Long Exposure calc when sun is going down

[dswiger]

While I have sort of figured out exposure & compensation in good light, when the sun is going down & I'm into reciprocity, I often am "behind" it & underexpose.

A typical scenario might be a few minutes before the sun sets on a cloudy day.
I might have a metered exposure of 30 seconds and say a reciprocity compensation that get's me to a minute.
In that minute, the light is decreasing.
I have tried "guessing" but want to know if there's a more reliable method.

I suppose I could push the film in processing depending on how much things changed over the exposure, but....
I shoot mostly color, trans & neg but I think this would be the same either way

Thanks

Dan

[BetterSense]

I built a compensating light meter once, which corrected for things like dusk, dawn, sun going behind a cloud, etc. I was having a heck of a time on partly cloudy days when making paper negatives which have little lattitude. It's basically a capacitor, photodiode, pot, and comparator, and worked really well, even for fill flash. It was pre-internet though so my notes are all on paper.

[dswiger]

Curious how you used this circuitry to control exposure.
Thanks
Dan

[BetterSense]

It wasn't linked to the camera, it just turned an LED off when it was time to close the shutter. So once you opened the shutter you pressed a button and the LED came on, and then the meter integrated the light until the LED turned off.

[Doremus Scudder]

While I have sort of figured out exposure & compensation in good light, when the sun is going down & I'm into reciprocity, I often am "behind" it & underexpose.
A typical scenario might be a few minutes before the sun sets on a cloudy day.
I might have a metered exposure of 30 seconds and say a reciprocity compensation that get's me to a minute.
In that minute, the light is decreasing.
I have tried "guessing" but want to know if there's a more reliable method.
Dan

Just meter during your exposure at, say, 3/4-way through. If your reading is significantly different, extend your exposure to match. This should get you really close; the extra exposure at the beginning will be balanced by the decreasing exposure at the end. If you are consistently overexposing doing this, then try 1/2-way. You can fine tune even more if you want.

Best,

Doremus

[ROL]