max aperture on lens

[Nicolae]

Hi All,

I want to compare a lens with max aperture F1.4 and one with max aperture
F3.8.
If I set the camera on F22, does it matter if I am using the first lens or
the second one? I know that a lens with F1.4 is faster (and better) than the
F3.8 one, but if I set the camera to a small aperture are there still
differences in quality between them?

Regards,
Nicolae

[Alan]

the chances are that the faster lens will have a poorer quality from f/4
down - and that at 5/22 it will noticeable worse!
You dont get owt for nowt! If you pay for speed, (=wide aperture) you are
almost certain to loose in quality.

The above is especially true of "cheaper" large aperture lenses, where
everything is sacrificed for speed.

[Nicolae]

Thanks, I understand now.

[Dadioh]

> I want to compare a lens with max aperture F1.4 and one with max
> aperture F3.8.
> If I set the camera on F22, does it matter if I am using the first
> lens or the second one?

No. Not as far as exposure goes.

A given f-stop on any lens lets in the same amount of *effective* light.
That is because f stops are ratios of the focal length of the lens
relative to the aperture....f=F/A

Say you have two lenses...one has a FL of 50 mm, the other 200 mm. Set
them both at f-22 and look at the size of the aperture. The opening on
the 200 mm lens is much larger than that on the 50 mm lens and much more
light (4X) passes through it yet the result on the film will be the
same. That is because the light from the 200 mm lens has to go 4X as
far to reach the film and once it gets to its destination it is weaker.
That "weakness" is defined by the inverse square law of light that says
"light varies inversely to the square of the distance". If it travels
twice as far, it is 1/4 (2*2); four times as far is 1/16 (4*4). That
is true for any light including flash.
__________________

>I know that a lens with F1.4 is faster (and
> better) than the F3.8 one,

Faster, yes; "better" not necessarily, depends on the lens. A good lens
with a max aperture of 3.8 can be better than a poor 1.4 lens at any
aperture.
_______________

> but if I set the camera to a small
> aperture are there still differences in quality between them?

It depends entirely on the lens...maybe yes, maybe no.

[Nicolae]

> I want to compare a lens with max aperture F1.4 and one with max
> aperture F3.8.
> If I set the camera on F22, does it matter if I am using the first
> lens or the second one?

No. Not as far as exposure goes.

A given f-stop on any lens lets in the same amount of *effective* light.
That is because f stops are ratios of the focal length of the lens
relative to the aperture....f=F/A

Say you have two lenses...one has a FL of 50 mm, the other 200 mm. Set
them both at f-22 and look at the size of the aperture. The opening on
the 200 mm lens is much larger than that on the 50 mm lens and much more
light (4X) passes through it yet the result on the film will be the
same. That is because the light from the 200 mm lens has to go 4X as
far to reach the film and once it gets to its destination it is weaker.
That "weakness" is defined by the inverse square law of light that says
"light varies inversely to the square of the distance". If it travels
twice as far, it is 1/4 (2*2); four times as far is 1/16 (4*4). That
is true for any light including flash.
__________________

>I know that a lens with F1.4 is faster (and
> better) than the F3.8 one,

Faster, yes; "better" not necessarily, depends on the lens. A good lens
with a max aperture of 3.8 can be better than a poor 1.4 lens at any
aperture.
_______________

> but if I set the camera to a small
> aperture are there still differences in quality between them?

It depends entirely on the lens...maybe yes, maybe no.

Thanks DadiOH for your comments, they are very interesting. I was reading
some articles and I had the impression that the faster the lens, the better
the quality of it. This also explain why some tiny cameras with a very small
lens still provide a good quality of the photos.

[Alan]

the main reason "why some tiny cameras with a very small lens still provide
a good quality of the photos" is to do with depth of field. Very short focal
length, and small aperture give a huge depth of field.

[James]

Thanks DadiOH for your comments, they are very interesting. I was reading
some articles and I had the impression that the faster the lens, the better
the quality of it. This also explain why some tiny cameras with a very small
lens still provide a good quality of the photos.


But the cause and effect loop here is due to the tolerances that the
more expensive fast lenses are built to, not some inherent benefit of a
fast lens being better in other respects. They are better because
they are designed to be a higher priced item. (And it could be that
not every manufacturer bothers to do that!)

[Chrlz]

"That is because the light from the 200 mm lens has to go 4X as
far to reach the film and once it gets to its destination it is
weaker."

Umm, I'm just nitpicking here, but isn't it because the 200mm lens
*spreads* the light over a larger area due to its higher magnification?
The light travels the same distance, within a few mm, for both
lenses...