Exposure length with a CLS filter

[Darren Protz]

Hi,

I was wondering, how can you calculate the ideal exposure length for my CMOS (M43 and APS-C) cameras with a CLS clip filter fitted?

For example, I have calculated my ideal sub length at all ISOs and optical focal ratios, using the formula of: 10x (read noise squared/LP in electrons).

Is there a standard multiplication factor I can apply? 2x, 2.5x, 3x?

I don't want to really have to rely on the camera's histogram.

Thanks,

Darren

[admin]

Hi,

By applying a filter like the CLS filter you are affecting the rate at which light pollution photons arrive at the sensor (reducing it) – this will reduce the light pollution rate figure that you put into the equation and therefore increase the recommended exposure length. You can probably work out how big the effect is by measuring the histogram level with and without the filter with the same camera settings to see how much the filter is cutting down the light pollution.

Cheers, Robin

[Darren Protz]

Hi,

Thanks, Roger. That's the way I would have gone about it.
I just thought there might be a way I could calculate a subexposure length mathematically without referring to the histogram.

Cheers.