Aperture is the opening that lets light pass through your lens to your sensor.  The iris is the series of blades that open and close to make your aperture larger or smaller.

Understanding the relationship between the aperture and the depth of field is key to achieving the artistic goal any shot you might take.  

Each lens will have a maximum aperture, this is the lowest f stop that the lens will operate. The more open the iris is, the shallower the depth of field and transversely, the more closed the iris is, the deeper the depth of field. A wider aperture has less depth of field. If you want to go deeper on this topic, we suggest you watch the segment on depth of field in our course titled camera controls and settings.

The maximum aperture also known as the speed of the glass. Its expressed as a ratio, like 1:1.2. The fast lenses are made up of more glass, excel in low light and allow for shallower depth of field.  

The maximum aperture of a zoom lens can change over the throw or zoom of its focal length. This is expressed as a ratio like 1:3.5-1:5.6. Typically zoom lenses with constant aperture across entire focal range are more expensive. But ideal for video. Their consistent aperture allows for the same exposure regardless of the focal length.

there is also a correlation between the maximum aperture and the physical size of the lens. Faster lenses tend to be physically larger.

A lenses minimum aperture isn’t typically important. Because most modern lenses have an aperture of f/16 or smaller, a lenses minimum aperture does not come in to play unless you’re trying to shoot in a situation without time to focus. In that situation having a wide depth of field, would offer everything in focus, making run and gun shooting possible. Minimum aperture is however important when operating a macro lens. We’ll cover more on macro lenses later on in the course in the section titled specialty lenses.

Most lenses provide sharpest results when used in the middle part of their aperture range. Lenses tend to have performance problems when used wide-open. Stopping down or increasing the f stop, helps a great deal, but once the aperture becomes too small then an optical phenomenon known as diffraction comes into play and the quality deteriorates once again. Because of this, most lenses work best in the middle of their aperture range. F/8 or f/11 usually fall into the middle of the aperture range.

So there you have it, understanding what the maximum aperture of your lens is will allow you to know if you are able to shoot in low light, or get a very shallow depth of field. As well, if you're looking to get everything in focus, shoot with a smaller aperture. And lastly, if you’re looking to get the sharpest picture out of your lens, f/8 or f/11 will be the ticket.