Wednesday, September 8, 2010

Exposure Metering

This article is the last one in a series of articles covering the subject of photographic exposure.
If you haven’t read Elements of Photographic Exposure and Exposure Modes, it’s a good time to do so now before moving on. In the above articles we talked about the factors that form the exposure (aperture, shutter speed, ISO) and the camera’s exposure modes (the programs that allow the photographer to take exposure control; Program Priority, Aperture Priority, Shutter Priority and Manual Priority). So, in order to keep things simple, lets summarize all the above. Depending on the scene you are photographing and your artistic vision, you choose an exposure mode:
·           Program Priority if you are in a harry: You let the camera set aperture and shutter speed based on what it things is best. You control the ISO.
·           Aperture Priority if depth-of-filed is important: You set the aperture and ISO and the camera sets the shutter speed.  
·           Shutter Priority if motion is important: you set shutter speed and ISO and the camera sets aperture.
·           Manual Priority: you have total control over the exposure by setting aperture, shutter speed and ISO.

As you might though so far, a piece of information is missing from the above decision workflow; how does your camera, or you, decide which the appropriate exposure is? Well, modern cameras have a build-in exposure meter which evaluates the scene and provide you (and your camera) with information about exposure.

The problem
Modern cameras have come a long way and they are becoming cleverer as time passes by, but still they are not perfect. There are some “tricky” situations that can fool your camera’s exposure meter. Even worsted, your camera can’t make artistic decisions. So how do cameras determine which exposure is right and which exposure is wrong? Well here’s the trick: your camera makes the assumption that the world is middle gray! (if you like the more technical term, middle gray is a subject with 18% reflectance). Please note: we are not talking about the actual color gray here, but the reflectance (tonality) of the subject.  
Middle gray looks like this:
The problem is that not all subjects have an average reflectance. Some subjects are either darker or brighter than average:
So let’s say you are photographing a black subject (e.g. a black jaguar – it happens all the time right? !) - . If you let your camera make the exposure decision, it will overexpose your black jaguar and turn it into a gray jaguar. Or, you are photographing a white subject (e.g. snow). Your camera will underexpose the snow and turn it into gray. 
So, what did the manufacturers were thinking when developing those meters? Well, as with most situations in life, you have to start with an assumption and build on that. On the other hand, if you sum the reflectance of an average scene, you come up with a mean that is middle gray.

Metering Methods
Nikon cameras have 3 metering methods for determining exposure:
·           Spot metering: it targets 1,5% of the frame, where your focus point resigns.
·           Center-weighted metering: it measures the entire frame but emphasizes on the middle area of the frame (exposure decisions are based 75% on the central area and 25% on the outer area).
·           Matrix metering: It measures the entire frame. This method is becoming more complex as the technology moves forward. Older camera models were measuring the entire frame and gave a little emphasis on the area that is behind the selected focus point. Modern camera bodies take into account a lot more factors; lens focus distance, highlights, skin tones, color, etc and compare the scene with an image database in order to make their exposure decision. Newer camera models are cleverer than old ones and have a tendency to do the right thing. However, they are not 100% accurate yet.

What to do (the easy method)   
If you are photographing a subject and you do not agree with your cameras’ exposure decisions, you can use exposure compensation to override your cameras’ decision. Use “-“ if your subject is darker than average, use “+” if your subject is brighter than average.

What to do (the accurate method)
1.            Set your camera to Manual Priority Mode and to Spot Metering Method.
2.            Set your ISO accordingly, depending on the situation and your lens aperture (if depth-of-field is     important), or shutter speed (if motion is important).
3.            Point your active focus point on a subject:
(a)          If the subject is middle gray, zero out the harsh mark of the metering bar (by adjusting accordingly ISO, Aperture or Shutter Speed):
 (b)         If the subject is completely white, put the harsh mark of the metering bar to +2 (e.g. if you are photographing a wedding, you can set the bride’s wedding dress to +2) .
(c)           If the subject is completely black, put the harsh mark of the metering bar to -2 (e.g. if you are photographing a wedding and the groom is a wearing a black suit, you should set the back suit to -1,5 in order to retain detail).
Note: Blue skies, light green and red in nature have an average reflectance. Yellow color is +1 stop, while brown and dark blue colors are -1 stop. Caucasian tone skin is +1 stop, while darker skin tones may fall under average reflectance.    
Tip: A good practice is to start using a dedicated gray card in the scenes you are photographing. Zero out the harsh mark of the metering bar on the card and then observe where the other tones in the scene fell. In no time you will gain a lot of experience and the card will no longer be useful to you!   

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