Focusing on Lenses

There are so many different types of lenses, when it comes to figuring out what you need, it can be a daunting shopping experience. Many people go through an experience similar to what I did… start with the kit lens your first DSLR comes with, buy an inexpensive telephoto zoom, pine for better results, hit up the used market, try 2-3 other lenses, and eventually sell everything you’ve acquired for professional-grade lenses. In the meantime, you’ve spent and wasted a lot of time and money, because you didn’t necessarily know what you were getting into because of the various terminology, options, and confusion.

I hope to cover most of the general topics regarding lenses in this post so that, hopefully, people can learn from my missteps without duplicating them, and make informed purchasing decisions in building a solid lens collection that serves their specific needs.

Focal length is defined as the distance between the lens and the focal point, or the point at which the rays of light projected through the lens converge.

There are two primary categories of lenses : zoom lenses and prime lenses.

Zoom Lenses

Zoom lenses are what most people are familiar with. These are lenses that can be mechanically adjusted, usually via an exterior ring on SLR lenses or via an electronic switch in point-and-shoot cameras, to cover a range of focal lengths. These will be marked by the range of focal lengths they cover (ie. 18-55mm, 55-200mm, etc.).

The advantages of a zoom lens are that a single lens can encompass multiple focal lengths, making them more convenient, and allowing them to replace multiple prime lenses.

The disadvantages of zoom lenses are that the design requirements of a single lens handling a range of focal lengths is more complex than a lens designed for a specific focal length, so there are inherently more compromises in the design, causing various image quality issues such as distortion or softness at certain focal lengths or apertures.

Prime Lenses

A prime lens is effectively the opposite of a zoom; simply put, it is a lens with a fixed-focal length. Prime lenses will be marked with the lens’ single focal length (ie. 50mm, 85mm, etc.).

The advantages of a prime lens are that they are typically smaller, lighter, sharper, and generally less expensive than zoom lenses. They can also be made with larger maximum apertures, offering more control over DOF.

The primary disadvantage of prime lenses are that they are not as convenient as zoom lenses, requiring multiple lenses to cover the same range of focal lengths as a single zoom lens.


Lens focal lengths can be broken down into several categories. Note that these examples are for 35mm or APS-C sensors found in most DSLRs. These categories encompass different focal lengths for other sensor sizes, such as medium format or large format.

Normal Lenses

Normal lenses are referred to as such because the perspective or angle of view they convey is similar to that of the bare human eye. Most typically, 50mm is recognized as being the closest focal length to give an equal perspective to “normal”. However, “normal” is a somewhat flexible term when it comes to how we see, so generally, the range of focal lengths for Normal lenses is between 35mm and 70mm.

Shrouded

Taken with a 50mm lens, this image is captured similar to the way it was seen with the naked eye.

Wide Angle Lenses

Wide angle lenses are called such because they convey a perspective that is wider than that of the human eye. Wide angle lenses are those that have a focal length shorter than 35mm. This allows you to include not only a lot of scenery in a single shot, but can also give skies a very sweeping effect.

Cabo Sunset

Shot at 17mm on a full-frame camera, this is an ultra-wide image that shows a wide, sweeping landscape

Telephoto Lenses

In contrast to wide-angle lenses, telephoto lenses have a narrower angle of view, making distant subjects appear larger in the frame. Telephoto lenses allow you to get full images of your subjects from farther away.

Lazy Days

A 400mm lens allows me to get up-close to this polar bear at the zoo without risk of being mauled to death

Ultra-Wide Lenses

Ultra-wide lenses are simply a sub-class of wide-angle lenses that have extremely short focal lengths, resulting in incredibly wide angles of view. While there isn’t a standardized distinction between wide angle and ultra wide, you’ll typically find ultra-wide lenses categorized as lenses with focal lengths of 12mm or less.

Super Telephoto

Likewise, super telephoto lenses are a sub-class of telephoto lenses with exceptionally long focal lengths, allowing full images of subjects from much further away. Super telephoto lenses are typically those with focal lengths greater than 300mm although, again, there isn’t a standardized distinction.


Focal Length vs. Field of View

It’s important to make the distinction between focal length and field of view, as this issue tends to cause a lot of confusion among photographers with cameras using smaller-than-35mm sensors and lenses designed for those smaller sensors.

Focal length is a physical property of the lens, and does not change depending on which camera it’s mounted on, whether that camera is using a full-frame sensor or an APS-C sensor.

Field of view changes, depending on the size of the sensor. A 50mm focal length lens used on a full-frame 35mm sensor will give a field of view appropriate for a lens with a 50mm focal length. However, that same 50mm lens used on a camera with an APS-C sensor will give a field of view equivalent to that of approximately a 75mm lens used on a 35mm sensor.

Field-of-view comparison

This image demonstrates the different field-of-view captured by a full-frame sensor vs. an APS-C sensor

Camera makers have released lenses that optimize the image circle they project onto the sensor for smaller sized APS-C sensors. This enables them to make the lenses smaller, lighter, and less expensive than their full-frame counterparts. Nikon designates these lenses as “DX”, while Canon uses the designation “EF-S”. When used on a full-frame camera, these lenses will project an image circle that will not fill the entire frame, resulting in a large portion of the outer edge of the image coming out black.

