09-24-2011, 11:35 PM
[quote name='Brightcolours' timestamp='1316866210' post='11813']
Another way to illustrate this:
Same aperture.
So, lets take again equivalent focal lengths (similar field of view) for FF and 4/3rds (easy to calculate due to 2x crop factor) as example.
100mm on FF, 50mm on 4/3rds. Both f2.
100mm / 2 = 50mm opening. 50 / 2 = 25mm opening.
The FF sensor can gather much more light per pixel, simply because the opening is 4x bigger. Then how come the exposure times are similar???
Because ISO is not ISO anymore. Not about sensitivity. For the 4/3rds sensor to reach that "ISO 100" level, the signal is amplified more than that of the FF sensor.
[color="#ff0000"]To get a similar total amplification. just use EQUIVALENT ISO settings[/color].
To get a similar field of view, just use equivalent focal lengths.
To get a similar depth of field, just use equivalent f-values.
[/quote]
ISO settings are based on a quantified density of light, i.e., a quantity of light (photons if you like) per surface area, and therefore independent of sensor or any medium for that matter.
The size of the medium only influences things like granularity for printed images of the same size from different media sizes, and things like DR and SNR.
Furthermore, in order to expose an image properly, the EV was invented, which is based on this principle. EV 0 is defined as the light density required to expose an image properly at F/1, for 1 sec at ISO 100. This is totally independent of the medium, other than that it is a starting point. This is why we can use the same exposure meter for all cameras, regardless of medium size or FL, and why we get exactly the same value for exposure time at the same ISO and aperture, for the same lighting conditions.
Of course with sensors the base ISO may well be different than 100 ISO, however, that is a little besides the point from an exposure POV.
The same is really true for aperture and FL equivalencies. Although F/2 on a smaller sensor may be equivalent for actual DoF to a much smaller F-stop on a camera with a larger sensor, it still is F/2, and therefore "catches" the same amount of light per surface area as does any other lens at F/2. And this is also totally independent of the FL.
Sensor surface area only comes into play when calculating the total amount of light captured is calculated, as obviously this depends on the surface area. And FL doesn't come into the equation at all, because again, it is all about the light captured.
I think this is what Rainer and Markus are trying to say.
HTH, kind regards, Wim
Another way to illustrate this:
Same aperture.
So, lets take again equivalent focal lengths (similar field of view) for FF and 4/3rds (easy to calculate due to 2x crop factor) as example.
100mm on FF, 50mm on 4/3rds. Both f2.
100mm / 2 = 50mm opening. 50 / 2 = 25mm opening.
The FF sensor can gather much more light per pixel, simply because the opening is 4x bigger. Then how come the exposure times are similar???
Because ISO is not ISO anymore. Not about sensitivity. For the 4/3rds sensor to reach that "ISO 100" level, the signal is amplified more than that of the FF sensor.
[color="#ff0000"]To get a similar total amplification. just use EQUIVALENT ISO settings[/color].
To get a similar field of view, just use equivalent focal lengths.
To get a similar depth of field, just use equivalent f-values.
[/quote]
ISO settings are based on a quantified density of light, i.e., a quantity of light (photons if you like) per surface area, and therefore independent of sensor or any medium for that matter.
The size of the medium only influences things like granularity for printed images of the same size from different media sizes, and things like DR and SNR.
Furthermore, in order to expose an image properly, the EV was invented, which is based on this principle. EV 0 is defined as the light density required to expose an image properly at F/1, for 1 sec at ISO 100. This is totally independent of the medium, other than that it is a starting point. This is why we can use the same exposure meter for all cameras, regardless of medium size or FL, and why we get exactly the same value for exposure time at the same ISO and aperture, for the same lighting conditions.
Of course with sensors the base ISO may well be different than 100 ISO, however, that is a little besides the point from an exposure POV.
The same is really true for aperture and FL equivalencies. Although F/2 on a smaller sensor may be equivalent for actual DoF to a much smaller F-stop on a camera with a larger sensor, it still is F/2, and therefore "catches" the same amount of light per surface area as does any other lens at F/2. And this is also totally independent of the FL.
Sensor surface area only comes into play when calculating the total amount of light captured is calculated, as obviously this depends on the surface area. And FL doesn't come into the equation at all, because again, it is all about the light captured.
I think this is what Rainer and Markus are trying to say.
HTH, kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....
