What's in a Picture?

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If you’ve ever taken a picture in your life (I bet you have) then you’ve heard the term “focal length” thrown around. Changing the focal length (by adjusting your camera’s lens) changes how the foreground appears against the background, and can make a big impact on how the image is perceived. Technically speaking, focal length is the distance between your camera’s sensor and what’s called the point of convergence, the spot inside your camera where the light is brought into focus. Simply put, focal length is a measure of how zoomed in your photos look.

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Measured in millimeters, focal lengths can be fixed or variable, most lenses being variable with a limited ideal range. From as low as 12mm to greater than 500mm, a lens’s focal length determines how your camera bounces around light and focuses it to the sensor while capturing an image. For reference, a “standard” lens range is 35-70mm, and a focal length of 45-50mm produces the closest match for how a human eye perceives distance.

Longer focal lengths compensates for the distance between the camera and object by zooming the background in. Faraway objects appear to be right alongside the objects in the foreground, flattening the scene and creating what’s called forced perspective. This technique can be very convincing and is used to great effect in movies like Lord of the Rings and Elf. Shorter focal lengths allow more context in the photograph, making them suitable for landscapes. The tradeoff for fitting more context in the photo is that individual details appear smaller, and seem to be farther away than they would if observing the same scene with the human eye.

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What’s important to note is that the perspective of the photo does not change; what’s changing is how the camera is handling the light entering the lens and recording it. Focal length compensates for the distance between an object and the camera with what is essentially selective zooming. Different lenses have their own ideal focal ranges and perform best in and at their own unique settings. Portraits are commonly shot in the 70-105mm range while landscapes and buildings are shot with “wider” frames, around 24-35mm. Wide angle and fish-eye lenses are notorious for their potential to warp spaces; their native habitat is found primarily in real estate photography.

Knowing what we know, we always check the metadata of photos we receive and make note of the focal length. This is huge for photogrammetry projects where we’re relying on matching our model to photos or surveillance footage to determine precise locations of people and objects, or accurately measure speeds of vehicles. Being able to check the camera settings at the time the photo was taken can save us hours of work trying to match our model’s digital camera to an unknown focal length.

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The main takeaway should be that good knowledge of the mechanics behind the photos you take is a valuable advantage when it comes to visual communication and coordinating across teams. There’s a whole lot more to focal ranges, perspective, and lens types than what we’ve discussed here, so feel free to check out these other resources if you want to learn more:

https://expertphotography.com/understand-focal-length-4-easy-steps/

https://www.shutterbug.com/content/check-out-cool-gif-showing-how-lens-focal-length-changes-peoples-faces

https://www.lensrentals.com/blog/2016/03/focal-length-guide-for-shooting-in-large-spaces/

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Jannine Hanczarek is a Technical Illustrator at Fat Pencil Studio.