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What is focal length? The focal length of a lens is measured in millimeters. A common misunderstanding is the belief that this lens measurement is the length of the lens from end to end or its overall dimensions. The focal length measurement actually begins at the optical center. A camera lens is made of many pieces of glass and combinations of elements, so as light enters the lens, it converges into “focus” at a point somewhere inside the lens among these elements. This point is the optical center. From the optical center this focused image is then sent to the camera sensor. Therefore to get the focal length, we take the measurement from the focal center to the camera sensor while the camera is focused at infinity (generally means that you are focused on something in the far away distance). Now when you are shopping for a lens and are comparing two different 50mm lenses, you will understand why one of them might be physically longer than the other.  This number or how it is calculated isn’t something you need to memorize or remember and knowing it isn’t going to make you a better photographer. What is important is to understand what focal length affects – which is your angle of view (how wide of a view or how much of your scene will be captured in an image) and magnification (how large subjects will appear). A shorter focal length, like 18mm, will capture a larger width or wider angle of view and subjects in the frame will appear smaller than they do viewing them with our eyes. Longer focal lengths, like 100mm, or even longer, like 400mm, have much more narrow angles of view. You will capture a much smaller width, but your subject will appear larger.  Angle of view is determined solely by the optics of the lens. It does not change if you place that lens on a different camera. On the other hand, field of view (which is often used interchangeably with angle of view even though they are not the same thing), is determined by the lens and the sensor of the camera. What type of camera you have – crop sensor or full-frame – will affect how much of a scene (field of view) ends up in your final image. A crop sensor camera will capture less of the subject or a smaller field of view. So how does a crop factor work? Let’s say I wanted to take a landscape photo with a full-frame Canon camera and a 50mm lens. It would look something like this:  If I took that same 50mm lens and put it on a crop sensor Canon camera, the resulting image would look more like this:  The sensor size is “cropped.” Since the senor is smaller, it is like trimming off a large border all around the image. Another way to think of it would be similar to using the cropping tool in a photo editing program where the image is cut, not shrunk. The crop factor is different for cameras from each manufacturer. A Canon EF-S camera has a crop factor of 1.6, while the Nikon DX models are a factor of 1.5. Going back to my example, both images were taken with a 50mm lens. To figure out how much “loss” of field of view we would get on the Canon crop sensor, we would multiply the 50mm x 1.6 to get 80mm. Therefore, the amount of image we get on the crop sensor camera with the 50mm is equivalent to 80mm if it had been on the full-frame we started with instead. Here is the same landscape again. This image was taken with the full-frame camera using a zoom lens set to 80mm. Notice how the field of view is basically the same as the 50mm on the crop sensor camera pictured above (and repeated here so you can see them side by side):
We can also work this equation in reverse. Let’s say I want to get the wider field of view of the full-frame camera, as we did with the 50mm, but by using my crop sensor camera instead. I would take 50mm and divide by 1.6 which would result in approximately 31mm. Therefore, if I put a 30mm on my crop sensor camera, it should “see” almost the same thing as the full-frame with the 50mm. You can witness this effect in the images below:
Should you learn how to do all these calculations? No, there are apps and online charts that have these calculations already completed for you! Like this one: https://mmcalc.com/ which is also available as both iOS and Android apps. So what is the take-away then for a beginning photographer? First, know that if you have a crop sensor camera that you will not be able to get the same field of view as a full-frame unless you go down to smaller focal lengths (in effect, zooming out). Second, understand that the focal length, as it gets larger, has the inverse effect on your field of view which gets smaller and smaller and brings the subject closer (in effect, zooming in). Third, know the basic classifications of lens focal lengths and their main uses so that you will have a basic idea of what lenses to use for different shooting situations (and make adjustments as necessary if you are shooting with a crop sensor camera). Lenses basically fall into five types of focal lengths; super wide-angle, wide-angle, standard, telephoto (zoom), and super telephoto. In each of the descriptions below I am speaking in terms of using a full-frame camera. Ultra wide-angle lenses have a 24mm or less focal length which allows them to capture a very wide scene. They can be useful for home interiors (real estate photography). Below 24mm, and especially super-wide fish eye lenses, images can become distorted and present an exaggerated perspective that can be artistic and fun to play around with.  Photo by sippakorn yamkasikorn on Unsplash Wide angle lenses have a focal length range from 24mm up to 35mm. These lenses are also good for confined spaces, such as home interiors, or capturing the whole table of guests at a family holiday or celebration. In addition, these lenses are good for large group photos, cityscapes, landscapes, and architectural photography. These lenses have large depth of field so both far away and near objects can have tack sharp focus and there will be visible distance between your subject and the background when taking portraits.  