Which Telephoto Lens and What for?

Which Telephoto Lens and What for?

Written by Marina Parha and George J. Reclos Thursday, 26 November 2009 00:00

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Article Index
Which Telephoto Lens and What for?
Page 2: Choosing a Lens (cont.)
Page 3: Using Teleconverters
Page 4: Notes and References
All Pages

Introduction

“In photography and cinematography a telephoto lens is a specific construction of a long focal length photographic lens in which the physical length of the lens is shorter than the focal length. In these lenses the optical centre lies outside of its physical construction, such that the entire lens assembly is between the optical centre and the focal plane. A regular lens of a focal length that is longer than what is considered a normal lens not necessarily a telephoto lens. A telephoto lens has to incorporate a special lens group known as a telephoto group [ … ]; nevertheless, non-telephoto lenses of long focal length are often informally referred to as telephoto lenses.”  (1)

A tele lens is used to magnify an object which is located far away from the camera. The degree of magnification depends both on the lens used and on the camera; in this presentation we will focus on DSLR cameras only.  To determine the degree of magnification of a given lens we need to divide the focal length of the particular lens by the focal length of the “normal” lens, which is supposed to cover exactly what we see. For example, suppose we use a camera with an APS-C sensor (smaller than 24 x 36 mm) for which the normal lens is the 35 mm. A 300 mm lens on this camera will magnify an object by 300: 35 = 8.5 times. The same 300 mm lens on a  full frame camera (sensor size: 24 x 36 mm) for which the normal lens is a  50mm lens will magnify the same object 300:50 = 6 times. Therefore if the subject is located at a distance of 30 meters, it will seem to you as if it is located at 3.5 meters if you are using an APS-C camera with a normal lens and at 5 meters if you are using a full frame camera.

The need for the magnification is obvious. A sparrow sitting on a tree 30 meters away is barely visible; the tele lens allows us to capture it, using the example above, as if it were standing 3.5 meters away.

Types of teles: Short – Medium – Long

Technically, telephoto lenses (teles for short) are classified as short, medium and long, depending on their focal length, which however is correlated to the size of sensor. Thus, anything from the normal focal length to double that is regarded as short, anything from double to quadruple is regarded as medium and anything over that as long. The table below shows this classification of focal lengths in correlation to the size of the sensor:

Choosing a Tele Lens

Choosing a tele lens requires some thought, particularly as these “toys” can be quite expensive. Below we briefly present and discuss the most important things to consider.

1. The focal length of the lens

This depends mainly on two factors:

a. The size of the subjects we are trying to shoot and the relative distance we expect them to be from our camera.

Taking the subject – camera distance into account, the larger the subject we are trying to shoot the shorter the lens can be. Therefore, if we want to shoot eagles and we expect them to be at a distance of 10 meters, a 200 mm lens will do. However if we try to shoot a sparrow sitting at the same distance, a 1600 mm lens is required to get an image of comparable size (thankfully sparrows normally sit closer to cameras than eagles).

b. The particularities of our subjects and the shooting location.

Photographers need also to consider the subject(s) they are wishing to shoot. One of the main advantages of using telephoto lenses is that they allow you to shoot a subject from a distance. You may wish to do so either because it is impossible to approach any closer (e.g. due to obstacles on the way)) or in order to avoid disturbing or frightening the subject or being hurt yourselves (e.g. in the case of wildlife). In the two shots below you can see a photo being taken with a Nikkor 70-200mm f/2.8 VR attached on a Nikon D2x camera. The telephoto lens allowed the photographer to stay partly hidden in the tall grass and take some interesting photos of the bird’s nest where the mother was taking care of her chicks. Moving any closer in this case was difficult as the nest was located in the middle of a rather deep and muddy water stream. However, even if the nest was easily approachable the photographer would be ill advised to try and approach closer; the moorhen would immediately get alarmed and leave the nest, followed by her chicks.

In the next photo a Nikkor 80-400mm is set to its maximum focal length (400 mm) on a Nikon D2x camera. The photographer is hiding amongst tall reeds near the edge of the water at Sale Water Park (Manchester, UK). This made the human presence almost invisible to the grebes which went on with their courting ritual. 

