Understanding DSLR Lens Requirements

You’ve got a little background in lenses now, and you’re ready to learn exactly what you should be looking for when choosing lenses for your digital SLR. After all, the lenses you own affect the quality of your images as well as the kinds of pictures you can take. The most important factors in choosing a lens are the quality of the lens, the resolution of the images it produces, the amount of light it can transmit (that is, its maximum lens opening), its focusing range (how close you can be to your subject), and the amount of magnification (or zooming, in a zoom lens) that it provides. Here are some of the things you should consider.

Image Quality

If you’re graduating from a digital point-and-shoot camera, one of the first things you notice is how concerned your colleagues are over lens sharpness. Most point-and-shooters don’t worry about it that much because there is little they can do about it other than purchasing another camera. The lens on a non-SLR is what it is; it may be sharp or it may be less sharp, and that’s it. The situation is similar to the horsepower question in an econobox automobile. Horsepower is not what you purchased the vehicle for. You’d be more interested in engine power if you had a sports car that, perhaps, could be souped up a little with some aftermarket components.

Lenses are the dSLR’s primary aftermarket component. If you frequent the dSLR newsgroups and forums, you’ll notice the attention paid to how sharp a particular lens is. Visit any such venue and you’ll find multiple postings inquiring about the resolution of this lens or that lens, and whether it has good or bad bokeh. A good rule of thumb is that most general purpose lenses produce good enough image quality for general purpose shooting. When you start to get into specialized areas—ultrawide lenses, extra long telephotos, super-fast optics with large f-stops—the compromises necessary to produce those expensive toys sometimes involve compromises in image quality. If you’re contemplating one of these lenses, it’s a good idea to read the magazines and Web sites that have formalized, standard testing, and check around among your friends, colleagues, and others who can provide you with tips and advice.

Lens Aperture

If you’re a veteran SLR user, you know all about lens apertures. If not, you need to know that the lens aperture is the size of the opening that admits light to the sensor, relative to the magnification or focal length of the lens. A wider aperture lets in more light, allowing you to take pictures in dimmer light. A narrower aperture limits the amount of light that can reach your sensor, which may be useful in very bright light. A good lens will have an ample range of lens openings (called “f-stops) to allow for many different picture-taking situations.

You generally don’t need to bother with f-stops when taking pictures in automatic mode, but we’ll get into apertures from time to time in this book. For now, the best thing to keep in mind is that for digital photography a lens with a maximum (largest) aperture of f2 to f2.8 is “fast” while a lens with a maximum aperture of f8 is “slow.” If you take many pictures in dim light, you’ll want a camera that has a fast lens.

Zoom lenses tend to be slower than their prime lens (non-zooming) counterparts. That’s because digital optics are almost always zoom lenses, and zoom lenses tend to have smaller maximum apertures at a given focal length than a prime lens. For example, a 28mm non-zoom lens for a 35mm camera might have an f2 or f1.4 maximum aperture. Your digital camera’s zoom lens will probably admit only the equivalent of f2.8 to f3.5, or less, when set for the comparable wide-angle field-of-view.

The shorter actual focal length of digital camera lenses when used with cameras that have a lens multiplier factor also makes it difficult to produce effectively large maximum apertures. For example, the equivalent of a 28mm lens on a full-frame camera with a camera having a smaller 1.6X multiplier sensor is an 18mm lens. There’s a double-whammy at work here. Although providing the same field-of-view as a 28mm wide-angle, the 18mm optic has the same depth-of-field as any 18mm lens (much more than you’d get with a 28mm lens). Worse, the mechanics of creating this lens complicates producing a correspondingly wide maximum f-stop. So, while you might have used a 28mm f2 lens with your film camera with a workable amount (or lack) of depth-of-field wide open, you’ll be lucky if your 28mm (equivalent) digital camera lens has an f-stop as wide as f4. That increases your depth-of-field at the same time that the actual focal length of your wide angle (remember, it’s really an 18mm lens) is piling on even more DOF.

What about the minimum aperture? The smallest aperture determines how much light you can block from the sensor, which comes into play when photographing under very bright lighting conditions (such as at the beach or in snow) or when you want to use a long shutter speed to produce a creative blurring effect.

Digital cameras don’t have as much flexibility in minimum aperture as film cameras, partly because of lens design considerations and partly because the ISO 100 speed of most sensors is slow enough that apertures smaller than f22 or f32 are rarely needed. A digital camera’s shutter can generally reduce the amount of exposure enough. So, your lens probably won’t have small f-stops because you wouldn’t get much chance to use them anyway. If you do need less light, there are always neutral density filters.

Zoom Lenses

A zoom lens is a convenience for enlarging or reducing an image without the need to get closer or farther away. You’ll find it an especially useful tool for sports and scenic photography or other situations where your movement is restricted. Only the least-expensive digital non-SLR cameras lack a zoom lens. Some offer only small enlargement ratios, such as 2:1 or 3:1, in which zooming in closer produces an image that is twice or three times as big as one produced when the camera is zoomed out. More-expensive cameras have longer zoom ranges, from 4:1 to 10:1 and beyond.

