by Gerald Boerner
Today we continue our examination of the process of choosing a set of lenses for your SLR (or dSLR) camera. More specifically, we examine several factors related to zoom lenses, lenses that allow us to vary the focal length of the lens. These lenses allow us to minimize the number of lenses that we carry in our camera bags. [In days past, we would carry a large number of fixed focal length lenses, called Prime Lenses, to meet the needs of our shooting requirements.] We will deal with some of these factors in application tomorrow.. GLB
“The camera is no more an instrument of preservation, the image is.”
— Berenice Abbott
“A film is never really good unless the camera is an eye in the head of a poet.”
— Orson Welles
“I still need the camera because it is the only reason anyone is talking to me.”
— Annie Leibovitz
“One should really use the camera as though tomorrow you’d be stricken blind.”
— Dorothea Lange
“My own eyes are no more than scouts on a preliminary search, for the camera’s eye may entirely change my idea.”
— Edward Weston
“I tried to keep both arts alive, but the camera won. I found that while the camera does not express the soul, perhaps a photograph can!”
— Ansel Adams
“The creative act lasts but a brief moment, a lightning instant of give-and-take, just long enough for you to level the camera and to trap the fleeting prey in your little box.”
— Henri Cartier-Bresson
“As I have practiced it, photography produces pleasure by simplicity. I see something special and show it to the camera. A picture is produced. The moment is held until someone sees it. Then it is theirs.”
— Sam Abell
Focal Concept/Technology: Why Use a Zoom Lens
A zoom lens is a mechanical assembly of lens elements with the ability to vary its focal length (and thus angle of view), as opposed to a fixed focal length (FFL) lens (see prime lens). They are commonly used with still, video, motion picture cameras, projectors, some binoculars, microscopes, telescopes, telescopic sights, and other optical instruments.
A true zoom lens, also called a parfocal lens, is one that maintains focus when its focal length changes. A lens that loses focus during zooming is more properly called a varifocal lens.
Simplified zoom lens in operation
Zoom Lenses on Digital Cameras
About.com provides good overview information on many photographic subjects. The following information is derived from the article referenced in the Reference Section below.
With most digital cameras, the zoom lens moves outward when in use, extending from the camera body. Some digital cameras, however, create the zoom while adjusting the lens only within the camera body.
Optical Zoom…
Optical zoom measures the actual increase in the focal length of the lens. Focal length is the distance between the center of the lens and the image sensor. By moving the lens farther from the image sensor inside the camera body, the zoom increases because a smaller portion of the scene strikes the image sensor, resulting in magnification.When using optical zoom, some digital cameras will have a smooth zoom, meaning you can stop at any point along the entire length of the zoom for a partial zoom. Some digital cameras will use distinctive stops along the length of the zoom, usually limiting you to between four and seven partial zoom positions.
Digital Zoom…
The digital zoom measurement on a digital camera, to put it bluntly, is worthless under most shooting circumstances. Digital zoom is a technology where the camera shoots the photo and then crops and magnifies it to create an artificial close-up photo. This process requires magnifying or removing individual pixels, which can cause image quality degradation.Most of the time you can perform functions equal to a digital zoom with photo-editing software on your computer after you shoot the photo. If you don’t have time for or access to editing software, you can use digital zoom to shoot at a high resolution and then create an artificial close-up by removing pixels and cropping the photo down to a lower resolution that still meets your printing needs. Obviously, the usefulness of digital zoom is limited to certain circumstances.
Understanding Zoom Measurement
When looking at specifications for a digital camera, both the optical and digital zoom measurements are listed as a number and an "X," such as 3X or 10X. A larger number signifies a stronger magnification capability.
Keep in mind that not every camera’s "10X" optical zoom measurement is the same. Manufacturers measure the optical zoom from one extreme of the lens’ capabilities to the other. In other words, the "multiplier" is the difference between the smallest and largest magnification measurements of the lens. For example, if a 10X optical zoom lens on a digital camera has a minimum magnification equivalent to 35mm, the camera would have a 350mm maximum zoom. However, if the digital camera offers some additional wide-angle capabilities and has a minimum 28mm equivalency, then the 10X optical zoom would only have a maximum zoom of 280mm.
