What's a good ad-hoc replacement for a gray card?

Question

I know that a real gray card is the best thing to use to determine exposure. But I'd like to know if there are other naturally available features that can be used in a quick and dirty way. Are fields of grass or road tarmac appropriate?

The reason for asking is that I suspect that my new used (manual focus) lens is exposing incorrectly using my camera body. I don't have access to a gray card, but I do have access to lawns and pavements, nicely and evenly lit by a cloudy sky.

Answer

Grass is generally considered to be 18% grey. I would guess if you shoot in grey scale mode and your exposure is said to be correct, it should come up 18% grey.

There's also the sunny 16 rule, which is on a sunny day, shoot at 1/ISO shutter speed, f16. This should produce a correctly exposed picture. You can check to see if this results in a correct picture.

Both of these were tricks I was taught back in film days.

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