Digital SLRs, of course, can be fitted with any zoom lens that is compatible with your particular camera, and you’ll find a huge number of them in all focal length ranges and zoom ratios. There are wide-angle and wide-angle-to-short telephoto zooms, which cover the range of about 18mm to 70mm (35mm equivalent), short telephoto zooms from around 70mm to 200mm (equivalent), high-power zooms in the 80mm to 400mm range, and lenses that confine their magnifications to the long telephoto territories from about 200mm to 600mm or more.

Digital SLRs generally rely solely on what is now called optical zoom, the relationship of the individual elements of the lens are changed to produce the changes in magnification. Because the lens elements can be finely tuned, this produces the sharpest image at each lens magnification.

For example, a typical zoom might be described as having 10 elements in eight groups. Each of the groups can be moved individually to provide the desired magnification and the best image. The optical science behind these relationships is complex, and we should be thankful that our spanking-new digital cameras have 50 years or more of research backing the optical component.

Thankfully, dSLR owners are spared the problems caused by that feature-without-portfolio found in point-and-shoot cameras: the digital zoom, in which the apparent magnification is actually produced simply by enlarging part of the center of the image. I tend to think of digital zoom as a feature-turned-bug (the exact opposite of a bug that’s promoted as a useful feature). Digital zoom is less sharp than optical zoom. Indeed, you can invariably do a better job by simply taking the picture at a point-and-shoot camera’s maximum optical zoom setting and enlarging the image in your image editor.

Because the elements of a lens are moving around in strange and mysterious ways, the effective aperture and focus of a lens may vary as the magnification settings change. A lens that has an f2.8 maximum aperture at its wide angle setting may provide only the amount of light admitted by an f3.5 lens at the tele position. Focus can change, too, so when you focus at, say, the wide-angle position and then zoom in to a telephoto view, the original subject may not technically still be in sharpest focus (although the huge amount of depth-of-field provided by digital camera lenses may make the difference impossible to detect). You’d notice the differences only when using the camera in manual exposure or focusing mode, anyway. When set to autofocus and autoexposure, your camera will provide the optimum setting regardless of zoom magnification.

Focusing

The ability to focus close is an important feature for many digital camera owners. One of the basic rules of photography is to get as close as possible and crop out extraneous material. That’s particularly important with digital cameras because any wasted subject area translates into fewer pixels available when you start cropping and enlarging your image. So, if you like taking pictures of flowers or insects, plan to photograph your collection of Lladró porcelain on a tabletop, or just want some cool pictures of your model airplane or stamp collections, you’ll want to be able to focus up close and personal.

What’s considered close can vary from model to model; anything from 12 inches to less than an inch can be considered “close-up,” depending on the vendor. Fortunately, those short focal length lenses found on digital cameras come to the rescue again. Close focusing is achieved by moving the lens farther away from the sensor (or film) and an 18mm wide-angle lens doesn’t have to be moved very far to produce an image of a tiny object that fills the viewfinder.

Add-On Attachments

Photographers have been hanging stuff on the front of their lenses to create special effects for a hundred years or more. These include filters to correct colors or provide odd looks, diffraction gratings and prisms to split an image into pieces, pieces of glass with Vaseline smeared on them to provide a soft-focus effect, and dozens of other devices. These range from close-up lenses to microscope attachments to infrared filters that let you take pictures beyond the visible spectrum. Add-on wide-angle and telephoto attachments are also available, along with slidecopy accessories and other goodies. If you’re serious about photography, you’ll want to explore these options.

Unfortunately, dSLRs come with lenses that have all different sizes of filter threads. One of Nikon’s strong selling points in olden times was that virtually all its general purpose lenses used a 52mm filter thread, so you could invest hundreds of dollars in filters and add-ons and be able to use them with a whole range of optics. Of course, a 52mm thread size is hardly practical for modern dSLR lens designs. You’re more likely to need 62mm accessories for many of your lenses, probably will require 67mm add-ons for many of them, and needn’t be surprised if your faster lenses, longer zooms, and widest optics require 72mm or 77mm filters.

Of course, you won’t want to choose a lens based on its filter thread, but it’s a good idea to look at how you plan to use your lenses before purchasing filters. If only one of your lenses requires a 72mm filter, but the lenses you use most use 62mm and 67mm filters, you might want to standardize on 67mm filters and use a step-down ring to mount those same filters on the lenses that accept 62mm accessories. Buy only those 72mm filters you actually need. Filters are so much more expensive in the larger sizes that you probably won’t need much prompting to make your plans carefully.

Construction Quality

The final consideration when choosing a lens is the quality of its construction. See if the key lens components are made of metal or plastic. Believe it or not, some lower-cost lenses have mounts that are made of non-metallic components. They’re less sturdy, and more likely to wear if you attach and detach them from your camera often.

Also check for play in the focusing and zooming mechanisms. You don’t want any looseness, stickiness, odd noises, or other qualities that signify cheap or poor construction. Your investment in lenses will probably exceed your cost for your digital camera body after a few months, so you want your lenses to hold up under the kind of use and abuse you’ll subject them to.

Remember that, most likely, the lenses you purchase after each bout with Lens Lust will probably work just as well with your next dSLR as with your current model, so you can consider them a long-term investment. I have lenses that I purchased early in my career that are still in use a dozen camera bodies later. So, don’t be afraid to spend a little more for lenses that are constructed well and have all the features you need. I own more than one lens that originally cost more than my dSLR body, and I don’t consider the expenditure extravagant.