The zoom range should be listed in the camera’s specifications, usually in a format similar to "35mm film equivalent: 28mm-280mm.” In most cases, a 50mm lens measurement is considered as "normal," with no magnification and no wide-angle capability.
Interchangeable Lenses on SLR Cameras
Digital cameras aimed at beginners and intermediate users typically only offer a built-in lens. Most digital SLR cameras, however, can make use of interchangeable lenses. With a digital SLR, if your first lens doesn’t have the wide-angle or zoom capabilities that you want, you can purchase additional lenses that provide more zoom or better wide-angle options.
Digital SLR cameras are more expensive than the point-and-shoot models, and they’re usually aimed at intermediate or advanced photographers.
Some Zoom Lens Drawbacks
Although choosing a point-and-shoot camera with a large optical zoom lens is desirable for many photographers, it sometimes presents a few minor drawbacks.
Some beginner-level, inexpensive cameras suffer from lower image quality because of noise when the lens is extended to the maximum zoom capability. Digital camera noise is a set of stray pixels that don’t record correctly, usually appearing as purple edges in a photo.
Maximum zoom also sometimes causes pincushioning, which is a distortion where the left and right edges of the photo appear stretched. Horizontal lines appear slightly curved toward the middle of the frame. Again, this problem usually is limited to beginner-level, inexpensive cameras with large zoom lenses.
When using the maximum zoom magnification, the shutter response time sometimes slows, which may cause blurry photos. You also could miss a spontaneous photo because of the slower shutter response. It simply takes the digital camera longer to focus on the image at the maximum zoom setting, which explains the slower shutter response time. Such problems are magnified when shooting at maximum zoom in low light.
Using a long zoom lens can cause increased camera shake. Some digital cameras can correct this problem through image stabilization. You also can use a tripod to prevent blurry photos from camera shake.
Reviewing the Selection Procedure
TheTechLounge presents some useful tips in selecting and using a SLR Lens. (See reference below for specific reference.)
What’s My Lens Mount? Different cameras use different lens mounts. Before you start looking for the perfect lens, you need to find out what lenses your camera is compatible with. This information can be easily found on the camera manufacturer’s website, usually listed under "Specifications." For example, I can see from Canon’s website that my EOS 5D DSLR uses the Canon EF lens mount and accepts EF (except EF-S) lenses. Likewise, the EOS 30D uses the EF mount and accepts EF (including EF-S) lenses. I should mention that there are lens mount adapters out there if you want to use a Canon lens on a Nikon camera, for example, but that’s not really within the scope of this article…
To make things a little more complicated, there is such a thing as a "Digital-Only" lens. In the case of Canon, these lenses are given the EF-S moniker. In the examples above, note that the 30D supports EF-S lenses while the 5D does not. Meanwhile, EF lenses are fully supported by both cameras. So pay attention when looking at a lens’ technical specs. This leads me to my next point.
What’s My Camera’s Crop Factor? Without going into too much detail, the crop factor (Canon refers to it as the Lens Focal Length Conversion Factor) will act as a multiplier to a given lens’ focal length as a result of having a digital sensor that is smaller than a 35mm negative. It’s common to hear the terms "cropped body" and "full-frame" thrown around amongst photographers; this is what they’re talking about. The 5D is a full-frame camera and so it follows that the crop factor is 1.0x, meaning your focal length is as stated on the lens. The 30D is a cropped body with a crop factor of 1.6x, meaning you will need to multiply a lens’ focal length by 1.6 to get the equivalent focal length.
Let’s say I have a Canon EF 16-35mm f/2.8L USM lens; the important part of the name for our purposes here is the focal length range, 16-35mm. On the 5D, the lens will act as just that, a 16-35mm lens. However, if you mount it on the 30D, what you’ll get is a 25.6mm-56mm lens. This multiplying effect has a greater impact with more telephoto lenses. For instance, take Canon’s EF 70-200mm f/4.0L USM lens. Again, the important part here is 70-200mm. We already know that the lens mounted on a 5D will give an actual focal length of 70-200mm. But on the 30D the lens give you an equivalent focal length of 112-320mm. It is for this reason that some people prefer to have a cropped body (long telephoto lenses are very expensive).
