Just to chime in, if you are using the in-camera meter there is no reason to adjust for the filter factor, at least in theory. When using an external meter tean lowering the ISO is called for. Each ISO designation is a 1/3 stop increment. Using a lower ISO will increase the exposure called for by 1/3 stop. The 1/3 stop convention was determined to be the least noticeable significant exposure change one can make. This was more important to those exposing chromes. If you have difficulty finding the third stop point on the aperture ring, or on those few older lenses with half stop clicks, the 1/2 stop point will do. The difference between 1/3 and 1/2 stop is negligible, especially when shooting negative film. Also my rule of thumb for shooting negative film is to err on the side of overexposure.
The ISO setting on the camera is only for the camera's meter. If the camera is in manual then the ISO setting on the camera meter has no effect on exposure, period. Only when a camera is in an auto mode does it matter.
In manual you set the ISO with your film choice, 160 here.
The other controls you have to affect exposure are aperture, time, and filters.
Your handheld meter will give you the aperture and time, set them then open up by whatever filter factor is in use.
Originally Posted by kbrede
Film: ISO 400
Filter: 2 stops compensation
Set meter at: EI 100
Film: ISO 160
Filter: 1/3 stop compensation
Set meter at: EI 125
Filter: 2/3 stop compensation
Set meter at: EI 100
Filter: 1 stop compensation
Set meter at: EI 80
To clarify for OP... How the ISO dial links to camera behavior seems to be the point of confusion...
The Pentax ME Super doesn't have an automatic aperture. The f/stop you click is the one you get.
Some cameras override the aperture in program and shutter priority mode (OM-Program comes to mind). Pentax ME Super doesn't mess with your f/stop once you set it.
The camera has an electronic shutter. When using Auto - Automatic mode, shutter speed varies automatically and the speed will change by 1/3 stop if you make 1/3 stop change to the ISO dial. (But don't do that for filters since it reads the light through the lens).
When using M - Manual mode, you get the shutter speed displayed in viewfinder - the shutter does not change 1/3 stop when you change the ISO dial.
Going to the original plan: Setting meter ISO "B" to 125 is a good idea. Leaving the camera at 160 is a good idea.
Thanks all for your help and sticking with it, until I got it. :)
Setting the ISO (on the meter) to 125 is convenient as you don't have to apply the filter factor to the reading. However, if you lenses can only be set at full stop or 1/2 stop and your shutter speed at full stop then the meter at times will give you a reading you can not set exactly.