09 September 2009

Light Metering - Is Spot Metering important?

When we take a picture in one of the automatic or semi automatic modes our camera knows how long to keep the shutter open and how wide to set the aperture in order to get the best exposure because they have an inbuilt gadget for measuring the available light.  They beep, blink and pop the flash up to tell us if they are not happy with the light conditions.  But how do they do this and why does it matter and do you really need spot metering to improve your photography? 

Light metering (or light measuring) options make an enormous difference to your photography.  Have you ever seen a professional photographer holding a little gadget up to a subject or maybe you had one stuffed in your face when you were getting married?  That was an external light meter and photographers pay alot of money for them because light metering is really important!

So take heed if you are purchasing a new camera, and investigate your manual if you aren't sure what options your camera has.
I'll explain why spot metering is a useful function and also why it is very useful for craft photography a little later but first here is a basic explanation of three of the most common light metering modes, some cameras have all three.

Centre weighted Average - This means that they measure the light from various parts of the scene but the central measurement is given priority.  This is fine in most situations for getting a well balanced exposure.

Matrix Evaluative –  As the name suggests it uses lots of points to determine the exposure required but it will often also adjust for the foucus point and distance, this varies from make to make and in most cases the less you pay the less refined the system will be.  Wikepedia has a good explanation if you'd like all the techno babble.  More cameras have this as standard now, it is the default setting on my camera.

Spot -  Again as the name gives it away, you are metering based on one point/spot in the frame.  Most cameras with spot metering will also have the ability to reposition the focus point and with this option the camera will meter from that chosen focus point.  The photographer can also choose the subject to focus on and then recompose the shot by keeping the shutter half depressed.   I use spot metering alot, especially when using my macro mode and for photographing flowers.
These pictures I took of a little spider (which was actively spinning it's web) show the difference very clearly.  The shot on the left using matrix metering has completely failed to identify this small subject matter and left the spider badly over exposed, it can be a bit of a gamble as to what the camera will determine as the main subject.  The image on the right, using spot metering, allows us to clearly see all the parts of the spider (not that I really want to!)

So why is spot metering important for photographing crafts?

Often when photographing objects we will use a white or light coloured background.  If there is a larger area of background reflecting lots of light  then your camera will compensate and this may result in the subject ,especially if is a small piece being underexposed.  By using spot metering you are telling your camera precisely which part of the image is important and needs to be correctly exposed.  This also saves on the amount of editing you need to do  as white backgrounds can often come out dull and grey after the cameras light sensor has attempted to give you an average reading. 
Here are a few examples, photos on left are with my cameras default setting (matrix) and on the right with spot metering - I haven't used  any editing software on these images so as to give you a true idea of the difference. 

These are not perfect photographs but they are designed to show you just how diffent the resulting image can be.
It's fairly clear from these bracelet shots that the camera is being a little blinded by all the white background, but you mustn't think spot metering will always help - look at this case where the point of metering has fallen on the shiny and reflective lobster clip in the centre of the frame, making the shot on the right which used spot metering underexposed.  be careful and give some thought to what you are focussing on.

If you are buying a new camera use I suggest using Jessops camera selector to check if the camera you like has these options available. At the time of writing this I found most cameras in excess of £150 and some less had spot metering as standard, I wouldn't buy a camera without it, even if it meant going second hand.

Please feel free to comment, ask questions, or add your own experiences.