What is a "fast" lens?

[bascom]

Here is what the Fuji S9000 review says:

"The lens isn't quite as "fast" (in terms of aperture) as some of the competition, though: the maximum aperture is F2.8 - F4.9. The Canon PowerShot S2 (F2.7-3.5), Panasonic Lumix DMC-FZ30 (F2.8-3.7), and Sony Cyber-shot DSC-H1 (F2.8-3.7) all have faster lenses. What this means in real world is that near the telephoto end of the lens those other lenses let in more light, which allows for faster shutter speeds, and thus less camera shake."

I thought a fast lens meant the aperture was big, which would be a small f-number. Here are the smallest f-stops for these cameras:
S9000 - 2.8
S2 - 2.7
FZ30 - 2.8
H1 - 2.8

So to me it looks like the S2 has the fastest lens and the rest are identical and close to the S2. Why does the review say the S9000 is slower than the other three?

[GNMink]

F2.8 - F4.9

Its saying that the SD900 is slower on the long end (telephoto) of the lens with a max apature of F4.9

[David Metsky]

The lens is only F2.8 at the shortest focal length. It may drop off quickly, and according to the numbers, by the time it reaches full zoom is it much slower than the competition.

If I understand it correctly, the two numbers given just represent the maximum apature at the two ends of the zoom. Both numbers are important, as well as the curve in between, which can't be represented by just 2 numbers.

-dave-

[bascom]

So the big f-number is the one for tele? I thought it was the small one. Comparing the max f-stops:
S9000 - 4.9
S2 - 3.5
FZ30 - 3.7
H1 - 3.7

The S9000 (not SD900) is much bigger than the others. So that means it's a relatively small opening which doesn't allow speeds as fast as the other cameras do? It that right?

[Nesster]

f/4.9 is just about 1 full stop slower than f/3.5, which means -all things being equal - a S2 exposure of 1/60sec will be 1/30 sec on the S9000.

[cdifoto]

This, my friend, is a fast lens:

[Riley]

maybe its better to think of the amount of light the lens design allows through, lousy design or narrow glass has an impact on this. notice how the truly fast canon long lenses for pro sports have enourmous front optics. the more glass elements a lens has the slower it will be. every stop gained/lost is double/half the light

longer lenses tend to be slower
wider lenses tend to be faster
fixed focals are the fastest.

and then this way
longer lenses can often use more depth of field (DoF) as the tele compresses
wider lenses already have more depth DoF, so a wider aperture is handy for separation or attempting to limit DoF
fixed focals are usually blessed with both high and low F stops, but ofcourse they dont zoom, less flexibility

to my mind F:2.8 like the Canon is acceptably fast for a zoom, which at the long end of 430mm is easier to arrange on a small sensor. the long zoom range means the light value gets less at the long end, F:2.8 wide, and F:3.7 long
F:2 Leica Vario Summicron is a fast zoom, but its limited to 28-90mm, so likewise shorter zooms tend to be faster

in the endgame, its generally easier to provide tighter apertures like F:22/32 etc, even though we dont see that much now, than to make a lens designed to be faster which has much tougher compromises. hence the cheaper zooms are around F:4, thats a long way from F:2, and the 2 stops difference cost good money to recover

Riley

[RichNY]

This, my friend, is a fast lens:

Now that's funny!

[Rhys]

Two things to avoid in life: slow film and fast lenses :p

Seriously, fast applies to any lens that has a very low f-number compared to the competition. For example, my Tamron 28-75 is f2.8 throughout the range. That's pretty fast. The Sigma 70-200 is f2.8 throughout the range which is pretty fast. The Canon f0.95 50mm lens is very fast. The Sigma 50mm f2.8 is slow by comparison.

[Vich]

Given X amount of light, how fast do you set the shutter speed to get correct exposure?

Correct exposure is a dance between Shutter Speed (faster allowing less light in) and Aperture (smaller also allowing less light in).

Therefore; a lens that allows for a bigger aperture (denoted by having a smaller number), by definition allows for a faster aperture.

This is where my ignorance begins...

Can someone help me on this? On the modern DSLR/SLR camera, where is the physical aperture? In the lens or in the body? Does it pre-position itself to the aperture size or is that something that happens during the photo?

