If someone could clear this up for me I would greatly appreciate it. Thanks


The answer is the ratio.




A 90mm lens images the same subject with higher magnification than a 50mm lens. That is, it spreads the light over larger area, which compensates for the difference in aperture diameter.



Well now it all makes sense. Light loses its intensity with distance travelled. Thanks to everyone for their input.



>Plus, longer focal lengths collect light from a narrower field, thus less energy


Well-Known Member
Not scientifically valid as applied to photo exposures. The only factors that count are the intensity of the light, the size of the aperture and the distance from the lens to the film plane. (assuming the lens is clean and clear and no filters are in the way, of