"ISO 25"

Another cracking new feature which I have just discovered in Magic Lantern is the ability to lower the ISO beyond the standard minimum setting of ISO100. It is now possible to reduce the ISO to ISO25, meaning a two stop reduction in sensor sensitivity. Why would you want to do this I hear you ask? Well, there are two reasons I can think of immediately:

1. Shooting outdoors in bright daylight. Unless one has a neutral density filter on the front of the lens, the only way to maintain a short depth of field (wide lens aperture), but avoid over exposure, is to decrease the shutter speed lower than the standard 180 degree shutter of 1/2n, where n = the frame rate. The main issue with doing this however, is that any movement in the shots can become very "staccato". This is often used in fight / action sequences for creative effect, but it is often not desired for normal shooting. Therefore, if one can reduce the ISO to ISO25, the aperture can be opened up two stops, which will help maintain the short depth of field which DSLR video so easily enables. Let's take an example where say the camera is set to ISO 100 and a shutter speed of 1/50s (i.e. standard 25fps shooting). To avoid over exposure, the aperture of the lens needs to be reduced from f/2.8 to f5.6 (two stops reduction). However, in doing this, the depth of field is increased beyond what is required. If the ISO is now changed to ISO25, the exposure will reduce by TWO stops meaning that the aperture on the lens can now be returned to f/2.8, giving the desired short depth of field. Therefore, this new feature acts in a similar way to using a two-stop ND filter.

2. Shooting timelapse sequences using the FRAME RATE OVERRIDE function. If shooting with say a frame rate of 2fps, the shutter speed is also greatly increased, meaning lots of light, meaning potential over exposure, even with the smallest of lens apertures (f/22 for example). However, with the ISO reduced to ISO25, again, one can open up the iris of the lens somewhat higher than its minimum setting.

The DOWNSIDE to this feature, is that some of the over exposed regions can become pink, but this can be corrected in post. 

A VERY nice feature!