Teleconverters: Does using them stop down apertures at all?
If I were to use a 1.4x teleconverter on a Sigma 70 - 200 2.8 lens, would the usage of the TC stop the lens down to 3.x or 4? Please advise, thank you...

Comments (6)

Yes. A 200mm F2.8 becomes 280mm F4. An F4 lens becomes F5.6, etc. Basically multiply the magnification of the converter by the lens F stop. A 1.4 converter is one full stop, so that halves the amount of light getting to the sensor. A 2.0 converter is 2 full stops so it quarters the light getting to the sensor.Some cool cats that can use your help

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Comment #1

A 1.4x teleconverter behind the lens costs you a full stop; f/2.8 is reduced to f/4.0..


Comment #2

When the teleconverter is placed between the lens and camera body, it reduces the effective aperture.A 1.4x converter loses one stop, so your f/2.8 lens becomes f/4..

Similarly, a 2x converter loses two stops, making the f/2.8 lens effectively f/5.6.Regards,Peter..

Comment #3

F/2.8 X 1.4 = f/3.92.

F/8 X 1.4 = f/11.2.

F/22 X 1.4 = f/30.8.

Here is image made with lens at f/8 + 2X teleconverter..

Take a look at exif - shows f/16. It was combined by camera from f/8 lens setting and 2X teleconverter

Comment #4

The aperture ratio is the ratio of focal length to effective aperture diameter. When you add a 1.4x TC, it leaves the size (diameter) of the aperture the same, but increases the focal length by 1.4x, so increases the ratio by factor 1.4x..

In practice, aperture ratio measures how bright an image one gets on the sensor (or film) from a subject of a given brightness. Adding a TC spreads the the light over a larger area, so gives a larger but dimmer image, and the higher aperture ratio indicates this dimming...

Comment #5

Stan_P wrote:.

F/2.8 X 1.4 = f/3.92.

Nope, it f/4. 2.8 and 1.4 are both rounded off..

2.8 is really 2^3/2, and 1.4 is really 2^1/2. To multiply just add the exponents....

2^3/2 * 2^1/2 = 2^2 = 4..

F/8 X 1.4 = f/11.2.

Same thing, 1.4 = 2^1/2. Result is typically expressed as f/11..

F/22 X 1.4 = f/30.8.

Ditto, f/32..

Seen in a fortune cookie:Fear is the darkroom where negatives are developed..

Comment #6

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