Hi All,This spot metering is only measured inside the circle you see in your viewfinder. However, the histogram is for the whole image, not just the central spot. So if you take the photo of course you'll see the histogram spike in many places not just in the middle unless your greycard fill the entire frame. The important thing is do get the correct exposure? Don't worry too much about spike in histogram.
I take lots of photos of dogs and I'm trying to get more accurate exposures of black dogs and white dogs and other dogs that tend to confuse me and my 50d's metering.
I thought I'd try to set my exposure using a 18% grey card, thus getting a correct exposure no matter what colour of dog I was photographing.
To do this I bought a grey card which, as I understand it, I hold in the same light as the dog and take a meter reading from the grey card. So far so good.
So I hold the card at arms length and take a meter reading using spot metering. From this reading I expect to see a histogram with a spike in the centre of the histogram and the rest should be blank.
My grey card fills at least 50% of my 50d's frame so a spot meter reading from just 3.8% of the frame should see nothing but my grey card
However this is never the case. My histograms have spikes all over the range, so my camera is getting metering information from much more than just the central 3.8% of the frame.
I thought I could get away with just a small pocket sized grey card but if the metering is not actually the central 3.8% of the frame I will have to get bigger pockets and a much bigger grey card that fills the entire frame and then it won't matter what metering I use.
Does anyone know if canon spot metering is supposed to just take a reading from just the central portion of the frame and nothing else?
Please try this for yourselves, hold anything with a solid colour up at arms length and take a spot meter reading and let me know what result you get.
Thank you. Simon