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A Zoom Lens Primer.

I cannot count the times I have been asked; which is better, a 55-200mm zoom or a 70-300mm zoom; or, what is the difference between a 12x and 15x zoom. To answer those questions, a little bit of discussion on focal lengths and lens relationships are in order.

Focal Length is the distance from the surface of the sensor (focal plane) in the camera to the optical center of the lens at infinity - or the furthest focus point. Generally, the reference is in respect to a 35mm camera, as all other camera types more or less specify their lens zooms in 35mm equivalent.

There are two types of lenses; zooms and prime. A zoom lens can change it's focal length/magnification, while a prime lens has a fixed focal length.

A second specification of some lenses describe the magnification, called the "x" power. A 4x zoom would be adjustable from one specific focal length at one end, and 4 times that length at the other. A 50-200 zoom lens would have a magnification of 4x. A 50mm prime lens (a non zoom lens) would have a magnification of 1x.

The Power of Two.

I like to say that photography is "living in a 2x world." Or more specifically, the Inverse Square Law applies to a lot of photo principles. Nature rarely binds itself in a linear fashion; almost nothing in nature is straight. Same applies to optics. Here is an example. Consider a lens that can zoom 100mm to 300mm. Refer to the photos to the right to see what that might represent.

Notice that from 100mm to 200mm, there is a significant change, but from 200mm to 300mm, the change is noticeable, but not nearly as significant. Why is this? The distance from 100mm to 200mm is the same as 200mm to 300mm - at least in linear terms. However, the magnification from 100mm to 200mm is 2x, and from 200 to 300 is 1.5x. This behavior is according to the Inverse Square Law. To be specific, the optical center of the 100mm to 200mm lens is 172mm, while the linear center of the lens is 150mm.

What does all of this mean? When comparing lenses, say a 28-270mm zoom vs. a 50-300mm zoom. Which provides a wider range of coverage? The 28-270mm zoom lens has a 9.6x magnification, while the 50-300mm zoom lens has a 6x magnification. This is a significant difference.

It's also surprising that while the linear distance of the 28-270mm lens is 242mm, and the 50-300mm lens is 250mm, it seems counter-intuitive that the lens with the shorter linear distance has a significantly larger magnification. Again, this is due to the Inverse Square Law, and you can confirm the differences by looking at the photos to the right. Indeed, the 28-270mm lens would provide significantly more coverage than the 50-300mm lens.

If you don't want to do the math, just remember that differences at the shorter focal lengths are more significant than at the longer focal lengths. In the example above, the 30mm difference between 270mm and 300mm is hardly noticeable, while the 22mm difference between 28mm and 50mm is a huge difference.

Super Zooms.

In the DSLR world, a zoom lens with a magnification more than about 4x is considered a "Super Zoom". Any lens that has a higher magnification can begin to have issues. This of course is highly subjective, and some fairly good examples of super zooms can be found. But the best optical quality zoom lenses will have limited zoom ranges.

Does this mean you should avoid Super Zooms? Not necessarily. I carry a Nikon 18-200mm zoom for my main "travel lens" when I am on a cruise vaction, which has an 11x magnification ratio. While it does not have quite as much sharpness as my other lenses, the advantages of an all-in-one lens for light-weight travel and packing justify the small loss of optical quality. If I am not on travel, I'll tend to use other lenses, but for travel - I prefer not having to lug around heavy lenses.

Crop Factor.

In the DSLR world, this is a very confusing concept, so it bears some explanation. Due to economic constraints, many DSLRs use smaller sensors than the 35mm film size. Nikon DSLRs of this type are called "DX" cameras. Canon and others use different terms for their versions. A crop camera is simply a camera that has a smaller sensor than the 35mm format. In contrast, Full-Frame cameras have the same sensor size as 35mm film.

So what does all of this mean? Generally speaking, when you use a cropped DSLR camera with a 35mm lens, the apparent focal length changes. Why this happens is rather involved, so I have a detailed web page for that topic. Different cameras have different crop factors. Nikon DX cameras are 1.5x, Canon cropped cameras are 1.29x or 1.6x, depending on model, and other cameras have different crop factors.

For example, if you have a Nikon DX camera, it has a crop factor of 1.5x. Therefore, a 50mm lens would appear to be a 75mm lens (50mm x 1.5 crop factor). In fact, every lens that can be mounted on on the camera will have the same 1.5x shift in magnification. One thing that must be noted however, is that although the lens has shifted in apparent focal length, the lens characteristics are still that of the original lens.

A 20mm wide angle lens, for instance, will have a certain perspective distortion characteristic. When that lens is fit on a cropped camera, it's effective focal length is 30mm. However, the perspective distortion characteristic of the 20mm lens has not changed. It is still a 20mm lens regardless of the crop factor.

This is a confusing principle, so review my crop sensor page for a detailed (and hopefully clearer) explanation.

Zoom with your feet.

Is it necessary to cover every millimeter of focal length with a lens? No; especially at the mid-range areas - from perhaps 30mm to 60mm. In that range, even a prime lens, located in the center of that coverage area will be fine, especially if you can move towards or away from your subject (zooming with your feet).

