Learnings

Exposure Metering – Uncle Ronnie explains how you can get perfect exposure every time

This write-up is designed for newcomers and intermediate users. We have, in the past, done similar write-ups but, since human memory is short and since there are always newcomers to photography, what’s mentioned here will be of help to one and all.

A newbie photographer has a lot on his mind – he has to worry about AF modes, AF-area modes, Release modes, what ISO to set, what aperture to use, what shutter speed to set and some more. This writeup, done with simplicity in mind, will ‘talk’ only about ‘exposure’.

So, to start with, what is exposure? It is the right dosage (quantity) of light that gives you an image that has the correct tones. If the tones are too dark, you say that the image is under-exposed and if they are too light, then you say it is over-exposed.

All modern cameras – whether a simple Point & Shoot, Bridge camera, D-SLR or Mirrorless – have what is called an ‘exposure meter’. Sometimes, this is also mentioned as ‘light meter’ or simply, as a ‘meter’. The purpose of the meter is to guide you while setting an ‘exposure’. An exposure is a combination of the ISO (sensitivity of the imaging sensor), aperture and shutter speed.

There are, usually, three types of exposure meters built into our cameras (some Canon cameras have four). They are (a) Evaluative (b) Centre-weighted (c) Spot. Evaluative metering is known by different names by different camera manufacturers, e.g. Matrix metering, Segmented metering, Multi-segmented metering etc.

What type of metering should you use?

My suggestion is that you use whatever type you are comfortable with. However, it’s good to know how the different meters work.

Evaluative metering divides the frame into several segments. The computer within the camera ‘reads’ the contrast and brightness amongst the various segments, then compares this information to an in-built databank of several thousands of exposures and selects the one that closely matches the contrast and brightness of the scene before you. Camera manufacturers generally suggest the use of Evaluative metering, especially for beginners.

Centre-weighted metering places greater emphasis on the centre of the frame. This means that, though the entire frame is metered, the central area is given more weightage. With some cameras, the user can have a choice to set the diameter of the weighted area from the ones which are available.

Spot metering reads a small area of the screen (usually about 2-5 percent) of the total area. It does not read the rest of the frame. With modern cameras, the user can move the ‘spot’ within the frame. Spot metering, we are told, is best suited for intermediate or advanced users, but as you’ll see later on, it’s not that difficult to use. Note: Do not confuse the in-built spot metering with the 1° spot meters which are available as a separate unit. A 1° spot meter reads only 1° whereas, the spot meter within the camera reads 2-5 percent of the frame area.

Some more on how exposure meters work

The in-built exposure meter measures the light reflected back from the subject. Hence, it is also referred to as a ‘reflected light meter’ (as opposed to the Incident light meter that measures the amount of light falling on the subject). Ideally, I repeat, ideally, we should measure only one reflectivity. But, let’s take a practical example. In a landscape scene, different elements within our frame have different reflectivity. When we point our camera to the scene, the meter reads the multitude of different reflectivities and averages them. Most of the time, this works well, but, if there is a predominance of a very bright or very dark area, the result is less than perfect. Note: Exposure meters are designed to turn whatever you point them at, to a mid-tone. If you point it to a very dark shade of blue for example, the result will be ‘mid-blue’ (not too dark, nor too light). If you point it to light yellow for example, the result will be ‘mid-yellow’ (not too dark, nor too light). If you point the meter to black, the result will be ‘mid-black’ (middle grey); if you point the meter to white, the result will be ‘mid-white’ (middle grey). And so on.

Why is it suggested that spot metering is best left to intermediate or advanced users?

The in-built spot meter, as mentioned earlier, reads about 2-5 percent of the frame area (which represents a very small area of the frame). As long as you meter off a mid-tone area of the frame, all is good. But what if you meter off an area that is not mid-tone? Well, if you know how much ‘exposure compensation’ to apply and in what direction (minus or plus compensation), you are okay. The problem is, most beginners do not know exactly how much exposure compensation to apply and in what direction. So here are some guidelines when using spot metering:

Subject Compensation

Mid-tone Nil

Black -2 stops

Dark (but not black) -1 stop White + 2 stops

Light (but not white) +1 stop

(You can also use exposure compensation during Evaluative or Centre-weighted metering).

When you look at colours, try to visualise them in shades of mid-tones. For example, dark blue is not a midtone (it is darker than a mid-tone); light blue is also not a mid-tone (it is lighter than a mid-tone). So what is a midtone blue? Mid-tone blue is blue that is neither too dark, nor too light. It sits in the middle. The same goes for all other colours.

How can a beginner be sure that he/she is pointing the spot meter to a mid-tone?

Once you start taking pictures and start to analyse the results, you’ll know. In the beginning, your judgement may not be perfect but, with practise, you’ll know a mid-tone when you see one. Believe me! (Please see photo on page 65)

If you take a spot meter reading from spots marked 1 and 2, you can use the indicated exposure since those areas are mid-tone.

Spot number 3 appears to be slightly darker than mid-tone. You could try -0.5 stop compensation. Spot number 4 covers an area that is much darker than mid-tone (but it’s not black). You could try -1.5 stop exposure compensation.

Spot number 5 covers an area that is much brighter than mid-tone. I would try +1 stop or even +1.3 stops exposure compensation.

So you can see that spot metering is not as difficult as it is often made out to be. Your judgement in deciding the metered tone could be wrong sometime, but with practise you’ll soon overcome all related issues.

OK. But what ‘exposure mode’ should you use?

Let’s talk of the most basic four exposure modes: Program (P), Aperture Priority (A, but for Canon users Av), Shutter Priority (S, but Tv for Canon users) and Manual (M). (We will not discuss ‘scene modes’).

Instead of discussing all the four modes (and thereby getting into unnecessary arguments with some people), I’ll explain my favourite exposure mode and why I use it.

Please set the ISO first. For a majority of shooting, that would be ISO 100.

My favourite exposure mode is Aperture Priority. In Aperture Priority mode, you opt for the aperture of your choice. You may like to use a narrow aperture like f/11 or f/16 for increased depth of field, or you may use a wider aperture like f/4 or f/2.8 for a diffused background. Alternately, you may use the ‘optimum aperture’ of your lens, for example, when doing ‘focus stacking’. Once you have set your aperture, when you half-press the shutter release button, the camera will automatically select the corresponding shutter speed and that shutter speed will be shown in the viewfinder.

If I feel that the automatically selected shutter speed is too low (which may cause subject movement or camera movement during exposure), I can either ‘open up’ the aperture (from f/16 to f/11 for example) or increase the ISO (from 100 to 200). So, for example, if at my selected aperture of f/16, I get a shutter speed of say 1/500 sec, but I need 1/1000 sec shutter speed, I could go from f/16 to f/11, or, stay at f/16 and increase the ISO from 100 to 200. Simple!

A note, especially to beginners

I have often noticed that beginners use only the Program mode. There is nothing wrong in using this mode – your exposures will be fine most of the time – but the Program mode has its limitations – it uses the algorithms (set of instructions) designed and set by the engineer who designs the metering system. As such, you may not be able to use the aperture of your choice (unless your camera also has a Flexible Program (P*) mode and you know how to use it!)

Another observation is that most beginners, when they use the Aperture Priority mode, they stick to whatever aperture is set when they pick up the camera. This is because they have not fully understood how apertures work and how they allow us control over the zone of acceptable sharpness (depth of field) in our pictures. Hence, it is imperative that you learn your basics well.

In closing, remember that photography is something to enjoy, something that is supposed to release the tension of daily life. If that does not happen with your photography, you are doing something wrong!