When to use ISO 50 on Canon SLR Cameras

[anonymous]

I've read some comments about the ISO 50 on the 5D being "fake", what's your take on this? Does it help improve the quality of some of your shots or not?

[McQ]

I would recommend against ISO 50 unless you do not have a neutral density filter and need a longer exposure (such as for a waterfall). It is "fake" in the same way that ISO 3200 is-- in both cases one can think of the camera as just manipulating the data from ISO 100 and ISO 1600, respectively.

ISO 50 has a limited dynamic range because it is the equivalent of just overexposing an ISO 100 shot by a full +1 stop (and therefore truncates the highlights). ISO 3200 does the opposite by adding exposure compensation to a lower ISO shot (and therefore also has limited dynamic range). On the other hand, both of these steps are still done in hardware so they produce slightly better results than if this were done in software using a RAW converter. If you need even lower noise than ISO 100 provides, I would consider averaging two ISO 100 exposures to create an ISO 50 equivalent (except with the full dynamic range of an iso100 shot).