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ƒ is in every practical use an indicator of how much light comes out of the lens, nothing else. Two lenses of identical diameter and identical focal length can

ƒ is in every practical use an indicator of how much light comes out of the lens, nothing else. Two lenses of identical diameter and identical focal length can at wide-open aperture have two completely different ƒ values (due to, among other things, the brightness of the glass used in the groups of lenses inside), yet produce identical DoF. This is why ƒ tells you not the state of the aperture, which is primarily what affects your DoF, but rather how much light reaches the film plane.

ghshephard 15y ago
F is not an indicator of light coming out of the lens. It is the ratio of the Focal Length to your aperture diameter. Nothing else. It says nothing about the amount of light coming out of your lens. And, indeed, cannot. Two Lenses of identical diameter [edit: aperture size] and identical focal length, by definition of the concept "F-Stop", will always have the same F Values. Period.
andrewjshults 15y ago
This explanation is going to confuse non-photographers because for modern 35mm format lenses the difference between the maximum f-stop (which is purely the ratio between the focal length and the diameter of the aperture) and the maximum t-stop (the actual amount of light transmitted through a lens) are minimal (no where near full t-stops of light loss). Two lenses with the same focal length at the same f-stop with the same distance to the subject are going to produce the same DoF[1] and same exposure. [1] To really understand how to calculate DoF for the format you are shooting with and the size you are viewing at, you also need to understand Circles of Confusion (CoC). The same (effective) focal length lens at the same aperture/focal distance and printed at the same size is going to give dramatically different DoF if one camera is 35mm format and the other is large format (8x10).
- sasha61 15y ago
  One thing we should also remember that because of the finite size of either silver halide particles in films or pixels in image sensors, it would not make sense to use very high F values - it would just not serve the purpose since once CoC is comparable with the size of the minimal light-sensitive particle/element, further increase in F will just reduce the amount of light without any increase in sharpness (e.g., a line which has the same width as the light-sensitive element would be recorded in the same way as a line which is only half as wide). For cameras with lenses that have very short focal length (such as, e.g., mobile phone cameras) pretty much everything is in focus (beyond a few dozen cm) even at low F (if you can control it, that is) - just because CoC diameter is so small it is comparable with pixel's dimensions of the image sensor. They are, in effect, "permanent focus" cameras.
  - ajkessler 15y ago
    Not only that, you'll introduce diffraction at smaller apertures as well, which will negatively impact image quality.

ghshephard 15y ago

F is not an indicator of light coming out of the lens. It is the ratio of the Focal Length to your aperture diameter. Nothing else. It says nothing about the amount of light coming out of your lens. And, indeed, cannot. Two Lenses of identical diameter [edit: aperture size] and identical focal length, by definition of the concept "F-Stop", will always have the same F Values. Period.

andrewjshults 15y ago

This explanation is going to confuse non-photographers because for modern 35mm format lenses the difference between the maximum f-stop (which is purely the ratio between the focal length and the diameter of the aperture) and the maximum t-stop (the actual amount of light transmitted through a lens) are minimal (no where near full t-stops of light loss). Two lenses with the same focal length at the same f-stop with the same distance to the subject are going to produce the same DoF[1] and same exposure. [1] To really understand how to calculate DoF for the format you are shooting with and the size you are viewing at, you also need to understand Circles of Confusion (CoC). The same (effective) focal length lens at the same aperture/focal distance and printed at the same size is going to give dramatically different DoF if one camera is 35mm format and the other is large format (8x10).

sasha61 15y ago
One thing we should also remember that because of the finite size of either silver halide particles in films or pixels in image sensors, it would not make sense to use very high F values - it would just not serve the purpose since once CoC is comparable with the size of the minimal light-sensitive particle/element, further increase in F will just reduce the amount of light without any increase in sharpness (e.g., a line which has the same width as the light-sensitive element would be recorded in the same way as a line which is only half as wide). For cameras with lenses that have very short focal length (such as, e.g., mobile phone cameras) pretty much everything is in focus (beyond a few dozen cm) even at low F (if you can control it, that is) - just because CoC diameter is so small it is comparable with pixel's dimensions of the image sensor. They are, in effect, "permanent focus" cameras.
- ajkessler 15y ago
  Not only that, you'll introduce diffraction at smaller apertures as well, which will negatively impact image quality.