Standard Focal Length
The standard focal length is that which provides a view matching our normal vision.
In order to find this standard view, I composed a scene where I knew branches of a tree would break the image boundary. Keeping both eyes open, with one looking through the cameras view finder, I adjusted the zoom lens until the image I was viewing through both eyes appeared to match.
Checking the zoom setting on the lens barrel, it was showing a focal length of just short of 50mm, however, checking the exif of the image revealed that the camera had recorded the focal length as 50mm.
Wide Angle Focal Lengths
Leaving the camera set in the same position, I reduced the same zoom lens down to the widest focal length of 17 mm:
To go wider still, I then swapped the lens to an ultra wide angle lens, selecting the widest focal length available of 11mm:
Telephoto Focal Lengths
In order to switch to the telephoto end of the focal spectrum, I again had to switch lenses using the longest lens I had available, a 70-300mm zoom lens. To provide comparisons with the standard focal length, images were recorded at both the 70 and 300mm focal lengths of this lens:
Comparing the Focal Lengths
Comparing the wide angle and telephoto images with the image taken at the standard focal length shows that:
- The wider focal lengths contain a larger amount of the scene than that shown in the standard view.
- Objects in the images from the wider focal lengths appear smaller than those of the standard view.
- Telescopic scenes show a lesser amount of the scene than that shown in the standard view.
- Objects in the images from the telescopic focal lengths appear much bigger than those shown in the standard view.
After capturing the images, I downloaded them on to an iPad so that the captured images could be compared at a size roughly equal to A4. Starting with the image taken at the widest focal length, and working through to the image taken at the longest focal length, I tried to adjust the distance of the iPad from my eyes until it matched the scene in front of me:
|Focal Length||Approximate Distance From Eyes|
|11mm||2cm – this distance was getting so close it was actually difficult to focus on the image in front of me.|
|50mm||40cm – not quite a fully stretched arm but it was further than I would consider a comfortable reading range if I was holding a book.|
|70mm||50cm – a full arms length.|
|300mm||This wasn’t possible to measure but it appeared it would have been many metres in front of me.|
The final comparison I did of the images was to merge them all into one image and mark the boundaries of each focal length:
The image shows a much greater difference between the outer edge, taken at 11mm, and the boundary of the image taken at 17mm than that of the next two boundaries taken at the 50mm and 70mm focal lengths.
Plotting the focal length against the relative pixel size in the merged image reveals that as focal lengths decrease there is an exponential relationship with the amount of a scene that will be included in the captured image: