Understanding camera lenses – L02

Making sense of different lenses.

Lens composition:
A camera lens is really a tube with lenses at either end, usually of glass for optical clarity but some lenses can be of composites. In most cases they are coated to reduce effects such as reflectance and flare, as well as protecting against abrasion. The number of lenses can vary depending on the type of lens. The amount of light let into the lens is controlled by an aperture adjustment ring usually mounted on the lens closest to the camera. This is composed of a diaphragm with moving blades. Again, the number of blades varies between lenses, the higher the number, the better quality the lens. Finally there is a focusing adjustment ring usually mounted furthest from the camera. Adjusting this will change the focal point on the subject.

lens elements diagramhttps://www.cambridgeincolour.com/tutorials/camera-lenses.htm

Sensor size:
Although not in/on the lens and in the camera the sensor size will affect the end image and depending on the camera and its sensor, may determine the type of lens fitted to the camera.
So a  little about the sensor size which was discussed in depth in N.C.F.E. Photography level 1, comparing full frame, APS and mobile sensor sizes. A full frame camera uses a sensor based on the old film format of 35mm film, i.e. 36mm x 24mm(approx). The general size difference between a full frame and APS-C gives a crop factor of x1.5 for Nikon and Sony cameras and x1.6 for Canon. Other cameras do vary slightly, and mobile phones have tiny sensors and vary greatly.

There is a formula for calculating Crop Factor for a Nikon:
c = √(a2 + b2)
a = vertical, b = horizontal, and c = diagonal
so a full frame camera will give:
√(242 + 362) = √ (576 + 1,296) = 43.27mm

So what is ‘Crop Factor’? It is the sensor’s diagonal size compared to a 35mm sensor. Using an APS-C camera (from now called an APS camera), as opposed to a full frame camera, will mean that the image will be cropped by x1.5 (I will use this figure throughout for simplicity), so reducing the image size as shown below:


This means that the APS camera is giving the impression of being closer to the subject, by a factor of 1.6 compared to a full frame camera.
How is this reflected in a camera lens? If using a 50mm lens on a full frame camera, then placing that same lens on an APS camera, on the APS it would be equivalent to using a 75mm lens on the full frame. This equates throughout all lenses. A 200mm = 300mm, a 300mm = 450mm etc. This will affect where the photographer is positioned with relation to the subject.
Another point about lenses. Not all lenses are interchangeable between full frame and APS cameras. Always check before buying a lens if different format cameras are being used, or there are thoughts of upgrading.

So to lenses. Most APS cameras are sold with ‘kit lenses’. These are a standardised zoom lens, usually an 18-55mm lens, although this can vary with different brands and models. For amateur photographers this is often their only lens, but there is a huge range of lenses available on the market for most brands.

Prime lens:
A prime lens is a specialist lens often used by professional and semi-professional photographers. It is a lens of a fixed focal length, so has no zoom. It’s big advantage is that it often has superior optical quality to zoom lenses and has a wider aperture. Due to these features it is smaller and lighter than a zoom lens. As the lens is optimised for a fixed focal length it means better clarity, as there is less optical aberration, as well as being able to be used in lower light situations. The potential downside is that, as there is no zoom, the photographer must select the best position to take the photograph. This can be restrictive but enhances a photographer’s abilities and expertise. They are particularly popular with studio photographers. The go-to lens is a 50mm or standard lens. This is deemed a lens that gives a similar perspective to human vision.

Here is the range of prime lens, according to focal length:
Ultra-wide: 12-22mm – A lens for very close subjects, distorting the background dramatically.
Wide: 24-35mm – A lens giving a wide field of view but may give a distorted perspective.
Portrait: 85mm – A lens known as a portrait lens. Due to its longer subject to camera distance, it is useful for image framing as well as giving a pleasant perspective.
Telephoto: 135mm – A popular lens for. Sport and action, as it is good for capturing distant objects.
Super telephoto: 200 – 500mm – A bulky lens that is very specialist, used for fast sport action and wildlife photography.

There are also specialist extreme telephoto lens but are usually restricted in use as they are classified as ‘Weapons’ due to their ability to focus over very large distances.

Wide Angle:
This type of lens is usually considered a lens of 35mm or less. It has a number of benefits, but for most photographers it serves two purposes. As it offers a wide field of view it is ideal for landscape, interior or architectural photography. Below is an example:
Image result for comparison standard wide angle lens building



The other is to exaggerate size. It enlarges objects in the foreground while widening the background as shown below:


However, a note of caution. Due to the focal length of the lens, it can create perspective distortion if the camera is not aligned properly. This is particularly noticeable when photographing a building when the camera is pointed upwards, creating the impression that the building is falling backwards and receding. This can be adjusted using software.

There are a range of wide angle lenses available, some going to extremes, known as ‘fish-eye’. This term was coined by Robert W Wood in 1906, as the image was likened to the way a fish is perceived to view its environment. These lenses zoom in on the centre of the subject, distorting the outer edges, giving a 180º field of view. There are two versions, a circular lens and a full frame. The former creates an image as viewed through the lens, so is circular, the latter crops the image so it fills the sensor. These lenses are very expensive, and have limited use, so many photographers use software to create fish-eye effects. The left image is via a circular fish-eye, the right is via a full-frame fish-eye.


Zoom lens:

What is the difference between a telephoto and zoom lens?
As mentioned previously, a telephoto lens has a fixed focal length, being a prime lens, while a zoom lens offers a variable focal length. The advantage of a zoom lens is it allows the photographer to take a photograph from a position, and adjust the focal length, where a prime lens could not be used, possibly because of the location or terrain.
It enables distant objects to be brought closer, useful with sports and wildlife photography. However it does foreshorten depth of field, altering the perspective of the background to the foreground. It can also be used to sharpen a subject that is close, but blurring the background.
There is a downside, apart from the poorer optical quality compared to a prime lens, and that is when used at maximum zoom, the image may be distorted, giving a slight fish-eye effect. Also if using a filter or a lens hood there may be vignetting, where the corners of a photograph are darkened.

Common zoom lens types are as follows:
70-200mm – Offers a wide variety of photographic options and the most popular lens.
85-300mm –  A relatively inexpensive zoom lens, used for sports and wildlife.
100-400mm – A more extreme lens, more specialist and expensive.

There are higher powered zoom lenses but these are heavy, requiring a  monopod for stability, and expensive.

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