Sunday, 9 March 2014

Understanding Camera Lenses

SUBJECT :- UNDERSTANDING   CAMERA   LENSES,

 by Des Wilkinson

There are big ones, little ones, long ones, short ones and lots in between.!


Before we discuss the various types of lenses we need to understand just what makes a lens and how they work.

DEFINITION :-

"A camera lens is an Optical lens or assembly of  lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically."

There is no major difference in principle between a lens used for a Still camera, a Video camera, a telescope a Microscope or other apparatus, however, the design and construction are different. A lens may be fixed or it may be interchangeable with lenses of different focal lengths, apertures and other properties.

A camera lens may be made from a number of elements : from one as in the Box Brownie or modern phone, to over 20 in the more complex lenses.

Glass is the most common material used, due to its good optical properties and resistance to scratching, other materials ie plastic acrylics. Plastics allow aspherical  lens elements which are difficult and or impossible to make in glass. Plastics are not used for the front or outermost elements of all but the cheapest lenses as they scratch easily.

 Plastic has been used for many years in disposable cameras and have acquired a bad reputation. Many manufacturers of high performance (and high priced) include molded or hybrid aspherical elements, so it is not altogether true that all lenses with plastic elements are of  low photographic quality.

Today, most lenses are multicoated in order to minimize lens flare and other unwanted affects, some have a UV coating to keep out ultraviolet light that taints colour, thus eliminating the need for UV filters.

A lens will most often have an aperture adjusting mechanism, usually an Iris Diaphragm which regulates the amount of light that passes to the sensor. They will also incorporate a shutter to regulate the time the light may pass.

Choosing the right lens for the task can be a complex trade-off between cost, size, weight, lens speed and image quality. We will try to improve our  understanding by providing a overview of concepts relating to, Image quality, focal length perspective, and Prime V Zoom lenses aperture or f / stop.

The focal length of a lens determines its angle of view, and thus also how much the subject will be magnified for a given photographic position. Wide angle lenses have a short focal length whilst zoom lenses have longer corresponding focal lengths.
Contrary to common belief, focal length isn’t a measure of how long or short a lens is physically, but the distance in millimetres from the optical centre of a lens to the imaging sensor when the lens is focused at infinity.
The two fundamental parameters of an optical lens are “Focal Length” and the “Maximum Aperture” The lens focal length determines the magnification of the image projected onto the image plane (or sensor) and the aperture the light intensity of that image.
Remember, the lower the f number, the higher the light intensity on the sensor, larger apertures , (smaller f numbers) give a much shallower depth of field than smaller apertures

It can be said that focal length also determines the perspective of an image, but strictly speaking perspective only changes with ones location relative to the subject. Example if you try to fill the frame with the same subject using both a wide and zoom lens the perspective must change because you are forced to move closer or further from the subject, Wide angle stretches perspective, zoom will compress or flatten the subject.

PRIME LENSES   V  ZOOM LENSES.

A PRIME lens has a fixed focal length whereas the ZOOM lenses can vary the focal length within its predefined range.

The prime advantage of the PRIME lens is cost, weight and speed.
An inexpensive Prime lens can generally provides as good (or better) image quality as a high end ZOOM. They also offer better light gathering ( larger apertures than the fastest ZOOM this is often critical for low light and when a shallow depth of field  Is necessary.
Once you’ve settled on a new camera, the next temptation comes from the lenses which will accompany it.
 Prices of digital camera lenses vary from the affordable to the eye-watering, and often it can be difficult to see why – so it’s important to know what you’re getting for your money.
The lens which will typically come with a new camera will suffice for everyday photography, but a new optic can quite literally broaden the scope of your picture taking.
Newer digital camera lenses are specifically designed for modern DSLRs, and as such help prevent unwanted side-effects such as internal reflections from ruining your images, but older optics equally have their charms, particularly simply-constructed prime lenses which can often last a photographic lifetime.
Whatever digital camera lens it is you need to elevate your photography to the next level, you should remember a few important points:
: Focal length
Firstly, most DSLRs have a  sensor, whose dimensions are smaller than the traditional 35mm size of a frame of film.
 This means the effective focal length of whatever lens you use changes;
The CROP FACTOR is the sensor's diagonal size compared to a  full frame 35mm sensor.
It is called this because when using a 35mm lens the smaller sensor effectively crops out this much of the image at it's exterior. It is the ratio of the focal length of a 35mm lens, Example a 18mm digital lens with a Crop Factor of 1.5 would be equal to 27mm 35mm lens.  
Sensors :- A subject unto it's self.
APS Sensors an old Film terminology.
CMOS Sensors, Complementary Metal Oxide semi conductor.
CCD Sensors, Charged Couple Device.
FULL Frame Sensor, = 24x36mm (the old 35mm film size.
FOUR THIRDS Sensors, = 17.3x 13mm Key feature, standard lens mounts allows interchange of lenses from different manufacturers.
1/2.3 Sensor, 6.17 x 4.55mm used in most compact cameras.
Compatability
Second, although a digital camera lens may be available in the right mount for your camera, it may not be fully compatible – again, your camera’s manual will explain which lenses can and cannot be used.
Many newer lenses are designed specifically for cameras with APS-C sensors, which means that they either cannot be used on full-frame cameras, or can only be used with limitations. It is even possible to damage your camera by using the wrong lens, so don’t leave this to chance.
Requirements
Finally, think about what kind of photography you practice, as this will determine which lens you should be looking for. ‘Fast’ digital camera lenses, whose aperture is fixed at around f/1.4 or f/1.8 let in a lot of light and allow shallow depth of field, attributes which lend themselves well to indoor photography and portraiture respectively.

