long exposure question

[Sebast]

I am shooting with a5100.  New to long exposure. I am using 8 + 4 stop ND filter.  In A mode shutter speed metered at 1/25 for scene.  ND timer app says 2:43 for this. Switched to M mode and bulb mode.  Shooting with ISO at 100.  Way overexposed.  Live view shows only a few seconds for appropriate exposure time but I am trying to smooth a waterfall and want longer time.

Problem I think is the camera keeps adjusting the exposure compensation.   In menu it is greyed out unless I change to Auto ISO.  I tried changing aperture from 11 to 16 to increase my shutter length.

Is there a way to shut off exposure compensation in manual mode. 

What am I missing?

[Pieter]

If you are in manual mode and have ISO set on 100, by definition can there be no auto exposure compensation by your camera: all three exposure variables are fixed by your manual settings.

Your math is correct: if you meter 1/25sec at f/11 ISO 100, if you add 12 stops of ND the same exposure requires 164 seconds (or 2 mins 44 sec).

Are you sure you stacked the two ND filters properly?

Was your ISO fixed at 100 when you did the metering in A mode? Unlikely to be your problem as this would have caused under- instead of overexposure. Just trying to isolate your problem.

[Sebast]

thanks for your reply.  yes ISO was fixed.  it seems when I switch from A to M modes the only thing that changes on the back display is the Exp.Comp changes from just a +/- number to "MM+/- number. 

then in M mode if I change the shutter speed, the MM+/- compensates for it.  very frustrating.  I just can't find a way to shut this off.  Maybe this is just a flaw in the a5100??

[Pieter]

I think I've found your solution: your ND filters are not 8 and 4 stops, but 3 and 2 stops respectively: to get the stop number, take the Nth root of the ND number, where the answer should be 2. So 8^(1/3) = 2 and 4^(1/2) = 2. Combined you only have 5 stops of ND, not 12. A correct exposure should then be 1/25 sec × 2^5 = 1.28 sec instead of 164 sec!

The numbers on ND filters are designed such that you can easily calculate the correct exposure time. Don't think of a number of stops, just multiply everything: 1/25 sec × 8 × 4 = 1.28 sec.

The MM+/- number is just a realtime exposure meter compared to a 'desired' value of 0. Point your camera somewhere dark or bright and you'll see this number go down and up. Nothing is compensated for, it's just a reading.

Edited September 20, 2018 by Pieter

[Sebast]

Ok... that makes sense.  Thanks Sherlock Holmes for deducing my issue.  Guess I need some stronger ND filters.