What is focal length?
The focal length of a camera, expressed in millimeters, also called focal length or simply focal length, refers to the distance between the camera sensor and the optical center.
In other words, the focal length determines the field of view of a lens, that is to say the environment that we see and the distinct subjects present in the photo. Depending on the focal length chosen, the angle of view may appear smaller or larger. The angle of view is linked to the value of the focal length used and the size of the sensitive surface (sensor surface). A short focal length (wide angle lens) will provide a wide angle of view, while a long focal length (telephoto lens) will provide a low angle of view.
The focal length of a device is therefore a determining element in the choice of a lens.
Understanding the focal length of a lens and knowing how to use it will help you shoot successfully.
In this article, we will see the essentials to know about the focal length of a lens and how to choose the right lens for your photographic practice.
The different types of objectives
Generally speaking, there are two types of objectives:
Fixed focal length lenses: 1 single value indicated on the lens (example: 70 mm). The focal length does not vary. Which means that you can neither zoom in nor zoom out. The only way to get closer to the subject is to move! At an equivalent price, a fixed focal length provides better image quality than a zoom. Fixed focal lengths also allow a larger maximum aperture, which can be very practical in difficult lighting conditions. Fixed focal length lenses also help you achieve shallower depths of field, which makes it easier to blur the background.
Variable focal length lenses or zoom type lenses: 2 values indicated on the lens (example: 24-70 mm). Focal length may vary.
We can take a 50 mm lens as a reference lens. It is a so-called “normal” lens because it is the one that comes closest to the human eye in terms of rendering. The different types of objectives can be classified into 3 categories:
- Standard lenses: 35-70 mm
- Lenses with a short focal length (Ultra wide angle: 10-24mm; Wide angle: 24-35mm)
- Lenses with a long focal length (Telephoto lens: 70-300mm; Super telephoto lens: >300mm)
Understand that getting closer to the scene or zooming in on a scene does not have the same impact on the image.
Understanding the focal length of a lens and knowing how to use it in the field:
For landscape or architectural photography, choose wide-angle focal lengths to obtain beautiful wide shots.
Benefits :
- Insert more elements into your image.
- Emphasis on depth of field.
- Increased sense of depth
- Ideal focal lengths for landscape or architectural photography: 10-35 mm
For portrait photography, choose telephoto lenses. Benefits :
- Isolate the subject or object by inserting few or no background elements
- Decreased depth of field
- Ideal focal lengths for portrait photography:
- 35-50 mm (full length portrait),
- 85 mm (American plan),
- 135 mm (face)
For street photography, opt for a small wide-angle lens. Benefits :
- Discretion, weight, bulk
- More background = more context = better composition
- Less compressed perspective than with a telephoto lens
- Ideal focal lengths for street photography: 35-50mm
For nature, wildlife, macro and sports photography, choose a telephoto or super-telephoto lens. Benefits :
- Target an element
- Emphasizing the subject in relation to the background
- photograph a distant subject
- Decreased depth of field
- More background blur
- Ideal focal lengths for nature and animal photography: 70-100 mm
- Ideal focal lengths for nature, animal, sports and macro photography: 135-200mm and beyond.
zoom
Zoom = angle of view associated with focal length. It modifies the perception we have of subjects or objects in relation to each other and particularly the distance between them. The advantage of zoom is above all its versatility.
Small focal length = wide viewing angle = large segment of the scene visible in the photo = reduced magnification
Conversely :
Long focal length = small angle of view = small segment of the scene visible in the photo = high magnification. It will therefore be easier to take a distant subject.
The more we increase the focal length, the fewer elements are visible in the scene and the greater the magnification effect we obtain.
Reminder: the viewing angle is linked to the focal length, but also to the size of the sensor.
In other words, the larger the focal length number, the greater the zoom.
Perspective
Perspective in photography is defined by the appearance of subjects or objects positioned on different levels in relation to each other.
If you take the same photo with a wide angle (20 mm) and with a telephoto lens (200 mm).
We see that the background is closer in the photo taken with the telephoto lens. This is called telephoto compression. This effect is not really due to the focal length, but more to the distance between the subject and the camera. In other words, the distortion of the subject is not due to the focal length, but to the distance at which you take the photo.
You can adjust the focal length and distance to the subject to create more interesting perspectives.
Depth of field
Reminder: Depth of field corresponds to the extent of the sharp area of your image.
For a subject positioned in the same place, you will have two different depths of field depending on your focal length
Small focal length = large depth of field
Long focal length = small depth of field
A wide-angle lens (for example: 35 mm) therefore has a greater depth of field than a telephoto lens (for example: 200 mm). If the maximum aperture of the aperture is too small, you will have difficulty achieving bokeh for background blur. It is therefore easier to produce bokeh with a telephoto lens than with a wide angle lens.
Conclusion
- the focal length is short, + the viewing angle is wide, + the magnification is reduced
- the focal length is extended, + the angular field is narrow, + the magnification is high
When you start, you may think that a long focal length will give more background blur and therefore reduce errors. It’s not false ! That said, shooting with a short focal length can provide more context and greater immersion for the viewer. Be careful though, because having less background blur, and therefore greater sharpness, you will have to pay more attention to all the elements present in the frame. In other words, take care of your composition!
The focal length clearly influences the viewing angle of your photos, namely the magnification.
The viewing angle depends on the size of your sensor (full format, APS-C, etc.).
Develop your creativity by playing with the focal length and distance of the subject or object in the foreground.
Test by taking the same photo with different focal lengths and changing the perspective.