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Focal Length, Visual Space, Crop Factor, Zoom vs. Prime
Learn the important facts in this easy to understand article.
Focal Length determines the field of view of our camera. The human eye has a field of view that is comparable to that of a 50mm lens on a 35mm film camera. We therefore consider 50mm the standard focal length.
Tip: Since our eyes are so used to the 50mm field of view, we usually find photographs shot with extreme telephoto or extreme wide-angle lenses much more interesting.
Transformation of the Visual Space
Telephoto lenses compress the visual space, causing foreground and background elements to appear closer together. In contrast, wide-angle lenses stretch the visual space. As a result, foreground elements appear greatly exaggerated, even distorted, while the background elements seem to recede.
We create compelling images by using the transformation of the visual space to our advantage.
Alternatively, we may decide to use a short lens and get really close to our subjects, to create an intimate perspective, exaggerating the subject and making it appear larger than life, while the background becomes diminishingly small.
With this awesome tool at our disposal, we can transform the world to our liking and create images that others will find pleasing without knowing why. Focal length is thus one of the most important creative tools in our arsenal. It is much more than deciding if we want to zoom in for the sake of getting closer to an otherwise out of reach subject.
Influence of Focal Length on Depth of Field
Most photographers believe that lenses with a shorter focal length (wide-angle) have a larger depth of field, while telephoto lenses have a shorter depth of field. Although this is incorrect in absolute terms, it certainly appears to be true. In reality, only aperture determines depth of field, but since the perspective is so grossly different, distant objects will appear much smaller on the image taken with a wide-angle lens. When we magnify distant objects to the same size, they are out of focus by the exact same amount. In the extreme case, the distant objects will become smaller than our pixel and we can truly say that wide-angle lenses have a larger depth of field.
Focal Length Multiplication (Crop Factor)
For decades, 35mm was the standard film format. When you bought a 50mm lens, you already knew what angle of view you would get with that lens. All was good, until the fabulous engineers introduced digital SLR cameras. Everybody was happy, but our world also got slightly more complex. We now had to consider something called crop factor.
Building sensors (CMOS chips) of the same size as 35mm film is still extremely expensive. Cameras with such sensors cost more than many photo enthusiasts are willing to spend. The simple solution is to use smaller sensors.
Using the exact same lens, the smaller sensor now “sees” a smaller portion of the image. Assume that I recorded the Image with a 35mm camera at a certain focal length (about 50 from the looks of it). Many prosumer and consumer Canon cameras have a crop factor of about 1.6 (shown with the red square). The narrower angle of view is similar to cropping the edges off, hence crop factor. Full frame cameras are cameras with sensors of exactly the same size as 35mm film. They do not have a crop factor (crop factor of 1.0)
In order to obtain the exact same field of view, a camera with crop factor therefore requires lenses with shorter focal lengths. A 30mm lens will give you about the same angle of view a 50 mm lens gives on a full frame camera. (50mm/1.6=31.25mm). A 30mm lens will have a wider apparent depth of field, which is good news for landscape photographers and bad news for portrait photographers.
As if the life of a photographer is not difficult enough will all this depth of field stuff, it gets even harder once we consider the other end of the scale.
Pro’s and Con’s:
Just remember the crop factor in your lens buying adventure. A wide-angle lens will behave like a normal lens when used on a camera with crop factor.
If you are uncomfortable with all this information, take comfort in the fact that it gets easier from here. At the end of the series, I will introduce some basic examples of lens buying decisions that will make it easy for you to pick. Things often get very technical, but fortunately you do not need to know all this to take good pictures.
Prime Lenses vs. Zoom Lenses
A prime lens has a fixed focal length whereas a zoom lens has a variable focal length. With prime lenses, you need to change the lens every time you want to change your focal length, and even then, you only have fixed steps available. For instance you have 30mm, 50mm, 80mm 135mm prime lenses.
Why Prime Lenses:
It seems that the zoom lens is the clear winner, but do not dismiss prime lenses just yet.
Why Zoom Lenses:
Zooms are much more popular with travel photographers such as myself, who need to keep the overall weight down (less lenses) while maintaining flexibility. Their versatility is very appealing, but keep in mind that nothing comes free.
Zoom lenses have a range. For instance 70-200mm or 18-200mm. While the latter seems much more appealing, keep in mind that image quality is often inversely proportional to zoom-range (for the same money). This is because it is harder to optimize a lens for a wider range.
Focal length determines our perspective, cost and artistic possibilities. We use a standard lens to mimic the human angle of view. We find it easier to accept these pictures with our subconscious mind as depicting reality, since we are used to this angle. Our own vision correlates well.
Tamron has a nice online tool designed to help you get a better feeling for focal length.
It is important do decide what you wish to photograph. People will not look flattering with a wide-angle lens. We need to move very close to our subjects, which magnifies the features closer to us, and thus, distorts facial and body features.
A good all around lens that I can recommend:
back to the Lens Buying Guide