### Sensor Size and Focal Length

At the beginning of this article, we touched a little bit on compatibility. As we learned, lenses made by one manufacturer are likely to be compatible with other cameras of the same brand. However, this comes with a bit of a qualifying statement due to the difference in camera sensors.

For the purposes of this article, there are essentially two camera sensor sizes: a full frame camera sensor is the same size as a 35mm frame of film. Full frame cameras, such as Nikon's D700 and D3 and Canon's 5D and Mark III, are considered to be in the upper echelon of digital SLR's. Most other cameras are "crop factor" or DX cameras. This means that the sensors are slightly smaller.

Because of mathematics, lenses on a crop factor camera are effectively longer than they would be on a full frame camera. If you put a 50mm lens on a crop factor camera and take a picture, then take a picture with that same lens on a full frame camera, you will find that the picture from the crop factor camera is "tighter" (meaning more zoomed in) due to this crop factor.

DX / FX Illustration - the entire circle represents the area that the human eye might see, while the red frame is what a full frame camera might capture. Inside of it is a blue frame that represents a crop factor camera's field of view.

Furthermore, camera manufacturers have been able to capitalize on manufacturing crop factor lenses. This is because less glass is required to project an image on to a smaller sensor. Therefore, smaller and more lightweight lenses are available for crop factor cameras. The downside to this is that these special crop factor lenses don't fully work properly on full frame cameras. (Full frame lenses work fine on crop factor cameras)

Canon calls crop factor specific lenses "EF-S" lenses, while Nikon refers to these lenses as "DX" models. Nikon calls full frame cameras "FX" and Canon simply refers to them as full frame cameras. These compatibilities are important when selecting both cameras and lenses. If you try to use a crop factor lens on a full frame camera, you will experience one of two things: one, the camera will automatically crop the frame to the "crop factor" frame and lose resolution, or two, you will get vignetting (black corners) around the edge of the frame because the "image circle" the lens makes is not large enough to project onto the full frame sensor.

What you need to basically need to know is that if you mount a 50mm lens on a crop factor camera it will look roughly the same as a 75mm lens on a film or full frame camera. The multiplier is usually 1.5-1.6. Inversely, if you shoot with a 200mm lens on your crop factor SLR, then you switch to a full frame camera, you'll need a 300mm lens to replicate the same field of view. Same goes for wide angle. If you loved shooting with a 28mm lens on your film camera, you'll need to get 18mm lens to get the same effect on your new crop factor digital SLR.

### Lens Ranges

There are different names for different ranges of lenses, based upon focal length such as wide angle zoom or telephoto zoom. These ranges aren't exactly scientific so one person's telephoto is another person's medium lens, and this is also affected by sensor size.

### Wide Angle

Wide angle lenses are often used for landscapes and any other type of shot where it's important to include a large scene. I typically think of wide angle lenses to under 35mm on a full frame camera and under 24mm on crop factor camera - much longer than that and they're more in the medium range.

Shot at approximately 20mm, a wide angle shot.

### Medium / Normal

Lenses in this range are tailored for walking around. An example of this is the classic 50mm lens that has been the cornerstone of photography for decades. Lenses in this range are often called "normal" because they are similar to the perspective of the human eye. Focal lengths around from 40-75mm if this bill on a full frame camera, while 35-50mm can work on a crop factor camera.

Shot at approximately 50mm, a "normal" range shot.

### Telephoto

Telephoto lenses are your long reach lenses. Typically, this classification is said to begin at around 85mm. If you find yourself needing to shoot tight portraits or anything far away, you might be in need of a telephoto lens. These lenses are the staple for wildlife, sports and event photographers.

Shot at approximately 130mm, a telephoto focal length.

Keep in mind that there are also lenses that defy these basic classifications by spanning across multiple ranges. Both Nikon and Canon have 18-200mm lenses that span the entire range.

### Aperture

Aperture is the another huge factor that I consider essential in describing a lens. The aperture, or more specifically the maximum (widest) aperture, refers to the amount of light that can pass through the lens. Better lenses allow more light to pass through.

As mentioned above, primes typically allow for more light to pass through and have a larger aperture. This is expressed, confusingly, by smaller aperture number. Some of the top primes will have apertures of f/1.8 or f/1.4. If you're using a kit lens, chances are that the widest aperture will be in the range of f/3.5-5.6.

There is also what we call a variable aperture lens and is typical of inexpensive lenses. This means that as you zoom in, the lens allows less light to pass through making the aperture number bigger.

Pro level zooms are usually f/2.8 lenses - an example is Nikon's 80-200mm f/2.8 lens. Lenses with wide maximum apertures are often referred to as "fast".

Aperture example

Don't get me wrong - variable aperture lenses certainly have their use. Typically, these lenses are more lightweight and inexpensive. It's great to have those lenses for travel or days out. While having f/2.8 lenses are great, you may find yourself not carrying the camera because it's inconvenient.

Aperture (along with shutter speed and ISO) is used to control exposure, how bright or dark your image is. It also controls the depth of field. A deeper depth of field allows more things to be in focus, say everything from 1 to 5 meters away, if you're focused 2 meters away. A deep depth of field is achieve by "stopping down," which mean make your aperture hole smaller by increasing the aperture number (f/8, f/16 or higher). A shallow depth of field may only allow things from 1.5  to 3 meters away to be in focus if you're focused at 2 meters

One other note about aperture: lenses usually aren't at their sharpest at max (widest) aperture. If I'm going for the sharpest image possible on my Nikon 35mm f/1.8 lens, I typically set the aperture to around f/3.5.

### Autofocus

Autofocus is another major characteristic of lenses. Nearly every modern lens is produced with autofocus, but there is some information about autofocus that is important the next time that you choose a camera or lens.

First, let's look at the Nikon system. Autofocus Nikon lenses have one of two labels: "AF" and "AF-S". Both of these labels mean that the lens is capable of autofocus, albeit in different ways. "AF" lenses do not have an autofocus motor built into the lens and instead rely on the camera itself to focus the lens. This is done through use of a screw-drive type motor, pictured below.

Here you can see the screw type autofocus motor used by Nikon "AF" lenses.

However, not all Nikon cameras are capable of focusing these lenses because some cameras lack the screw drive motor. Entry level Nikons such as the D40, D5000, and D3000 all lack this screw drive motor. (Note: this does not mean that these cameras do not autofocus, only that they do not autofocus with non "AF-S" type lenses) This is a huge reason to step up to the next camera class (D90, D300, etc) because it opens the door to a much wider range of lens compatibility.

The other autofocus Nikon lens, designated AF-S, is much simpler to understand. These lenses have motors built in directly to the lens and will autofocus on any modern Nikon camera. Also, some AF-S systems are faster than a screw driver system could ever be, so many of the new Nikon lens versions are all "AF-S" type lenses.

Canon lenses on the other hand are much simpler to navigate. Every modern Canon "EOS" lens autofocuses on every Canon EOS camera body. "USM", or ultra sonic motor lenses, mean that the motor is built into the lens, but every Canon EOS camera can still focus every Canon lens.

Each camera and lens manufacturer has a little different system. If you shoot a brand like Sony or Pentax, make sure to do some reading both in the manual and online to explore the various autofocus options. There's nothing worse than dropping a good chunk of change to find that your lens doesn't autofocus on your camera body.

### Conclusion

Lenses are one of the most exciting parts of owning an SLR camera. With many options available for covering every type of subject, it's important to understand the characteristics that make each lens different before making a major purchase. Stay tuned for part two when we'll dive into even more details about lenses.