This new mini-series is about camera functions that photography novices tend to confuse, even though they have little or nothing to do with each other. In this first part, we talk about spot metering and the autofocus area and why it’s best not to confuse them.
Where does the confusion come from?
That’s a good question. Simone and I aren’t quite sure about that. If you’ve been taking pictures as long as we have, then it’s a mystery to you why newcomers to photography make a connection between focus and metering. Perhaps it’s because of the belief that the focus should normally be on the most important part of the subject, and therefore the exposure should be metered at that point. First of all, this does not lack a certain logic. If the main subject is sharp and correctly exposed, then from a purely technical point of view we are normally satisfied with the shot.
However, this assumption presupposes that the exposure metering will result in a correctly exposed image under all circumstances. However, anyone who has understood the functional principle of exposure metering knows that this can often not be the case. This is because, unfortunately, the exposure meter does not meter the brightness of the lighting, but rather the light reflected from the subject. And that makes the exposure metering dependent on the subject. In other words, different subjects lead to different results of the exposure metering with the same lighting. And that often leads to incorrectly exposed images.
Enter “Spot Metering”
The default setting for the exposure metering is usually matrix metering. In this setting, exposure metering and focusing are largely decoupled* Exposure metering is done via several individual fields distributed across the viewfinder, resulting in differently weighted exposures depending on the subject.
With spot metering, however, only a small part of the viewfinder image, usually exactly in the center of the image, is used for exposure metering. In combination with the center autofocus point, it is now actually the case that focusing and exposure metering are performed on the same part of the subject as soon as the shutter release button is pressed halfway (provided you have not intentionally decoupled the two functions!). You might think that this is actually the optimal solution for sharp and correctly exposed photos (and maybe that’s where the confusion comes from). Unfortunately, in most cases you are exactly wrong. That’s because spot metering only measures the light reflected from the subject. For the exposure to be correct, the subject would have to be exactly medium gray at the metering point. Our cheetah would therefore have to hold a gray card in front of its face:
If the subject at the (very small!!) measuring point is brighter than middle gray, the image is too dark. If it is darker than middle gray, it is too bright. But how likely is it that the part of the subject that is suitable for the spot meter is exactly the same as the middle gray? That was a rhetorical question that I can’t really answer!
With matrix metering, which uses all parts of the subject for exposure metering, you therefore have a higher probability of a correctly exposed image in most cases. And of course, you can use the center autofocus point in combination with matrix metering. Precisely because exposure metering and focusing have nothing to do with each other. Which brings us back to our initial question …
* In fact, with many types of cameras, the exposure metering point(s) corresponding to the active autofocus metering point is/are weighted more heavily. Personally, I think this is nonsense. See above.
As always, I’m looking forward to your comments and thoughts.
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