by Eugene Struthers
Before you set out to purchase a new lens, you should first have some idea of what you want to photograph. This will assist you to determine which "Focal length" you require to achieve the best results.
What is a Focal length?
The technical definition: The distance between the optical centre of a lens and its focal point. In practice, the focal length is a measure of the magnification and angle of view of a given lens or zoom setting. The focal length of an optical system is a measure of how strongly it converges (focuses) or diverges (diffuses) light. It is measured in millimetres (mm) and it represents the distance from the optical centre of a lens to the digital camera sensor when the subject of the photo is in focus.
I know what you are thinking. That's quite a lot of technical information to remember. Okay, let's break it down so it is easier to fully understand and apply to our photography. Don't worry if you can't get your head a round this topic. I cover it again next month in the "Depth of field" course.
But for now. To clear up any confusion.
When you use a short focal length you have to be close to your subject for a closeup. Anything that is up to around 35mm has a short focal length and is often described as being wide. These are classified as being a low power lens. Which basically means it is great for capturing images close to you. Reason being, if you were to try and capture an image of a subject far in the distance. The subject would appear really small. This is because the lens doesn't have the power to magnify the subject over that distance. The lens will actually exaggerate the distance, and the subject will appear further away than they actually are.
What is happening:
A wide-angle lens is merely creating the illusion of exaggerated perspective. This is because there is a greater distance between the subject in wide angle photography and the closest subject to the camera will always appear bigger.
Whilst with a telephoto lens, the distance between subjects shrinks, causing the difference in the size of the subjects to decrease.
When we use a long focal length the photographer can be far away and still get up close to the subject. The focal length of the lens increases, whilst the depth of field gets shallower. A long lens will visually compress space to make the subject in the scene appear and seem closer than they actually are.
A Zoom lens has a variable focal length
A Prime lens has a fixed focal length
So how do you decide which focal length to choose. Well, you have to decide what type of images you want to capture. Basically different genre's of photography require different lenses to get the job done relative to their perspective. Plain and simple.
A lens is designed for a specific purpose in mind. They are grouped into four primary categories based on their focal lengths.
Lens Type Focal Length
Wide Angle 28mm or lower
Standard Anything from 35mm to 85mm
Telephoto Anything from 100mm to 300mm
Super- Telephoto 300mm or higher
Lens Type Focal Length Best use
Wide Angle 28mm or lower Landscape / Interiors
Standard Anything from 35mm to 85mm Portraits
Telephoto Anything from 100mm to 300mm Portraits / Sports
Super- Telephoto 300mm or higher Wildlife / Sports
The manufacturer of lenses have designed them to have a specific purpose and function. Each lens will have its own specific focal length.
You will need to determine what focal length you require before you make a decision on what lens to purchase.
The focal length will dictate the best pratical use relevant for that particular lens.
It kind ofexaplains why, when you purchase a new camera. A lot of the manufacturers will give you a kit lens of EF-S 18-55mm f/3.5-5.6.
Full frame camera: Click here
Lens: 10 - 28mm
Ultra wide angle lenses are mostly used for architecture and landscape photography. They are ideal if you need to capture everything in the scene as you actually see it. This is one of the reasons a lot of real estate photographers use them. As they allow the photographer to capture everything within a confined tight space whilst giving off the impression of an exaggerated large space from the ceiling to floor. The key to using an "Ultra wide angle lenses" is to know how to compose your image correctly. So that you can get everything in your image. It is important to have a focal point to capture the sure expansiveness of an area or scene. As this can assist in capturing the true scale of an object or scene. So it pays to fill the frame. However; you have to be conscientiously aware that your perspective could be exaggerated. The characteristics of this are an increase in sizes of objects in the foreground as you move into towards them. And a dramatical shrinkage in objects in the background. So whilst you are taking an image of a huge mountain some distance away. An Ultra wide lens can make the mountain seem really small unless you are are making the compositional focal point in your image (filling the full frame) with just the mountain. So you will need to arrange and organise the components within your image to achieve clear composition. Whilst being aware of image distortion. Image distortion takes on two forms. Barrel distortion is when straight lines will appear bulged if they don't pass through the centre of an image. Edge distortion is when objects at the edges of the frame appear to be stretched leading away from the centre of the image. It is an art in itself using these types of lenses.
Lens: 18 - 55mm + 50 - 100mm
When we think of standard lenses we think of our kit lens that came with our camera in the box. So although this section is defined as being 35 - 100mm, our kit lens would fall into this category as well. Reason being our kit lens is 18 - 55mm and the 55mm falls midway between 35 - 100mm.
This is an underestimated lens and isn't really given due credit. The range that it covers means that it is very useful and versatile for photographing a wide range of subjects. As this lens covers wide angle and standard and edges on towards being a telephoto lens slightly. They can be used for closeup "macro", landscape and portrait photography. However, their main strength lies when you need to take portraits. When we do photography workshops here a glamour magazine. We find that a lot of our students use them to capture great street life and documentary style images. Hence, why they are given to you when you purchase a new camera as they serve a variety of purposes. Unlike the wide angled lens, these lenses are free of facial and image distortion. They are a great lens for newbie photographers. Especially when you consider the different genres of photography you can cover with just one lens.
