Understanding Camera Lenses: Objective Focal Lengths Explained

Camera lenses are an essential component of photography, as they are responsible for focusing light onto the camera's image sensor or film to create an image. The lens's focal length, typically measured in millimetres, determines the angle of view and magnification of the image. Different types of lenses, such as zoom, prime, wide-angle, and telephoto lenses, offer photographers versatility in capturing various subjects, from landscapes to wildlife. The lens's aperture, indicated by the f-number, also plays a crucial role in determining light transmission and depth of field. Lenses with larger apertures (lower f-numbers) are generally brighter and more expensive, offering greater flexibility in low-light conditions.

Zoom lenses vs. prime lenses

Camera lenses are typically divided into two types: zoom lenses and prime lenses. Both have their advantages and disadvantages, and the right choice depends on the photographer's needs and preferences.

Zoom Lenses

Zoom lenses offer a variable focal length, allowing photographers to change the angle of view by turning the zoom ring. This means that they can make objects appear larger or smaller and include more or fewer objects in the frame without physically moving. Zoom lenses are incredibly versatile and are commonly used for a wide range of photography, from street photography to wildlife. They are a good choice when the photographer needs to adapt quickly to different subjects and situations. Zoom lenses are generally heavier and more expensive than prime lenses, and they might not be as sharp, especially near their maximum focal length.

Prime Lenses

Prime lenses, on the other hand, have a fixed focal length, which means that the field of view cannot be changed. To enlarge or reduce the subject, the photographer must physically move closer or farther away. Prime lenses are often smaller, lighter, and less expensive than zoom lenses. They usually offer better low-light performance and shallower depth of field, resulting in better image quality and background blur ("bokeh"). Prime lenses are ideal for photographers who want to specialise in a particular type of photography and are willing to sacrifice versatility for better image quality.

Focal length

There are two types of lenses: prime and zoom. Prime lenses have a fixed focal length, while zoom lenses offer variable focal lengths. Zoom lenses are more versatile, allowing photographers to capture a variety of subjects without changing lenses. Prime lenses, on the other hand, tend to be more compact and lightweight, and offer larger maximum apertures, making them ideal for low-light conditions and achieving a shallow depth of field.

The choice between prime and zoom lenses depends on the photographer's needs. Zoom lenses are convenient for dynamic subjects and situations where changing lenses is impractical. Prime lenses, with their specialised focal lengths, often deliver superior image quality and are ideal for specific types of photography, such as portraits with a 50mm lens or landscapes with a 35mm lens.

In addition to the type of lens, the camera's sensor size also affects the field of view. Smaller sensors, often found in crop-sensor cameras, result in a narrower field of view compared to full-frame sensors. This is known as the 'crop factor' or 'focal length multiplier', and it can be used to calculate the effective focal length for a given camera and lens combination.

Aperture or f-number

The f-number, also known as the focal ratio, f-ratio, or f-stop, is a measure of the light-gathering ability of a camera lens. It is calculated by dividing the focal length of the lens by the diameter of the entrance pupil (or "clear aperture").

The f-number is typically expressed using a lower-case hooked f with the format f/N, where N is the f-number. For example, if a lens has a focal length of 100mm and an entrance pupil diameter of 50mm, the f-number would be 2, or "f/2".

The f-number is key in determining the depth of field, diffraction, and exposure of a photograph. A lower f-number means a larger aperture and more light entering the system, while a higher f-number means a smaller aperture and less light entering the system.

Most lenses have an adjustable diaphragm, which allows the user to change the size of the aperture and, consequently, the f-number. Increasing the f-number by a factor of the square root of 2 will halve the aperture area, reducing the light entering the lens by half.

Lenses with lower f-numbers are considered "fast" lenses, allowing more light to pass through the system. They are better suited for visible and NIR imaging, as they result in increased light sensitivity and enable the camera to capture more accurate images in lower light levels.

On the other hand, lenses with higher f-numbers are considered "slow" lenses, featuring reduced light throughput. They are commonly found in long-range cameras due to their very long focal lengths.

The f-number is also related to the numerical aperture (NA) of the system, which measures the range of angles over which light can enter or exit. The numerical aperture takes into account the refractive index of the medium, while the f-number does not.

In summary, the f-number plays a crucial role in photography, influencing the depth of field, exposure, and image quality. Lenses with different f-number capabilities are chosen based on the specific photographic requirements, such as the need for low-light performance or a specific depth of field.

Perspective

Photographers can change the perspective of an image by moving the camera to gain a better viewpoint than eye level. Moving the camera left, right, up, or down can have an immense effect on the image, especially in the foreground. Changing the camera angle also allows photographers to see and show the world from a different aspect.

There are various types of perspective that photographers can use to create illusions of depth in their images. These include:

• Vanishing Point: This refers to a small area in the image where things get small and cramped and seem to converge and vanish.
• One-Point Linear Perspective: This is a specific case of linear perspective where the lines converge into one vanishing point at the horizon.
• Two-Point Linear Perspective: This is a case of linear perspective with two vanishing points, creating two sets of lines that go off in opposite directions.
• Forced Perspective: This involves creating a composition that forces the observer to doubt what they are looking at, viewing something that is not actually there.
• Distorted Perspective: This type of perspective arises from the lens itself and is also known as optical lens distortion. It refers to the change in shape of objects and lines in the photo, often appearing as highly magnified, wavy, and curvy.

In addition to these visual perspectives, physical characteristics such as height, length, lighting, contrast, and colours can also affect the perspective of an image.

Image quality

The objective lens is the primary lens closest to the object being observed or photographed. The size of the objective lens is typically measured in millimetres and refers to the diameter of the front lens element. The objective lens size is crucial in obtaining the best possible image quality.

A larger objective lens allows more light to enter the optical system, resulting in brighter and clearer images. This is particularly beneficial in low-light conditions, such as during dawn or dusk, or when observing distant objects. The increased light-gathering capability enhances image quality and clarity, making it advantageous for photographers, birdwatchers, hunters, and other outdoor enthusiasts.

However, larger objective lenses can introduce distortions and aberrations if not properly designed. They also have some drawbacks, including increased weight and bulk, making the optical device less portable and more challenging to handle and carry. Therefore, it is essential to strike a balance between image quality and portability based on individual needs and specific use cases.

The objective lens size also influences the field of view provided by an optical instrument. A larger objective lens tends to provide a wider field of view, allowing users to observe a greater area without constantly repositioning the device. This is beneficial for observing fast-moving objects or capturing panoramic landscapes.

Additionally, the objective lens size affects image brightness. With a larger objective lens diameter, more light reaches the camera sensor, resulting in brighter and more vibrant images with greater detail.

The quality and clarity of the glass used in the objective lens also play a crucial role in image quality. High-quality glass can produce clearer and sharper images, while lower-quality glass can result in blurry or distorted images. Lens coatings, such as anti-reflective coatings, can further improve image quality by increasing light transmission and reducing glare.

When choosing the right objective lens size, it is important to consider factors such as the intended use, environment, budget, and personal preferences. For example, hunters in wooded areas may prefer a smaller objective lens for better manoeuvrability, while those in open fields may opt for a larger lens for improved visibility.

In summary, selecting the appropriate objective lens size is essential for maximising image quality and overall performance. By considering factors such as light-gathering capability, field of view, image brightness, portability, and cost, individuals can choose an objective lens that best suits their specific needs and ensures optimal image quality.

Frequently asked questions