Focal Length

Focal length is the fourth variable of five that we will have discussed after today.  The others, Aperture, ISO, and Shutter Speed we’ve previously discussed and I encourage you to review them as often as necessary to maintain a keen understanding.  Next week we’ll discuss “focal distance” and from then on we’ll know and understand the five main variables and be able to refer to them as we discover exciting new techniques and methods of photography.

The “focal length” of a lens is something you encounter when perusing the specifications for any camera.  On the most basic level focal length refers to the ability of a lens to magnify and image distant objects.  The greater the focal length the greater the magnification.  The standard measurement for focal length is millimeters.  It’s important to understand that as the physical size of the film plane, or the physical size of the sensor changes, that the magnification factor for a given lens changes.  There are all sorts of technical reasons for this, but for now it’s only important to understand that a 50mm lens on a 35mm film camera, crop sensor digital camera, medium format digital back, or a large format film camera will have a different level of apparent magnification.  Because of this we use a “equivalent” standard when talking about cameras, reading specifications, and interpreting photography books.

Almost universally we use a “35mm equivalent”, which refers to the most popular film camera, a 35mm.  In this case a 35mm camera refers to the measurement of the film, not the lens.  However, we use this “35mm equivalent” so we can reference our perception of say a 100mm lens.  Once we know through experience and practice how much magnification a 100mm lens on a 35mm camera provides, how far away from a standard size subject (a person for instance) we need to stand to get them all in the frame, then we can use this mental reference to refer to the focal length of other lenses on cameras with different size sensors and/or film planes.

For instance, a Nikon D300 crop sensor camera has a magnification factor of 1.5x when compared against a 35mm film camera.  The same 100mm lens on a Nikon 35mm film camera, now becomes a 150mm lens when used on the D300 crop sensor digital camera.  A 50mm lens now becomes a 75mm lens, and a 200mm lens now becomes a 300mm lens.  When I say “becomes a XXXmm lens”, I’m talking about magnification factor only, not other characteristics of the lens.

You will notice for instance if you read the specifications for Canon’s G9 compact camera here, that it lists the focal length of the lens as “35-210mm (35mm equiv)”, but if you go to Canon’s site here it will provide the true mathematical focal length of 7.4-44.4mm!  Because there are so many different sized sensors used in point and shoot cameras, we could never mentally keep track the actual perspective without a reference perspective, and universally we use the “35mm equiv” standard.  Review sites, photography magazines, and other photographers will almost always refer to a lens in “35mm equiv” perspective, while often a manufacturer lists the mathematical focal distance instead.

Why are the different focal lengths?  Can’t you just walk backwards and forwards from your subject and achieve the same thing?  With magnification yes.  But not in perspective.  The focal distance (the physical distance from the film plane to the subject) used will provide a different perspective of the subject, and from one extreme can make them look wide and distorted, and the other extreme of very compressed.  IT IS THE FOCAL DISTANCE THAT CONTROLS PERSPECTIVE, NOT FOCAL LENGTH.  Focal length will allow you to get closer to your subject, or further away, for a given perspective.  Let’s use an example.  If I stand 10 meters from a human subject and using a 24-70mm zoom lens, without moving my feet and changing the distance, take a shot at 24mm, and 70mm, and bring them up on my computer screen we’ll see the 24mm image has the subject very small in the frame compared to the 70mm image.  However, if we zoom in on that subject until the subject on the 24mm image fills the same screen space as the subject on the 70mm image, you’ll see the exact same perspective.

So why use a 70mm focal length when we could just zoom in on the 24mm focal length image and get the same perspective?  Because the 70mm image will fill up more of the sensor, giving us more pixels on the subject, and therefore greater clarity, greater resolution, more contrast, and over all a much higher quality picture.

This makes some focal lengths “ideal” for certain uses.  You might have already heard that a 85mm lens (35mm equiv) makes a perfect portrait lens?  Some might say 100, or even 135mm rounding out the top three portrait lens focal lengths.  I have all three, and if the only lens characteristic I’m worried about is focal length, then I’d choose one over the other based only on how far I wish to work from my subject.  For a head and shoulders shot, a 85mm might provide 10 feet of working distance, a 100mm 15 feet, and a 135mm 20 feet.  A 300mm (often used in swimsuit/bikini shoots outdoors) lens would give me an even greater working distance of almost 50 feet for the same head and shoulders framing.  Of course there are other lens characteristics to be concerned with, characteristics which are easier and/or cheaper to achieve with one focal length over the other, but we’ll talk about them at a later time.

So.. 85/100/135mm lenses make good dedicated portrait lenses, and of course you can use them for other purposes as well.  You’ve probably heard of a “wide angle” lens?  This could be any lens from 35mm down to 12mm or less.  A “telephoto” lens normally starts at 135mm and extends to 300mm.  A “super telephoto” from 300-800mm and beyond.

Choosing the best lens for the task at hand can be very easy if you’re only concerned with magnification, but if you’re concerned with all the characteristics of a lens then deciding which lens can become very complicated.  For instance, I have five lenses that  cover the ranges from 12-35mm.  Which of the five I choose for the tasks concerns everything but magnification.  That I have five lenses to choose from at a given magnification, should give you some hint as to the importance of the other lens characteristics.  We’ll get to that later on in this series.  For now, just know that universally the focal length of a lens is referenced to a “35mm equiv” and that the “focal length” refers to the magnification of the subject related to the distance from the subject.