When you got your first DSLR camera, chances are it came with a lens as part of a kit. This lens is typically one of the variety optimized for the smaller APS-C sensor, most commonly 18-55mm. Even though the equivalent field of view when using these lenses on an APS-C camera gives you an approximate field of view of 27-83mm in 35mm equivalence (Nikon’s crop factor of 1.5x) or 29-88mm (Canon’s crop factor of 1.6x), the focal length of the lens itself is still 18-55mm because, again, focal length is a physical property of the lens that does not change.

It’s important to understand this distinction, because many novice photographers wonder why they’re getting a lens marked 18-55mm if it gives then an equivalent field of view of 27-83mm and why the lens isn’t simply just marked 27-83mm. Again, this is because there is a difference between the physical measure of focal length, and the perceptual measure of field of view.

Now that you understand this distinction, it’s critical to understand that, while you may be using a normal 50mm lens, expecting the equivalent of the perspective of the human eye, when using that lens on an APS-C camera, you’re getting the 35mm full-frame equivalent of a 75mm lens, which would make it result in an image with the perspective of a short telephoto lens with a narrower angle of view than what you might be expecting. If you are using an APS-C camera, and looking to purchase a lens, you MUST take the crop factor of the sensor into account, and adjust accordingly. If you want a normal lens for an APS-C camera, you might want to consider a 30mm or 35mm lens instead, or a zoom lens that encompasses that range.


Variable Aperture

Variable aperture lenses are zoom lenses whose maximum aperture (widest opening) changes as the lens moves through it’s zoom range. They will have a wider aperture when zoomed to their widest focal length, and as the lens is zoomed in (increasing the focal length), the maximum aperture will gradually narrow.

For example, you might have a lens that, at it’s widest setting, has a maximum aperture of f/3.5. If you set your camera to that widest aperture setting, you’ll notice that, as you zoom in on your subject by turning the zoom ring, the aperture will slowly decrease.

You can tell that a lens has a variable aperture by looking at the markings on the lens itself, or in the description of the lens. The lens will be marked with it’s focal length range, followed by it’s aperture range.

Variable Aperture

A variable-aperture Nikon 18-70mm f/3.5-4.5 lens, as marked on the bottom of the lens barrel, near the mount

Since you’ll remember from the post about aperture, aperture is a physical property of the lens, based on the focal length, so that as a lens’ focal length increases, the aperture increases in physical size proportionally. Because of this, variable aperture lenses can be made smaller by leaving the opening a smaller size at it’s longest focal length, and simply changing the aperture as the focal length increases. Since the lens can be made smaller, they are also typically less expensive.

However, since the aperture changes as the lens is zoomed in or out, so does the exposure. In some cases, the aperture can change by as much as 2 stops or more. This is something that must be kept in mind when using a variable aperture lens, as you may encounter situations where the meter reading at the wide end of the lens gives you a shutter speed that allows you to counter camera shake (based on the reciprocal rule), but when you zoom in, the shutter speed becomes too slow, and you end up with blurry photos.

Constant Aperture

Constant aperture lenses are zoom lenses whose maximum aperture remains constant throughout the zoom range. They will have the same maximum aperture (widest opening) at the widest focal length as they do at the longest focal length.

Constant aperture lenses will be designated similarly to variable aperture lenses, but instead of a range of apertures appearing on the barrel or in the description, it will simply have a single number referencing the maximum aperture.

Constant Aperture

A constant aperture Nikon 80-200mm f/2.8 lens, as marked at the top of the lens barrel, near the front lens element

Constant aperture lenses need to be physically as large as they must to be to accommodate the maximum aperture at it’s longest focal length, so constant aperture lenses will typically be much larger and heavier, and therefore more costly, than variable aperture lenses encompassing the same or similar range of focal lengths.

However, because they allow a constant aperture throughout the entire focal range, constant aperture lenses are generally more desired by professional photographers because they will not change the exposure as they are zoomed through the focal range, not to mention better control over depth-of-field.

Note that, since prime lenses have no zoom range, they’re not discussed in terms of constant or variable aperture… they simply list the maximum aperture for their focal length.


Specialty Lenses

There are lenses that fit into other categories outside the general purpose lenses discussed above. They include, but are not limited to:

  • Macro Lenses – These lenses allow high degrees of magnification, allowing you to photograph your subject at life-size (for example, an object measuring 1mm across would be projected onto the imaging sensor or film at 1mm across) or greater magnification. While some general purpose lenses might tout macro capability, they typically cannot reach the same magnification rates as dedicated macro lenses.
  • Fisheye Lenses – These lenses can achieve an angle-of-view of 180 degrees, giving you a full horizon-to-horizon view. However, they achieve this with an incredible amount of distortion in the image, which can sometimes be corrected for, or are sometimes left as-is for effect.
  • Tilt-Shift Lenses – These lenses have the ability to bend away from being perpendicular to the focal plane, allowing for selective focus and perspective control. They are most often used for architectural photography because they allow the photographer to correct the perspective distortion that occurs when looking up at a building. They are also often used to accentuate selective focus, giving an image the appearance of being a scene rendered in miniature.

 

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