Understanding Aperture and Depth of Field In Photography
Understanding Aperture In Photography
Aperture Basics
To understand aperture in photography, one must understand the relationship between the light reflected from everything in the camera’s frame and the amount of light allowed to enter the lens and hit the camera sensor. Essentially, everything we see with our own eyes is due to the light that hits, and is then reflected, from everything around us. If something is too bright, we squint our eyes to cut the amount of light hitting our eyes. Conversely, the aperture on your camera’s lens allows your camera to either squint when it’s bright, or go wide open in low light.
The aperture is the size of the opening of the diaphragm of the lens. The wider an aperture of a lens is, the more light it can allow into the camera’s sensor. A series of f-stops denotes the aperture size of the lens. Lenses are described by two criteria, the focal length and the largest aperture the lens is capable of opening to.
The aperture ring on this lens is manually adjusted by twisting the ring to the desired aperture.
The standard f-stops found on lenses go from small to large (see diagram above): f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8, f/11, f/16 and f/22. Counter-intuitively, the numbers of aperture get larger as the opening gets smaller. A very large opening would be an f/1.4, while a very small aperture would be f/16 or f/22. The “speed” of a lens also gets faster as the numbers get smaller. A lens with an aperture of f/1.4 is a very fast lens, and is capable of shooting under low light conditions with a faster shutter speed.
Although these numbers don’t seem to make much sense at first glance, they are actually quite logical. The f-stops of a lens are tied to a ratio of the focal length of the lens.
Depth Of Field
Understanding depth of field in photography is knowing essentially the distance between the closest and furthest objects that are in acceptable focus in your image. Depth of field is determined by the size of the aperture opening and the focal length of the lens, in addition to the distance at which the camera is focused. The larger the aperture opening or the longer the focal length, the shallower the depth of field your image will have. To keep most of your image in focus from close to far, you will need to have a long or deep depth of field by using a wide lens and small aperture opening. To have a shallow depth of field where only a specific object is kept in focus, you’ll use a longer lens with a larger aperture opening.
This image was made with a Fujifilm X-T2 at 200mm with an aperture of F/9.
The butterfly image above has a very shallow depth of field, and everything other than the subject is smoothly out of focus and blurred.
This image was taken with the Rokinon 12mm at F/5.6 with the focus set on infinity. Everything in the image is very sharp, and the image has a large depth of field.
How Camera Sensor Size And Lenses Effect Depth Of Field
It has long been held that full frame sensors produce a shallower depth of field than crop sensor cameras. However, it appears that this may have been a somewhat false narrative, based on a faulty comparison. In fact, when comparing sensor size and shallower depth of field, it appears that the smaller crops sensors do indeed shallower depth of field.
Typically, this comparison is done with a full frame and crop sensor, using say, a 50mm at f/2.0. Both cameras at the same distance to the subject. But think about it, if both cameras are at the same distance from the subject, you’re not even getting the same image, or frame of view. In order to get the same frame of view on a crop sensor, you need to increase the distance of the subject. This added distance is what increases the depth of field on the crop sensor.
The focal length of the lens appears to have a an impact on depth of field as well, with longer lenses producing much more blur. A 200mm lens focused at 15 ft will have a super-thin depth of field compared to a 20mm lens focused at 15 ft.
The reason longer lenses appear to produce a shallower depth of field is thanks to their narrow angle of view: compared to a wide lens, a telephoto will fill the frame with a much smaller area of background, so any blur appears magnified too. Use this characteristic to add a professional bokeh to your portraits.
Hopefully this article has given you a better understanding of aperture and depth of field in photography.
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