Focal length and perspective

Everything you need to know about focal lengths - and how to use them effectively

High-quality photographic equipment not only includes a camera with a powerful sensor, but also a well-coordinated range of lenses. The selection ranges from wide-angle to telephoto lenses, from travel zooms and fixed focal lengths to specialists such as macro or tilt-shift lenses. An important distinguishing feature is the focal length, which is specified in millimetres (mm). The longer it is, the closer you can get to your subject and the larger the image. The shorter the focal length, the more surroundings are captured in the image.

Wildlife and sports photographers often use a telephoto lens of 200 mm or more to photograph shy or dangerous animals or action-packed scenes from a great distance. Landscape photographers, on the other hand, prefer wide-angle lenses under 24 mm to capture wide panoramas. A focal length of 50 mm is considered a "normal focal length", as it comes closest to the way the human eye sees things. And portrait photographers prefer focal lengths between 85 and 105 mm because they hardly distort faces.

The influence of the image sensor on the focal length

The size of your camera's image sensor has a direct impact on the lenses you need. We would therefore like to show you the differences at this point and explain what the so-called "crop factor" is all about.

A full-frame sensor has the same size as the image of an analog 35 mm film, namely 36 x 24 mm. The term "full format" was coined at the beginning of digital photography and occasionally leads to confusion, especially among newcomers to photography, as sometimes the full format and sometimes the 35 mm format are mentioned. However, the size ratio is identical. Both terms refer to sensors with an area of 36 x 24 mm. In comparison, APS-C sensors are significantly smaller at around 23.5 x 15.7 mm and sensors of the Micro Four Thirds (MFT) system at 17.3 x 13 mm.

In order to fully expose the rectangular sensors right into the corners, the round lenses of a lens must cover a correspondingly large image circle. The image circle of a full-frame or 35 mm sensor is therefore larger than that of a comparatively smaller APS-C or MFT sensor. As a result, full-frame lenses generally have a larger diameter than lenses that have been specially developed for the smaller sensor formats. In principle, full-frame lenses can also be used on APS-C cameras from the respective manufacturers, as the bayonets of both camera classes have the same dimensions. However, as the image circle of a full-frame lens is significantly larger than that of the APS-C sensor, the edge areas that you would see in a full-frame image are no longer visible when shooting with an APS-C camera - it looks as if the original full-frame image has been cropped smaller.

What is the reason for this? The smaller image circle of APS-C sensors leads to a so-called "crop factor", which extends the effective focal length of the lens or reduces the angle of view. An example: A 50 mm lens on a mirrorless full-frame camera has an angle of view of approx. 47 degrees, while the same focal length on an APS-C camera has a smaller angle of view of approx. 31 degrees due to the crop factor and therefore a smaller image section. With APS-C cameras, the crop factor is usually around 1.5x (1.6x for Canon). With the MFT system, the crop factor is 2x.

What does this mean for the focal length? Let's stay with the example of the 50mm full-frame lens on an APS-C camera. The crop factor of 1.5x results in the aforementioned smaller angle of view on the APS-C camera, which corresponds to that of a 75 mm lens on a full-frame camera. Manufacturers often state this focal length extension as "converted to 35 mm" in the lens data sheet. The calculation is simple, as the original focal length is simply multiplied by the crop factor.

This information in the manufacturer's data sheets is particularly interesting if, for example, an APS-C photographer is looking for a lens that creates the look of a 35 mm full-frame lens for reportage or street photography. If you now mount a 35 mm full-frame lens on the APS-C camera, the focal length is extended to 50 mm due to the smaller sensor with a crop factor of 1.5x. This makes the image section smaller than desired. In order to achieve the desired 35 mm look, the focal length must first be divided by the crop factor. With a factor of 1.5, a full-frame lens with a focal length of 23.33 mm is therefore required to achieve the desired 35 mm full-frame look with an APS-C camera. As there are no lenses with a focal length of 23.33 mm, a 24 mm full-frame lens would be used in this case.

Extending the focal length for smaller sensor formats has advantages and disadvantages. Due to the extension in the telephoto range, cameras with APS-C and MFT sensors are particularly recommended for wildlife photography. Thanks to the crop factor, you can achieve the same image detail with a 300 mm full-frame focal length on an APS-C camera, for example, for which you would need a long 450 mm telephoto lens on a full-frame camera - without the use of a teleconverter. However, there are disadvantages for landscape and architectural photography. The focal length extension turns a 24 mm full-frame lens on the APS-C camera into a 35 mm focal length converted to the 35 mm format. Part of the wide angle of view is lost in the process.

