Difference between lenses

[Funboy]

Hi guys,

I am confused about F2 lenses. From what I understand, zoom lenses have different focal lengths (like the 18-50mm right?), and prime lenses have a fixed focal length (like the 30mm f1.4), so then what are these F2 lenses (like the sigma 65mm F2 DG DN). Are prime lenses not considered autofocus lenses?

I tried searching before posting, but I think the question is so basic I couldnt really find anything

 

[cgw]

Hi guys,

I am confused about F2 lenses. From what I understand, zoom lenses have different focal lengths (like the 18-50mm right?), and prime lenses have a fixed focal length (like the 30mm f1.4), so then what are these F2 lenses (like the sigma 65mm F2 DG DN). Are prime lenses not considered autofocus lenses?

I tried searching before posting, but I think the question is so basic I couldnt really find anything

Nonsense. Google aperture, f/stop, how photo lenses work...

 

[petrochemist]

f-numbers are calculated using the focal length divided by the diameter of the iris opening. As this ratio changes more light gets through to make a brighter image.
Yes, zooms have variable focal length, while primes are fixed. This is a completely different thing than the f-numbers.
Autofocus is again another thing, both zooms & primes can be either autofocus or manual focus, or even non focusing...

 

[480sparky]

Autofocus has nothing to do with a len's focal length.

 

[Rickbb]

The F2 just stands for the maximum aperture for that particular lens. For instance, a 105mm f4, or 50mm f 1.4, etc. Some manufactures, like Sigma, include the F as part of their model of lens.

 

[wfooshee]

The aperture stated on a lens is that lens's maximum available aperture, for f: setting. It's not the only available aperture, simply the largest the lens is capable of. A fixed-focal-length lens (i.e. a prime) will only have one f-number specified, which as stated, is the maximum the lens is capable of. The lower the number, the "faster" the lens, meaning it can let more light into the camera in a given amount of time: an f:1.4 lens can let in more light than an f:4 lens, so it can expose the frame in less time, i.e. faster.

Zoom lenses might have 2 f-numbers specified, and that would be because of the way the lens is built. The physical barrel of the lens could accommodate a certain size iris, but that iris might be a different fraction of the lens's focal length. A 70-300 f:3.5-5.6, for example, could open up to f:3.5 at 70mm zoom but only be capable of f5.6 at 300mm zoom.

 

[Funboy]

Nonsense. Google aperture, f/stop, how photo lenses

The F2 just stands for the maximum aperture for that particular lens. For instance, a 105mm f4, or 50mm f 1.4, etc. Some manufactures, like Sigma, include the F as part of their model of lens.

Nonsense. Google aperture, f/stop, how photo lenses work...

Nonsense. Google aperture, f/stop, how photo lenses work...

f-numbers are calculated using the focal length divided by the diameter of the iris opening. As this ratio changes more light gets through to make a brighter image.
Yes, zooms have variable focal length, while primes are fixed. This is a completely different thing than the f-numbers.
Autofocus is again another thing, both zooms & primes can be either autofocus or manual focus, or even non focusing...

The aperture stated on a lens is that lens's maximum available aperture, for f: setting. It's not the only available aperture, simply the largest the lens is capable of. A fixed-focal-length lens (i.e. a prime) will only have one f-number specified, which as stated, is the maximum the lens is capable of. The lower the number, the "faster" the lens, meaning it can let more light into the camera in a given amount of time: an f:1.4 lens can let in more light than an f:4 lens, so it can expose the frame in less time, i.e. faster.

Zoom lenses might have 2 f-numbers specified, and that would be because of the way the lens is built. The physical barrel of the lens could accommodate a certain size iris, but that iris might be a different fraction of the lens's focal length. A 70-300 f:3.5-5.6, for example, could open up to f:3.5 at 70mm zoom but only be capable of f5.6 at 300mm zoom.

Thank you for all the nice responses. I was just confused by a prime lens that had a manual aperture ring on the outside.

But since I’m still new to all this, your responses really helped solidify my understanding of f numbers, thank you.

 

[phkc070408]

To farther elaborate on what was stated above, the Lowest F-Stop on a Zoom Lens, when in full Zoom Mode is a higher number / smaller opening. This is because the angel of spread from the sensor going through the iris and out to the subject is narrower when fully zoomed. While the Iris might be able to fully open, any light coming in from outside the areas in black won't hit the sensor, so it's moot. See my highly professional art work for an example of the difference in spread.

Also, as I stated above, Higher F Stop, is a Higher Number with a Smaller Opening.