How do teleconverters affect aperture?

A 1.4x teleconverter loses 1 stop of light (aperture multiplied by 1.4). A 2x teleconverter loses 2 stops (aperture multiplied by 2). A 200mm f/2.8 becomes 280mm f/4 with a 1.4x TC, or 400mm f/5.6 with a 2x TC. This affects both exposure and autofocus performance.

Will autofocus still work with a teleconverter?

Most cameras require f/5.6 or faster for all AF points to work, and f/8 for the centre point. A 2x TC on an f/4 lens gives f/8 — only centre point AF on most cameras. Modern mirrorless cameras often autofocus at f/11 or even f/22, making TCs more practical.

Do teleconverters reduce image quality?

Yes, teleconverters add optical elements that can reduce sharpness, especially at the edges and at wide apertures. Quality varies greatly — first-party TCs designed for specific lenses perform best. Stopping down 1-2 stops from maximum helps. A 1.4x TC typically loses less sharpness than a 2x TC.

When should I use a TC vs cropping?

A TC increases reach with maintained resolution but loses aperture and some sharpness. Cropping maintains aperture but loses resolution. For high-megapixel cameras (45MP+), heavy cropping can match TC results. For lower resolution cameras, a quality TC often produces better results than cropping.

PhotoCalcs

Teleconverter Calculator — Effective Focal Length & Aperture

Calculate effective focal length, aperture loss and autofocus impact when using teleconverters.

Lens Focal Length

Base lens focal length

Lens Aperture

Lens maximum aperture

Teleconverter

TC multiplier

How We Calculate This

Teleconverters multiply both focal length and f-number:

Effective Focal Length

Effective FL = lens FL x TC multiplier

Effective Aperture

Effective aperture = lens aperture x TC multiplier

Light Loss

Loss (stops) = 2 x log2(TC multiplier)

1.4x TC = 1 stop loss. 2x TC = 2 stops loss.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard optical and photographic formulas. Results may vary with specific equipment.