What to consider with APS-C and MFT lenses

With lenses that have been specially developed for APS-C and MFT cameras, the focal length specifications engraved on the lens mount cannot be compared one-to-one with the focal length specifications of full-frame lenses. The manufacturers specify the focal lengths before conversion with the extension factor. A lens specially developed for APS-C cameras with a focal length specification of 16-55 mm, for example, therefore does not provide the same angle of view as a full-frame zoom with this focal length. The specifications for APS-C and MFT lenses must always be multiplied by the corresponding crop factor for comparison. For example, an APS-C lens with a focal length range of 16-55 mm engraved on it provides an angle of view which, with a crop factor of 1.5x, corresponds to 24-82.5 mm in full format or 35 mm format. An MFT lens with a focal length of 12-100 mm specified by the manufacturer provides a focal length of 24-200 mm converted to 35 mm format by the 2x crop factor.

How to recognize APS-C lenses

For APS-C cameras, manufacturers who also have full-frame cameras in their portfolio bring their own lenses onto the market, which have been specially developed for the smaller image circle and are correspondingly more compact, lighter and cheaper. At Canon, they bear the designation "EF-S" for digital SLR cameras and "RF-S" in the mirrorless R system. At Nikon, APS-C lenses for both DSLRs and mirrorless system cameras (DSLMs) are labeled with the abbreviation "DX". In Sony's mirrorless E system, APS-C lenses start with the letter "E", while full-frame lenses are labeled "FE". Sigma distinguishes between "DC" lenses for APS-C cameras and "DG" lenses for the full-frame format. And at Tamron, APS-C lenses can be recognized by the abbreviation "Di II" for DSLRs and "Di III-A" for mirrorless cameras.

Can APS-C lenses be used on full-frame cameras?

In principle, APS-C lenses can also be used on the full-frame cameras of the respective manufacturers, as the bayonets of both camera types have the same dimensions. This can be useful, for example, if you are switching from an APS-C camera to a full-frame model and want to continue shooting with the existing APS-C lenses for cost reasons. The APS-C lenses for digital SLR cameras from Canon, which are marked with the abbreviation "EF-S", are an exception. These cannot be used on full-frame DSLRs from Canon. With the mirrorless R system, Canon has lifted this restriction so that the "RF-S" lenses specially developed for EOS R DSLMs with an APS-C sensor can also be used on full-frame R system cameras.

As APS-C lenses are designed for a smaller image circle, the sensor area is not fully exposed when used on a full-frame camera. This results in dark corners in the image. To avoid this, Canon, Nikon and Sony have equipped their mirrorless full-frame cameras with the option of switching between full-frame and APS-C format - and even automatically. So if an APS-C lens is attached to a full-frame mirrorless system camera, it automatically uses a smaller area in the middle of the sensor that corresponds to that of an APS-C sensor. The crop means that no black corners are visible. This also results in the crop factor already mentioned.

The ability to switch manually between the large area of the full-frame sensor and a crop in APS-C format offers further interesting advantages. For example, if the focal length of the full-frame lens is not sufficient because you cannot get closer to the subject due to an obstacle. The focal length can then be extended by manually switching to the smaller APS-C sensor area using the crop factor. Small disadvantage: If only the smaller area of the full-frame sensor is used, only a reduced sensor resolution is available.

Extending the focal length with a teleconverter

If you want to extend the focal length of your lens on a full-frame camera without having to switch to the smaller APS-C format and thus accept a loss of resolution, you can connect special converters between the camera and lens. Manufacturers generally offer two converters with extension factors of 1.4x and 2.0x. With the 2x extension, for example, a full-frame focal length of 200 mm on a full-frame camera achieves an angle of view that corresponds to that of a 400 mm lens - with full sensor resolution. However, there is one disadvantage. By extending the lens using a converter, light is lost on the way to the sensor. With a 1.4x converter, you lose 1.4 f-stops. With a 2.0x converter, the loss is two f-stops. In practice, this means that if you use the 2.0x converter in combination with a 70-200 mm telephoto zoom with a constant aperture of F2.8, for example, you can extend the focal length in the telephoto range from 200 mm to 400 mm - but the loss of two f-stops means that the aperture can no longer be opened to f/2.8, but only to a maximum of f/5.6.

Conclusion: The right focal length for the perfect moment

Choosing the right focal length depends on the subjects you mainly photograph and the look you want to achieve with the image. Wide-angle lenses with a short focal length are ideal for landscape and architectural photography, while telephoto lenses are preferred for portrait, sports and wildlife photography. As the sensor size of the camera and the resulting crop factor have a major influence on the focal length, it can make sense to use an APS-C or MFT camera for wildlife, sports and travel photography. The extension factor of 1.5x to 2x makes it possible to photograph distant subjects without having to resort to longer focal lengths in often larger and heavier lenses from the outset. In landscape and architectural photography, on the other hand, you are better off with full-frame cameras. Otherwise the crop factor will "steal" important focal lengths.

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