In a previous blog post, we talked about exposure and how f/stop affected exposure by adjusting the amount of light enters the camera. The f/stop adjusts the opening (the aperture) with a diaphragm (groups of leafs that adjust to make a larger or smaller aperture). However, that is not all that an f/stop does. There are three functions of an f/stop and we will discuss them here.

1)f/stops are half of the control of exposure (the shutter speed is the other half).

2)f/stops allow the photographer to adjust the aperture for the best definition of the lens.

3)f/stops control depth of field.

As we mentioned before, exposure is determined by the quantity of light (intensity) and the length of time light is allowed into the camera. The shutter speed adjusts the time and the f/stop adjusts the quantity.

f/stops are half of the control of exposure (the shutter speed is the other half).

We have also discussed that when light passes through a different medium it changes speed. This means that when light passes through glass it moves slower than when it moves through the atmosphere. If the glass is crafted properly, the light bends in a very predictable way and you can recreate the light to form a focused image on the film plane.

When the light does not directly focus on the film plane that part of the image is represented as fuzzy spots. These fuzzy circles will translate to the image as out of focus. If the lens is focused on a particular point of a scene, there will be an area in front of and behind that focus point which will represent itself as “in focus” and everything closer and farther away will be “out of focus.” The distance of area that is in focus is called “depth of field” or DOF.

What does this have to do with the f/stop? Well, the f/stop determines the angle light that comes into the lens. With a small aperture there is less competition for light to get into the lens, and that will increase the depth of field. The larger the aperture the more light can come into the lens and the shallower the depth of field.

Now remember that the larger the aperture, the smaller the f/stop number. So f/4 will be a larger aperture than f/8. That also means that if you are focused on a single object at f/8 and open up the aperture to f/4 you will cut your depth of field and less of the image will be in focus.

f/stops allow the photographer to adjust the aperture for the best definition of the lens.

Before I go into detail on depth of field, I want to address what I mean by best definition of a lens. While lenses may have an indicated range of f/2.8 to f/32, that doesn’t mean that the lens will perform at its best at all of those f/stops. Some lenses lose their quality when you go to their extremes. While it is not true of all lenses, a good rule of thumb is to close a lens down or open it up at least one stop to start getting the best performance from a lens.

There are some newer lenses that perform amazingly well opened wide open, but you will have to research which lenses these are. Some lenses are designed only to work at optimum performance wide open.

f/stops control depth of field.

Depth of field is defined as the distancefrom the nearest point of acceptably sharp focus to thefarthest point of acceptably sharp focus of a scene beingphotographed. The depth of field isn’t the same at all f/stops and it also changes as the focal length of the lens changes. When you shoot at f/4 on a 50mm lens and a 180mm lens, the depth of field will be different.

Depth of field also changes with distance. The farther the subject is from the camera, the larger the depth of field. There is a focus point (called the hyperfocal distance) when everything behind the focus point will be in focus for infinity.

The depth of field can be measured with a DOF calculator. CLICK HERE

Adjusting the depth of field is an important to photography. It gives you control over your environment and isolate or integrate your subject with its environment.

Using a large depth of field with 14mm wide angle to integrate the environment.

Using a shallow depth of field with an 85mm lens to separate the subject from the environment.


There are several factors that must be included when you are considering the characteristics of lenses. To perform well as a photographer, you must recognize the effect of these lens characteristics. Realize also that it is the recognition and use of these various lens features and/or qualities that can make the difference between a good and a poor photograph. By understanding the advantages and disadvantages beyond just the field of view of a lens opens an entirely new level of photography that is an essential, but often overlooked.

“let me be clear on this the focal length of a lens DOES NOT CHANGE PERSPECTIVE”

Focal Lengths of lenses is all about (what I call) “perception of the angle-of-view .” The human eye sees in three dimensions because of binocular vision (two optical cameras with overlapping images). A camera does not have the optical ability to be stereoscopic on its own and therefore does not allow for the sense of depth. What is considered “normal” focal lengtha for a lens gives the similar field of view that our vision covers (which is 45-55 degrees field of view), and the relative DOF (Depth of Field) of these lenses gives the familiar sense to feel the “depth” of our normal vision.

If you close one eye and look with the other, you can still register in your brain the sense of depth, because our brains have been programed to view the sense of Depth of Field through what is in focus and what is not.

However, when you use wide or telephoto lenses, the field of view and the characteristics of depth change properties. The “angle of view” and the perception of depth are changed– hence the “Perception of Angle-of-View” has also changed.

Any lens with an angle of view less than 45 degrees with a given film size has a longer focal length.

However, just as we know that we want/need more than just a pinhole camera to create the images we want, when you start using the studio lights, you will find that you need more from your flash than just a bright light.

Now…let me be clear on this the focal length of a lens DOES NOT CHANGE PERSPECTIVE. If you stand in one spot and change lenses, you get the same perspective no matter what lens you use. It is constant, because perspective is the relationship between the subject and the camera.

There are three classes of lenses that effect “angle-of-view perception:” Normal, wide, and telephoto. Within these catagories there are often subcategories of ultra-wide, zoom lenses, macro/extreme close-up to name a few. Regardless of the nomenclature– the following characteristics apply to the sub-categories.

Normal— as we said, the normal lens gives the same angle of view and feeling of depth of field as your normal vision. For most 35mm film cameras a 50mm lens is considered “normal.”

“ … you should have a good spectrum of lenses in your bag, and know the characteristics of each …”

Wide— Wide angle lenses have a greater than normal field of view. This change of FOV also changes the characteristics of compression between objects. These lenses create an exaggerated sense of distance. For example: You can shoot a close up of a subject model’s face and have a group of people who are close by seem very far away.

Wide-angle lenses have their own qualities, causing apparent, repeat, apparent, distortion and foreshortening of perspective, so objects close to the lens appear large, while background objects diminish in size dramatically

Obviously, there is the advantage of being able to get more of your scenery in your photo with a wide angle lens. You will able to get closer to your subject. You will be able to get steadier shots at lower shutter speeds. You will get greater Depth of Field at almost all distances.

Telephoto— Telephoto lenses have a smaller field of view, but they also have more reach. Just like binoculars or a telescope, they use the science of light refraction through glass to be able to see objects at a far distance up close. The reduced angle of view has the opposite characteristics of the wide angle lense. With the telescopic view, the angle of view is decreased. However, telephoto lenses characteristics also compress their images making subjects seem much closer than they actually are. (remember that this apparent change of distance is because our brains look at things through our “normal” lens. Thus there may seem to be a change in prespective, but the perspect hasn’t changed at all).

Telephoto lenses are great for: Shallow DOF. Making images look closer to each other and the compression often gives a sense of equal sizing. Compression of distance, the ability to see objects far away.


Lots of photographers find a lens they like. Or a zoom they rely on. In fact, you should have a good spectrum of lenses in your bag, and know the characteristics of each.

The difference between a good picture and a great picture is often a matter of matching the images in your head to the image on your print. Very often, people will get lazy, or at least comfortable, and try to make a wide angle zoom do the job of a long telephoto and then wonder why the photo doesn’t cry “brilliance” to them later.

Here are some photos of lenses fromn 10mm to 180mm. I moved between one foot to twelve feet away from the bottles to fill the frame. See how shooting a very similar shot differs greatly as the compression, field of view and DOF change. Note the house next door behind the bottles and how it seems much closer with the telephoto lenses. This is an important consideration in choosing a lens.