How do I know how fast the lens speed is?

[sdm19]

I see alot of articles talking about fast glass. What determines the speed of the glass? Thanks for the help!

 

[PJL]

The f-stop. The smaller the number, the wider the aperture, which in turn means the "faster" the lens. "Faster" means that you can use faster shutter speeds to get correct exposure in low light vs. a lens with a higher f-stop.

For example, the 50 f/1.8 is "faster" than the 70-200 f/2.8, which is "faster" than the 24-105 f/4.

 

[sdm19]

Thanks so much!

 

[Big Mike]

That's right.

This is a bit of a weird one because it's actually referring to size, not speed. Lenses that we call 'fast' are the ones with a large maximum aperture. As mentioned above, F1.8 is 'faster' than F2.8, which is 'faster' than F4 and so on.

We call it 'faster' because a larger aperture lets in more light, which in turn allows us to use a faster (shorter) shutter speed.

Another characteristic of lenses is the Auto Focus speed...how fast the lens can auto focus. Some lenses are faster than others, but the camera also plays a part in this.
Just don't confuse the two types of 'faster'.

 

[KmH]

For the sake of accuracy, the f-stop is a fraction of the lens focal length, so f/2 is a bigger number (and larger lens opening) than f/4.

Also, the f-stop is describing the area of the lens opening, not the diameter of the opening, so an opening that is twice as larger or half as small is a function of the square root of 2 (√2 - 1.41421), which is why the full stop steps (a doubling or a halving) of lens aperature are:

• f/1
• f/1.4
• f/2
• f.2.8
• f/4
• f/5.6
• f/8
• f/11
• f/16
• f/22

Most dSLR's are set to a default that alters lens aperture in 1/3 stop steps:

• f/1
• f/1.1
• f/1.2
• f/1.4
• f/1.6
• f/1.8
• f/2
• f/2.2
• f/2.5
• f/2.8
• f/3.2
• f/3.5
• f/4
• etc.

 

[kundalini]

I don't think of 'faster' lenses as letting me shoot faster shutter speeds, but more about how much faster it will let in the light.

Poor analogy (using lens max aperture as a guide):
Garden hose - f/4
Firemans hose - f/2.8
Water mains - f/1.4

Which one fills a swimming pool 'faster'?

 

[Big Mike]

I don't think of 'faster' lenses as letting me shoot faster shutter speeds, but more about how much faster it will let in the light.

Light always travels at the same speed though. :er:

Poor analogy (using lens max aperture as a guide):
Garden hose - f/4
Firemans hose - f/2.8
Water mains - f/1.4

Good analogy.

 

[o hey tyler]

Think of aperture like the battle of Thermopylae.

The sensor counts as the 300 Spartans, and the Light acts as the Persians. The bottle neck is your aperture value, and the shutter speed is the time that it takes for your army to get demolished by Xerxes. It's really quite simple actually.

ISO has direct control over your phalanx and your ability to take photos while the sun is blocked out by arrows (so you'll take photos in the shade).

 

[KmH]

Light always travels at the same speed though. :er:

The speed light can travel, depends upon the medium it is travelling through.

Light doesn't travel as fast through glass, or air as it does in a vacuum. The ratio between the speed of light in a vacuum and the speed at which light travels in a material is called the refractive index of the material.

The refractive index of visible light wavelengths in glass is about 1.5, or the speed of light in a vacuum divided by 1.5.

 

[RockstarPhotography]

I don't think of 'faster' lenses as letting me shoot faster shutter speeds, but more about how much faster it will let in the light.

Light always travels at the same speed though. :er:

Poor analogy (using lens max aperture as a guide):
Garden hose - f/4
Firemans hose - f/2.8
Water mains - f/1.4

Good analogy.

speed of light is 186,282.397 miles per second......don't ask my why I know this.