Prime versus Zoom Aperture

[Anteaus]

I know that a wider aperture generally means a faster lens. If a prime lens and a zoom lens are set to the same aperture setting, is the prime lens faster than the zoom by virtue of simpler optics or are they the same.

i.e. If both are set at f/8, is my 35mm 1.8G faster than my 18-105mm zoom adjusted to ~35mm.

I'm just curious as to whether performance is comparable in low light situations.

 

[RobDickinson]

Theshould be the same. i.e. zoom and prime both at f4

But they likely wont be.

Theres another rating used more in the movie industry of T value, this measrues how much real light gets through the lens. A zoom tends to have a higher ( darker) t value than a prime due to more optical elements.

Also Remember that a prime at f1.4 will vignette a lot , and by f2.8 much less, whereas a zoom may vignette more at 2.8 (max aperture).

 

[finbarqs]

f/8 is not faster han f/8. It will be the same "speed". When people are talking about speed, they're referencing to shutter speed. So exposure wise, f/8 is f/8.

The prime, however is able to open up to f/1.8, thus allowing you to hit that faster shutter, while the 18-105 can only allow you to go up to 3.5 @ 18mm.

So the f/1.8 lens is capable of faster shutters, hence, it's a faster lens

 

[RobDickinson]

f/8 is not faster han f/8. It will be the same "speed". When people are talking about speed, they're referencing to shutter speed. So exposure wise, f/8 is f/8.

No its not. In simple consumer terms its all you really need but its quite possible your fast prime at f8 is 1/3rd of a stop faster than your zoom at f8.

 

[JohnnyRebel]

F-stops are purely geometrical, the ratio of aperture to focal length, regardless of actual light transmitted. Since all lenses absorb some portion of the light passing through them (particularly zoom lenses containing many elements), f-numbers do not accurately correlate with light transmitted. F-numbers corrected to measure light transmission rather than aperture ratio, called T-stops (for Transmission-stops), are sometimes used instead of f-stops for determining exposure.[5] A real lens set to a particular T-stop will, by definition, transmit the same amount of light as an ideal lens with 100% transmission at the corresponding f-stop.

- Wikipedia