What’s My Subject? Photographically speaking, what is your typical subject? There is no wrong answer here, and you may not even have a single subject that you typically photograph. Below I’ve laid out some typical scenarios to give you a general idea of what lenses may be a good fit for you.
Do I Want Zoom or Fixed Focal Length (Prime)? You might have noticed that I didn’t use the word zoom once in the above section, and this was for a reason! As pertaining to focal length, there are two types of lenses: zooms and primes. The difference is basically that zoom lenses zoom while prime lenses, well! they don’t zoom. So if you wanted to have similar focal lengths to what you’d get with a 24-70mm and 70-200mm zoom lens combo, you might get a 20mm, 50mm, 135mm, and/or 200mm prime lens.
Whether or not prime lenses are for you is really a matter of preference. Those who prefer primes usually do so because they have a slight edge in image quality, they’re often faster (that is, they have a wider maximum aperture), they’re lighter, and they can be less expensive than a zoom which covers a similar range. What they gain in image quality, they sacrifice in convenience, however. If you want to get a different perspective you have to either get some exercise (move those legs) or switch lenses. Furthermore, some of the really high-end primes aren’t much less expensive (if at all) than a comparative zoom lens. With that said, if you aren’t under any pressure while shooting, and you’ve got plenty of time to move around and switch lenses when necessary (i.e. landscape, interior, city) then you might want to seriously consider the benefits prime lenses have to offer.
Do I Want Constant or Variable Aperture? Variable aperture may be a non-issue for some, but for others it can be the cause of great annoyance. But before I go off on a tangent, let’s talk about what it means! I would say that probably the majority of mainstream lenses (that is, affordable for the casual photographer) have a variable aperture. For example, a lens I once owned (and loved) was Canon’s EF 28-105mm f/3.5-4.5 USM. Notice how it says f/3.5-4.5 instead of just f/3.5? What that means is that when the lens is at its widest (28mm) the maximum aperture is f/3.5, and as you zoom in it gradually increases until you’re at full telephoto (105mm) and your maximum aperture becomes f/4.5. So what, right? Well, for some of you at least, that may not matter.
Conclusions
In the previous six posts in this series “Choosing your Lens”, we’ve dealt with most of these factors. Some additional comments are appropriate here:
-
Optical zoom is the Solution…
This is the only solution for SLR cameras with interchangeable lenses; it is the only good solution for compact and prosumer digital camera. Digital zoom is a “BAD” solution since it “creates” the extra pixels used to create the zoom. -
Newer Zoom Lens Technology is often Better than older Prime Lens Technology…
We have seen a refinement in the processing of the lenses that go into the modern batch of zoom cameras. In most cases, this provides us with better lenses than a whole set of “Prime” lenses. Building your lens kit with zoom lenses will save you both money and space. -
Buy the Best “Glass” that you can Afford…
Most camera manufacturers provide a premium series of lenses. In the case of Canon, these are called the “L” series and they generally are only available with the “EF” mounts. That means that these lenses can be used on both full-frame and smaller sensors. -
Select Constant Aperture whenever possible…
Since photography is “drawing with light,” it makes sense that to carry lenses that admit the most light possible, if that feature is appropriate to your photographic subjects. The same applies to the rotation of the lens when the zoom factor is adjusted; this is important especially when using polarizing and variable Neutral-Density filters.
References:
Barbara London, Jim Stone, & John Upton. (2008) Photography. Pearson, Prentice-Hall
Background and biographical information is from Wikipedia articles on:
Wikipedia: The Camera…
http://en.wikipedia.org/wiki/Camera
Wikipedia: Zoom Lens…
http://en.wikipedia.org/wiki/Zoom_lens
Web Sites and Blogs:
Bainy Quote: Camera Quotes…
http://www.brainyquote.com/quotes/keywords/camera.html
The Tech Lounge: 5 Things You Need to Know About SRL Lenses…
http://www.thetechlounge.com/article/379/
About.com: Understand Camera Zoom Lenses…
http://cameras.about.com/od/buyingguide/a/zoom_lenses_und.htm
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