In the original (pre mid 1960's) SLR, I believe the curtian was a circular blade-system that opened to a specified size. I seem to recall believing it was in the lens (it would have to), and all that moved in the camera was a flipping mirror. Then Nikon came out with today's curtian system (2 curtians) in the camera body so the lens could pre-set the aperture size. That way if you wanted shutter speed greater than 1/500ths (or 1/60th on slower consumer model cameras) it would just start closing the 2nd curtian before the first curtian was done (creating a slit). This emulates a "faster shutter", but it still moves at 1/250th (or whatever constant speed the camera has) A slower shutter speed would be emulated by delaying the 2nd curtian..

However; where is the aperture (lens or body) and did it / does it open during the photo, or does it just pre-set itself? If in the body, is it behind the mirror or in front of it?

I've just assumed it must be in the lens and must set itself to it's pre-set aperture width before the shutter started moving, then releasing itself after it's done. Now I'm realizing I'm completly ignorant of actual fact in this matter.

[timmciglobal]

The apature is in the lens, aka, the rings which stop down the lens.

A given lens has a "maximum" apature which is always kept at until time of exposure or pressing the DOF preview button.

The kit lens for example is 18-55 F3.5>5.6

That means @ 18 mm the lens is at its most open F3.5. At 55 it's F5.6

So let's take the 55 mm end and compare it to a 50 F1.4 lens

Given the same lighting if a 50 1.8 is shot at an apature value of F1.4 and correctly exposed the shutter is open 1/100th of a second. This would be enough to stop most slow motion objects. Now if the kit lens was used since it's apature is F5.6 the exposure would be 1/6th of a second.

Why? Because starting @ f1.4 at 100th you would need 1/50th at F2.0, 1/25th at F2.8, 1/12th at F4 and 1/6th at F5.6 because every full apature stop is half the original light value of the previous stop.

Tim

[Vich]

The apature is in the lens, aka, the rings which stop down the lens.

A given lens has a "maximum" apature which is always kept at until time of exposure or pressing the DOF preview button.

The kit lens for example is 18-55 F3.5>5.6

That means @ 18 mm the lens is at its most open F3.5. At 55 it's F5.6

So let's take the 55 mm end and compare it to a 50 F1.4 lens

Given the same lighting if a 50 1.8 is shot at an apature value of F1.4 and correctly exposed the shutter is open 1/100th of a second. This would be enough to stop most slow motion objects. Now if the kit lens was used since it's apature is F5.6 the exposure would be 1/6th of a second.

Why? Because starting @ f1.4 at 100th you would need 1/50th at F2.0, 1/25th at F2.8, 1/12th at F4 and 1/6th at F5.6 because every full apature stop is half the original light value of the previous stop.

Tim
So then, the lens has the circular-bladed eye, it's normally wide open, then narrows as specified by "f-stop" setting during the shot. Correct so far?

I assume it completes it's movement prior to the camera ever beginning it's shutter movement. It could be moving during the mirrior flip-up.

Then, the 1st shutter curtian moves. Etc.

Thanks!

Funny about surfing these forums. Here I thought I had a good handle on things, then some fundamental basic like this shows up and I realize my ego has gotten in the way of education.

I can still hear the echos of my 3rd grade teacher "Hey Vich, back to the 2nd grade with you"! lol (oh wait, that was for being the class cut-up - for that, we can get universally praised until the day we're banned hehe)

[timmciglobal]

Yes, the apature stops down before the curtain opens.

Tim

[BonjiB]

Fast refers to the shutter speeds that are possible with the aperature of the lens. If a lens has a low f number then the shutter speed can be increased thus the lens is considered fast. The reason the fuji isn't considered fast is because that 2.8-4.9 means that when the lens is zoomed all the way in... the f number is only 4.9 which isn't "fast" because it doesn't let a lot of light in meaning the shutter speed or iso has to be bumped to compensate.... Does that help?

[bascom]

Yes thanks for the replies...and the cartoon.

[Razr]

With "pro" glass, "fast" is usually a lens with f/2.8 (or faster) glass.
With "pro" zooms, that means f/2.8 throughout the entire zoom range.

Other "Fast" lenses are f/2 or faster prime lenses. Almost without exception, a variable aperture lens is not considered "fast", no matter what the wide end aperture is.

There is a "catch" however. F/3.5, f/4, f/4.5, f/5.6 constant aperture lenses can also be considered "fast" if:
1. They have constant apertures and
2. They are considered "pro" lenses and
3. These constant aperture "pro" lenses usually start at 300mm and go up.

So a 600mm f/5.6 is also considered "fast".