How can I estimate the focal length I’ll need for a given subject size and distance?
Asked 9/26/2010
4 views
2 answers
0
Before heading out with an APS-C DSLR, is there a practical way to estimate what focal length range I’ll need based on how far away I expect to be from the subject and how tightly I want to frame it? For example, if I’ll be photographing animals at roughly 10–20 m away at a zoo and want anything from wider environmental shots to tighter close-ups, can I calculate the focal length needed from subject distance and subject size? Is there a simple formula or rule of thumb?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
23
If you know or can estimate the distances, use this equation:
Focal Length = Sensor Dimension * Distance / Scene Dimension
Where you match the dimension of the sensor and the scene. e.g:
Focal Length = Sensor Width * Distance / Scene Width
Note that the the advertised size of your sensor is typically not the width, height, or even the diagonal, which are the appropriate dimensions you can use. See this wiki article for the size.
For non-Canon APS-C, the width is 23.6mm. Say your scene / subject is 20m away and you want a width of 5m at that distance. You can use feet as it is just a ratio. This equation says to use a focal length of 93.6mm.
If you are out in the field already and want to know what lens to put on, it might be good to give yourself a good hand calibration:
Hold your fist in front of your face with your elbow bent at a right angle; use your knuckles to select the lens you want for the scene behind your hand
4knuckles:50mm, 2:100, 1:200.
The trick is figuring out where to hold your fist so the above rule holds. Try it a few times.
Take a photo of a scene with a 50mm lens (or look through the camera's viewfinder with a 50mm lens in place.) Put the camera down & hold your fist in front of your face such that the 4 knuckles just fill that scene height. Remember where to hold your fist next time you want to select a lens.
Originally by user389. Source · Licensed CC BY-SA 4.0
user389
15y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. If you can estimate both subject distance and the size of the scene you want to fit in the frame, you can estimate focal length.
A useful approximation is:
focal length = sensor dimension × distance ÷ scene dimension
Match dimensions correctly: use sensor width for scene width, or sensor height for scene height. For APS-C, use the actual sensor size, not the marketing format name.
Example: if your APS-C sensor is about 23.6 mm wide, your subject is 20 m away, and you want to frame a 5 m wide scene:
f = 23.6 mm × 20 m ÷ 5 m ≈ 94 mm
A related rule of thumb for normal-to-telephoto lenses is:
angle of view (radians) ≈ sensor size ÷ focal length
So if the framed scene width is one-tenth of the distance, you need about 10× the sensor width as focal length.
This works well as an estimate, especially for normal and longer lenses; it’s less accurate for very wide-angle lenses. In practice, estimate your nearest/farthest distances and smallest/largest framing, calculate both ends, and choose a zoom that covers that range.
Recommended products
UniqueBot
AI15y ago
Your Answer
Related Questions
How can I estimate an object's real size from one image if I know the camera-to-subject distance?
How do I calculate the focal length needed to frame a subject of a given size at a given distance?
How can I estimate an object's real size from pixel size, focal length, and subject distance?
How do I calculate the smallest detail a camera can resolve at a given distance?
How can I estimate an object's real size from photographs if one object in the image has a known size?