Digital SLR Camera Lens Guide

Everything you'll ever need to know regarding selecting a new lens for your DSLR camera, no matter if it's macro, ultra-wide, macro or super super wde-range zoom lens. Camera lenses where the focal lengths exceed 35mm on APS-C DSLR cameras or those over 50mm on SLR film and Full Frame Digital cameras are dubbed telephoto lenses. These lenses allow you to capture close-up images of your subject without getting too close.

Regrettably, selecting lenses for a DSLR is somewhat complex. That's due to the image sensor that captures pictures is usually not as large as a 35mm film frame. Although all lenses are still identified as for 35mm SLR film cameras. Although you can buy a digital-only lens that may not function correctly on an SLR film camera. Camera manufactures are now creating an abundance of these lenses for digital only.

As digital image sensors are usually not as large as film frames, images become magnified to what's known as 35mm focal length equivalent.

A cropped field of view is also labeled as the multiplication factor or focal length multiplier. customary crop field of view numbers start from 1.3 up to 2. Multiplying the focal length of the lens by the focal length multiplier to determine the 35mm equivalent.

To locate the focal length multiplier number for your particular camera look it up in in your camera manual, or search the name and model number on my review section for that specific camera.

As an example, if the crop field of view value for your camera is 1.5. Employing a 50mm lens is going to provide a 75mm focal length.

For a DSLR camera, you'll probably prefer lenses that feature shorter focal lengths. For wide-angle lenses this becomes especially true. A 28mm or 24mm lens may not create wide-angle images on a DSLR (28 to 35mm has become the new normal for DSLR cameras).

Lenses come in distinctly two different categories: fixed-length (or prime) and zooms. Don't mix up zooms with telephotos. Zoom merely means that the lens barrel extends to adjust the focal length. There are both wide-angle and telephoto zooms.  updated article Mar 2, 2011

Angle of View
Which is better? A zoom lens will give you more versatility; it is also cheaper than buying many different lenses. But some people believe zoom results are less crisp.

If you're just starting out, master the camera kit lens first. About 28-105mm (18-70 digital) is good for beginners. This allows you to take pictures from a variety of perspectives. Of course, you'll probably miss out on wide-angle shots.

Pick up a wide-angle lens for landscapes. A focal length shorter than 16mm (10mm digital) will give you the fisheye effect, though. This distorts the image, causing it to curve around the edges.

When you pick out a lens, you'll also see f-numbers. They refer to aperture, or how wide the lens opens. Aperture affects shutter speeds and field of view.

F-numbers are confusing; they're really a fraction, but rarely expressed that way. You'll see lenses labeled f/2.8-4, for example. This means it has aperture openings ranging from 1/2.8 to 4.

A large f-number gives you a wide depth of field. Shutter speeds are slower, as the aperture doesn't open very far. Small f-numbers give you a shallow field of view and fast shutter times.

Ideally, the lens should have a wide range when it comes to f-numbers. But it is more important to have a lens with a small f-number (for example 1/1.4 as opposed to 1/2.8). The smallest f-number is often referred to as the lens' speed. That's because it will dictate the fastest shutter speed you can get with the lens.

Lens speed

The speed of a lens is determined by size of the lens opening, known as aperture. The aperture controls the amount of light that reaches a digital camera sensor

Lens F Stops The diameter of an aperture is measured in f-stops. A lower f-stop number opens the aperture to admit more light onto the sensor. Higher f-stop numbers close the lens opening so less light gets through. A lens with an f-number of f/1.8 has a larger aperture than one with an f-number of f/4.5.

The aperture, or aperture range, is indicated on the front of a lens

Fast Lens

A fast lens is one with a large maximum aperture; the larger the aperture, the faster the lens. A lens is called fast because the larger aperture lets more light pass through during a given time span. When more light falls upon a subject, pictures can be shot with faster shutter speeds .

The aperture of a lens can be reduced if desired by the user of a camera with manual and/or semi-automatic controls. The process of reducing the aperture size is called stopping down.

Itís important to note that a lens is usually not at its sharpest when wide open, nor when stopped down too much.

One interesting effect of using a large aperture is it greatly reduces the depth of field in an scene. This is very useful to isolate a subject from the background such as when taking portrait and some marcro shots. A photographer desiring a large depth of field (for instance when photographing landscapes) will have to stop the lens down by using a smaller aperture.

Photographers who do a lot of low light photography prefer fast lenses.

Advantages to using faster f2.8 zoom lenses .. vs .. slower amateur lenses :

An aperture of f2.8 allows you shallower depth of field than f4.5 or f5.6 would. So you'd be able to isolate the subject much better against the background.

The faster f-stop allows for faster shutter speeds, which means you can stop action more effectively or conversely, handhold the camera in lower ambient light levels.

The f2.8 zooms have constant apertures, and the slower zooms mostly have variable apertures. The variable apertures are not such a problem with automatic metering, but where you need to meter manually or set a specific f-stop, such as in studio applications, it can be a problem. Whether a variable aperture is a problem, also depends on the specific camera make and how the camera body interfaces with the lens though.

The faster optics will improve focusing abilities in dimmer light over a slower lens. This is a huge help whether you need to focus manually in dim light, or with auto-focus if you want to confirm that the camera didn't mis-focus.

An 80-200mm f2.8 will be noticeably sharper and have no light fall-off, compared to the 80-200mm f4-5.6 when shooting at f5.6. The f2.8 lens will be very close to optimum optical performance, because it is stopped down 2 stops already .. but the budget lens will still be at wide open aperture.

Some magazine articles suggest that you could use a higher ISO to compensate somewhat for the lack of the wider aperture (and implied slower shutter speed), eg. 1/250th @ f2.8 = 1/60th @f5.6 .. and they therefore suggest the use of say 800ISO instead of 200ISO to get that 1/250th shutter speed that the f2.8 user would have had. But this kind of logic is flawed .. for in that situation, the user of an f2.8 lens could still have the benefits of using a lower ISO - less grain if you're using film, or less noise of you're shooting digital.

The disadvantages to faster optics are that they are heavier and bulkier, and are more expensive than amateur zooms. (I don't see the wider zoom range of amateur lenses as an advantage, since this nearly always comes at the cost of optical quality.)

But in the end it all comes down to the fact that you simply can't beat faster lenses. There are very good reasons for wanting to use and own top-end and professional spec lenses .. their build quality is better and they give superior results to amateur equipment in a number of ways. But whether we can afford them .. that is unfortunately the spill around which these questions are usually resolved. Oh well...

But there is another way to look at it - it is unlikely that you'll ever hear someone wistfully saying, "if only I had bought a slower lens".

Slow Lens

A slow lens is one with a small maximum aperture, such as F/4.5. A slow lens lets less light pass through towards the sensor, and exposure times will be longer.

Longer zoom lenses are generally slower and have an aperture range. They are slower at the telephoto end of zoom and faster at the wide end.

A slow lens delivers a deeper depth of field. The same is true for a fast lens sopped down. A deeper depth of field can be desirable depending on the visual effect a photographer wishes to capture in a give scene.

Slow lenses are far less expensive than fast ones.

Make sure the lens will fit your camera. Manufacturers such as Sigma, Tamron and Tokina make lenses that fit other manufacturers' cameras. If you buy a third-party lens, make sure it is compatible.

Spend a little more looking for a lens made by the camera manufacturer. You shouldn't have compatibility issues, and all the features should work. And, if you have problems, you only have to deal with one company.