Crop factor and f/stop

[HiFiGuy1]

Daniel,


Does this mean that the iris diameterchanges when zooming a 70-200 @ f/2.8 from 70 to 200?


If I were to shoot an evenly illuminatedwhite screen (16 ft/L)at 70mm and at 200mm, both shots at f/2.8, it seems like there would be less total light available at 200mm since I would be shooting a smaller area of screen, thus less available Lamberts. For the total light to be equal, given the same ISO and shutter speed, it seemsit would make sense ifthe focal length and iris diameter correlate inversely to maintain an equivalent f-stop. If the iris diameter is constant, then wouldn't something have to give? Shutter speed?

[George Slusher]

I'm not Daniel--nowhere near his technical knowledge, but this is an easy one: Yes. The definition of "f/stop" is the lens focal length divided by the effective aperture diameter. Thus, the effective aperture diameter of the 70-200mm lens (or any zoom lens) changes with the focal length for a particular f/stop. Most zoom lenses have a range of maximum apertures (lowest/fastest f/stop). My 100-400mm lens has maximum aperture ranging from f/4.5 to f/5.6, for example. (It's probably hard to make a lens with that wide a zoom range--4:1--with a constant maximum aperture.) Those with a constant maximum aperture tend to be expensive and large, especially if the aperture is fast, like f/2.8.

[HiFiGuy1]

Thanks George. I felt like it must, but my concept of f-stop and irisup until now has been simply "lower f-stop means larger iris". I guess it does hold true, but obviously focal length must be considered when comparing f-numbers. I never really thought about it in the context of zooms, but now this thread is making me!!

[Colin]

In terms of taking the picture and setting exposure within a single camera, the f/X number is really what we want to know. If the F/X number is the same, the same amount of light hits the sensor. the actual iris diameter will be larger at a longer focal length, but the angle of view is less, so it collects less light from an angular sense, so makes it up with a larger iris diameter.

[Jon Ruyle]

For non-macro focus distances, I find that the crop factor does provide equivalent depth of field to a very close approximation, so I find it highly useful.

Daniel, are you saying that it is *not* true for macro?

[Daniel Browning]

Yes, that's what I'm saying; at a minimum, bellows factor has to be considered as well.

[Colin]

would you mind explaining bellows factor?

[Jon Ruyle]

I *thought* bellows factor refered to the fact that effective f number is f number times (1 + magnification). (Ie, if you're shooting at f/10 and magnification 1:1 then you must expose as if you were shooting f/20). With ttl metering, we don't have to worry about that much, except to know we'll need more light for macros.


Effective f number also gives rise to more diffraction. I don't think you use this "effective f number" in dof calculations. dof is proportional to f number (the real one, not effective f) and inversely proportional to magnification.


But I don't see how this changes the fovcf rule. That is, it seems to me that a 100mm lens on a 1.6fovcf camera shooting at f/10 and 1:1 magnification still acts like 160mm f/16 at 1:1 would on ff (in terms of exposure and dof).


I'm probably wrong, though [^o)]