Getting it right – Exposure – Tip 2

To follow on from my last “getting it right” post we will continue looking at exposure. Please understand that I am really giving some basic tips about exposure, you can go into serious detail about exposure, in fact, whole books have been written on the subject. You can look at things like Ansel Adams Zone System (mostly applicable to negative film, but can be used in digital) this is a complex system, but can give some amazing results. Many photographers swear by light meters, which are great and add tremendous value, if you have some spare cash. Others will talk about a grey card, a less expensive solution, but will also work pretty well (If you arent sure what a grey card is let me know)

I am not going to debate the merits of each of these ways or measuring light, what needs to be said is that you need to choose a system that works for you and then make sure you use it.

How to control Exposure

We saw in the previous tip that you as the photographer need to control exposure. The best way to do this is to ensure that your camera is in Manual mode (See image below)

Yes, I know, you will HAVE to come off “GREEN” mode and no…green mode has nothing to do with your carbon footprint. Photography is all about control and if you continually shoot on “Green” or auto mode, you will never really have any control of how your image is exposed. Now, when you turn that dial to “M” and look through the viewfinder, you will see information that you may never have seen before. The first thing you will see is a graph like this…

You will also see numbers that will tell you your shutter speed (normally a number like 1/150, which means the shutter will open for one one hundred and fiftieth of a second…really fast, but most SLR’s can go up to 1/4000) and then another number that will have an “F” in front of it i.e. F 8. The F number is your aperture and the other number is your shutter speed. Now these two functions work in tandem to determine exposure. The aperture determines how much light is allowed to fall onto the sensor, while the shutter speed determines how long that light is allowed to fall onto the sensor. Pretty easy so far. It is this relationship that determines how your image ultimately turns out. Remember as photographers all we have to do is control how much light reaches the sensor…simple.

Aperture is important in determining depth of field. Depth of Field speaks about which part of an image is in focus and which part isnt. If the subject that you are shooting is in sharp focus and the background is totally blurred, then that is referred to as SHALLOW depth of field, an image like this would be seen to be having a shallow depth of field (Images courtesy wikipedia)

This effect is achieved by having the aperture wide open. The lower the F number is, the wider the aperture. The higher the F number the smaller the aperture. Now, the same image shot on a small aperture would give deep depth of field, that means that everything from the flower to the items in the background will be in focus or will be visible, like this..

A quick little diagram to help you understand how wide the aperture is a what read is below:

As you can see, the higher the F number goes, the smaller the aperture is….

So when do you use this technique? Well, if you are doing macro photography and want to make your subject matter stand out, then use a large aperture, like F 2.8 or F3.5. This is determined by the lenses you have and the more expensive lenses have wider apertures or fixed apertures (see my post on lenses) If you are shooting landscape images then you want to have as much in focus as possible and so you will go up to F22 or F25 again lens dependant. 

What about Shutter Speed?
It depends what you are shooting. If you are shooting sports, then you will have to push your shutter speed up higher and probably have a wide open aperture. Thats how the photographers get those great sport shots we see when Bryan Habana flies over the tryline and he is in perfect focus and the background is soft. If you are doing landscapes you will want your shutter speed to be quite slow and you will need a tripod to avoid camera shake.

So take a look at the image below, each of them has a different effect based on longer shutter speeds (Photo courtesy Wikipedia)

As you can see, the longer the shutter stayed open, the more light was allowed to fall onto the sensor and the result was the image got brighter. This is how dramatic landscape shots are taken, especially of a seascape where the water starts looking creamy and soft and the waves arent seen anymore. Bear in mind though, you need to make sure that you are measuring the light correctly (Your lightmeter will tell you this) otherwise it will be easy to overexpose a scene.

So the combination between aperture and shutter speed is crucial to exposure. It is also important to taking creative images as we saw in the first tip. So to summarise, tip number 2….get onto Manual mode and start experimenting with aperture and shutter speed.

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