the Lens

There is a lot to know about lenses, but not a whole lot you need to know. Lets cover the basics.

Some terminology:

Focal length:
This is defined in mm and will look like 55mm for a prime lens or 18-55mm for a zoom lens as shown above. The focal length determines the field of view (FOV) and magnification of an image.

Here's an interactive FOV program on Canon's website which really demonstrates well how different lenses "see things".

This defines the maximum size of the opening in the lens for passing light to the sensor. The minimum aperture is not shown. The aperture rating will look like 1:1.8 for a constant aperture lens or 1:3.5-5.6 for a zoom lens with a variable aperture as shown above. In the case of a variable aperture zoom lens the smaller number applies to the shorter focal length (i.e. f/3.5 at 18mm) and the larger number applies to the longer focal length (i.e. f/5.6 at 55mm).
A smaller number after the 1: means the lens is "faster" and can transfer more light to the sensor. Wider apertures also give a shallower depth of field. Faster is "better", but no lens is going to give it's maximum sharpness at the maximum aperture setting. There's much more on this topic under Exposure>Aperture

DX or EF:
This shows that the lens was designed for a small sensor body. DX is Nikon speak and EF is Canon Speak. The other manufacturers use different designators. You shouldn't buy a lens designed for a small sensor body to use on a full frame body. Some will not even mount on a full frame body. 

AF-S or USM:
This means the lens has a built in autofocus motor. AF-S (Nikon "autofocus- silent wave) and USM (Canon "ultra sonic motor") will autofocus with any camera body they will mount on. Other manufacturers use different designators. If the lens does not have a built in motor then the camera body must have a motor in order to provide autofocus. The built in motors are generally faster and quieter.

VR or IS (not shown):
VR is Nikon for "vibration reduction" and IS is Canon for "image stabilization". This helps eliminate vibration blur when handholding at slower shutter speeds. It does not help with subject motion blur. It helps but it is not a replacement for a tripod.

Filter size:
This is not usually stated but will be in the manual or on the box. It's the size of the filter threads at the objective end of the lens and will be given in mm. 

Objective lens: This is the lens element that is closest to the "object" being photographed.

the "Kit Lens": This just means the lens came with the camera as part of the package.

a "Fast Lens": This just means the lens has a large maximum aperture which passes more light and therefore allows for faster shutter speeds to be used. A "slow" lens is just the opposite.

Types of Lenses

Your camera probably came with a lens. If your camera is a point-and-shoot or super zoom the lens is not interchangeable. That can be considered a limitation or a benefit. The main benefit is versatility. The main limitation is that when your reach the limits you have no other option. If your camera has interchangeable lenses you have more options.

The Zoom lens

Your camera probably came with one of these. A zoom lens has a variable focal length and less expensive versions also have a variable aperture. The greater the zoom range a lens has the more optical compromises were made in it's construction. That's just the way it is. But that doesn't mean they don't perform well when used correctly (see Know your Gear). My favorite travel lens is the "super zoom" Nikon 28-300mm f/3.5-5.6. I use this lens anytime I don't have a particular subject in mind because it allows me to take so many different types of images.

There are also constant aperture zoom lenses. These are the choice of professionals. They have a shorter zoom range, have fewer optical compromises. They have a faster aperture at all focal lengths, they are built better, they weigh more, and they cost more. I use my Nikon 70-200mm f/2.8 whenever I want maximum image quality in that focal range. It's also my choice in that focal range if the light levels are going to be lower.
(shown: Canon 17-85mm f/4-5.6

The Prime Lens

A prime lens has only one focal length and only one maximum aperture. Because of this they are usually smaller, they weigh less, they tend to be "faster", and they make few optical compromises. Because of this there is great popularity for the "nifty fifty" and the fast 35mm. I don't personally own either of those lenses, but they are a very economical way to get a fast lens for low light photography and blurring the background (shallow depth of field). 
(shown: Nikon 55mm f/1.8 G


The Macro Lens

A macro lens is a special purpose lens. A macro lens can be either a prime lens or a zoom lens. A "true macro" will have a reproduction ratio of at least 1:1. This means if the object being photographed is physically 3mm wide it will occupy 3mm on the camera sensor. Many lenses listed as "macro" are not true macros, this is particularly true if the lens is a zoom lens. When choosing a macro lens a longer focal length lens allows you to work from further away which helps with lighting. There are other ways of taking macro images that cost less than a dedicated macro lens.
A macro lens is not limited to taking macro images. Because they are optimized for very accurate and very sharp reproduction of fine detail they can be a great choice for general photography.

(shown: Sigma 150mm f/2.8 macro the lens I use) 

The Fisheye Lens

The circular fisheye lens is a specialty type of prime lens. It's specialty is in capturing a VERY WIDE field of view; usually around 180 degrees. But with this capability comes severe distortion. The rectangular field of view is distorted into a circular image. I don't own one of these; I prefer to use an UWA.
(shown: Rokinon fisheye for Canon


The Wide Angle and Ultra Wide Angle (UWA) Lens

A wide angle lens does exactly what the name implies. It captures a wide field of view. The UWA is like a fisheye lens except the image isn't compressed into a circular form. These lenses can be either prime lenses or zoom lenses.
Wide angle is generally considered any focal length shorter than 28mm on a full frame camera or 20mm on a crop sensor body. In a wide angle lens, a few mm shorter in focal length can mean a MUCH wider FOV.
(shown: Nikon 14-24mm f/2.8 the lens I use on full frame) 


The Telephoto Lens

The telephoto lens is a specialty lens for long range or small subjects. This is the tool for action, sports, and wildlife photography. Technically "telephoto" means the physical length of the lens is shorter than the lens's focal length. The telephoto lens can be either a prime lens or a zoom lens. My favorite general purpose telephoto lens is the Sigma 300-800mm f/5.6. If you want to get serious about action photography it's going to be expensive.
(shown: Nikon 400mm f/2.8


Tele Converters (TC's)

Tele converters are not really lenses, they are specialty tools which multiply the focal length of the lens. They have the negative affects of reducing the maximum aperture and reducing image quality. The results will vary between the different TC and lens combinations and some TC will only mount on certain lenses. A 1.4x will reduce the light by 1 stop, a 1.7 will reduce the light by about 1.5 stops, and a 2x will reduce the light by 2 stops. A 2x TC will turn a f/4 lens into a f/8 lens. Many cameras have a very hard time autofocusing at f/8 and almost no camera will autofocus above f/8, at least not well. Personally I use a Kenko TelePro-Plus 1.4x and the Nikon 2x shown, but only when I have to.  

(shown: Nikon TC-20E III)

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