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In the recent thread, "why olympus have f2 zooms":
http://forums.dpreview.com/forums/read.asp?forum=1022&message=37211902
the controversy of Equivalence once again appeared when discussing "fast" lenses. Thus, I thought a thread dedicated to the symantics might be educational.
Why do we care about the speed of the lens? Two reasons:
Well, the f-ratio of the lens is the quotient of the focal length and the aperture diameter (entrance pupil diameter), so a smaller f-ratio means a larger aperture diameter, which means more light will fall on the sensor for any given shutter speed.
If we simply increase the shutter speed by raising the ISO or setting the shutter speed manually, the aperture diameter does not change, so less light will fall on the sensor, which will result in more noise.
Thus, a "faster" lens will allow for a less noisy photo for a given shutter speed on a given format .
On the second point, DOF, we can always get a more shallow DOF by either framing tighter, or using a longer lens with the same f-ratio and stepping back. So, again, what does the f-ratio have to do with it?
Well, getting closer changes our perspective and framing, and stepping back with a longer lens will change the perspective as well, even for the same framing. So a faster lens allows us to get a more shallow DOF for a given perspective and framing than a slower lens.
Now, when we compare across formats , what we find is that the same f-ratio has a very different effect, just as the same focal length has a very different effect. In fact, the effect of both the focal length and f-ratio scale the same. For example, just as 50mm on 4/3 results in the same AOV as 100mm on FF, f/2 on 4/3 results in the same aperture diameter as f/4 on FF for a given AOV.
That means that the same amount of light will fall on the sensor for a given scene, perspective, AOV, and shutter speed at f/2 on 4/3 as will at f/4 on FF. For equal sensor tech, that means the same noise. In addition, the DOFs will also be the same.
So, is f/2 on 4/3 the same "speed" as f/4 on FF? Well, that's merely semantics. The effect of f/2 on 4/3 has the same effect as f/4 on FF, so I don't know what the utility is of saying "f/2 on 4/3 is 'faster than' f/4 on FF" would be.
Does it matter? Only if you are comparing formats. For sure, f/2 is faster than f/4 on the same format. But when comparing across formats, it's rather misleading to say that f/2 is "faster" than f/4 if they result in the same DOF and noise.
In other words, saying "f/2 on 4/3 is faster than f/4 on FF" is like saying a 2L engine revving at 8000 rpms is "faster than" a 4L engine revving at 4000 rpms. Sure, it's revving faster, but that doesn't mean it's accelerating faster.
In other words, f/2 on 4/3 is no more "better than" f/4 on FF than is 100mm on FF "better than" 50mm on 4/3. In both cases, they are simply "equivalent".
http://forums.dpreview.com/forums/read.asp?forum=1022&message=37211902
the controversy of Equivalence once again appeared when discussing "fast" lenses. Thus, I thought a thread dedicated to the symantics might be educational.
Why do we care about the speed of the lens? Two reasons:
- Faster shutter speed
- More shallow DOF
Well, the f-ratio of the lens is the quotient of the focal length and the aperture diameter (entrance pupil diameter), so a smaller f-ratio means a larger aperture diameter, which means more light will fall on the sensor for any given shutter speed.
If we simply increase the shutter speed by raising the ISO or setting the shutter speed manually, the aperture diameter does not change, so less light will fall on the sensor, which will result in more noise.
Thus, a "faster" lens will allow for a less noisy photo for a given shutter speed on a given format .
On the second point, DOF, we can always get a more shallow DOF by either framing tighter, or using a longer lens with the same f-ratio and stepping back. So, again, what does the f-ratio have to do with it?
Well, getting closer changes our perspective and framing, and stepping back with a longer lens will change the perspective as well, even for the same framing. So a faster lens allows us to get a more shallow DOF for a given perspective and framing than a slower lens.
Now, when we compare across formats , what we find is that the same f-ratio has a very different effect, just as the same focal length has a very different effect. In fact, the effect of both the focal length and f-ratio scale the same. For example, just as 50mm on 4/3 results in the same AOV as 100mm on FF, f/2 on 4/3 results in the same aperture diameter as f/4 on FF for a given AOV.
That means that the same amount of light will fall on the sensor for a given scene, perspective, AOV, and shutter speed at f/2 on 4/3 as will at f/4 on FF. For equal sensor tech, that means the same noise. In addition, the DOFs will also be the same.
So, is f/2 on 4/3 the same "speed" as f/4 on FF? Well, that's merely semantics. The effect of f/2 on 4/3 has the same effect as f/4 on FF, so I don't know what the utility is of saying "f/2 on 4/3 is 'faster than' f/4 on FF" would be.
Does it matter? Only if you are comparing formats. For sure, f/2 is faster than f/4 on the same format. But when comparing across formats, it's rather misleading to say that f/2 is "faster" than f/4 if they result in the same DOF and noise.
In other words, saying "f/2 on 4/3 is faster than f/4 on FF" is like saying a 2L engine revving at 8000 rpms is "faster than" a 4L engine revving at 4000 rpms. Sure, it's revving faster, but that doesn't mean it's accelerating faster.
In other words, f/2 on 4/3 is no more "better than" f/4 on FF than is 100mm on FF "better than" 50mm on 4/3. In both cases, they are simply "equivalent".
?
