DEPTH OF FIELD- Focal Length

  the depth of field.

In short, depth of field is simply the space in the image that is acceptably sharp and in focus.

Focusing consists of adjusting the lens to find the maximum sharpness, contrast, and resolution for a chosen subject.

There are two ways to focus in digital photography:

• Manual focus: Using your hands to tweak the focus ring until you get to the best focus.

• Automatic focus: Using the camera’s and lenses’ internal motors to focus on a given subject.

Using manual focus or autofocus will depend on the type of photography that you do. 

For instance, in some genres like macro and night photography, it’s better to use manual focus. Yet, in other genres, like wildlife or sports, autofocus is faster and will make things easier.

In photography, we can say that there’s

 a shallow depth of field when just a narrow portion of the frame is acceptably sharp, 

whereas 

a large depth of field when a big portion of the frame is considered to be in focus.

Depth of field is affected by many different factors:

• Aperture: The wider the aperture, the shallower the depth of field
• Focal length: The longer the focal length, the shallower the depth of field
• Focusing distance: The closer the subject to the lens, the shallower the depth of field
• Sensor size: The smaller the camera sensor size, the shallower the depth of field .

Aperture

Aperture is the opening in your lens that lets light pass through to the sensor. 

Large apertures, which correlate to small f-stop numbers, produce a very shallow depth of field. On the other hand, small apertures, or large f-stop numbers, produce images with a large depth of field.

• Large apertures, which correlate to small f-stop numbers, produce a very shallow depth of field. On the other hand, 
• small apertures, or large f-stop numbers, produce images with a large depth of field.

Focal length

Focal length is related to the photography lens basics. 

The focal length works by describing each lens in terms of millimeters from the optical center of the lens to the sensor. 

Focal length, usually represented in millimeters (mm), is the basic description of a photographic lens. 

It is not a measurement of the actual length of a lens, but a calculation of an optical distance from the point where light rays converge to form a sharp image of an object to the digital sensor or 35mm film at the focal plane in the camera. The focal length of a lens is determined when the lens is focused at infinity.

Depending on how short or long the focal length is, this will have a direct impact on the field of  view of your images and in other aspects like the depth of field.

For example, 

shorter focal lengths like wide-angle lenses will have a wide-angle of view, whereas 

longer focal lengths like telephotos will have a narrower angle of view.

The focal length will also create different effects, 

    like distortion in wide angles of view, and 

    magnification in narrower angles of view.

WHAT IS A LONG FOCAL LENGTH 

long focal length. 

An image taken with a 100mm telephoto lens will have a much smaller section of coverage than a normal or wide-angle 

Lens focal length tells us the angle of view—how much of the scene will be captured—and the magnification—how large individual elements will be. The longer the focal length, the narrower the angle of view and the higher the magnification. The shorter the focal length, the wider the angle of view and the lower the magnification.

Zoom vs. Prime Lens

There are two types of lenses-prime and zoom. Prime lenses have a fixed focal length and zoom lenses have variable focal lengths.

Zoom Lens Benefits

The advantage of a zoom lens is versatility(ability to adapt). They are ideal when you are photographing a variety of subjects such as landscapes and portraits, and you just want one lens for both situations. Using a zoom lens also reduces the number of times you need to change the lens which saves time .

Prime Lens Benefits

The main advantages of prime lenses or fixed focal length lenses are their size and weight as well as their maximum aperture or f/stop. Prime lenses tend to be more compact and lightweight than zoom lenses.

Prime lenses also tend to have a larger maximum aperture (f/1.4 to f/2.8). This is an advantage when shooting in low light conditions as it will increase the possibility of hand holding the camera and freezing the subject without shake or blur caused by the longer exposures. Photographing using prime lenses with large apertures also means you can get a shallow depth of field which is useful for portraiture where you might want a softer or blurred background .

Lens Focal Length Comparison

https://www.youtube.com/watch?v=4CoEsqePADw&list=RDCMUCJFMWWscsO-msPiLj42eXiQ&start_radio=1&rv=4CoEsqePADw

Wide-angle Lens

FX format approx. 14 – 35mm / DX format approx. 10 – 24mm

Wide angle lenses are popular lenses for landscape photography, interiors, large group photos and when working in confined situations.

Standard Lens

FX format approx. 50 – 60mm / DX format approx. 35mm

Standard lenses are popular as they are closest to the angle of view we humans see. These lenses have minimal distortion, which can be flattering to the subject. They tend to use large apertures and allow a lot of light to enter the lens which makes them fast in low light conditions. Large apertures (f/1.8 – f/1.4) also produce a pleasing out-of-focus effect to the background which concentrates the attention of the viewer on the subject. Standard lenses are the popular choice for a wide range of photography including portraiture, nature and low light situations where the photographer can not use a flash or is looking to capture the scene with available light.

Standard Lens

FX format approx. 50 – 60mm / DX format approx. 35mm

Standard lenses are popular as they are closest to the angle of view we humans see. These lenses have minimal distortion, which can be flattering to the subject. They tend to use large apertures and allow a lot of light to enter the lens which makes them fast in low light conditions. Large apertures (f/1.8 – f/1.4) also produce a pleasing out-of-focus effect to the background which concentrates the attention of the viewer on the subject. Standard lenses are the popular choice for a wide range of photography including portraiture, nature and low light situations where the photographer can not use a flash or is looking to capture the scene with available light.

Super Telephoto Lens

FX format approx. 300 – 600mm / DX format approx. 200 – 600mm

These lenses provide a good range for wildlife and sports photography where the photographer is limited as to how close they can get to the subject.

Camera-Subject Distance

Another important factor affecting depth of field is the distance between the camera and the subject. The shorter that distance, the smaller the depth of field. 

Sensor Size

Sensor size also affects depth of field. The size used as a reference in digital photography is the classic 35 mm sensor size, also known as Full-Frame. If the sensor is smaller than this size, it’s said to be “cropped”, and if it’s bigger, it’s considered “medium format.“

For example, a basic rule in night photography is to use a sensor with larger pixels, since these are better at capturing light and will allow you to take a better-quality image with less digital noise.

The depth of field is the last effect of the camera sensors. Using the same field of view, the depth of field will be narrower in cameras with larger sensors and larger in cameras with cropped sensors. An example is portrait photography, where photographers usually shoot with larger sensors to decrease the depth of field and bokeh effect.

In a nutshell, cameras with smaller sensors have larger depths of field. 

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