I've always known that in photography, the aperture affects the field depth. A small f-stop means the aperture is larger, and a large f-stop means the aperture is smaller. A large f-stop, therefore, necessitates reduced exposure time. With a small f-stop, the shutter has to stay open longer for a proper exposure.
I like things to be simple. Especially when I'm taking pictures, I don't like to think about aperture size, f-stops, etc. My thinking has been that I could set the control on my camera to "Tv" (Shutter priority), choose my speed, and let the camera intelligence make all the decisions for me. I believed, because a higher f-stop lets in less light, that depth of field was governed by how long the film was exposed by light. Although incorrect, that line of reasoning usually worked pretty well for me, because most of my pictures are taken with a zoom lens. Then I took this picture (speed 1/160, f. 5.6):
With my reasoning, it would make sense that at 1/160 the depth of field would be greater. Instead, note how the bird 'pops' out of the background! That made me curious, so I decided to explore this further. I had also taken this (speed 1/160, f. 8.0):
I realized my reasoning must be faulty. So I researched it on the internet (thanks, Internet!). I discovered that it's the angle of light hitting the film that affects depth of field the most. When using a zoom lens, the light's angle is smaller the more the subject is magnified. So the depth of field is already going to be shorter than it would be if you were using a shorter lens. The aperture size also affects the angle of light hitting the film. The larger the aperture (and smaller the f-stop), the wider the angle of light allowed to hit the film. The smaller the aperture (larger f-stop), the shorter the light angle. So shutter speed has absolutely nothing to do with it!
My equipment is the best I can afford. But it's not good enough to take the pictures I want to take in every situation. I need to make up for that by being better at using the equipment. So I will be spending some time in the next couple of weeks exploring the f-stop.
To help me remember what the f-stop is, I "borrowed" a diagram from Wikipedia.*
The smaller the hole the higher the f-stop number. The higher that number, the tighter the angle. The tighter the angle, the shorter the depth of field.
One could think of the f-stop as the "Angle Tightness Indicator." The higher the number, the tighter the light angle. When you want a short depth of field, you need a high angle tightness.
* Wikipedia image came from Wikimedia Commons. It is licensed under the GNU Free Documentation Licence. The picture can be found here.
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