Teleconverters and Depth of Field: What Actually Changes
Attach a 1.4x teleconverter to your 300mm f/4 and you get 420mm f/5.6. That part is straightforward. What confuses photographers is how depth of field changes, whether bokeh looks different, and why AF sometimes slows down. This article covers all three.
What a Teleconverter Actually Does
A teleconverter is a magnifying optical element placed between the lens and the camera body. It multiplies the focal length of the lens by its magnification factor. A 1.4x TC turns 300mm into 420mm. A 2x TC turns 300mm into 600mm.
The cost is aperture. Teleconverters reduce the amount of light reaching the sensor by the square of the magnification factor. A 1.4x TC costs exactly 1 stop. A 2x TC costs exactly 2 stops. This is a fundamental optical relationship, not a design flaw.
📐 Teleconverter Maths
Effective focal length = lens FL x TC factor
Effective aperture = lens aperture x TC factor
300mm f/4 + 1.4x TC = 420mm f/5.6
300mm f/4 + 2x TC = 600mm f/8
500mm f/5.6 + 1.4x TC = 700mm f/8
600mm f/4 + 1.4x TC = 840mm f/5.6
How DOF Changes with a Teleconverter
Attaching a 1.4x TC increases focal length and reduces aperture simultaneously. Both changes affect DOF in opposite directions:
- Longer focal length reduces DOF (shallower)
- Smaller aperture increases DOF (deeper)
The two effects do not cancel perfectly. The focal length increase dominates slightly, so the net result is a marginally shallower depth of field at the same shooting distance.
📐 DOF Comparison: 300mm f/4 vs 420mm f/5.6 at 20m
300mm f/4, subject at 20m: DOF approx. 5.8m
420mm f/5.6, subject at 20m: DOF approx. 3.0m
Same distance, same subject. The TC gives shallower DOF despite the smaller aperture because the focal length increase outweighs the aperture loss.
If you compare the TC combination to shooting from a closer position with the base lens to achieve the same framing, the DOF is essentially identical. The TC is optical shorthand for moving closer, and the DOF result reflects that.
The Equivalent Comparison
Say you want to fill the frame with a bird at 30m. You can use a 500mm lens at 30m, or a 300mm lens with a 1.4x TC (giving 420mm) and move closer to 20m to get the same framing. The DOF at the same framing will be nearly identical between these approaches, because the subject fills the same proportion of the sensor in both cases.
For equivalent framings, the DOF is roughly equivalent regardless of whether you achieve the magnification through a longer lens, a TC, or closer proximity. What the TC primarily changes is convenience and minimum focus distance, not the fundamental DOF rendering.
Bokeh Character with a TC
Even though total DOF depth is similar at equivalent framings, the character of the out-of-focus areas can shift with a TC attached. The TC introduces additional optical elements between lens and sensor. In some combinations this softens bokeh slightly. In others the effect is imperceptible.
High-quality OEM teleconverters on the lenses they are designed for (Canon RF 1.4x, Nikon Z TC-1.4x, Sony 1.4x TC) show minimal bokeh degradation. Third-party TCs and mismatched combinations can show more visible changes. If bokeh quality is critical, test your specific combination before a shoot.
Autofocus and the f/8 Limit
Many cameras limit autofocus performance when the effective aperture falls below f/8. This directly affects TC use:
| Lens + TC | Effective Aperture | AF Impact |
|---|---|---|
| 300mm f/4 + 1.4x | f/5.6 | Full AF on most bodies |
| 500mm f/5.6 + 1.4x | f/8 | Works but may be slower |
| 400mm f/5.6 + 2x | f/11 | Unreliable or disabled on many cameras |
| 600mm f/4 + 2x | f/8 | Functional on modern mirrorless bodies |
Modern mirrorless cameras handle f/8 AF much better than older DSLRs. Sony A9 series, Canon R3/R5, and Nikon Z9/Z8 maintain strong continuous AF at f/8 in good light. In dim conditions or with fast-moving subjects, the 2x TC at f/8 remains challenging on any body.
💡 The 1.4x Is Almost Always the Right TC
The 1.4x TC gives you 40% more reach at a 1-stop aperture cost with minimal quality degradation. The 2x TC doubles reach but costs 2 stops, often drops into f/8 AF territory, and shows more image quality reduction on all but the sharpest prime lenses. In most situations, a 1.4x TC used regularly produces better results than a 2x TC. Unless you genuinely need to double focal length and light is abundant, the 1.4x is the better tool.
Minimum Focus Distance with a TC
Teleconverters reduce the minimum focus distance of the host lens, so you can focus closer than normal. This is an often overlooked benefit. A 300mm lens that normally focuses to 1.4m can focus to around 1.0m with a 1.4x TC attached. Maximum magnification increases as a result, making TCs useful for close-focus shooting of insects and flowers with telephoto lenses.
| Lens | Normal MFD | With 1.4x TC | Result |
|---|---|---|---|
| 300mm f/4 | 1.4m | approx. 1.0m | Higher magnification at close range |
| 500mm f/5.6 | 3.0m | approx. 2.1m | More frame-filling shots of medium subjects |
| 100-400mm at 400mm | 0.98m | approx. 0.7m | Useful for larger insects and small birds |
When to Use a Teleconverter
A TC makes sense when you need more reach than your lens provides and cannot get physically closer, when you are shooting in adequate light where the aperture loss is manageable, and when a lighter kit matters more than the next focal length up.
A TC is the wrong choice when you are in low light and the aperture loss will push ISO too high, when you are tracking fast erratic subjects where AF at f/8 is unreliable, or when you could simply move closer and achieve the same framing without the optical penalty.
📐 Calculate DOF with Your TC CombinationQuick Reference: TC Effects Summary
| Effect | 1.4x TC | 2x TC |
|---|---|---|
| Focal length | 1.4x | 2x |
| Aperture loss | 1 stop | 2 stops |
| DOF at same distance | Shallower | Shallower |
| DOF at same framing | Similar | Similar |
| Minimum focus distance | Reduced | Reduced |
| AF at effective aperture | Usually fine | Often limited |
| Image quality impact | Minimal with OEM TC | Noticeable |
Final Thoughts
Teleconverters do not fundamentally change the DOF relationship of your lens at equivalent framings. They change the effective aperture, the minimum focus distance, and the AF behaviour. The 1.4x TC is one of the most cost-effective reach upgrades in photography, giving you 40% more focal length for a fraction of the cost of the next lens up. Understand what it costs and what it delivers, and it becomes a very useful part of the kit.