Aperture

As the title says, I will focus on Depth of Field (simply called DoF) and the relationship between aperture and shutter speed values. Depth of Field = the area of a photograph where the subject(s) is in clear focus. Playing with Depth of Field values makes a lot o sense when you what to isolate your subject from distracting areas of the background or foreground, thus making it a great thing to play, but not restricted to portraiture, commercial and macro photography.

The amount of Depth of Field in a photograph is given by 3 factors. First, and most important, the aperture value. Aperture is the opening of your lens when you press down the shutter button. Inside every lens you’ll find set of circular blades that move together to control the amount of light entering your camera. DoF values on your camera are represented as “f” followed by a number. That “f” comes from “f-stop” or light stop.

The bigger the f number, say f22 or f32, the less light enters your camera and DoF values of final picture are greater (more field is in focus). In the same way, for smaller apertures (f1.2 / f1.4 / f2.0) more light enters your camera and the DoF is very shallow or narrow (less field is in focus). To conclude:smaller f values => lens aperture is wide open (more light) => shallow DoF (less in focus), and the opposite,bigger f values => lens aperture is smaller (less light) => greater DoF values (more in focus).

The second factor in controlling DoF is the focal distance of your lens. DoF narrows with the increase of focal distance. For example, a 100mm lens opened @ f2.8 will produce a narrower DoF than a 50mm lens with the same f2.8 aperture (opening). In the same way, using an extension tube, which is virtually modifying the focal length of your lens, the DoF values narrows (for the same aperture). DoF values are maximal for wide angle lenses and minimal for telephoto lenses.

The 3rd one is your distance to the subject: the closer you are, the shallow the DoF. If your subject is far away, even if you select a smaller aperture value, your subject and the background will be in clear focus.

In today’s digital cameras, unlike the old passive (non electric lenses) where you had a special ring for controlling the aperture, the lens aperture is controlled electronically, which means that if you look at your lens, you’ll always see it at it’s greatest opening. If you, however, are curious to see the aperture blades in action, there are 2 ways for this: set the aperture values you’d like, and press the DoF preview on your camera body. If a DoF preview function is not available, simply set a longer shutter speed, and look at the lens while taking a shoot. You’ll see that by using different aperture values, the blades opening will be wider or narrower.

But enough talking and let’s see how this translate in images. For the images below, the camera was mounted on a tripod with a 28mm f1.8 lens plus one 12mm Kenko extension tube attached. ISO set to 100, focus was set to manual, almost at infinity. Complementary aperture and shutter values were used to achieve the same overall exposure.

@ f2, the DoF is only 2, maybe 3 millimeters. Judge for your selves: the pistil size was about 3 millimeters.

@ f2.8, more details of the pistils are revealed.

@ f4, the 3 pistils from foreground are almost in complete focus.

@ f5.6, details in the petals texture from the margins start to appear.

@ f8, background becomes clearer. You can see where the stems are ending.

@ f11 all of the pistils are in clear focus.

@ f16, petals texture is clearly revealed.

@ f22, almost all frame from foreground to background is in complete focus. Another interesting thing to watch for is the small dot of light from the background. Look at the ghosting from f2 or 2.8 and see how becomes clearer and clearer. Click the image for slide. Use arrow keys to navigate.

You may wonder why I’ve moved from f2.0 at f2.8? What about f2.2 and f2.5 that’s on my camera? Your camera may be capable of stopping light by one stop going through intermediary levels. If your camera has 1/2 intermediary level between f-stops, will move from f2.0 to f2.8 passing through f2.5. For 1/3 intermediary level, both f2.2 and f2.5 will be selectable. The same goes for the shutter values. Anyway, what is important is that f2.8 lets in half the light of f2.0, f4.0, half the light of 2.8, you got the picture.

Also, looking at the above series of images it is clear that same exposure resulted in each frame. That means if you compare the light distribution (histogram) across each shoot, it will be pretty much similar. To achieve the same exposure when you modify the aperture value, a complementary shutter speed must be selected.

That is, if I cut down in half the amount of light entering the camera by stepping from f2.0 to f2.8, I have to compensate this with a longer exposure of camera sensor to light otherwise, the resulting picture will be too dark. You see on the above images both the aperture and shutter values.

The initial image was metered @ aperture f2.0 and 1/250 (0.004) seconds exposure time. When the aperture moved to f2.8, the light was cut down in half. To compensate this, the exposure time must be doubled, therefore the value of 1/125 (0.008) seconds. Below is a table with corresponding values:

It’s really that simple. You cut one in half, you compensate by doubling the other. Same goes when moving from f2.8 to f2.0, the amount of light entering the camera has now doubled, I have to cut down on the exposure time otherwise I’ll have an overexposed shoot.

The 3rd factor that comes into play when we talk about aperture and shutter speeds (exposure times) is the selected ISO speed (sensibility to light of you camera sensor). But that will be the subject of another story.

Here you can find some more of my shoots with a “twist of DoF”:

http://www.flickr.com/photos/dragosz/sets/72157604267013515/

May DoF hide you

Zet

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