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DSLR News - August 2008 This month's Digital SLR Guide newsletter is all about an essential component of any digital SLR system: the lens. Without a lens to focus the outside world onto the camera's sensor, a digital SLR is essentially a blind paperweight (or at least a very nearsighted paperweight). When you pop a lens onto your camera, everything snaps into focus and you're able to start taking pictures. Putting the cheapest lens you can find on your digital SLR is equivalent to buying the cheapest pair of glasses you can find to correct your vision. While you might see some improvement over not having the glasses at all, chances are you could see things a whole lot better if you paid the extra cost for glasses with better optics. The same applies to a camera lens: its whole purpose in life is to focus a clear image straight onto the digital sensor inside your camera. Lenses with cheap optics don't focus the image nearly as well as lenses with more expensive components. If you've been disappointed with the sharpness and clarity of the photos coming from your digital SLR camera, chances are that it's the lens that's causing the problem, not your camera. You might even have experienced a moment of sheer panic when you realized that the photos captured by your cheap compact digital camera looked better than the ones from your ultra-expensive SLR. The reason is simple: compact digital lenses have been optimized for the camera bodies they're attached to. Since the lens is actually part of the camera, manufacturers can ensure that the lens is focusing a nice sharp image onto the digital sensor. With an SLR, it's not so simple. There are some notable exceptions to this rule, but just about any SLR lens that you can put your hands on for less than $100 will hinder your images more than it will help. After years of ongoing research, I've found that there's a sweet spot for lens prices: a place where you're paying enough to get a lens with great optics but not exceptional optics (since the difference between these two can be hard to tell). This sweet spot is anywhere between $300 to $600 - although a lot will depend on the TYPE of lens that you're considering. For example, standard zoom lenses are pretty common these days, and competition means that you can find a great lens for closer to the $300 price point. But if you're considering the purchase of a nice big telephoto zoom you're going to have to part with $600 or more. Regardless of what you decide, it's important to do research first. Start with my 6-step guide to buying a digital SLR lens, and use online resources like Fred Miranda's web site to read actual user reviews of how a lens performs.

In This Issue Avoid Cheap Lenses Focal Length Defined Cheat Sheet: Portraits With Blurry Backgrounds Do You Need a Constant Max Aperture? Alternative Kit Lenses Photo Links Latest SLR Guides Canon Rebel XSi Pentax K20D Canon 40D Olympus E-510 Nikon D80 Canon Rebel xTi Nikon D40 Digital SLR Terms Megapixels Stabilization ISO / Image Noise Sensor Dust Crop Factor RAW vs. JPG Continuous Photos Autofocus Points Aspect Ratio Lens Terms Focal Length Prime vs. Zoom Maximum Aperture First vs. Third Party SLR Lens Features Canon Lens Glossary Nikon Lens Glossary

Digital SLR Q & A

Question: What is focal length and why does it matter?

Answer

The focal length is a property of your camera's lens and different focal lengths allow you to capture either more or less of the scene or subject in front of you.

Here's the summary information:

• Focal lengths are measured in millimeters
• Short focal length lenses (10-20mm) are also called wide angle
• Medium focal length lenses (28-100mm) are also called regular
• Long focal length lenses (200-400mm) are also called telephoto
• Lenses with only one focal length are called prime lenses
• Lenses with variable focal lengths are called zoom lenses

All well and good - but what type of focal length should you use when taking photos? The answer to this question depends entirely on your subject, and the subject-to-camera distance.

EXAMPLE #1

You want to take some nice photos of the kids to send to their grandparents. Assume that there are 3 kids you're trying to capture, and that you'd like some photos with all three in addition to individual headshots.

For this photo session you'll want to use a regular zoom lens: something with a variable focal length between 28 and 100mm. The 28mm (wide angle) setting will allow you to capture all three kids standing next to each other, while the 100mm (telephoto) length will capture some nice closeups, even if you're some distance away.

EXAMPLE #2

You'd like to take some shots of a sporting event, but you'll have to take them all from the edge of the playing field.

In this case, you need a telephoto zoom lens with focal lengths anywhere between 200 and 400mm. These long focal lengths will allow you to get up close and personal with your subjects, even though you're a fair distance away from all the action.

EXAMPLE #3

This one's topical: you're a photographer at the Olympics taking photos of the event from the press box.

If you've watched any Olympic coverage, you may have seen a cluster of photographers, all with these absolutely enormous lenses on their cameras. Most of those lenses are extreme telephoto primes: lenses with focal lengths around 600mm.

These long focal length lenses allow the photographers to capture the events and expressions of the competitors, even from a considerable distance.

