FStop function for DSLR cameras explained

We’ve touched on the subject of f/stops, and seen enough examples to understand that they affect both exposure and depth of field. Now it’s time to explain what the numbers mean and how they affect the quality of our pictures and lens design. Then we will talk about how to use them creatively.

Inside a lens is a set of thin metal plates, called the iris or diaphragm. These create a variable opening in the center of the glass. When the iris is wide open, more light is let in; as it closes, the intensity of the light reaching the sensor is dimmed. The opening itself is called the lens aperture.

An f/number is the ratio of the focal length of the lens divided by the diameter of the aperture. Don’t worry about the math. All we need to know is that the formula can be used to determine the relative intensity of light reaching the sensor.

A “full” f/stop is an aperture opening that cuts the intensity of the light exactly in half (stopping down), or doubles it in intensity (opening up), compared to the next full stop in the series. That’s the same change in exposure value as when we halve or double the shutter speed. So we can easily choose an exposure combination to suit our needs.

The next image shows the f/stop adjustment ring and DOF calculator on a 50mm manual lens. With most new DSLR lenses the f/stops are adjusted using camera controls, so they don’t have an f/stop ring.

F/Stops, Maximum Aperture, and the Need for Speed

The widest f/stop (smallest number) on a lens is its maximum aperture. The lower the number, the more light it lets in, which can be very important when working in low-light conditions, or when you need to use the fastest shutter speed or the lowest ISO setting possible. Maximum aperture and build quality are major factors in the cost and available features of a lens.

They also affect the technical quality of the pictures it takes. Let’s look at some extreme examples: this picture was taken with an extremely fast f/1.2 55mm prime lens under marginal shooting conditions.

The exposure was 1/5th of a second at f/1.2 with an ISO of 1600. The lens weighs twice what the much less expensive f/1.4 50mm model prime. It also let me get that shot under extremely adverse conditions.

The longer the lens, the more it costs to increase the maximum aperture, and the more it weighs.

The 70-200mm is classed as a professional lens, and is built to a higher standard with more durable construction and a more-developed optical formula that produces sharper images with less distortion. It is also designed to work with full-frame sensors. The 55-200mm lens can be used only with cameras sporting a smaller 1.5 crop-factor sensor.

Is the $1,700 model worth that much more? It depends on what your needs (and desires) are. I shoot weddings and sports. The 70-200mm is one of my favorite lenses. I’ve considered buying an inexpensive zoom as a travel lens. They are lighter and easier to pack than faster zooms. There are models for most DSLRs that cover moderate wide-angle to telephoto.