Thursday, 16 October 2014

Focal Length of Camera Lens

What is Focal Length?

Focal Length of Camera Lens
Focal Length of Camera Lens

When light rays pass through the camera lens there is one point where they all converge before going on and making an inverted image on the sensor; this point where the rays converge is called Nodal Point. Focal length is the distance from the Nodal point to the Image sensor.

Why is Focal Length Important?

It is the focal length of a lens that determines how wide its field of view is or in other words how wide a lens is. Shorter focal lengths create a wide angle of view and so lenses with short focal lengths are called wide angle lenses. Similarly lenses with longer focal lengths create a narrow angle of view and so they are called telephoto lenses.

Effect of Focal Length on Subjects

In lenses with shorter focal lengths (wide angle lenses) the light rays converge closer to the image sensor and this in turn makes subjects appear smaller. And in longer focal length lenses light rays converge further away from the image sensor thus making the subject appear much bigger (greater magnification).

Focal length of Lens and Perspective

Creative control of perspective is without doubt a powerful compositional tool in photography and is often the number one factor based on which the photographers choose which focal length lens to use (assuming there is no space limitation and the subject could be photographed from any position).

It is a common misconception in photography that focal length determines the perspective of an image. However this is not true; perspective only changes with one's location relative to their subject (perspective changes if you move closer or farther from the subject).

If the photographer is filling the frame with the subject; then wide angle and telephoto lens will produce different perspectives because in order to fill the frame with a wide angle lens the camera need to be positioned much closer to the subject than when shooting with a tele photo lens and this changes perspective, the wide angle lens will exaggerate or stretch perspective, whereas the telephoto lens will compress or flatten perspective.

If one is photographing a subject or scene from the same distance both wide angle and telephoto will produce the same perspective. If you crop your wide angle frame to match the output of your tele photo lens then you will have similar results.

There are some other factors influenced by the focal length of a lens.

  • Telephoto lenses are more susceptible to camera shake as even minute hand movements get magnified as the focal length increases, the longer the focal length greater this effect. 
  • Wide angle lenses are typically more resistant to lens flare than tele photo lenses. This is because lens designers assume that sun is more likely to be in the frame when using a wide angle lens due to its wider angle of view and so they design them to reduce the effect.
  • Medium and telephoto lenses generally yield better optical quality for similar price ranges.

Focal length and Sensor Size

Traditionally SLR camera lenses were made to fit the 35mm film format, it is still the standard with which most lenses are manufactured, recently many camera manufacturers have introduced lenses especially for the crop sensor cameras. Canon’s lenses for the crop sensors are called EF-S lens and Nikon’s lenses for crop sensors are called DX lens.

It should be remembered that in this article we are only referring to lenses designed for the 35mm format cameras.

When a lens designed to work with the 35 mm format camera is used with a full frame DSLR (which has a sensor size equivalent to the 35mm film) it produces the intended angle of view which could be referred to as 1.0x. So a 100mm lens will produce an angle of view of 

100 * 1.0 = 100mm

But when the very same lens is mounted on a crop sensor camera with a crop factor of 1.6 the angle of view of the lens changes accordingly as we are only capturing the center portion of the image formed by the lens and cropping out the borders. To find out the effective focal length all we need to do is to multiply the focal length of the lens with the sensors crop factor so our angle of view will be

100 * 1.6 = 160mm

A lens with the same focal length when used with different sized sensors produces different angle of views. So focal length is not the same thing as angle of view, they are two separate distinct things. So whenever focal length is discussed the size of the sensor should also be considered as the same lens will act differently on different cameras.

Focal Length and Rule of Thumb for Handholding the Camera

Focal length of the lens used has a significant impact on how easy it is to achieve a sharp handheld photograph. Longer focal lengths require shorter exposure times to minimize blurring caused by camera movement as vibrations are magnified greatly with distance.

The one over focal length rule is a rule of thumb used by photographers to calculate the minimum shutter speed required for getting sharp handheld shots. It states that for a 35 mm camera, the shutter speed needs to be at least as fast as one over the focal length (of the lens used) in seconds.

So when shooting with a lens of 200 mm focal length on a full frame camera, the exposure time needs to be at least 1/200 seconds.

However it is just a general rule, some people have much steadier hands than others and will be able to take sharp pictures while handholding the camera at much lower speeds. Also when calculating the focal length of the lens one needs to consider crop sensors and effective angle of view.

So when same 200 mm lens is used on a crop sensor camera with a crop factor of 1.6x the angle of view now becomes 200 * 1.6 = 320mm and thus minimum shutter speed for handholding becomes 320mm.

Zoom Lenses and Prime Lenses

When lenses were first made, they we all fixed focal length lenses. Changing lenses were one big hassle especially when shooting dynamic scenes or subjects such as when doing photojournalistic work or when shooting action – sports / children. Thus lens designers came up with a lens that could vary its focal length within a pre-defined range thus allowing the photographer to achieve a variety of compositions or perspectives without changing lenses. This new breed of lenses that could vary their focal length was called Zoom lenses and the original fixed focal length lenses were now addressed as Prime lenses.

The major advantages of zoom lenses were that they enabled the photographer to change both composition and perspective. The photographer could change the composition without shifting his / her position with relation to the subject by simply zooming in or out. And he / she could change the perspective by either zooming out and getting closer to the subject (exaggerating subjects near to the lens in relation to background) or by zooming in and moving farther from the subject (compress the scene).

Although Zoom lenses offered many advantages, most of the earlier zoom lenses were no match to their prime counterparts when it came to image quality. But advances in lens design and technology has now made it possible to make Zoom lenses that could deliver similar results to that of prime lenses.

Prime lenses still have certain advantages over Zoom lenses:

  1. They are fast (wider apertures) making them the ideal lens for low-light sports/theater photography, and in situations where a shallow depth of field is necessary.
  2. They weigh much less than a zoom lens of similar focal length, if only a small portion of the focal length range of a zoom lens is necessary to make the shot, then a prime lens with a similar focal length will be a significantly smaller and lighter choice.
  3. Prime lenses are also cheaper when compared to Zooms. Even an inexpensive prime lens could deliver equal or better image quality to that of a high end zoom lens.

One common mistake a lot of people make is to take zoom for tele photo. A zoom lens just means it changes from one focal length to another. So if you say I really need a zoom lens to take a picture of the bird that’s really far away, actually what you mean is that you need a tele photo lens. As a zoom lens could also be a wide angle zoom. E.g. Canon 10-22mm.

Compact Zooms

Many point and shoot cameras released recently to the market have powerful zoom lenses. But one thing to be understood is that when a camera is said to have 10x zoom, it actually is referring to the ratio of its longest and shortest focal lengths. So larger zoom designations like 15x or 20x need not necessarily mean narrower angle of view, they could simply be wider when fully zoomed out.

Also digital zoom does not mean that the camera has more zoom range. Digital zoom works by taking a small portion of the sensor image and enlarging it using interpolation it is just like cropping your images and enlarging them in Photoshop.