UNDERSTANDING LENS APERTURES

Updated: May 22

You have no doubt heard about how aperture affects your exposure and depth of field but have you ever asked yourself why??


Every lens has a diaphragm opening which allows light to pass through it. This is what we call the aperture. It works very much like the pupil of an eye. In dark lighting your pupils will widen to allow more light to pass through. In bright light the pupils will narrow to restrict light. Your camera will use the aperture of a lens the same way. Fortunately this does not have to be an automatic reaction. In manual and TV priority modes you have to think and decide how to control it. Which is a marvellous thing to help you get more creative with your photography.


Aperture is measured in F stop numbers. The lower the number the wider the aperture. The range varies depending on type of lens you use. Most detachable lenses for DSLR cameras will cover a range between f4 to f20. Some go lower and some higher depending on their intended design


Take a look at the three examples below which demonstrate how changing your aperture alone can change your exposure. All three images were taken at ISO 100, with a shutter speed of 1/3 sec.



EXPOSURE ONE

This image was taken with an aperture of f8 which was metered in camera as being a correct setting for the ISO and shutter speeds mentioned above.







EXPOSURE TWO


Here I changed the aperture to f16.

As you can see the image is now underexposed due to less light passing through the lens.




EXPOSURE THREE



I then widened the aperture to f3.2. The result is a much brighter image due to more light passing through the lens.









Typically ranges below f6 are classified as a wide aperture. Ranges of f16 and above are know as a narrow aperture. The sharpest aperture settings for most lenses often referred to as the "sweet spot" will be in the middle range of f8 or f11.


You may have notice a change in sharpness and detail on the dice in the background of the three images. This is because changing your aperture also has an impact on depth of field both in foreground and distance.


Why is this? The aperture creates a light cone. The narrower the cone the smaller the circle of light confusion. A wide aperture creates a larger circle of confusion.


Take a look at the three images below which demonstrate how changing the aperture affects depth of field. They were all taken with a 40mm lens at ISO 100. I changed both the aperture and shutter speed to keep a consistent exposure. In all the images `I had my focal point set on the counter with three bars in the middle of the image.



SETTING ONE


The first image was taken at f4 with a shutter speed of 1/8 sec. Notice the foreground and background are both out of focus.






SETTING TWO


Second image was taken at f8 with a shutter speed of 1/2 sec. You can see the counter in the foreground and the background are now a bit sharper but still not in clear focus.



SETTING THREE

Third image was taken at f16 and a shutter speed of 2 seconds. You can see this is a sharper image front to back.








You will have noticed from the settings of the three images that I had to compensate with a slower shutter speed to make up for the reduced light created as the aperture was narrowed.




To get a better understanding on how apertures work I really recommend practice and experimentation.


A suggested exercise for you:-


Pick a static subject which you can get close to.

Take three photographs keeping your ISO and shutter speeds the same.

Take one at f8 with a suitable shutter speed to give you a correct exposure.

Take the second shot with the same ISO and shutter speed settings but widen the aperture to f4

For the third shot just move your aperture to f16 and compare the results.


You should have a correctly exposed image along with an under exposed and over exposed one.


Repeat the exercise again but this time adjust your shutter speed for all three shots to keep a good exposure. Compare the three images to see how it's impacted your depth of field.


I hope this has now left you with a better understanding on what an aperture is and how to control it to get the best out of your photography.


For more information on my workshops and tuition courses please visit my website at https://www.louiseehubbardphotography.com

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