Turning on the Spot Meter
If you have a digital SLR, you already own a SPOT meter. Typically all cameras have 3 metering options â€“ averaging or evaluative metering, center weighted metering and then the SPOT mode. The evaluative meter takes light readings off multiple points in the view finder and averages them out to give you the exposure reading. The center weighted as the name suggests, gets these readings from the center area of the composition. A SPOT meter on the other hand gives you an accurate exposure reading from ONE POINT or SPOT in the view finder. This is what the spot meter symbol looks like on your cameraâ€™s LCD selector/ metering button, so look for this to turn on the spot meter mode â€“
Make an Exposure
There is really no right and wrong where exposure is concerned. A wrong exposure for the sky could only mean a right exposure for a shadow area. To understand this better, I am going to take you through a few images, explaining where I took my spot readings from. Spot metering is tricky business, a slight change in camera angle can give you an extremely different reading.
For this image, I got the spot reading off the blue part of the sky, and the reading was 1/500 at f16 (ISO 200 kept constant on all exposures). As you can see, this has prevented over exposure of the sky area, but it has also underexposed the shadow area â€“ the house and the foreground detail.
Opening up the Lens a Little
Opening up the lens by one stop gives a far more acceptable result at f11. Look for the mid tone â€“ the second hill from the right. A spot reading from this region would give you this exposure. Can you now work out how you need to isolate the MID TONE of your image and take a spot reading from that area? You need to identify the tonal difference between different areas of the composition, and visualize the shift in every area, when you make a change in exposure. A good way to go about this would be to get multiple spot readings off multiple areas. WHATEVER exposure you choose, THAT area would become the mid tone of your image, and cause an understandable shift of high and low key areas that are above and below in terms of exposure. In this image, we see the clouds starting to burn out, but the shadow areas bringing out a lot more detail.
One stop further down, exposure reading from the yellowish wall of the house, and we are now shooting at 1/250 f11. This exposes the walls just right, and causes a clear burn out of the highlight areas â€“ the clouds. Notice the roof of the house bordering on over exposure as well. But is this an overexposed image on the whole? No it isnâ€™t!
I decided to cause a purposeful error by taking the spot reading off the window of the house, and this gave me a reading of 1/90 second at f11. The brown window is exposed just right now, with clear burn out of the sky and details of the roof of the house starting to show obvious signs of over exposure. I would call this a wrong exposure, keeping in mind that the sky does play a major part in this composition, occupying a good 50% of the frame.
This exposure (1/45 at f11) is thrown in just for good measure, demonstrating clear over exposure on most areas of the composition except for the grey wall on the bottom right corner of the frame. A spot reading from this area would give you his exposure!
The Right Exposure
Personally, I prefer low key images and hate burning out any part of my photographs with over exposure. Keeping this in mind, I needed to make a choice between the first two images â€“ 1/500 f16 and 1/500 f11. I decide to go with the latter, because there is minimum burn out here, and a majority of the image area is exposed for detail, and a very small part of the sky has a burn out.
Heres another image made using spot metering. Can you identify the area off which the meter reading was taken? Try shooting silhouettes to understand spot metering better.