How To Find Focal Length Of Mobile Camera

What is Camera Focal Length?

Focal length is a number that is vital to photography and photogrammetry only often misunderstood.

What is focal length?

A strict technical definition of focal length is difficult without providing a lot of background in lens theory, so we will utilize a simplification. You lot can remember of focal length equally the distance between the imaging plane (eastward.k. the image chip in a digital camera) and a betoken where all calorie-free rays intersect inside the lens (the 'optical heart'). And so a focal length of 20mm means that the distance from the optical center to the imaging plane is 20mm long (about ¾ of an inch).

What does the focal length number hateful?

The focal length number tells usa how much of the scene is captured in the movie. The lower the number the wider the view, and the more nosotros can run into. The college the number, the narrower the view, and the less nosotros tin see. This is illustrated below – where the camera is stationary and the focal length (in white numerals) changes:

focal length illustrated

A photographic camera typically has focal length in a range of 10mm to 500mm. Different types of camera tin can accept unlike ranges and speciality lenses can extend outside this range as well. A 10mm focal length would exist a very wide lens (capturing a lot of the scene), and 500mm would exist a very narrow lens (capturing merely a minor role of the scene – giving a big magnification like binoculars or a telescope).

Cameras can have fixed lenses (sometimes called 'prime number' lenses) which have but i focal length, or zoom lenses which let the focal length to be varied (for example between 18mm-55mm, or 55mm-200mm).
For high accurateness photogrammetric work in PhotoModeler, a fixed (or prime) wide lens (such as a 20mm lens on an APS-C frame camera) is recommended as the main choice, but different applications may require unlike focal lengths, and cameras with adjustable zoom lenses can still be used to attain very skillful results with some extra procedural intendance over the focal length.

Note: A technical photogrammetry term that yous may come across is the "Principal Distance". Strictly, the Principal Altitude is the altitude mentioned higher up (i.e. altitude from imaging plane to the lens optical sensor), and the focal length is the master distance when the lens is focused at infinity. Meet below for more data on focus vs focal length. When PhotoModeler lists focal length for a camera, information technology is actually the Master Distance that is shown.

Format Size

When you buy a digital camera you lot volition frequently see the specification "equivalent 35mm focal length". What does this mean? Near digital cameras have imaging fries that cover much less area than a standard 35mm film frame. Since 35mm film cameras were the standard for then long in photography, much of the techniques and methods were adult around them. A 35mm film camera has a negative that is about 36mm broad by 24mm high (the "35" comes from the physical width of the motion-picture show stock that is exactly 35mm wide). A 'normal lens' (has a field of view that appears 'natural' to humans) on a 35mm film camera has a focal length of 50mm.

Modernistic digital cameras tin can have imaging chips that are every bit pocket-sized every bit 6mm by 4mm; some Smartphone cameras are even smaller, and then up to full 24mm by 35mm size. A very common size is the APS-C format at 16mm by 24mm. This smaller size affects what is considered to be a 'normal' focal length.

Let'south say you take a picture of an automobile with ii cameras, a 35mm moving-picture show camera and a smartphone camera. You stand in the same spot and take ii photos, one with each photographic camera. In both cases you want to accept a photo of the machine that fills the frame. If the 35mm picture camera lens has a 50mm focal length, the digital camera's focal length might exist 4mm. So even though they are very different numbers they produce the same event because of the size of the imaging surface. So the "equivalent 35mm focal length" for this smartphone camera at 4mm is 50mm.

Photographic camera manufacturers sometimes list these equivalents because some photographers are more than familiar with 35mm cameras and they want to go far easier to understand. It also gives us a standard of reference for all the different format sizes. They may also list the multiplier factor. For case, the APS-C multiplier is around one.6x. And so a 32mm lens on an APS-C photographic camera (like the Nikon D3200) would deed like a 50mm lens on a 35mm film photographic camera.

Does focusing touch the focal length?

In a higher place we mention that focal length is related to focus altitude. Focal length is the principal distance of a camera when it is focused at infinity. In photogrammetry we are interested in the photographic camera's internal geometry at the fourth dimension photos were taken – and so information technology is the principal distance that we want to know precisely in photogrammetry.

The answer is 'yes focusing a lens changes its chief altitude'.

All lenses take a stated or specified focal length value (or range of values for a zoom lens). This printed number is actually its nominal length or the primary distance when the lens is focused at infinity. Equally you lot focus on objects that are closer to the photographic camera, the principal distance changes. So for case, a 50mm lens focused on an object a few feet away might take a principal distance of 55mm lens at that time. The most farthermost example of this is with a macro setting (a lens setting that allows you to focus on very close, very small objects, under 5″ in size for example). A lens that has a 50mm nominal focal length (and so a 50mm principal altitude when focused at infinity) might in fact have a 100mm main altitude when focused at a few inches! This is why it is skilful with photogrammetry (where precise geometry is needed) to calibrate a camera at the distance you will exist working with.

There is some ability to calibrate a camera (which solves the principal altitude) at one focus and execute your photogrammetric project at another focus. The actual discrepancy that is acceptable depends on your accuracy requirements and how much the focus changes. Generally a calibration done at 2m/6ft focus distance is acceptable for projects up to space focus (once again depending on accurateness requirements), just may not exist adequate for a projection where the focus distance was 50cm/20in.

In many cases though, the advantages of using focus (i.e. crisp targets and singled-out features) with subtle effects on principal distance/focal length, outweigh the advantages of keeping focus and primary distance constant (i.e. potentially causing blur in some photos taken at a different distance).

Focal Length and Photogrammetry

There are three main items to remember in relation to focal length and photogrammetry / PhotoModeler:

PhotoModeler needs to know the principal altitude of the camera/lens at the time the photos were taken and so that it can solve geometry and summate the camera positions and create 3D coordinate points. This means a) calibrate the camera to constitute the verbal focal length/principle distance for more accuracy (or at least employ a reasonably close focal length estimate), and b) if using a zoom lens, take photos with the camera set at the same focal length (zoom setting) at which the camera was calibrated (for repeatability, use the widest angle or fully zoomed).
The format size and the focal length are both important. Knowing just the focal length is not enough.
PhotoModeler has 4 methods for calculating the focal length/principle distance and the format size. These are a) Camera Scale, b) the quick Gauge Photographic camera method using the images' EXIF header, c) the Changed Photographic camera method (for projects using photos from an unknown source), and d) Field and Auto Scale (which is camera calibration executed at the same fourth dimension equally the modelling project). For the most consequent and authentic project results, calibrate your photographic camera and so always use information technology at the same focal length that y'all calibrated it at.

PhotoModeler and Photogrammetry

PhotoModeler is ane of the leading tools for photogrammetry (the science of generating measurements and accurate 3d information from photography).

For a brief explanation on how PhotoModeler works:
www.photomodeler.com/products/how-it-works.html

For information on cameras for PhotoModeler:
www.photomodeler.com/products/about_cameras.html

Larn how to use PhotoModeler with your camera to create detailed digital models:
www.photomodeler.com/products/why.html