April 25, 2011

Food Photography Basics #4: It’s All About Light – Exposure


In the next few posts in this series, we will be exploring the various aspects that control the amount of light in photography. Do remember that food photography, like any other kind of photography, is about photography first and food second so the basic concepts of photography apply here too. All the photographs in this post are going to be "egg-centric" for no other reason than that we've just been through an Easter weekend!
There are various factors that go into making a good photograph but it all boils down to light and well we get to know it and learn to use it. In photography, this means understanding exposure.


So, what is exposure?


As Wikipedia explains it, “Exposure is the total amount of light allowed to fall on the photographic medium (photographic film or image sensor) during the process of taking a photograph. So exposure is a measure of how bright your photograph can be, and in DSLRs the camera usually sets this automatically by default using an inbuilt metering system. If too much light comes into your camera you will have a blown out or overexposed image. If there’s too little light, the image will be dark or underexposed.




Talking specifically about shooting food photographs, most photographers prefer to shoot in “Aperture Priority” mode (where you decide the aperture, and ISO setting, and the camera calculates and sets the shutterspeed for optimum exposure). Yet sometimes one finds that the photograph is either overexposed (too much light) or underexposed (not enough light) by some degree.

The easiest way to correct this brightness, or lack of it, in your picture is by making exposure compensation adjustments. The exposure compensation slider is usually indicated on the camera as a graduated line (in my Canon 450D screen it is below the shutterspeed and aperture readings) progressing from a –ve value on the left, a “0” in the middle to a +ve value on the right. “0” is the base (the setting I’m assuming you shot the image you want to correct for exposure) from which you can move to the left (darker) or to the right (brighter), one stop at a time.

So if you move to the right one step from “0” to “1”, then your image will be twice as bright, and the next step to “2” will make it twice as bright as it was at “1”. Conversely your image will be half as bright at "-1” and half as bright as this at “-2”.

If this sounds confusing, just take a look at the photograph below and you will understand what I’m saying. You can see the image in the middle of the tile ("0") is the the one at optimum exposure (as decided by camera) on those on the left are overexposed and those on the right are underexposed. These phothographs were taken at aperture setting f /3.2 and ISO 100 using a 50mm f/ 1.8 II lens. The shutterspeed values changed according to exposure compensation levels.
This means that adjusting Exposure Compensation really is useful mainly in semi-manual modes like Aperture Priority, Shutterspeed Priority modes. In Manual mode you can adjust your settings to give you the exact exposure you're looking for.


1/30             1/60            1/125          1/250           1/400


This will correct exposure levels to some extent but however might not always give you the particular exposure level you desire. For this you have to also take into consideration which light metering mode will work best for your particular situation.

First let’s just take a look at what a metering system is.
A metering system measures the amount of light in your area of composition (the picture that is in your viewfinder/ frame) and calculates an average amount of light to decide the “correct” exposure for your photograph. This means that your final photograph is exposed just right, neither too bright nor too dark. However, the camera's metering system is designed to meter for about 18% grey with which it calculates the shutterspeed or aperture or exposure depending on your mode of shooting. While this is fine for exposure in average light conditions, the camera can miscalculate in a situations where there's a lot of white (or black in your frame. In such situations one can use a grey card to adjust exposure.

Then there are small hand held devices called light meters which will measure the light in the area of your frame or focus, and based on this you can calculate best shutterspeed and aperture settings for optimum exposure. A light meter is a good thing to have but not absolutely necessary and the DSLR’s inbuilt light meter does a reasonably good job most of the time.




Depending on the situation you are photographing you can select the most suitable metering mode for it. DSLRs usually come with 4 metering mode choices – centre weighted metering, spot metering and evaluative metering. Canon DSLRs also have a partial metering mode which is really a slight variation of the spot metering mode.

What you read might sound a bit confusing but if you try shooting in each metering mode, you will come to understand the differences. As I keep saying in every post, it takes practise and exploring each metering more to become comfortable with them and this will eventually help you figure the best metering mode for your photographs.


