Wondering what “wide open” stands for in photography? Many photographers get confused by this term, but it is really easy to understand, as it has to do with the lens aperture. The term does not refer to a specific aperture, because it is lens-specific.
Wide Open Definition
“Wide open” literally means the maximum aperture of the lens. You will often hear photographers say something like “shoot wide open”. In this case, they always refer to the maximum size of the aperture available on your lens, such as f/1.4 or f/2.8. For example, if you are shooting with a 24-70mm f/2.8 lens, its maximum aperture is f/2.8. If I said something like “The center of the frame looks sharp wide open”, referring to the same 24-70mm f/2.8 lens, you can rephrase this sentence to “The center of the frame looks sharp at the maximum aperture of f/2.8”.
Take a look at the below illustration:
As you can see, the largest aperture in the above illustration represents the “wide open” aperture of f/2.8.
Maximum Aperture is Lens-Specific
It is important to point out that the term wide open does not refer to a specific aperture. The reason is that the maximum aperture of a lens is different from lens to lens. For example, while the maximum aperture of a 24-70mm f/2.8 lens referenced above is f/2.8, the maximum aperture of another lens like the Nikon 50mm f/1.8G is f/1.8. This means that wide open in the latter example would refer to an aperture size of f/1.8 instead.
Wide Open on Variable Aperture Lenses
If one uses a variable aperture lens, such as the Nikon 18-55mm f/3.5-5.6, the maximum aperture of the lens is going to vary at different focal lengths.
For example, at 18mm, the maximum aperture of the lens is going to be f/3.5, whereas at 55mm it will be reduced to f/5.6. This means that the wide open aperture on the 18-55mm f/3.5-5.6 lens is going to highly depend on its focal length.
With Macro Lenses
Many macro lenses are capable of focusing at extremely close distances. Because of the way they are designed, their maximum aperture might shift depending on the subject distance. For example, the Nikon 105mm f/2.8D Macro has a maximum aperture of f/2.8, but only when shooting at longer distances. As you increase the magnification ratio by focusing closer, its effective aperture also decreases to a smaller f-stop.
At 1:4 magnification, the maximum aperture on the 105mm f/2.8D Macro is reduced to f/3.3, while at 1:1 magnification, it drops all the way down to f/5.0. See NIKKOR Micro Lens information for more details about this behavior when using different focal length macro lenses.
When using longer lenses and coupling them with teleconverters, the maximum / wide open aperture is going to depend on how much light loss takes place as a result of using the teleconverter. For example, when using a 1.4x teleconverter, the amount of light loss is typically around one exposure stop, whereas using a 2.0x teleconverter loses two full stops of light. For example, if one uses a 300mm f/4 lens with a 1.4x teleconverter, it essentially becomes a 420mm f/5.6 lens. This means that if you wanted to take a picture wide open, you would be limited to f/5.6, even though the lens has a maximum aperture of f/4. If you couple the same lens with a 2.0x teleconverter, it would become a 600mm f/8 lens.