What makes glass "Fast"

[Vautrin]

Hi,

So I keep seeing people talking about slow glass and fast glass, and fast glass being best.

What makes glass slow or fast? Should I throw my camera lens and see how long it takes across the room to determine whether it's a speed demon or a slow poke? (j/k)

What are the advantages and disadvantages to using either? Is fast glass better quality and I should prefer fast glass in all situations if my budget can afford it? Or for still life is slow glass better?

-Dan

 

[RyanLilly]

"fast" refers to lenses that have a large Maximum aperture, allowing more light to enter, thus affording the use of faster shutter speeds.

f/2.8 is considered fast for a zoom, and many primes are faster like 1.8, 1.4, etc.

 

[RyanLilly]

Advantages of fast glass- Faster shutter speeds possible, better low light shooting, ability to use lower(slower) ISO, because the glass is faster, shallow DOF with wide apertures, Generally faster glass is designed with serious photographers in mind so, the optics, build and the general quality are better than consumer lenses.

Generally fast glass has overall good quality, but that does not mean that slower lenses are poor quality, there are some very good, and even inexpensive slower lenses available.

 

[Steph]

"fast" refers to lenses that have a large minimum aperture.

Surely you meant large maximum aperture.

 

[Steph]

What are the advantages and disadvantages to using either? I

Some disadvantages of fast lenses: they cost more and are often bulkier and heavier.

 

[Mike30D]

Some disadvantages of fast lenses: they cost more and are often bulkier and heavier.

The zoom lenses can be (heavier and bulkier), but not the primes. Besides, I'll take the bulkiness of a lens if it gets me the shot I need. Fast lenses are a must if you are ever gonna shoot weddings.

 

[Vautrin]

What's a prime?

 

[usayit]

prime = single focal length lens.

 

[davebmck]

Prime means it has a fixed focal length. In other words, not a zoom.

 

[Vautrin]

What's the point of that?

 

[davebmck]

Primes are generally thought to have better image quality than zooms. Modern zooms are getting much better though, so there is not so much difference these days.

 

[Joves]

What's the point of that?

You actually have to move around to get the shot you want as well. It is good old fashioned photography.

 

[usayit]

Optimized for single focal length.
Fast aperture
Smaller size
Image quality
Focus on a single focal length without the distraction of many focal lengths

 

[Steph]

The zoom lenses can be (heavier and bulkier), but not the primes. Besides, I'll take the bulkiness of a lens if it gets me the shot I need. Fast lenses are a must if you are ever gonna shoot weddings.

As I said in my post, fast lenses are often bulkier and heavier than slower lenses (I did not say always as they are probably some exceptions but there must be few and I have yet to find one). And that is also true for prime lenses. Some examples from Canon (weight and maximum diameter x length of lenses):

EF 28mm f2.8: 185g, 67.4x42.5mm
EF 28mm f1.8: 310 g, 73.6x55.6mm

EF 85mm f1.8: 425g, 75.0x71.5mm
EF 85mm f1.2L II: 1025g, 91.5x84.0mm

EF 300mm f4.0L IS: 1190g, 90.0x221mm
EF 300mm f2.8L IS: 2550g, 128x252mm

So clearly, from these examples (and from many other examples from Canon, Nikon, Pentax, Olumpus...), the faster the lens, the bulkier and the heavier it is.

 

[usayit]

Oh thats usually a given but I think Mike was referring to the bulkiness of a fast zoom versus prime.