Hey guys ok really numpty question..?

[FoG]

SLow Lens Fast lens ..... what on earth is a slow lens compared to the fast lens ? I am trying to get a grip of a video tutorial and keep coming up with this reference I guess from what they say it has to do with Aperture through the zoom range but what do they mean exactly?

[kencaleno]

A so-called fast lens has a large maximum aperture and a constant aperture throughout the zoom range, like 70-200 f 2.8; as opposed to a slower lens, where the maximum aperture reduces in size as you zoom to full range for example: 70-300 f3.5-f5.6 ( f3.5 at 70mm reducing to f5.6 at 300mm)

How zoom lenses can affect metering in manual shooting mode:

There are two distinct types of zoom lenses
a) Constant aperture zooms, that maintain a fixed aperture when zooming
b) variable aperture zooms, where the aperture changes as you zoom

Therefore metering with a type b) zoom lens, is trickier than a type a) where the aperture remains constant, when in manual mode
So when you zoom away from the original metered scene, you will need to change the settings, because the aperture will have changed while zooming
So, with an 18-55 zoom lens, which aperture varies from F3.5-F5.6 as you zoom, If you get a meter reading at the 55mm (F5.6)setting, then zoom back to 18mm, your maximum aperture will have changed to F3.5 ,and those two stops difference in exposure will make a noticeable effect on the final image.

Regards, Ken

[FoG]

Originally Posted by kencaleno

A so-called fast lens has a large maximum aperture and a constant aperture throughout the zoom range, like 70-200 f 2.8; as opposed to a slower lens, where the maximum aperture reduces in size as you zoom to full range for example: 70-300 f3.5-f5.6 ( f3.5 at 70mm reducing to f5.6 at 300mm)

How zoom lenses can affect metering in manual shooting mode:

There are two distinct types of zoom lenses
a) Constant aperture zooms, that maintain a fixed aperture when zooming
b) variable aperture zooms, where the aperture changes as you zoom

Therefore metering with a type b) zoom lens, is trickier than a type a) where the aperture remains constant, when in manual mode
So when you zoom away from the original metered scene, you will need to change the settings, because the aperture will have changed while zooming
So, with an 18-55 zoom lens, which aperture varies from F3.5-F5.6 as you zoom, If you get a meter reading at the 55mm (F5.6)setting, then zoom back to 18mm, your maximum aperture will have changed to F3.5 ,and those two stops difference in exposure will make a noticeable effect on the final image.

Regards, Ken

See I just knew the font of all knowledge here would make me know what it was in seconds!!

Thanks Ken

[Mr Guy]

It's also important to note the context. All constant aperture zoom lenses are not necessarily fast. How fast a lens is considered to be is also how wide the widest aperture is. The breaking point between fast and slow is more a matter of opinion than well defined nomenclature, but is generally considered to be right around f/2.8. Fast zoom lenses tend to top at f/2.8 unless they become very very large and very very expensive, while fast prime lenses are often in the 1.X category, with a very small number of f/1.0 lenses out there, some f/1.2s, a reasonable selection of f/1.4s, and quite a variety of f/1.8s.

[Preacherman]

May I just say thanks to FoG for asking the question, and to kecaleno and Mr Guy for very readable answers. Posts like this are great for me. God bless for now. PAUL

[FoG]

Originally Posted by Preacherman

May I just say thanks to FoG for asking the question, and to kecaleno and Mr Guy for very readable answers. Posts like this are great for me. God bless for now. PAUL

No problemo I get meself all confuzzled with this new DSLR lol one day I might even get out and shoot the darn thing instead of reading about what it can do and maybe juuuuust maybe they will turn out ok rather than underexposed crappola (scuse the language) that happened recently lol ....

[FoG]

Originally Posted by Mr Guy

It's also important to note the context. All constant aperture zoom lenses are not necessarily fast. How fast a lens is considered to be is also how wide the widest aperture is. The breaking point between fast and slow is more a matter of opinion than well defined nomenclature, but is generally considered to be right around f/2.8. Fast zoom lenses tend to top at f/2.8 unless they become very very large and very very expensive, while fast prime lenses are often in the 1.X category, with a very small number of f/1.0 lenses out there, some f/1.2s, a reasonable selection of f/1.4s, and quite a variety of f/1.8s.

Ok so any that have a high f number through would be slow and those with constant lower numbers are seen as high makes sense I suppose will look carefully when getting me next lens then as was wondering why some 70-300/300 were sooo much more than others leaving out the whole VR thing now it makes sense why they command higher prices

[Digidave]

Don't count out slower lenses completely. I have the Nikkor AF-S DX NIKKOR 16-85mm f/3.5-5.6G ED VR. Great lens for outdoor use or if your going to use a flash.

[fletch]

Slow lenses are generally considers to be any lens with a maximum aperture of f/3.5 or smaller.

[inkista]

On more tiny footnote. "Fast" and "Slow" refers to the shutter speeds you can achieve with the lens. A wider aperture lets in more light all at once, so you can get a faster shutter speed and shoot faster moving subject matter.

Also, if a lens has a variable aperture it will always be smaller at the long end. This is because the aperture opening actually (probably) stays the same size, but with the focal length getting longer, the f-number will become larger, since the f-number is the ratio of the focal length to the aperture diameter:

f-number = focal_length/diameter