DVPhoto

As with all tutorials you may find in this blog, this is put here to help both me, as well as you the reader, learn. Reading something is a great start to learning, but doing is much better; and with something like photography, learning and understanding the science behind it will help you even more. So write down notes, practice taking photos with what you have learnt, and record the results. It requires effort, but the benefits will be immense

The Basics

So you want to know what aperture and f-stop are in a couple of lines? Well aperture governs the amount of light that a lens lets through to the film/digital sensor, and aperture is indicated by the f-stop. f-stop numbers available vary by lens, but most will have a series of f-stops somewhere in the following range: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45.

The smaller the f-stop, the larger the aperture; so f/1 is a very large aperture, letting in a lot of light, while f/45 is extremely small letting in almost no light. Each increase in f-stop (i.e. from 1 to 1.4, or 4 to 5.6) halves the amount of light going through the lens, and thus to maintain the same exposure you must also slow the shutter speed down by half (i.e. 1/250th sec will need to be 1/125th sec). So when a photographer says to “stop down”, you need to increase the f-stop number; and using a lens “wide open” means using the lens with the smallest f-number available to it.

So why was this strange and seemingly random set of numbers chosen? Well first we need to look at how light works…

Distance & Light Intensity

There is a law in science called the Inverse Square Law: Any point source which spreads its influence equally in all directions without a limit to its range will obey the inverse square law. The law describes, to put it simply, the effect that when distance is doubled from such a point source (like a flash) the intensity of the light is reduced by four.

To determine the distance change when doubling or halving the intensity of light, we multiply or divide the distance by √2, which is 1.41 (rounded off for ease). As you may have figured out already, when doing flash photography, one can easily change the distance of the flash to achieve the same effect as stopping up or down.

f-stops

So now you can see, the f-stops on a lens follow the sequence of the powers of the square root of 2 (rounded off for ease). Each stop down (large f-number) halves the light, each stop up (smaller f-number) doubles it. Now these numbers are not the only f-stops you can have, and most modern cameras allow increments of 1/2 or 1/3.

But what do these numbers actually represent in the lens?

Aperture size and Focal length

Well the “f” in f/1.4 stands for focal length. So a 50mm lens with a f-stop of 1.4 would be 50/1.4 which equals 35.7. This is the diameter of the aperture, and as you can see the diameter for a lens with a longer focal length would have to be even larger to gain the same f/1.4 (an f/1.4 85mm lens for examples would have an aperture of 60.7). This explains why you don’t get 600mm f/1.4 lenses that fit onto a 35mm body.

Just as an experiment, calculate the aperture diameter for a 50mm lens at f/1.4 and f/2. Then calculate the area of each aperture circle (πr²). Assuming you did the calculations correctly, you’ll find that the are of the aperture at 1.4 is twice that at 2 (with some errors due to rounding).

What aperture affects

Well now that you know what aperture does, many may be asking why don’t you just always use the lens wide open? Well aperture affects many more things than just the amount of light entering the lens. I’ll briefly list them here, though they will be covered in more detail at a later date.

• Aperture effects DOF (depth of field: the area that appears to be in focus), with large apertures having very shallow DOF and the DOF increasing as the lens is stopped down.
• Using a lens wide open will often result in vignetting and aberrations.
• Stopping the lens right down will result in diffraction, thus effecting sharpness.

So what f-stop should you use? Well in the end it’s all about what you want from your shot; if you need as much of the photo to be in focus as possible, stopping down is the answer; to narrow down the DOF to help emphasise the subject of the photo, you’ll want to use the lens wide open. However, the sharpest pictures will generally come from the mid ranges, f/8 - f/11.

Don’t worry about it too much though, it’s better to get the photo at a less than perfect f-stop than to not get it at all!

Tags: Tutorials

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