7. How to Take Photos 2: Aperture
Almost all this information comes from lessons by Chris Parker and Chris Bray, including the photo examples. Information on sweet spots comes from Simon d'Entremont on Youtube.
Aperture (the size of the hole in the camera’s diaphragm) controls depth of field, which is the amount of space in the image that will be in focus (sharp, crisp). A shallow depth of field keeps only one precise distance in focus:
A large depth of field shows everything in focus:
Aperture can be difficult to learn because it isn’t intuitive – you can’t always see the difference when you look through the viewfinder. You just have to get to know what the different numbers do, when you change the setting – through practice. You should also know, your aperture setting isn't the only factor affecting depth of field. It also depends on your distance from the subject, your subject's distance from the background, and the type and length of the lens you use. Every lens will have a different minimum and maximum F number. In fact, some people are willing to pay thousands of dollars more for a lens that's almost identical to a cheap lens, but with a lower F number:
Aperture is measured in F numbers, the F referring to "focal length". The bigger the number, the smaller the hole, and the longer the exposure must be. With the smaller the number, the larger the hole, and the shorter the exposure time needed.
Remember, for smaller depth of field, use a smaller F number. For larger depth of field, use a larger F number. Don’t worry about what the hole looks like, just remember small for small and large for large. F22 and 32 are pretty large, so you’ll get more detail in a big depth of field. At F2, you’ll have a very small depth of field, meaning anything in front or behind the subject will be blurry. Things close to you may be blurred out completely. This is great for annoying things that get in the way of your shot, like a fence at a zoo:
When Your Camera's Settings Start Flashing
If you try a very large F number in low lighting conditions, the shutter speed (that you see through the viewfinder) will start flashing. This is warning you that there isn’t enough light to get a good exposure at that aperture setting. When it’s flashing, that’s bad. Same with too much light. If you photograph towards the sun, even at the fastest shutter speed, you might see it flashing because there is too much light coming in, and the camera knows it’ll be over-exposed – so it warns you by flashing. If you take a shot while the shutter speed is flashing, the photo will either be underexposed or overexposed, depending on the light. In general, if you adjust the aperture one way and the shutter speed (or ISO) the other way, you can come to a compromise point where the numbers will stop flashing and you'll get a good exposure.
Lens Speed, Distortion & Sweet Spots
Photographers will often talk about the “speed” of a lens. Lens speed refers the maximum aperture size. The smallest possible F number (the largest opening) is the "speed" of that lens. It lets you know how quickly you can get lots of light through the lens. Photographers will often spend lots of money on a "fast" lens.
Having said that, you might not want to use the largest aperture on your lens - because it might be outside of the sweet spot. Kind of like how a tennis racket has a sweet spot, lenses have a smaller range of apertures where you get the sharpest image, with the least amount of distortion. Lenses get all kinds of distortion (or aberration):
Large apertures can cause:
Chromatic Aberration: This is when the lens bends the light at extreme angles into the camera, and in the process different colors (with different wavelengths) bend at different rates, separating from each other:
In a photo, it looks like this (note the orange/red edges(:
Spherical Aberration (light dispersion): this is due to the rounded shape of a lens. When light hits the edges of the lens, it doesn't quite line up perfectly as it travels through the lens to your sensor. The result is that the center of the picture will be sharp, but the edges of the photo will be blurrier, even at fast shutter speeds.
Small apertures can cause:
Diffraction: any time light goes through a very narrow gap, or hole, it bends. This simply leads to blurrier images.
Vignetting: This is when the light hitting your sensor is so narrow that it doesn't evenly hit the corners, making the corners darker. Photographers often like the effect of vignetting - you can add it to a photo later on, in a darkroom or digitally in programs like Photoshop and Lightroom.
Coma: this is when light-emitting objects like stars in the edges of the photo get pulled and stretched, like a piece of taffy:
Barrel Distortion: this is a distortion in magnification so that things in the center seem larger than what's outside the center. This is different from lens compression, and is an effect of the curvature of the lens - Fish Eye lenses use extreme barrel distortion on purpose.
You can find your lens's sweet spot by checking this website here: dxomark. It shows charts like this one:
If you click on "Measurements" and "Sharpness" you will get a helpful chart (that's easier to understand):
In general, a shallow depth of field is better for portraits, because you don't get distracted by details in the background. Which of these emphasizes the person better?
and a large depth of field is generally better for landscapes, when you want to see all the background details:
Bokeh
Using a shallow depth of field is a very powerful compositional tool to force viewers to focus on what you want – mastering aperture really helps make you a more professional photographer. When you master this, you will find that blurry areas of your image form little circles of light. These are called bokeh, and can be quite beautiful. Different lenses form different bokeh shapes.
For landscapes, even with a high depth of field, try to focus primarily on what’s near you – far objects will still be well in focus. But, if you focus on what’s in the background first, what’s in front will be blurry. If you want a really sharp, in-focus landscape image, the aperture will be so small (the F number so big) it may need a longer exposure, in which case you may need to put the camera on a tripod.
Aperture and Light
One thing to note, when you use a large F number (small aperture), and take a shot with the sun in it, or any bright light, you’ll get a beautiful starburst pattern of light from the sun – showing rays of sunshine (this may vary from lens to lens).
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