a DSLR with an 85mm lens, notice the 7 bladed aperture.

           An aperture or Iris is a diaphram made up of usually 5 or more metal leaves. as shown above. I find that aperture is easy to relate to because it operates similarly to the way the human eye works. Just like when you walk into a dark room your iris expands your pupil to allow in more light, so too setting a larger aperture on your lens will yield a brighter exposure with more light being recorded to the film or digital sensor.

            Every lens has an aperture which is made up of a series of metal blades overlapping each other to create a circle. Similar to the human eye, the aperture allows in more light when the aperture is wider and less light when the aperture is smaller. When you press the shutter release to snap a photo, an instant before the shutter opens, the lens aperture closes down to a predetermined size called an Fstop. (Fstop is set by the camera in auto mode,  and set by the user in Manual or A mode.)

Fstops

           Just like with Shutter Speeds, Fstops are adjusted in 1/3 stops, ("Stop" means halving or doubling the amount of light being recorded) so 3 clicks of the aperture dial will result in exactly half or double the amount of light (one full stop).  The lower the f/number the Larger the aperture, the higher the f/number the Smaller the aperture. see the diagram below.

Fast Glass

     Not all lenses can go to very wide apertures like f/2.8 or f1.4, in fact a zoom lens with the ability to open up to f/2.8 vs a zoom lens that can only open up to 5.6 can easily have over 1500$ price difference. A lens that has the ability to open up to wide apertures like 1.2 or 2.8 are known as "Fast Glass" due to there being able to gather more light with a faster shutter speed. Dont have 2000$ to blow on a lens? your not out of luck, you can buy a f/1.8 50mm prime lens for about 200$!(Nikon) (B&H) (canon version) this lens does not zoom but it can take photos in low light using a shutter speed more than 4x as fast as an 18-55 kit lens! Kit lenses are the cheap lenses that generally come for free or in a kit with low end and midrange prosumer dslrs. Kit lenses such as the 18-55, 55-200, 18-200, generally have a variable aperture of f/3.5-5.6, this means that when zoomed out the lens apertures max setting is f/3.5 but if you zoom in the max aperture automatically stops down to f/5.6. for all intents and purposes many pros refer to variable aperture lenses by there max fstop when zoomed in ie: 3.5-5.6 lens = f5.6 lens. or in pro slang its called a "five six, lens" AKA "Slow zoom"

Depth of Field (DOF)

            We learned earlier that shutter speed affects two things, Exposure and Motion. Aperture also affects two things, the first one we know already is exposure the second thing is something called Depth of Field.

             The concept of Depth of Field can be a bit of a confusing subject, I will do my best to explain it in as clear a way as I can. The simple explanation is that DOF refers to how large the in focus area in a photograph will be. Large apertures (low F numbers) will result in a Shallow depth of field. Wide Fstops such as 2.8 are commonly used for portraits when you want to isolate the subject by throwing the background out of focus. Using small apertures (high fnumbers) such as f/8, or f/11 can help you get a DEEP depth of Field resulting in more area in focus, this can be great for photographing landscapes or large group photos where you will want everything sharp. Aperture is not the only factor that affects Background sharpness, other factors such as subject to background distance, Focal length, sensor size and distance to the subject also have great impact on background sharpness and/or DOF. when a large distance from subject to background, a large aperture, a long focal length, a large sensor and a close camera distance to the subject is combined you have a recipe for a great OOF (Out Of Focus) background

A wide aperture, a long focal length, a relatively close subject to camera distance, a large subject to background distance. On a DSLR sensor allowed me to focus selectively on the babys eyes eliminating all distractions. f/2.8 1/250 of a second, @ ISO 200, 190mm.
           

Balancing aperture and shutter speed

             Shutter speed affects how much light hits the sensor by the amount of time that the shutter is open, Aperture affects how much light hits the sensor by how wide of a hole the light has to pass through. If for example you were outside taking photos of squirrels, lets say its an overcast day and your camera is giving you correct exposure at f/5.6, 1/60 of a sec. at ISO 200. If you wanted the background to be more out of focus you can "open up" the aperture to F/4 (double the size of f/5.6) thereby achieving shallower DOF. However you must also increase the shutter speed so as not to overexpose the photo. Increasing the shutter speed speed to 1/125 (half the light of 1/60) of a second would give equivalent exposure as 1/60 of a second, at f/5.6. For the photo below i wanted to really isolate my subject with a shallow DOF so i used an aperture of f2.8 my camera mode was set to aperture priority so the camera automatically increased the shutter speed one more stop to 1/250 of a sec. for correct exposure.

This squirrel was photographed with a 50mm lens. 1/250 of a sec. f/2.8, @ ISO 200

What mode should I shoot in

            Aperture Priority (A mode or Av mode) allows you to set aperture and ISO and the camera selects the shutter speed to obtain correct exposure, in this way you can control your depth of field without having to concentrate as much about obtaining correct exposure, leaving your mind free to think about your subject, composition or framing etc. Experimenting with your camera set to Aperture Priority is a great way to start learning how your camera works and how DOF impacts an image.

Sharpness

         "In  a photograph, A sharp subject is in focus, but an in focus subject is not necessarily sharp." While f5.6 on a kit lens will give u the same Depth of field as f5.6 on a fixed focal length lens (prime lens), the in focus area in a photograph taken with a prime lens will be noticeably sharper than that of a "slow zoom". Most lenses will produce there optimum sharpness about 2 stops below there maximum setting or near there mid range. for instance an 18-55 f/5.6 lens will be sharpest at about f/11 (this is not DOF this is referring to the sharpness of the in focus area), a 50mm 1.8 will be sharpest at about f5.6. Lenses such as the Nikon 70-200 f/2.8 are amazingly sharp even when shot "wide open" at f/2.8. On the other end of the spectrum using high Fstops such as f/16, f/22, f,36 etc. you will start to lose sharpness due to diffraction. Diffraction happens at high Fnumbers b/c the light has to be squeezed through such a tiny aperture this causes some of the light rays starting to interfere with each other which result in a loss of sharpness.

using the Nikon 70-200 "Wide open" at f/2.8 allowed me to get a super shallow Dof so that the subject "pops", notice the beautiful soft bokeh.

Bokeh

    Bokeh pronounced [Bow-kuh] or [bow-keh] reffering to the aesthetic quality of the out of focus areas in an image, there is "good bokeh" where the out of focus area (oof) looks smooth and soft, and there is "bad bokeh" where the out of focus area may have a bit of a texture or look harsh and distracts from the subject. More on Bokeh and DOF in a future blog post.

Good Bokeh. 200mm, 1/250 of a sec, f/3.2, ISO 400

    In conclusion Aperture affects two things
1: Exposure (amount of light being recorded by the sensor)
2) Depth of Field (How large the in focus area is)
Feel free to ask a question or leave a comment, stay tuned for the next Blog post where we will be talking about ISO and its affect on photography. Be sure to check out the previous blog post "What is shutter speed"