Lens: 18 - 28mm
When a lens has a focal length of less than 35mm it is considered to be a wide angle lens. Similar to the ultra wide lens in character. A wide angled lens field of view means that both the relative distance and size will be exaggerated when comparing far and near objects. This will cause objects in the foreground to appear huge whilst objects in the background will appear distant and really small. These lenses are great for landscape photography as they capture what we actually see from our feet up into the skyline. And this is useful when we want to draw our viewing audience into our image. These types of lenses create greater depth of field within an image. And just like the ultra wide angle lens, they are also great for architecture. A basic rule I use all the time is. Ultra wide angle lenses for indoor architecture and wide angle lenses for outdoor architecture. This lets me define there specific use and individual characteristics. The only draw back to using wide angle lenses for portraits. Is how they distort facial features. A wide angle lens will cause your model's nose to enlarge and her eye's to widen apart and sometimes bulge. Which isn't at all flattering.
Lens: 100 - 300mm
A telephoto lens offers less depth of field so you can use selective focus to make your subject stand out distinctly from the background. They are great for isolating an interesting subject. They allow you to photograph a subject without being noticed, as you are able to zoom in some distance. Photographers are able to stand further away from there subjects due to a longer focal length. Which changes perspective and draws the background in closer to the subject. The lens compresses the apparent distance between objects so that a line of telephone booths will appear right next to one another when actually they are 20 feet apart. They are great for creating intimacy between objects that are widely separated. They let you get right in the middle of all the action. Especially if you are photographing sports. It can make sports images appear as though the photographer was actually right there on the field with the players. It is easier to frame a subject when you are not able to move any closer to them. Ideal for filling the frame with just the subject. Which allows you to be more creative from a distance. They are usually very expensive as a result of their high-quality optical glass lens, which produces a lot of detail. The only down-side is the fact that they can be very big and weigh a lot. And you may struggle to hand hold them. A rule of thumb is when you have a longer focal length, you will also need a faster shutter speed to avoid camera shake. Longer focal lengths make you a distant observer, whilst a shorter focal length can make you an active participant. As you have to get up close to your subject and interact with them. To capture them in an image. The longer you set your focal length the shallower the depth of field. (This is covered in more detail in the Depth of field course). Great for capturing action sports shots. If you need to isolate your subject, or would like to take a picture of a wild dangerous animal at a distance. As this lens allows you to capture your subject at a safer distance without it realising that you are there.
Lens: 300 - 600mm
These lenses are a lot similar in character and function to that of a telephoto lens. The only main difference is the size and strength of the magnification qualities of the lens. The only drawback being, that they are super expensive as well. And that they require a high degree of training and industry knowledge to be proficient in using them correctly. They are mainly used by professional photographers who make an income from selling their images. They are great for bringing far objects in closer so that the subject fills the frame. They allow the photographer to pick distant subjects out from their environment. Ideal if you capture wildlife photography images. They have a longer focal length and a shallower depth of field. So this makes them great for capturing action shots, whether this be sports or of wildlife. They are bulky and may require you to attend the gym in order to fully operate and use them. As most of them are really heavy. Classified as a specialist lens.
What factors determine the focal length of a lens?
The degree to which a lens refracts the light is the power of the lens, measured in dioptres. A Dioptre is a unit of refractive power, which is equal to the reciprocal of the focal length (in metres) of a given lens.
The focal length of a lens is determined by these factors:
1) The refractive index of the material of the lens.
2) The refractive index of the surrounding medium.
3) The radius curvature of the lens surface closest to the light source.
4) The radius curvature of the lens surface farthest from the light source.
5) The thickness of the lens between the curved surfaces.
To fully understand this topic in more detail.
Why not follow our online intensive Photography course.
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Architecture Ultra Wide angle 10 - 28mm
Landscape Wide angle 18 - 28mm
Covers 1/2/4/5 Kit lens Standard 18 - 55mm
Portraits Standard 50 - 100mm
Sports Telephoto 200 - 300mm
Street / documentary Normal 35 - 70mm
Wild life Super Telephoto 400 - 600mm
Katja has recently purchased a new Canon 80d digital SLR camera that came with an 18- 55mm f/3.5 - 5.6 kit lens.
At her local pub, a group of glamour models have found out about her new purchase. And have asked her to photograph them for a charity topless football calendar.
The calendar is for a local charity associated with a local hospice. To help send terminally ill patients to the USA for a holiday.
Katja know's how important this calendar will be to the girls and she doesn't want to let them down.
She only has her kit lens and knows to capture a proper set of images for their calendar. She will require a better lens. One more suited with the capabilities and functions to make the day a success.
She know's that she will need to be in the mix with the glamour models as she photographs them from the sidelines.
After reading Glamour-Photography magazine. Katja decides that as she has a Canon camera, the lens must be canon as well. She also knows that she will require a zoom lens so that she can get up close to the models as they play topless football on the field.
Katja decides on: Lens mount: Canon
Lens focal length: Telephoto