The same kinds of digital camera lenses with smaller maximum apertures, such as f/3.5 or f/4 tend to be cheaper, but will suffice if you only ever take landscapes, for example, where the likelihood is that you will be requiring smaller apertures to maintain sufficient depth of field.:
Types of lenses :-
Wideangle zoom lens
A wideangle digital camera lens typically covers a focal range from around 12-24mm or 16-35mm, and allows you to fit landscape scenes, architecture or anything else where you need a wide angle of view.
Some suffer from a little barrel distortion at wider settings, where the image takes on a slightly barrelled appearance, but this is only likely to be objectionable when shooting close up to your subject, or when the scene has defined linear details.
 It’s advisable to use a lens hood with such lenses (which are often supplied) to prevent unwanted light from entering the frame which can cause flare and a drop in contrast.
Standard lens
The standard digital camera lens offers a focal length of around 50mm, which translates to around 75mm on many DSLRs. As such, a 50mm camera lens lends itself perfectly to portraiture, particularly as such lenses often offer wide maximum apertures to create shallow depth of field.
Thankfully, despite being classed as ‘fast’ lenses, their simple construction makes then an inexpensive and lightweight addition to your kit bag.
Super Zoom lens
Superzoom digital camera lenses are a popular choice for keeping on your camera, as they span a wide focal range, from wideangle right through to telephoto. Typically this will begin at around 28mm and culminate at close to 270 or 300mm.
Many contain image stabilisation to help at longer focal lengths, though the maximum aperture available can be restrictive in order to achieve such an expansive focal range.
Although they are less suited for specific types of photography, they are ideal for travel photography where both flexibility and portability are key considerations.
The primary advantage of the ZOOM is that it is easier to achieve a variety of compositions or perspectives
Obviously as lens changes are un necessary
Keep in mind that by using a Zoom does not mean that you no longer have to change your position, ZOOMs simply increase flexibility.
Super Zoom/ Telephoto, Lens's
Telephoto digital camera lens gets you closer to the action, and so is ideal for sports and wildlife where you may need to keep your distance. These can either been prime or zoom lenses, and usually cover a focal range of between 100mm to 400mm.
Their construction makes them costly, and only the most expensive lenses can offer relatively wide apertures of around f/2.8. Even so, for any kind of action photography, where you need to maintain fast shutter speeds, there’s no real alternative.
Macro lens
Macro lenses can focus closer to your subject than a conventional lens allows, which allow you to capture plenty of intricate details.
Commonly used in nature photography, but well suited for portraiture too, macro lenses typically offer wide maximum apertures for shallow depth of field, and are renowned for their high optical qualities.
Other specialist lenses
Fisheye lenses
Fisheye lenses offer an exceptionally wide angle of view but are purposefully distorted to create the ‘fisheye’ effect. They come in two different varieties: full-frame and circular, which respectively capture an image to fill the whole frame and a circular image contained within the frame.
Although they are commonly used for landscape photography, they can be fun to use within other genres, such as in portraiture and architecture.
Tilt-and-shift lenses
Tilt-and-shift lenses, meanwhile, are used most commonly to correct for the effects of converging verticals in architectural photography, but their control over depth of field makes them ideal for any kind of photography where the plane of focus needs to be altered. In other words, the photographer can use these to vary depth of field without having to change the aperture.
They can also be used to make subjects appear as though they are miniatures, although their price restricts them to those who need to use them on a professional level.
I find that the more you become engrossed in photography the more there  is to learn and if you let it can become very technical.

Try to remember the old  K I S S principle.