Though long teles have considerable advantages in allowing us to shoot from a distance they do have their downsides, too. One of the main ones is the shutter speed required for shooting when using such lenses. The minimum shutter speed which should be used for any lens is 1/ focal length (so for a 200 mm lens the minimum speed is 1/200 seconds). Thus, a 200 mm lens should not be handheld if the shutter speed is not 1/200 or higher (1/300 etc). Keep in mind that APS-C sensors result in a cropping factor, essentially increasing the focal length of the lens by an equal amount. Thus a 300 mm lens on a Nikon D3, D700 or D3x becomes a 450 mm lens on a Nikon D300, D90, D2x etc. In the case of the APS-C sensors, the lowest speed you can hand hold your lens at becomes 1 / focal length x cropping factor in seconds (in the case of the 200mm indicated the speed then becomes 1/350 seconds). You should also keep in mind that this is the absolute minimum and requires really stable holding of the lens. Most of the times you will need to double the minimum shutter speed to be on the safe side.  (2)

The reason for this limitation is the blur caused by camera vibration. The severity of blur for a given vibration is proportional to the focal length of the lens. Lets say when we handhold a camera we get a tiny bit of motion at the front of our lens, maybe moving the camera 0.1 degree of arc during the time the shutter is exposing (this would be pretty good actually). If we have a 50mm lens mounted on a full frame camera, the field-of-view of our image is 39 degrees, so we would blur the image about 0.1 / 39 or .02% — a very small amount. If we blur the image the same amount (0.1 degree of arc) when using a 500mm lens, which has a field of view of 4 degrees, we would blur the image 2.5%. So for the same amount of motion while handholding, the 500mm image would appear to be blurred more than 10 times as much than an equivalent image taken with a 50mm lens. (3)

2. The maximum aperture of the lens

This can be of paramount importance when it comes to shooting with less than ideal lighting conditions, for example inside a forest, in the evening or simply during winter in an overcast day. As a general rule the less light available, the larger maximum aperture you want your lens to have.  Our Nikkor AF 80-400 mm f/ 4.5-5.6 D is a fantastic lens in full day light but mostly unusable during overcast winter days in the UK, as are most of our lenses with  a maximum aperture of f/4/5.

3. The speed of the lens

This is related to the point above.  A lens with a larger maximum aperture (e.g. f/2.8) is faster by comparison to a lens with a smaller maximum aperture (e.g. f/4)  because it allows you to get a higher shutter speed under the same light conditions. In practice this means that such a lens has two main advantages over slower lenses of equivalent focal length:

• you will be able to hand hold it under lower light conditions and
• you are more likely to be able to get the shutter speed required to stop the action of a flying bird instead of getting some blurred wings.

Speed is required when shooting fast moving subjects but a fast focusing lens is also a blessing when it comes to capturing an “unexpected” scene.

Unfortunately fast telephoto lenses are usually expensive and bulky. The fast long telephoto lenses are actually so expensive that are also called “exotic” lenses to signify that they are not an everyday convenience. What they give you in return is much faster focusing (an f/2.8 lens will always focus faster than a f/4 lens everything else being equal) and excellent image quality, if properly used, which is available even when the lens is used wide open.

Lens manufacturers usually offer a fast and a slower telephoto lens with the same focal length, so you always have a choice. As fast, we refer to 200mm f/2 – f/2.8, 300mm f/2.8, 400mm f/2.8, 500mm f/2.8-4, 600mm f/4 and 800mm f/5.6. If you can get one of these (with or without image stabilization / vibration reduction) you are getting the best glass available. Slower (and considerably cheaper) alternatives include the 180 f/3.5, 200mm f/4, 300mm f/4-5.6, 400 mm f/5.6 and 500mm f/5.6. In some cases, cheaper does not mean worse image quality – just a slower lens, slower in every aspect. Slower in focusing with a slower shutter speed under the same conditions. If the only thing you need a telephoto for is to shoot landscapes on a tripod, then there is no reason to go for a Nikkor 300mm f/2.8 lens, the Nikkor 300mm f/4 lens is equally sharp and costs one third of the price.

