A tip for beginners today, about a subject that can confuse.
I constantly hear people confuse focus with exposure. I hear things like “I focused on the face”, when they mean “I exposed for the face”, and vice versa. Or “I used one focus point to get the right exposure”.
Clarity of language leads to clarity of thinking and hence, to better understanding. So here for beginners are a few definitions – you will find these helpful if you are just getting into photography.
- “Focus” means “what is sharp”, particularly “what distance is the sharpest”, and also “what range of distances is acceptably sharp” (we call the latter the “depth of field”). Your camera cannot make everything, from 5cm in front of your lens to the infinite distance, sharp. That is why we talk about it.
- You focus by aiming focus spots, or preferably one chosen focus spot, at your subject and then pressing half way down before clicking. Your camera now sets its focus distance to the object you point at.
This does have anything to do with, or affect, exposure. They are entirely separate.
- “Exposure” means, in practical terms, “how dark or how bright is my picture”.
- Exposure is also measured when you press half way down – but it still has nothing to do with focus.
- You can base your picture’s exposure on an average of the entire scene (we call this “average metering”) or intelligently (“evaluative metering”, or “3D Color Matrix metering”), or in one small area only (“spot metering”).
The fact that the camera measures and decides on focus and exposre at the same time is what leads to the confusion. But realise that they are different, and independent.
“How sharp a picture is” has nothing to do with “how dark or light a picture is”. One is set by the lens moving its elements, the other is set by adjusting ISO, aperture or shutter speed.
If these things are not clear, ask me!
From the manual:
OK, it is not a beginner’s camera but, the 1Ds Mk III manual on page 198 says: Metering modes: (1) Evaluative metering (linkable to any AF point)
Hummm… Page 226 of the Rebel T2i manual has the same text.
It is not just Canon manuals either, the manual for the Nikon CoolPIX 8800 on page 87 says: Spot AF area: Links spot metering to active focus area when auto or manual focus area selection is in effect. Use instead of autoexposure lock to meter off-center subjects. Not available if focus-area selection is off.
Since exposure is the result of metering, I can see how “I used one focus point to get the right exposure” comes to be uttered.
The problem for beginners using this mode is they may not get the exposure result they expect if the focus area happens to be much brighter or darker than the rest of the scene.
Oh indeed. Certainly, you can set the spot metering spot to be the same as the focus spot. They are still very different things, though. And yes, the camera makers do not help things by being so unclear.
“preferably one chosen focus spot”, could you explain why you prefer one focus spot and when you would use 9, 21 and so on focus points.
I certainly will – I’ll do that as a post in the next couple of days!