DSLR Cheat Sheet

This month's cheat sheet covers a topic that's simple to re-create but that takes a long time to master.

The topic is this: how to capture portraits with very blurry backgrounds.

Once you've taken enough photos of people, you begin to realize something: the background behind the person is as important - if not more so - than the person you're photographing.

A great background can create a sense of space: it provides the viewer with context about the location of the portrait. Additional information you want to convey about the subject - social status, personality type, profession - can be conveyed by a well-selected background.

On the flip side, a background that's too busy competes with your primary subject, drawing attention away from the portrait and toward the clutter in the image.

This is why many portrait photographers deliberately blur the background so substantially that specific details become unrecognizable.

There are three steps you need to take in order to capture these types of portraits:

1. Set your lens to its telephoto setting (a longer focal length)
2. Open the lens aperture as wide as it will go (in aperture priority mode, select a small f number)
3. Get up close and personal with your subject

All three of these variables have an impact on something called depth of field: this is the distance in front of and behind your primary subject that appear clear in your photo.

When you're working with a shallow depth of field (as is the case when you follow the three steps above) only the subject of your portrait will be crystal clear, and the background - whatever it may be - will blur out into a wash of color that doesn't draw attention away from your primary subject.

What Do You Need?

This month I'll answer the question: do you need a lens with a constant maximum aperture?

Before I dig into the answer to this question, it helps to have a better understanding of the alternative: variable maximum aperture.

The maximum aperture of your digital SLR lens represents the widest opening the lens can achieve, which allows the greatest volume of light to land on the sensor. Lens maximum apertures are always marked on the lens itself - if you hear someone chatting about a 28-135mm f/2.8, the 2.8 number is the maximum aperture of that particular lens.

If you have a kit lens for your digital SLR, you might have noticed that there are actually TWO numbers instead of one. This is because all kit lenses have variable maximum apertures.

Simply put, the maximum aperture of the lens is not the same at the wide angle setting as it is at the telephoto end. The maximum opening of the lens narrows the more you zoom.

In the case of an 18-55mm f/3.5-5.6 (the most common kit lens), the maximum aperture at 18mm is f/3.5 while at 55mm the max aperture narrows to f/5.6 (the larger an aperture number, the smaller the opening in the lens).

Now consider the following lens: a 28-75mm f/2.8. Since only one aperture number is listed, this means that the lens has a CONSTANT maximum aperture throughout the zoom range. Whether you're at 28mm, 75mm or somewhere in between, the maximum opening in the lens never changes.

Now that we've got a better understanding of what a constant maximum aperture means, let's talk in more depth about whether or not you really need it.

Constant maximum apertures are most useful if you take photos with the camera set to manual exposure mode.

If you only ever use your camera in full AUTO mode (something that I don't recommend) it decides what the aperture and shutter speed settings should be. If you use manual mode, you are in control of both settings.

Let's say that you're using the camera in manual mode with a lens that has a variable maximum aperture (an 18-55mm f/3.5-5.6). With the lens set to 18mm, you open the aperture as wide as it will go (f/3.5). Using the camera's light meter, you figure out that a good shutter speed setting is 1/60th of a second.

With aperture and shutter speed set, you're ready to take some photos of your subject. But let's say that in the middle of the photo session, you zoom your lens to 55mm, which narrows the aperture to f/5.6. If you don't immediately change the shutter speed setting, you're going to get a photo that's under exposed (too dark).

If your lens has a constant maximum aperture, there's no need to worry: just dial in the right aperture and shutter speed and zoom the lens as much as you want. So long as the available light remains constant, the settings that you've selected don't have to change.

Alternatives to the Kit Lens

Visitors to the Digital SLR Guide often ask me: what lens would you recommend as an alternative to the kit lens?

After all, the kit lens that comes bundled with a digital SLR camera isn't top of the line - if it were, it would drive the price of camera and lens higher and make the overall price of the camera less appealing to consumers.

So what if you're looking for something just a TAD bit nicer, but without spending an obscene amount of money on a pro-grade lens?

The answers lie in the following selection of lenses - my choices mind you - but lenses that have received overall good reviews and that present a step up in image quality from just the basic SLR package.

Links

The following collection of links will help to keep you posted about what's new at the Guide and in the world of digital SLR cameras.

In Conclusion

Be prepared - the big Photokina trade show is next month and there are plenty of new camera announcements are right around the corner.

Canon just announced the 50D, Nikon's out with the D90 (see above) and you can definitely expect to hear more from Sony very soon. Looks like I'm going to be quite busy as the year wraps up here.

Until next month...happy picture taking!

--Chris Roberts, Your Digital SLR Guide