For example, in my "pro" kit, I have a Tokina 11-16mm f/2.8, a Nikon 35mm f/1.8, and Nikon 80-200mm f/2.8 lenses. The lenses at the two extremes are zooms, but there is a gap from 16mm to 80mm. This is filled adequately with the 35mm lens and zooming with my feet.

At first look, you might not think that the 35mm lens would be the best choice, but the lens is optically in the exact center of the two zooms. The Tokina zoom tops out at 16mm, and the Nikon's zoom low end is 80mm. But, isn't 48mm at the center of this gap? Yes and no. Yes, it is in the linear center, but no it is not in the optical center.

True, 48mm is half-way between 16mm and 80mm, so it would suggest using a 50mm lens. But a 35mm lens is the optical center due to the Inverse Square Law. In magnification power, 35mm is 2.2x from 16mm, and it is also 2.2x from 80mm. Therefore, optically, 35mm is in the center, and that is the lens I chose for my "kit".

Long Zooms.

OK, my lens coverage is adequate to 200mm. But what about 300mm, 400mm, and 500mm? We have already seen that the power of 2, or the Inverse Square Law results in a marginally significant difference between 200mm and 300mm, so many pros that have expensive 70-200mm f/2.8 lenses like it's superior optics, and can easily make up for the 300mm reach by again, zooming with their feet.

There are times when that is not good enough, so they have to go with longer focal lengths. A 400mm zoom or prime is the logical choice. There is enough difference between 200mm and 400mm to justify the expense, but they pay dearly for purchasing a 400mm lens that is anywhere close to the performance of their 70-200mm f/2.8 lens.

While there are plenty of extreme zoom lenses on the market that are longer than 300mm, there is a lot of compromise unless you are willing to spend many thousands of dollars. In a lot of cases, you might be better off with a superior f/2.8 zoom and cropping the photo as the less expensive (but not cheap) lenses tend to be optically "slow" at the longer telephoto lengths.

Teleconverters. These have a pretty limited application, and work best with fast (f/2.8 and faster) prime lenses. If you try to use a teleconverter with a zoom lens that tops out at f/5.6 (or worse), you won't achieve the results you hoped for.

Variable Apertures.

The typical consumer-grade zoom lens will have a variable aperture. That means the aperture changes as the focal length changes. For instance, a consumer 55-200mm zoom might have a f/4~f/5.6 aperture, meaning that the aperture is f/4.0 at the 55mm end and f/5.6 at the 200mm end.

In contrast, "pro" caliber lenses tend to have constant apertures, and a 70-200mm f/2.8 lens will have a constant f/2.8 along the entire zoom length. Constant aperture lenses are harder to make, and much more expensive than consumer grade lenses - but their performance warrants the extra cost.

Does that mean that consumer-grade variable aperture lenses are no good. Of course not; it just means those lenses tend to be "daylight" only; in that they require adequate lighting for the best results.

Vibration Reduction.

Some of the modern zoom lenses (and a few primes) have Vibration Reduction, wherein a small motor and sensing electronics are built into the lens. The motor is connected to a lens element(s) internal to the lens, and compensates for "camera shake". Camera shake is user induced, and comes from not being able to hold the lens steadily enough for a sharp photo.

Prior to Vibration Reduction, the photographer had to either use a high-shutter speed or a tripod to keep from moving the camera. Camera shake becomes more significant as the zoom length increases, and if you have a lens more than 200mm, for maximum flexibility, you should make sure it has Vibration Reduction.

Vibration Reduction usually adds some cost to the lens, so some camera manufacturers build Vibration Reduction into the camera body; Sony is one such manufacturer. Vibration reduction is achieved in these cameras by moving the camera's sensor rather than in the lens. Which is better? Consider that both Nikon and Canon use in-body Vibration Reduction in their less expensive Compact Cameras, but in-lens Vibration Reduction in their DSLRs.

If in-lens Vibration Reduction was no better than in-camera Vibration Reduction, why do Nikon and Canon go to the additional expense of using it in-lens? My cameras are Nikon so I suppose I am a bit biased in this regard, but it seems to me that if there was no difference, Nikon and Canon would be using in-camera Vibration Reduction as they do in their Compact Cameras.

Fast Focusing.

Historically, most autofocus lenses relied on a focus motor built into the camera body, but newer lenses tend to have focus-motors in the lens. Of course, this does increase the cost of the lenses somewhat, but in most cases, it results in faster focusing lenses. Nikon calls the lenses that require the focus motor to be in the camera body "AF" lenses, and those that have focus motors in the lens "AF-S". Canon uses "USM", and other lens manufacturers have their own designation.

Some cameras in fact; typically Nikon's entry-level DSLRs, have no built-in autofocus motors in the cameras, and will only autofocus with the AF-S lenses.

In conclusion, there are a lot of factors to consider when choosing your zoom lens; it's focal range, speed, Vibration Reduction, internal focus motors, and so on. What is best for you will ultimately be the lens that meets your needs within your budget.