Guides · Updated 1 November 2026

Canon RF 100-400mm for bird photography: the numbers, worked out

What the Canon RF100-400mm F5.6-8 IS USM puts on a bird at 400 mm, worked out from the focal length and the sensors of five popular bodies: pixels across birds from chickadee to heron at 10, 25 and 50 m, what the Extender RF1.4x and RF2x add, how far you can crop and still meet iNaturalist and contest sizes, and how much the wingtips blur in flight.

Pixels on the bird at 400 mm: 10, 25 and 50 m

Pixels across the bird from beak to tail at 400 mm on the Canon EOS R10, Canon EOS R7, Canon EOS R6 Mark II / R8 and Canon EOS R5, using the bird lengths in our reach calculator:

“Won't fit” means the bird is longer than the frame is wide at that distance. The sum: the bird's length × focal length ÷ (distance − focal length) gives its size on the sensor, divided by the pixel pitch (Canon EOS R10 3.7 µm, Canon EOS R7 3.2 µm, Canon EOS R6 Mark II / R8 6.0 µm, Canon EOS R5 4.4 µm).

What the camera body changes

The lens puts the same image of the bird on every sensor: at 400 mm, an American robin at 25 m is 3.90 mm long on the sensor. The body decides how many pixels cover that image. In pixels per millimetre of sensor: Canon EOS R10 269, Canon EOS R7 312, Canon EOS R6 Mark II / R8 167, Canon EOS R5 228. So on the same lens, at the same distance, the Canon EOS R7 puts 1.87 times as many pixels on the bird as the Canon EOS R6 Mark II / R8: 1,218 against 652 across that robin. A smaller sensor adds pixels on the bird only when its pixels are smaller (crop sensor vs full frame).

What an extender adds

Canon lists the Extender RF1.4x and Extender RF2x for this lens. An extender multiplies the focal length: 400 mm becomes 560 mm with the 1.4x and 800 mm with the 2x. On the Canon EOS R7, that takes an American robin at 25 m from 1,218 pixels to 1,716 or 2,476:

Pixels across the bird from beak to tail, worked out the same way as the table above. Try it in the reach calculator with the 1.4x.

How far you can crop and still meet real sizes

Nearly every size rule counts pixels on the long edge. Here is the smallest share of each body's frame width that a horizontal crop can keep for each:

An American robin at 25 m with 400 mm is 1,218 px on the Canon EOS R7, so you can crop until it fills 41% of the width and still have Audubon's 3,000 pixels; on the Canon EOS R6 Mark II / R8 (652 px), 22%. Crop tighter and the file drops below the size; enlarging it to reach the number adds no detail, and the contests that ask for full-size files say so (crop rules and minimum sizes for 11 contests). For the general case, see how much you can crop a bird photo. The rule that always holds: crop from the full-size original, never from a downsized export, and never upscale.

Birds in flight: wingtip blur at 1/2000 and 1/4000

From our shutter-speed calculator, with 400 mm on the Canon EOS R7, panning with the bird, wingtips measured mid-stroke where they move fastest:

Wingbeat rates for the osprey and bald eagle are estimates (est.), so treat these as close guides rather than exact figures. Holding the osprey's wingtips within 3 pixels at this distance would take about 1/16000; for the bald eagle, about 1/12800. The wings slow to a stop at the top and bottom of each stroke, which is why a burst catches sharp wingtips that a single frame usually misses.

Try it with your own body and birds

The reach calculator and the shutter-speed calculator open pre-set to 400 mm on the Canon EOS R7; change the body, focal length, distance or bird to see your own numbers. Both run in the browser. For one body in more depth, see the camera guides: Canon EOS R10, Canon EOS R7 and Canon EOS R6 Mark II.

BirdFrame for Mac finds every bird in a folder or Photos album and crops each from the full-size original, thousands at once.

Questions

Is the RF 100-400 long enough for bird photography?

It depends on the bird and the distance. At 400 mm it puts about 1,218 pixels across an American robin at 25 m on the Canon EOS R7 and 604 at 50 m; a black-capped chickadee at 25 m gets 685. On the Canon EOS R6 Mark II / R8 the same robin at 25 m gets 652. Crop the Canon EOS R7 frame to iNaturalist's 2,048 pixels and the robin still fills 59% of the width.

Does the RF 100-400 take an extender (teleconverter)?

Yes. Canon lists the Extender RF1.4x and Extender RF2x for this lens. They take 400 mm to 560 mm and 800 mm. On the Canon EOS R7, an American robin at 25 m goes from 1,218 pixels at 400 mm to 1,716 with the 1.4x and 2,476 with the 2x.

Does an APS-C body give the RF 100-400 more reach?

Only when its pixels are smaller. In pixels per millimetre of sensor: Canon EOS R10 269, Canon EOS R7 312, Canon EOS R6 Mark II / R8 167, Canon EOS R5 228. On the same lens, at the same distance, the Canon EOS R7 puts 1.87 times as many pixels on the bird as the Canon EOS R6 Mark II / R8. An APS-C sensor doesn't change the focal length; it records a smaller frame around the same image of the bird.

What shutter speed freezes a bird in flight at 400 mm?

On the Canon EOS R7, with an osprey at about 46 m, wings across two thirds of the frame, the wingtips blur about 20 pixels at 1/2000 and 10 at 1/4000 mid-stroke when you pan with the bird. Holding them within 3 pixels would take about 1/16000, so a burst is what catches the wings sharp near the top or bottom of the stroke. Wingbeat rates for the osprey and bald eagle are estimates.