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

Native focal length printed on the lens, e.g. 200 for a 200mm

Lens Aperture

Lens maximum (widest) f-number, e.g. 2.8 for an f/2.8 lens

Teleconverter1.4× = 1 stop loss, 2× = 2 stops loss

How We Calculate This

A teleconverter (TC) sits between the camera body and lens. It magnifies the image, multiplying both focal length and f-number by the same factor while the physical aperture diameter stays the same:

Effective Focal Length

Effective FL = lens FL × TC multiplier

Effective Aperture

Effective aperture = lens aperture × TC multiplier

Light Loss

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

Because the same light is spread over a larger image, the loss scales with the square of the TC factor: 1.4× TC = 1 stop loss, 1.7× TC = 1.5 stops, 2× TC = 2 stops.

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.