Saturday, June 10, 2006

Photography Concept 2

In the first post of this series I covered Exposure. Now we'll go into the basics of focal length and how it relates to camera lenses and your photos.

Photography Concept 2: Focal Length And Lenses

The 'focal length' of a lens is the distance from the optical centre of the lens to the film or sensor plane, measured in mm. The "f" in "f-stop" is the focal length. Typical 35mm SLR lens focal lengths range from fisheye lenses which have approximately 10mm focal lengths up to super-telephoto lenses with 1200mm focal lengths - that's almost 4 feet(!) to you imperial system holdouts.

Now before we wade any deeper into the lens pool, I think we better clear things up straight away when it comes to DSLR (Digital Single Lens Reflex camera) vs 35mm film lenses. Unless you buy a 'full frame' DSLR, the sensor in your camera is somewhat smaller than that of a traditional 35mm film frame - which incidentally is 36mm x 24mm. I believe Canon (and possibly Kodak) are the only manufacturers who offer full-frame DSLRs - in fact I'm not sure if Kodak sells these full frame DSLRs anymore. Anyways, what this means is that if you're a peasant like me, you're dealing with a "crop factor". On my Canon Rebel XT this crop factor is 1.6, but it can vary between camera models. Simply put, a lens that has a focal length of 200mm on a 35mm film camera (or full-frame DSLR) will behave as if it has a focal length of 200mm x 1.6 = 320mm on my DSLR camera. A 50mm lens on my old film camera will behave like a 50 x 1.6 = 80mm lens on my DSLR.

I'm not going to delve into the dirty details of why this happens but do remember that the actual focal length of the lens is unaltered. What is actually happening is that the sensor is taking only a 'cropped' portion of what a full frame sensor would capture - hence the term 'crop factor'. It behaves like a longer lens in some respects but not in others - more on this in a few moments.

The important thing to remember for the remainder of my discussion here (and any time I state the focal length of a lens) is that I'm describing the 35mm focal length of the lens - not the length modified by the cropping factor.

Also note that there are fixed lenses and zoom lenses. Zoom lenses allow for a range of focal lengths while fixed lenses are, well... fixed.

Enough with the fiddly bits. Here are the primary reasons why you care about the focal length of the lens: Image Composition and Perspective Characteristics.

Image Composition: The length of the lens will determine your angle of view. A relatively short lens will give you a wide angle of view and a longer lens will give you a narrower angle of view. The length of the lens will also determine the size of your subject in relation to the entire frame of the picture. So with a longer focal length a subject will take up more of the available frame than with a shorter focal length lens - assuming the distance between you and that subject remains constant. If your subject is far away, or if you want your subject to fill the frame, that crop factor we talked about actually helps (remember, that 200mm lens is behaving like a 320mm lens). Of course on the other end of things, that 28mm lens is behaving like a 45mm lens, so you're losing out on the wide angle end of things. I tend to shoot with longer focal lengths more often, so I happen to like the crop factor. If you tend toward wide angle shots, you'll have to pony up for wider lenses or a full-frame DSLR to get your fix.

Perspective Characteristics: The focal length of the lens also affects how perspective is represented in a photograph. Shorter focal lengths will tend to accentuate depth while longer focal lengths will tend to flatten things out. There will be times when you want to accentuate depth (to enhance close-up action shots or emphasize shape or size). And of course there will be times when you need to flatten things out - like when you're taking a portrait shot of your Aunt Bessy, the one with the huge nose. In fact you will find that a focal length of 80mm - 120mm will yield much more flattering portraits of people's faces than if you shoot with focal lengths in the 28-50mm range - and that goes for people with normal sized noses as well. Refer to the comparison photo below to see the kind of difference perspective can make in a portrait. Note however that the crop factor on an DSLR camera DOES NOT change the perspective characteristics of the lens. So the way my 50mm lens represented perspective on my film SLR is unchanged on my DSLR. The magnification of the subject is greater, but the perspective characteristics are no different.

Now you might think that someone writing a post about understanding photography might want to showcase his best work. Here's where I surpise you. Instead of hitting you with a quality photograph, I'll hit you with self-deprecation. Take a look at the comparison shot below. After you're all done saying 'Man that guy's sure got some stones to post that!' you will undoubtedly notice that while both portraits are undeniably horrifying, the one taken at a focal length of 88mm is significantly more appealing in terms of proportion and perspective... relatively speaking of course. Have I illustrated the point? I hope so. And for the record, it was 11:30pm after a long day at work. And if you tell my daughter I was wearing her hat, I'll hunt you down.

My intent with this series of posts is to illustrate the most basic and most important concepts regarding photography. I didn't necessarily want to get diverted into providing a series of quick tips for people - because sometimes giving people shortcuts to an answer prevents them from learning the basis for the solution which is much more valuable in my opinion.

However I am nothing if not easily dissuaded. There are a few basic tips that can make a huge difference in your photography. First, before pressing the shutter button, look around to all the corners and edges of the viewfinder (or LCD screen) and all the places that are not your subject. It is absolutely amazing how easy it is to miss that telephone pole extending out of Uncle Leo's head, or those 16 empty beer cases propped up in the background of the group portrait at your family barbecue. Second, zoom in (with your lens or your feet) to get you subject large enough in the frame. Once you feel your close enough, move in some more. Don't be afraid to really fill the frame with your subject, you'll be surprised at the difference. And finally (for now anyway) remember that people portraits will usually look more attractive at focal lengths of around 80 to 120mm - or even longer. So even with a point and shoot (non-SLR) camera, it is sometimes wise to step back a bit and zoom your lens to a longer focal length before shooting. You might be pleased with the results - but not at 11:30pm on a weeknight ;)

As you might be able to tell, this post was not about describing all the different types and relative merits of different lenses (wide-angle vs telephoto, zoom vs fixed, etc.). There are a huge number of online sources for good information on those subjects - much more detailed than would be appropriate for this series of posts. Along the way I will try to point you to good sources for information that extends past the scope of what I'm writing. Good additional information on lenses can be found here and here. Additional articles on crop factor and focal length can be here and here respectively.

As always, post your comments or email me if you've got an suggestions or complaints.

1 comments:

Portraits from photos said...

Thanks Richard. These are excellent tips. I've learned some good stuff here...