Long exposure miscalculation

I took a shot of a gray card for 300 sec at f4.0. Then I halved the aperature to f8.0 whilst doubling the shutter speed to 600 sec. The two pictures in theory should match, but the second shot is underexposed. Can this be a result of some sort of shutter speed inverse square law that throws off the math.

Film sensitivity isn't linear with such long exposures, but in this case it ought to be. Sure illumination was the same?

Reseach Reciprocity here:
http://en.wikipedia.or​g/wiki/Reciprocity_(ph​otography)

The light was constant, but he results weren't linear. I use th 5DMarkii

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Proper exposure for twice as much time is an aperture 1 stop further closed, which would have been f/5.6, rather than f/8.0. This passes half the light. f-stop designations do not halve or double the same way that shutter speeds do.

texshooter wrote in post #9285709
I took a shot of a gray card for 300 sec at f4.0. Then I halved the aperature to f8.0 whilst doubling the shutter speed to 600 sec. The two pictures in theory should match, but the second shot is underexposed. Can this be a result of some sort of shutter speed inverse square law that throws off the math.

EdZep wrote in post #9286257
Proper exposure for twice as much time is an aperture 1 stop further closed, which would have been f/5.6, rather than f/8.0. This passes half the light. f-stop designations do not halve or double the same way that shutter speeds do.

^^ +1. Should be f/5.6 @ 600 seconds.

I'm going senile.

texshooter wrote in post #9286400
I'm going senile.

No, you're not. It's an understandable mistake most of us make at one point or another.

Simple brain lapse. Heh, how many people read your post and didn't catch the slip?

texshooter wrote in post #9285709
I took a shot of a gray card for 300 sec at f4.0. Then I halved the aperature to f8.0 whilst doubling the shutter speed to 600 sec. The two pictures in theory should match, but the second shot is underexposed. Can this be a result of some sort of shutter speed inverse square law that throws off the math.

You goofed with your understanding of f-stop settings. One full stop from f/4 is f/5.6, not f/8.

Full-stop F-stop setting changes progress by a factor of 1.414, not 2. 1.414 is the square root of 2. This is because we are dealing with the area of the "hole" in the lens.

PS - others caught on too. I replied after reading only the first post.

EdZep wrote in post #9286504
Simple brain lapse. Heh, how many people read your post and didn't catch the slip?

This guy.

I was thinking to myself -- f4 to f8 is two stops, so that means twice the light and so 300s to 600s should be good. What am I missing?

I don't get it, 8 is 2x 4, so it should be double....



no, I understand exposure

My first thought when reading the OP, was "wow, what a geek". I didn't even catch the simple error.

That is the cool thing about digital photography though, it costs you nothing to try.

I let my light meter tell me what's what. Never did a pic that was longer than 25 seconds or so:

What are you crazy people talking about??????????