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DSLR Guide News - All About Lenses
August 31, 2008
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Digital SLR Q & A
Question: What is focal length and why does it matter?
AnswerThe 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:
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.
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.
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.
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 SheetThis 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:
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.
Focal Length:140mm | Lens Aperture: f/4.5 | Lens to Subject Distance: about 5 ft.
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 LensVisitors 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.
For CanonCanon EF-S 17-85mm f/3.5-5.6 IS USM.
Its primary advantages over the kit lens include a wider zoom range and image stabilization, which can be quite useful if you don't want to lug around a tripod all the time.
About the only drawback is that is has the same variable maximum aperture as the standard kit lens and a fairly expensive $488 USD price tag.
A second alternative for those looking for a wide maximum aperture is the Tamron 28-75mm f/2.8 XR Di LD which lacks the image stabilization but makes up for it with exceptional optics and a wide constant max aperture of f/2.8 for the relatively affordable price of $360 USD.
For NikonNikon 18-200mm f/3.5-5.6G ED IF AF-S VR (about $645 USD).
This lens not only covers a much greater zoom range (from wide angle to telephoto) it also includes Nikon's image stabilization technology, called Vibration Reduction (VR).
Since the lens has an internal focusing mechanism, it is compatible with the Nikon D40, D40x and D60.
For OlympusOlympus 14-54mm f/2.8-3.5 Zuiko ED ($400 USD).
Offering a wide variable maximum aperture (great for low light photography) and producing incredibly sharp images, this lens also features weatherproofing for those who enjoy taking photos in less-than-ideal conditions.
For Pentax/SamsungPentax DA 18-250mm f/3.5-6.3 ED AL IF.
This is one of those lenses that you can attach to the camera body and leave on at all times, whether you want to take scenic landscapes or close-up portraits.
The only drawback is the relatively narrow maximum aperture (eespeciallyat 250mm) which does limit your ability to use this lens in low light conditions (unless you have it attached to a K20D with its built-in image stabilization).
For SonySony Alpha DT 18-250mm f/3.5-6.3.
Covering the exact same zoom range as the Pentax lens mentioned above (14x zoom to be exact) this is a good lens to leave on your camera at all times, especially if you're not sure what types of photos you'll want to take.
LinksThe 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.
Other Photography Sites
In ConclusionBe 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
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