Evaluative Metering:




Depending on your camera, there will be numerous metering zones within your frame (mine has 35 metering zones). Evaluative metering reads the entire scene, and chooses the best overall exposure. This is a good metering mode to use for general photography and also if you’re not too sure what mode to use.


Spot Metering:




In this mode, the camera meters the light from a “spot” you decide on within your frame to give the desired exposure. Here, only about 1 - 5% of the frame is metered to calculate the exposure and the rest of frame is ignored. This is useful in a setting where your subject is backlit (light comes from the back) and would be silhouetted with the subject detailing showing up dark in your photograph. You can use spot metering to light up your subject better.


Partial Metering:




Partial metering is a variation of spot metering in some sense, as the camera uses only about 10-15% of the entire frame to calculate exposure. If there are very bright or very dark areas on the edges of your frame you can avoid that affecting the exposure of your photograph with this metering. This is also good for macro shots and backlit subjects except the camera meters a slightly larger area than in spot metering.


Centre-weighted Averaging:




Centre weighted average metering is somewhere between evaluative and spot metering. In the centre weighted average metering, the camera meters light from the centre as well as the other areas in the frame. It tends to concentrate between 60 - 80% of light sensitivity towards the centre of the image resulting in a good exposure.
This mode is less influenced by small areas of varied brightness at the edges of the viewfinder and good for a large variety of shots. This mode of metering is somewhere between partial and evaluative metering. This is a good mode to use when you want more control over what’s in the frame, especially around your area of focus.


Automatic Exposure Bracketing (AEB)

Sometimes the light situation can make it difficult it difficult to decide the settings which could give you the best exposure. It could be a situation, like photographing a frozen dessert perhaps, where you need to work quickly to get the perfect shot. At such times you could use the “Automatic Exposure Bracketing (AEB)” feature which most DSLRs have. To use this, you turn the AEB function and set the function. Once you depress the shutter button, the camera will take three shots at the same time of the same frame - one optimally exposed image, an underexposed and one overexposed. If your camera is in single shot mode you have to depress the shutter three times for the the three shots but just once if in continuous/ burst mode.

If you use AEB in either Aperture Priority mode or Shutter Priority mode you can have a little more control with the exposure as you can set the aperture or shutterspeed and the camera calculates the exposure depending on your settings.


So that’s all there is to exposure? Unfortunately or fortunately, depending on how you look at it, no!

Exposure (the amount of light in your photograph) is dependent on 3 other factors. These factors are equally important and as often referred to as points of the “Exposure Triangle”. These factors are Aperture, Shutterspeed and ISO. You can make changes in one or more of these to ensure a better or desired exposure for your image. Apart from affecting the amount of light in your images, changes in each of these factors will also affect your image in other ways.


(Source : Robert Ellis)


A larger aperture (smaller f-number and bigger opening) means more light but gives you a shallower depth of filed (DOF) or blurriness and vice versa.

A lower shutterspeed results in more light in the camera but can capture motion blur. A faster shutterspeed will mean less light but you can freeze motion in your images such as falling/ pouring motion of liquids.

A low ISO means low sensitivity to light which you can use in a well-lit situation. Low light conditions require higher ISO values but can end up making your photographs “grainy” or “noisy”.

In the following posts, we will see each of these three factors in detail and how they can make a difference in photographs.

If you have any questions about what has been covered in this post or otherwise, please leave a comment here or send me a mail and I will do my best to answer your questions.



Useful Links :


DSLR Metering Modes Explained

Metering Modes Explained

Understanding TTL Metering

Metering

The 18% Grey Card

Using A Grey Card For Correct Exposure

The Exposure Triangle

Automatic Exposure Bracketing



So Far In This Series


Food Photography Basics #1 : Do I Need A DSLR To Get Good Photographs?

Food Photography Basics #2 : Which Camera? What Lenses?

Food Photography Basics #3 : Getting Started


Finally, just a mention that I'm giving away a couple of cookbooks and you might like to give your luck a swing. All you need to do is leave a comment.

1 comments:

Shweta Pandey said...

Hi Arpana,

I am very impressed by your write up and blog. The way you have tried to explain every aspect of food photography shows your sincerity. It’s so wonderful to see how you want to share your knowledge.

Thank you!!