I found out that the relation doesn't hold anymore if the area is very bright i.e in the beach or desert.
Let say the correct exposure is 1/500 f5.6. Theoritically and equivalent exposure would be 1/250 and f4. But if the latter is sett would result more light going to the camera and result in hazy like shoot or worst, overexpsose picture.
FYI, this is test using Manual mode with center weight metering and the shoot is bracket.
Please advice why is this happening?

