Hello everyone. Today I would like to talk about how I use the exposure triangle to calculate the my exposures. If you are not familiar with the exposure triangle it is a common way of associating the three variables that determine the exposure of a photograph: aperture, shutter speed, and ISO. you must balance all three of these to achieve a desired result, an adjustment of one requiring adjustments of at least one of the others. You must also remember what changing each of those will do to your image, and decide which changes you can make to create the image your mind’s eye is trying to create.
So let’s take a look at the three sides of the triangle and what each one will affect.
Side 1: Aperture
The Aperture number is a measure of how open or closed the lens’ iris is. A wider aperture means more light will be let in by the lens (f/2.8), simply because the opening is larger. A narrower aperture (f/22) allows less light to reach the sensor. But this does not explain what effect it has on the image.
The lens aperture is mainly used to control the depth of field of your image. The depth of field is the distance in front of, and behind your focus point that is in focus. The larger the aperture (f/2.8) the smaller this distance will be. For wildlife photography you would usually want a narrow depth of field to allow you to isolate your subject from the background, so I am normally at the smallest aperture I can. SInce I normally have the aperture set to the largest I have available, I don’t often use this to make changes to the exposure.
One positive of using a small aperture is since it allows the most light to hit the sensor, you are able to use a faster shutter speed or lower ISO.
Side 2: Shutter Speed
Shutter speed is a measure of how long the shutter remains open or how long the sensor is exposed to light. you might want to use a higher shutter speed is to stop motion, whether that be camera shake or a subject that is moving, allowing you to take the sharpest image you can.
To me this is a very important consideration. If your shutter speed isn’t fast enough to give you a sharp image, nothing else will save the image.
When I am working out an exposure I will take into consideration what shutter speed I need to create the effect I am looking to achieve. If I am looking to freeze the wings of a hummingbird, then I will need @ 1/2500 sec., but if I am taking a portrait of the same hummingbird sitting on a branch, then 1/250 would be more then fast enough. I always try to use as slow a shutter speed as I can so that I can keep the ISO as low as possible.
Side 3: ISO
ISO measures the sensitivity of the image sensor. Higher numbers mean your sensor becomes more sensitive to light which allows you to use your camera in darker situations. This sounds great! Why don’t we just shoot at the highest ISO are camera has? Because when you increase the ISO you also increase the noise in your images. This is where knowing your gear comes into play. You, and only you, can decide what is an acceptable amount of noise, and what the highest ISO you are willing to use.
This side of the exposure triangle is where I make most of my exposure adjustments for bird photography.
Now to put it all together
let’s look at an example of how I work through an exposure. For this example I will be trying to take an image of a hummingbird in flight. I want to catch it as it just gets to the flower, but before its beak enters the flower. I want the wings to be slightly blurred so there is a sense of motion to the image, but I want the rest of the bird to be tack sharp. It is a slightly overcast late July day @ 11;00 am. I will be using the camera in Manual Exposure.
First I will set my lens to the largest aperture available because I want to really blur the background and get the bird isolated in the image. This will probably be f/5.6. Then I will set the shutter speed to 1/1250 sec. as I know from experience this will give me the wing blur that I am looking for. So now I have two sides of the triangle set and I just have to figure out what ISO is needed to create the proper exposure.
At this point it is vitally important to remember what your camera is trying to do to create an exposure reading. It is trying to make the scene 18% grey. So knowing this I find a patch of green grass or leaves that are in the same lighting as the hummingbird will be in and take my exposure reading off of that. When you convert green grass or green leaves to grayscale they are close to 18% grey. I then set the ISO to a number that will give me a correct exposure meter reading.
Since I have used the largest aperture, and lowest shutter speed I can to create the look I am trying to achieve, I have also enabled me to use the lowest ISO possible to achieve the correct exposure. This will give me the sharpest image of the bird, isolated from the background, with nice wing blur, and the lowest noise possible.
Now I just need to wait for the humming bird to visit the one flower that I want it to! Most times the waiting is the hardest part!
A couple of reminders. My 2020 calendars are ready to order at: https://www.wpotrebkaphotography.com/2020-calendar
and registration for my October 7th Goose Flight Workshop at FortWhyte Alive is now open at:
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