Teleconverters and Depth of Field: What Actually Changes
Attach a 1.4x teleconverter to a 300 mm f/4 and you get a 420 mm f/5.6. That part is easy. What confuses people is what happens to depth of field, whether the background blur changes, and why autofocus sometimes slows down. This page covers all three.
What a teleconverter does
A teleconverter is an optical unit between lens and camera that magnifies the image. It multiplies the focal length by its factor, and the f-number by the same factor. A 1.4x turns a 300 mm f/4 into 420 mm f/5.6, losing one stop. A 2x makes it 600 mm f/8, losing two. A 500 mm f/5.6 becomes 700 mm f/8 with a 1.4x, and a 600 mm f/4 becomes 840 mm f/5.6.
Depth of field at the same distance
Depth of field is proportional to the f-number and inversely proportional to the focal length squared. With a teleconverter, the f-number rises by the factor and the focal length squared rises by the factor squared, so the zone shrinks by the factor. At 20 m, a 300 mm f/4 has a zone about 1.0 m deep. With a 1.4x it is 420 mm f/5.6 and the zone is about 0.72 m, and with a 2x it is 600 mm f/8 and about 0.50 m. So at the same distance, the teleconverter makes depth of field shallower, by one over the factor.
At the same framing
You rarely compare at the same distance. You compare at the same framing, which means standing further back with the longer combination. Then the answer changes. A teleconverter combination behaves exactly like a native lens of the same focal length and aperture. Take a 500 mm f/5.6 at 30 m and a 300 mm f/4 with a 1.4x (420 mm f/5.6) at 25.2 m. The framing is the same, both are f/5.6, and the zone in focus is 1.14 m in both cases. Against the bare 300 mm f/4, the combination at the same framing is 1.4 times deeper, since its f-number is 1.4 times higher. So the converter is not a shortcut for moving closer, because moving closer with the bare lens gives a shallower zone.
Background and bokeh
Because the combination acts like a longer lens, background blur grows with it, as long as the f-number allows. The character of the blur can change a little, since you have added optical elements. With a manufacturer's own converters on the lenses they were designed for, such as Canon's RF extenders, Nikon's Z teleconverters or Sony's, the change is usually small. Third-party converters and mismatched combinations can be more visible, so test yours before a shoot where it matters.
Autofocus and the f/8 limit
Many cameras limit autofocus when the effective aperture is slower than a certain value, often f/8 on recent bodies and f/5.6 on older ones. This decides what you can use:
| Lens and converter | Effective aperture | Autofocus |
|---|---|---|
| 300 mm f/4 + 1.4x | f/5.6 | Full on most bodies |
| 500 mm f/5.6 + 1.4x | f/8 | Works on many recent bodies, may be slower |
| 400 mm f/5.6 + 2x | f/11 | Limited or unavailable on many cameras |
| 600 mm f/4 + 2x | f/8 | Fine on modern mirrorless |
Recent flagship mirrorless cameras, such as the Sony A9 line, Canon R3 and R5, and Nikon Z8 and Z9, handle f/8 well in decent light. In dim conditions or with fast, erratic subjects, a 2x at f/8 or f/11 remains hard on any body. The 1.4x gives 40 percent more reach for one stop with little loss of quality, and the 2x doubles reach for two stops and usually costs more in image quality, so in most situations the 1.4x is the better tool. Check compatibility, since not every converter fits every lens.
Close focus
Teleconverters are often said to shorten the minimum focus distance. They do not, since the lens still focuses no closer than before. What they do is multiply the magnification you get at that closest distance. A lens whose maximum magnification is 0.25x becomes about 0.35x with a 1.4x and 0.5x with a 2x. That makes them useful for insects and flowers with a telephoto, even though the closest focusing distance stays the same. See minimum focus distance for how it is measured.
Converter or crop?
On a high-resolution camera, a 1.4x converter gives about what you would get by cropping a 45 MP file to 23 MP, but it puts more pixels on the subject, at a cost of one stop of light. If your lens and light are fine, a converter is worth it. If the light is poor, or autofocus slows, cropping may do as well. For the sums on how much reach a bird needs, see the bird lens calculator post.
When to use one
A converter makes sense when you need more reach and cannot get closer, when the light leaves room for the aperture loss, and when a lighter kit matters more than the next lens up. It is the wrong choice in low light, with erratic fast subjects when autofocus at f/8 is unreliable, or when you could simply move closer.
To work out the depth of field for your combination, enter the combined focal length and aperture in the calculator.
Related reading: Full Frame vs Crop Sensor: The Real DOF Differences, Mirrorless vs DSLR: Does It Affect DOF?, Aperture vs Distance: What Affects DOF More?.