Lens Qns: f1.4, f1.8 , f2.8

[cichlid]

Good afternoon everybody

I wana ask, those lens with fixed f numbers, they are called fast lens? And they are supposed to be very sharp also?

Why? I thought "f" means aperture.

And these lens stay at f1.4, 1.8, 2.8 all the time? So it always have shallow depth of field?

[adamadam]

I think they say fast because it has a large aperture, so you can use faster shutterspeed. And having a large aperture, may help with the focussing.

They don't stay at those, you can stop it down. Make it smaller - to have a larger depth of field.

[gooseberry]

The aperture number shown for a lens is usually the largest aperture that the lens can open up to. You can stop down the aperture of these lenses to a smaller aperture or f stop.

These lenses are termed "fast" lenses as they have a large aperture and this let in more light than lenses with smaller apertures (thus for the same lighting condition, you can achieve faster shutter speeds)

[ExplorerZ]

Originally Posted by cichlid Good afternoon everybody I wana ask, those lens with fixed f numbers, they are called fast lens? And they are supposed to be very sharp also? Why? I thought "f" means aperture. And these lens stay at f1.4, 1.8, 2.8 all the time? So it always have shallow depth of field?

shallow DOF or not depends on how you shoot it... with a 12-24 or etc, you can get a lot of DOF even at f4.

as for fixed f number = fast lens... not necessary... (unless you talking about those in the title.)
sharpness... no necessary as well, it depends very much on the lens.

[cichlid]

Originally Posted by gooseberry The aperture number shown for a lens is usually the largest aperture that the lens can open up to. You can stop down the aperture of these lenses to a smaller aperture or f stop. These lenses are termed "fast" lenses as they have a large aperture and this let in more light than lenses with smaller apertures (thus for the same lighting condition, you can achieve faster shutter speeds)

oic, thanks...my kit lens have f3.5-5.6, but sometimes cannot get the lowest f3.5 due to the focal length. Can't remember if is during wide angle or during zoom.

So my next qn is these fast lens f stop are also affected by the focal length? We may not get the lowest f number all the time right?

[Francis247]

Originally Posted by cichlid oic, thanks...my kit lens have f3.5-5.6, but sometimes cannot get the lowest f3.5 due to the focal length. Can't remember if is during wide angle or during zoom. So my next qn is these fast lens f stop are also affected by the focal length? We may not get the lowest f number all the time right?

I believe it should be during your zoom, you cannot get f3.5. For example, a 18-70mm f3.5-5.6, you should be getting f3.5 @18mm and f5.6 @70mm.

For prime lens where you usually get f1.4 or f1.8, the focal length is fixed eg. 50mm f1.8, 85mm f1.4. (except 105mm f2.8 micro lens where the aperture will change due to light loss)

For those zoom lens with constant f2.8, you will get f2.8 at all zoom range, eg. 28-70mm f2.8.

[Artosoft]

Originally Posted by Francis247 For prime lens where you usually get f1.4 or f1.8, the focal length is fixed eg. 50mm f1.8, 85mm f1.4. (except 105mm f2.8 micro lens where the aperture will change due to light loss)

All the micro/macro lens (regardless prime or zoom lens) have that light loss behavior.

And I prefer to called it aperture will change or smaller because the front element of lens will move away from aperture leaf (to get closer distance focus), thus make light loss.

Regards,
Arto.

[Francis247]

Originally Posted by Artosoft All the micro/macro lens (regardless prime or zoom lens) have that light loss behavior. Regards, Arto.

Thanks for clarifying.
Was just using the 105mm micro as an example so that TS or others won't post another thread and claim that their 105mm f2.8 micro aperture is not constant f2.8.

[gooseberry]

Actually all lenses have the "light loss" (effective aperture vs theoretical aperture), it's just that it doesn't become significant until you start approaching macro focusing distances.

[Static]

Got noobie qtn from me. Does the Bokeh effect dependant on the number of blades in the lens ?

[zac08]

Originally Posted by Static Got noobie qtn from me. Does the Bokeh effect dependant on the number of blades in the lens ?

Yes... with a larger number of blades, you would have a better aperture curvature rather than sharp angles.

[Artosoft]

Originally Posted by Static Got noobie qtn from me. Does the Bokeh effect dependant on the number of blades in the lens ?

One of the factors for good lens' bokeh. There are another factors like quality of elements (glass or plastic), quality of coating, optical design of lens, etc.

Regards,
Arto.