Photo by Khiem Tran on Pixabay Standard lenses have a focal length range of between 35mm to 70mm. These lenses “see” the world in much the same range and way our own eyes see it. There is little distortion of the subject so they make flattering portrait lenses. The shallower depth of field allows the photographer to separate the subject from the background as well. These lens are excellent, not only for portraits, but for nature, “on the street” shots, and low light conditions (or when you do not want to use a flash or only want natural light). The 50mm lens is in the standard lens range, and is such a popular lens choice, that it has earned the nickname – The Nifty Fifty.  Photo by Alexei Chizhov on Pixabay Telephoto lenses are focal lengths of 70mm up to 300mm. These lenses are very popular for wildlife/nature photographers because it allows shooting from a distance without encroaching on the subject (which might be skittish). These lenses bring the subject closer. A lens that is 70-135 is considered a short telephoto and one that is 135-300 is a medium telephoto. Telephoto lenses have a shallower depth of field so crisp focus on the subject is a must. In additional to wildlife, these lenses are also used for any activity or subject where distance is required or unavoidable, such as shooting from the sidelines during a sporting event. Super telephoto lenses are much like telephoto lenses, except their focal lengths go beyond 300mm. They provide a telescope type magnification which brings the subject and the background closer. Objects behind your subject will look much closer than in a similarly framed shot using a smaller focal length lens. These lenses are very heavy and can’t be used for handheld shooting. A sturdy tripod is needed to support these lenses. Some even include additional support brackets built into the lens. Any type of photography where you don’t want to fight through a crowd for a position or you are limited on how close you can get benefits from this type of lens: birding, wildlife, sports, astrophotography, moon photography (or any other small distant objects), air and boat shows, car races, and more.  Photo by smarko on Pixabay Macro lenses are not a type of focal length. They are specialty lenses that come in various focal lengths. They are used for photography of small objects, flowers, products, and insects in amazing detail. Macro lenses create 1:1 or life size reproductions on the camera sensor. (If you have an interest in Macro photography, check out my Macro Photography class) Your camera may have come with a “kit” lens. An 18-55mm is a common wide angle zoom lens. This lens give the photographer the whole range of lens focal lengths from 18mm – 55mm. An 18-135mm lens would give a photographer the range of focal lengths from wide angle all the way up through short telephoto. Zoom lenses like these can be great lenses because you don’t have to change your lenses as often and they cover a wide variety of photographic situations. Prime lenses on the other hand have one fixed focal length, like a 50mm. Here is a series of images from a local garden showing the range you can achieve with a zoom lens. In this particular instance, the lens used was the Tamron 18-400mm f/3.5-6.3 Di II VC HLD for Canon APS-C cameras.  What is the advantage of a zoom vs. a prime lens? The main advantage, as demonstrated in the photos above, is versatility. With one zoom lens in your bag you reduce the weight of your gear and you can shoot everything from wide scenery shots to close ups, people, or details without changing lenses. Additionally, when shooting sports or other fast action, you can re-frame your subject without having to move closer or farther away. Finally, less lens changes means you save time and protect the camera sensor from dust and moisture exposure! One disadvantage of a zoom lens is that they often have narrower maximum apertures than prime lenses and/or variable apertures which means less and less light makes its way to your image sensor as you zoom in. A prime lens can open up wider and let in more light, as well as achieve a shallower depth of field. Another disadvantage of a zoom lens is their size and weight. Zooms are usually larger and heavier than a prime; however, if one zoom can replace three or four primes in your bag you will still come out ahead with the zoom. Lastly, zooms are usually not as pristine when it comes to image quality, but that should not stop you from purchasing a zoom if it fits your needs. So, now that you know a bit about focal length and which lenses are best for different photographic scenarios, you can buy gear that will serve your needs best! If you’d like some suggestions for some “can’t go wrong with one of these” lenses, check out my recommended lenses chart or the shortened list below. These are affiliate links, so if you make a purchase, we make a small commission at no cost to you (full disclosures). Also be sure to check out my blog post Understanding Lens Abbreviations for help deciphering all those letters and numbers on the lens barrel. It will help you buy lenses with confidence! Recommendations Copy and paste these descriptions into an online seller site. I would recommend KEH for used and B&H Video for new - see my Buy Camera Gear page for links to these sellers and IMPORTANT advice before buying any gear. Some sellers are not authorized resellers, so buyer beware! Prime, Nifty 50: Canon EF 50mm f/1.4 Nikon AF-S 50mm f/1.8G^ Mid-Range Zoom: Canon EF 24-70mm f/2.8L II USM Sigma 24-105mm f/4 DG OS HSM Art Telephoto Zoom: Canon EF 70-200mm f/4L USM Tamron 70-300mm f/4-5.6 Di LD Macro Mega Telephoto Zoom: Nikon AF-S 200-500mm f/5.6E ED VR^ (available from used sellers like KEH - see my Buy Camera Gear page) ^Check compatibility for Nikon lenses here.  
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