How can I estimate how much of the frame a subject will fill at a given distance and focal length?
Asked 8/24/2011
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I’m choosing a longer lens for wildlife and want a simple way to estimate how much of the frame a subject will occupy at a certain distance. For example, I currently use a 70-300mm lens and am considering a 400mm prime.
Is there a formula or practical rule of thumb to calculate frame fill based on focal length, subject size, distance, and sensor size? A way to compare what 300mm vs 400mm will look like would also be helpful.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
17
The formula for the percentage of image filled is
focal_length x subject_size x 100
_________________________________
distance x sensor size
All units are millimeters. Use the width of the subject/sensor to work out the horizontal fill % and the height of the object/sensor to work out the vertical fill %
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
15y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. A useful approximation is:
fill % = (focal length × subject size × 100) / (distance × sensor dimension)
Use consistent units, typically millimeters. For horizontal fill, use subject width and sensor width; for vertical fill, use subject height and sensor height.
Rearranged for distance at which the subject exactly fills the frame:
distance = (focal length × subject size) / sensor dimension
A practical rule of thumb is to pick the direction you care about (horizontal or vertical), divide focal length by that sensor dimension, and use that ratio to estimate framing.
Also, focal length changes framing in direct proportion. So going from 300mm to 400mm gives 400/300 = 1.33× more subject size in the frame. In other words, a 400mm view is like taking your 300mm image and cropping each dimension to 75% of the original.
This is a good way to preview the difference: shoot at 300mm, then crop to 75% width and 75% height to simulate 400mm.
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