4.  Minimum focus distance

This indicates how close a subject can be to the nodal point of a lens before the lens can focus on it. (4) Most reputable lens manufacturers indicate the minimum focus distance of a lens in the manual but photographers can calculate this themselves by placing an object in front of the lens and measuring the minimum distance it should have from the front of the lens before the lens can focus on it.

Again, knowing what we will be trying to shoot and what distance we are likely to find it at from ourselves is a blessing. In the last photo above the photographer is using a Nikkor 80-400mm set to its maximum focal length (400 mm). The minimum focus distance of this lens is 1.5 meters. As anything which is nearer to the nodal point of the lens than 1.5 meters is practically invisible to the lens when the lens is wide open, the lens does not focus on the reeds close to it but on the main subject, which is the courting grebes about 10 meters away.

While, in this respect, photographers can count their blessings, the minimum focus distance can get in the way if the main subject comes too close to the lens: in this case the lens will simply be unable to focus on it.

5. Lens sharpness

The sharpness of a lens refers to its ability to render contrast, in other words its ability to shoot for maximum definition. There is a lot of discussion amongst photography enthusiasts (and professionals) about sharp(er) lenses; there is also a lot of information, including lab tests, regarding the relevant performance of tele lenses in simulated conditions. On the flip side of the coin it is occasionally suggested that all the talk about lens sharpness is gobbledygook when it comes to tele lenses: all modern teles are sharp provided one knows how to use them. (5) The proviso is most important: different lenses have different optics and will yield different results when used in the field. It is important to know the particulars (or limitations) of a lens prior to acquiring it to see how well these match the shooting conditions preferred or expected.

Indeed under the “right conditions” all lenses are sharp. Imagine shooting a still subject in perfect lighting; any two lenses will yield excellent results. The differences between two lenses start becoming obvious when conditions are not ideal. For instance, when light levels fall, the expensive lens will still be able to take fantastic shots while the cheaper one will start to reveal its weaknesses. It will hunt more, it will perhaps not be able to focus lock and finally the shutter speed may not be enough to “freeze” the subject. For moving subjects, especially birds in flight, focusing may be so slow that you will completely lose the shot, again and again. This is true for zoom lenses, too. The Nikkor 70-200mm f/2.8 VR II and the Nikkor 80-200mm f/2.8 will focus almost instantly, even if the phase contrast is minimal, even under low light conditions. In contrast, the Nikkor 80-400 f/4.5-5.6 VR and the 70-300mm f/4.5-5.6 VR II will focus fast under high light levels but as soon as the first clouds appear in the sky they will start hunting, sometimes indefinitely. This results in losing the shot.

6. Number and quality of lens elements

This is important to minimize optical aberrations.  “Aberrations are departures of the performance of an optical system from the predictions of paraxial optics. Aberration leads to blurring of the image produced by an image-forming optical system. It occurs when light from one point of an object after transmission through the system does not converge into (or does not diverge from) a single point. Instrument-makers need to correct optical systems to compensate for aberration.” (6)

Contemporary technology does not yet allow a lens free of all optical aberrations. Good quality lenses, however, balance these out. The quality of lens elements is usually indicated by a number of acronyms following the lens name, for example: ED (Extra dispersion glass), APO (Apochromatic), N (Nanocrystal coating) etc. (7)  Characteristics such as these indicate the ability of the lens to handle better (or correct) chromatic, spherical and / or other optical aberrations such as lens flare, vignetting etc. 

The construction of the lens does not just aim to reduce optical aberrations.  It also determines the way in which the out-of-focus area of the photo is captured. This, in photography, is referred to as bokeh.  The importance of the out-of-focus areas is obvious: a busy or poorly captured background attracts the eye and therefore detracts it from the main subject in addition to being aesthetically unpleasant.  Good quality lenses have built-in controls to manage the way in which the out-of-focus areas are rendered. These lenses are indicated as having good or excellent bokeh.

 

 

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