Guides · Updated 4 November 2026

Sony FE 100-400mm GM for bird photography: the numbers, worked out

What the Sony FE 100-400mm F4.5-5.6 GM OSS puts on a bird at 400 mm, worked out from the focal length and the sensors of six popular bodies: pixels across birds from chickadee to heron at 10, 25 and 50 m, what the Sony 1.4x and 2x teleconverters 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 Sony A6700, Sony A7 IV / A7 V, Sony A7R V and Sony A1 / A1 II, 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 (Sony A6700 3.8 µm, Sony A7 IV / A7 V 5.1 µm, Sony A7R V 3.8 µm, Sony A1 / A1 II 4.2 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: Sony A6700 266, Sony A7 IV / A7 V 195, Sony A7R V 266, Sony A1 / A1 II 241. So on the same lens, at the same distance, the Sony A7R V puts 1.36 times as many pixels on the bird as the Sony A7 IV / A7 V: 1,039 against 762 across that robin. The APS-C Sony A6700 and the full-frame Sony A7R V have pixels of almost the same size (3.76 and 3.76 µm), so they put almost the same pixels on the bird (1,037 and 1,039 across that robin); the Sony A7R V records a wider frame around it. A smaller sensor adds pixels on the bird only when its pixels are smaller (crop sensor vs full frame).

What a teleconverter adds

Sony lists its 1.4x and 2x teleconverters (SEL14TC and SEL20TC) for this lens. A teleconverter multiplies the focal length: 400 mm becomes 560 mm with the 1.4x and 800 mm with the 2x. On the Sony A6700, that takes an American robin at 25 m from 1,037 pixels to 1,461 or 2,108:

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 1,037 px on the Sony A6700, so you can crop until it fills 35% of the width and still have Audubon's 3,000 pixels; on the Sony A7 IV / A7 V (762 px), 25%. 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 Sony A6700, 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 1/12800; for the bald eagle, about 1/10000. 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 Sony A6700; 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: Sony A6700, Sony A7 IV, Sony A7 V, Sony A7R V, Sony A1 and Sony A1 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 FE 100-400 GM long enough for bird photography?

It depends on the bird and the distance. At 400 mm it puts about 1,037 pixels across an American robin at 25 m on the Sony A6700 and 514 at 50 m; a black-capped chickadee at 25 m gets 583. On the Sony A7 IV / A7 V the same robin at 25 m gets 762. Crop the Sony A6700 frame to iNaturalist's 2,048 pixels and the robin still fills 51% of the width.

Does the FE 100-400 GM take a teleconverter?

Yes. Sony lists its 1.4x and 2x teleconverters (SEL14TC and SEL20TC) for this lens. They take 400 mm to 560 mm and 800 mm. On the Sony A6700, an American robin at 25 m goes from 1,037 pixels at 400 mm to 1,461 with the 1.4x and 2,108 with the 2x.

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

Only when its pixels are smaller. In pixels per millimetre of sensor: Sony A6700 266, Sony A7 IV / A7 V 195, Sony A7R V 266, Sony A1 / A1 II 241. On the same lens, at the same distance, the Sony A7R V puts 1.36 times as many pixels on the bird as the Sony A7 IV / A7 V. An APS-C sensor doesn't change the focal length; it records a smaller frame around the same image of the bird. The APS-C Sony A6700 and the full-frame Sony A7R V have pixels of almost the same size (3.76 and 3.76 µm), so they put almost the same pixels on the bird (1,037 and 1,039 across that robin); the Sony A7R V records a wider frame around it.

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

On the Sony A6700, with an osprey at about 44 m, wings across two thirds of the frame, the wingtips blur about 18 pixels at 1/2000 and 9 at 1/4000 mid-stroke when you pan with the bird. Holding them within 3 pixels would take about 1/12800, 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.