On3legs | What the f stop…
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What the f stop…

Understanding Exposure part 4

In yesterdays blog post I explained why you may want to be able to make adjustments to shutter speed… you can read it HERE if you missed it. If you have just tuned in you may want to start at part 1, the Bermuda Triangle of Photography otherwise things might get confusing.

In today’s blog post, we’re going to look at aperture, or ‘f-stop’ if you want to sound like a real pro! Aperture relates to the amount of light you’re letting in through your lens, just like the pupil of your eye. You can open or close your aperture depending on how much light you want to let in. Remembering, you need a specific amount of light to get the correct exposure, so if you open the aperture up to let more light in, then you would need to use a faster shutter speed to make sure you didn’t over expose your image.

Aperture is represented by an ‘f’ number. The confusing thing about ‘f’ numbers is that the smaller the number, the bigger the opening and the more light you are letting into your lens. You may of heard of the term ‘stopping down’, all this is referring to is closing the aperture and letting less light in, and to make it confusing, stopping down would mean you were using a higher f number. Every lens is different, and some will go as low as f1.4, typically you will use from f2.8 up to f22. A kit lens will probably start around f3.5 whilst a more expensive lens will go as low as f2.8, we will look at lenses and f numbers of lenses in a later blog post.

If you read yesterdays blog post you should be able to see the relationship between aperture and shutter speed. At this stage, all you need to remember is the bucket theory I used in part 2, if you let more light in, you will need to make the shutter speed faster, let less light in and you will need a longer shutter speed. The same works in reverse, if you decide you want to show motion blur and decide to slow down your shutter speed, you will also need to let less light in or you will over expose your image.

Adjusting your aperture has a couple of other side effects. In particular you need to understand how it affects the ‘depth of field’ of your image. That is, how much of your image is in focus. Have you ever seen a portrait where the background is a nice soft out of focus? This is known as bokeh and is a side effect of using a large aperture (small number). If you use a small aperture (large number) then your depth of field will increase, making more of your image in focus.

Tomorrow we will look at what ISO does and how to use it to your advantage. We will also look at how to use the histogram to know whether or not we have got the perfect exposure.

The Daily Pic – Creamy Bokeh

This is a photo of one of my dogs, his name is Rocky! Tough hey! Anyway, in this image I have used a small f number to blur the background, this is known as a shallow depth of field as it has very little depth in focus, when taking portraits I like to blur things that aren’t important to the image, and that otherwise would detract from the main subject. Using a low f number and blurring the background has made Rocky stand out.


A small f number (large aperture) gives me a nice bokeh

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