What makes a fast lens a fast lens?

[Candi lynn]

Lenses are really starting to confuse me! I understand the focal length of a lens and the maximum aperture of a lens, but when people talk about a fast lens what number are they talking about. Or is is even a number? What makes a fast lens a fast lens? And when a lens has USM, does that mean it is quieter? Sorry for so many dumb questions all at once.
Thanks
Candi

[blue_max]

In essence, lots and lots of glass!

A really big hole for the light to travel through. Simple as that. USM is the focussing mechanism and has nothing to do with it, although having it can make it quicker to focus.

Graham

[AjP]

aperture (bigger is better 1.4, 2.8...), quiality of glass, motor..... L
more light comoing through, faster lense see the point of focus the rest up to mecanical stuff

[Keiffer]

Normally a fast lense is 2.8 or below. A wide aperature allows for a "faster" shutter speed in turn allowing you to shoot in low light and also handhold instead of using a tripod.
USM is quieter but also faster focusing.
No such thing as a dumb question, That's what everyone is here for, the exchange of information and ideas.
Oh yeah, Welcome to the forum

[ddelallata]

Fast refers to the Aperture. ex (f/2.8 ) etc. I would say that fast is any number lower than f/2.8 .
the lower the f/number the faster the shutter speed. this allows the light to record the image on the sensore faster.

[ayotnoms]

It usually refers to lens performance in low light conditions
f/1.2 = Fast
f/5.6 = Slow...at least compared to the f/1.2

Allowing more light through the lens means faster shutter speeds while still getting an acceptable exposure.

...the Cliff Notes version.

[gasrocks]

Fast and Slow are relative terms. What was fast years ago is probably slow now. What is fast for an 8X10 camera is slow for a 35mm camera. F/8 is faster than F/11 but slower than F/5.6. I see people complain about their f/4 lens not being fast - sigh, you can tell they aren't very old. Not too long ago f/4 was fast. Get the fastest lenses you can afford - is the real answer.

[CorruptedPhotographer]

Originally Posted by Keiffer

Normally a fast lense is 2.8 or below. A wide aperature allows for a "faster" shutter speed in turn allowing you to shoot in low light and also handhold instead of using a tripod.
USM is quieter but also faster focusing.
No such thing as a dumb question, That's what everyone is here for, the exchange of information and ideas.
Oh yeah, Welcome to the forum

I also h ad this Q in mind, so an aperture of 2.8 or below is considered a fast lens, allowing for faster shutter speeds and hand held shooting. But when would you really need an aperture of 2.8 or below? Isnt the DOF going to be very minimal? Woulndt using an aperture of between f/4 and f/8 the best?

[Keiffer]

When your in low light your choices are limited. F4 or 8 would be great for outdoors, not indoors.

[ayotnoms]

Originally Posted by gasrocks

I see people complain about their f/4 lens not being fast - sigh, you can tell they aren't very old. Not too long ago f/4 was fast.

Huh?
The FD mount lens that came with my Canon-AE1 25 years ago was an F/1.8

I'm fairly certain that fast lenses in the sub-f/2.8 range have been around for long time and f/4 was as slow then as it is now--for some circumstances, of course.

discuss amongst yourselves....

[Candi lynn]

So the L means better quality of lens right? And if i was shooting a picture at let say an f-stop of 8 (outside) then the 2.8 lens would/not matter?

[ayotnoms]

Originally Posted by Candi lynn

So the L means better quality of lens right? And if i was shooting a picture at let say an f-stop of 8 (outside) then the 2.8 lens would/not matter?

It depends.

If I'm using a long focal length lens to photograph a baseball pitcher from a good distance, the F/2.8 may allow me the shutter speed I need to freeze his/her arm motion; and at that distance, the f/2.8 will still allow for an acceptable Depth of Field.

Someone here at in the POTN forum used the phrase "The Trinity" to describe the components of a good photograph; Focal Length, Aperture, and Shutter Speed.

Check out the Stickies at the top of this forum. There are tons of reference/resource sites where you can learn of other factors such DoF, FOVCF, etc.

Cheers

[blue_max]

Originally Posted by Candi lynn

So the L means better quality of lens right? And if i was shooting a picture at let say an f-stop of 8 (outside) then the 2.8 lens would/not matter?

The L moniker refers to the professional range. It has met certain quality standards that generally exceed those of the consumer range. A faster lens will give you the option of shooting at a faster speed. It is not compulsary! If you always shot at f8 for example, there might not be such an advantage in using an L lens. Other factors such as saturation and detail may still make the image better with an L though.

Graham

[SkipD]

Originally Posted by Candi lynn

So the L means better quality of lens right? And if i was shooting a picture at let say an f-stop of 8 (outside) then the 2.8 lens would/not matter?

You are correct in understanding that if you are shooting in bright light where f8 is your normal setting there would be little advantage in a "fast" lens. BUT..... There are other characteristics that the "fast" lenses often have. Many are of the "professional" caliber, such as Canon's L series lenses, and that brings with it a number of things.

Typically the sharpness, contrast, and color of images produced with the professional grade lenses will be better than those made with a cheapie lens. The physical build and handling properties are usually better as well. Are these factors worth the (usually) significant price difference? That's something you will have to decide. There are also many middle-of-the-road lenses that are much better than the cheapies and not quite as good - when considering all the variables - as the professional grade lenses.

Bottom line - huge maximum aperture alone is not the only measure of a great lens. There are many variables to consider. Keep on reading here and elsewhere and keep on asking the questions. That's the only way to be able to make an intelligent decision that you'll be happy with when it comes to buying glass for your cameras.

[Candi lynn]

Thanks for all the help. I'm sure i will have more questions furture on down the road.
candi