Bird Photography: How to Calculate If Your Lens Is Enough
Before you head out for a species, you can work out how large it will be in your frame at a given distance, and whether your lens will give a usable picture or a tiny speck. The sums are simple.
The calculation
The bird's height on the sensor is the focal length times the bird's real height, divided by its distance, all in the same units. Double the focal length and the bird doubles in size. Double the distance and it halves. For a robin 14 cm long at 10 m with a 500 mm lens: 500 times 140 mm divided by 10,000 mm is 7.0 mm on the sensor. A full-frame sensor is 24 mm tall, so the robin fills 29 percent of the frame height.
As a guide, a bird filling about half the frame height, 12 mm on full frame (7.8 mm on APS-C), makes a strong picture with little cropping. At a quarter of the frame height you can crop and still print or share well on a modern high-resolution sensor. Under about 10 percent, the result is usually a record shot.
What focal length you need
This table shows the focal length that fills half the frame height on full frame. A quarter of the height needs half as much. For APS-C divide by 1.5 for the same framing. I have used the height of a perched or standing bird, which is the dimension that fills the frame.
| Bird (height) | 5 m | 10 m | 20 m | 30 m | 50 m |
|---|---|---|---|---|---|
| Robin (14 cm) | 430 mm | 860 mm | 1,700 mm | 2,600 mm | 4,300 mm |
| Blackbird (25 cm) | 240 mm | 480 mm | 960 mm | 1,400 mm | 2,400 mm |
| Grey heron (90 cm) | 70 mm | 130 mm | 270 mm | 400 mm | 670 mm |
| Perched eagle (85 cm) | 70 mm | 140 mm | 280 mm | 420 mm | 710 mm |
Small songbirds are brutal. A 500 mm lens fills half the frame with a robin at about 5.8 m, and at 10 m it gives a quarter of the frame. A reed warbler 13 cm long at 15 m with a 400 mm lens is only 3.5 mm on the sensor, about 15 percent of the height. Beyond about 10 m, songbirds are generally record shots unless you accept heavy cropping. Large birds are far more forgiving: a heron at 20 m needs only about 270 mm to fill half the frame.
APS-C and reach
An APS-C camera gives the same framing from a lens 1.5 times shorter, which is part of the appeal. The larger benefit for reach is pixel size. A 26 MP APS-C sensor has pixels about 1.6 times smaller than a 24 MP full-frame sensor, and a 40 MP one about twice as small, so the same lens at the same distance puts a bird across 1.6 to 2 times as many pixels. That is why APS-C is a good choice for birds. The costs are more noise at high ISO and less low-light range. For a head-to-head on equivalent framing, remember that the f-number needs a similar adjustment, as the full frame versus crop sensor post shows.
Teleconverters
A 1.4x teleconverter turns a 500 mm into 700 mm and costs one stop of light. A 2x gives 1000 mm and costs two stops. A 400 mm f/4 becomes 560 mm at f/5.6 with a 1.4x, a 500 mm f/5.6 becomes 700 mm at f/8, and a 600 mm f/4 becomes 840 mm at f/5.6. A 500 mm f/5.6 with a 2x is 1000 mm at f/11. Many recent mirrorless bodies autofocus at f/8, and some at f/11, but older ones do not, so check yours. Autofocus speed and sharpness can drop, most of all with the 2x.
Lenses by budget
A 100-400 mm zoom on APS-C frames like a 150 to 600 mm lens, and a 150-600 mm frames like 225 to 900 mm. Either handles large birds well at 20 to 50 m and medium birds at 10 to 20 m. A 500 mm prime or 200-600 mm zoom opens up medium birds at greater range. At the top end, 600 mm f/4 lenses are the reference for serious work. Light, relatively affordable options include the Canon RF 800mm f/11 and the Nikon Z 800mm f/6.3, though the Nikon is considerably dearer. Slow apertures need good light and a high ISO.
Using the calculator
The calculator works in both directions. After a shoot, measure the bird's pixel height in a photo, enter its known size, and it tells you how far away you were. Before one, enter the species size, your focal length and the fill you want, and it gives the distance you would need, which you can compare with how close the bird will let you get. You can also run two lenses through the same bird and distance to see the difference before buying. The same formula is used for drone altitude in the post on estimating a drone's height from a photo.
Distance also has an ethical side. Nests of many birds are legally protected, and you should never push closer for a picture, as the wildlife distances guide explains. More often than not, the answer to "is my lens enough?" is to get closer through a hide, patience and good fieldcraft, not to buy a longer lens.