Should lenses be allowed to claim a specific f-stop speed if they vignette, horribly?

On Sat, 23 Mar 2013 01:28:39 -0700 (PDT), RichA
wrote:

IMO, if a lens wide open exhibits more than 0.7 stops of vignetting at
the corners, it should not be allowed to call itself what it claims
its f-stop to be. It's like a lens that can only resolve to a claim
in the very centre saying it is capable of "X" resolution.

It's got nothing to with speed, vignetting, image quality or anything
else. It's merely the diameter of the aperture expressed as a fraction
of the focal length. The expectation that this says something about
usable speed is a figment of your own expectation. Next thing you will
be wanting a limit set to f numbers on the grounds of diffraction.

If you want a number more directly related to speed use 't' stops but
that doesn't say anything about vignetting either.
--

Regards,

Eric Stevens

On Sun, 24 Mar 2013 06:11:32 -0700 (PDT), RichA
wrote:

On Mar 23, 10:47*pm, Eric Stevens wrote:
On Sat, 23 Mar 2013 01:28:39 -0700 (PDT), RichA
wrote:

IMO, if a lens wide open exhibits more than 0.7 stops of vignetting at
the corners, it should not be allowed to call itself what it claims
its f-stop to be. *It's like a lens that can only resolve to a claim
in the very centre saying it is capable of "X" resolution.

It's got nothing to with speed, vignetting, image quality or anything
else. It's merely the diameter of the aperture expressed as a fraction
of the focal length. The expectation that this says something about
usable speed is a figment of your own expectation.

Vignetting IS a reduction in lens speed at the edge, which is why you
end up with it being darker than the rest of the image. The way to
combat it is with better lens design (more expensive).

It still doesn't affect the ratio of lens diameter to focal length.
The f number remains the same.
--

Regards,

Eric Stevens

On 24/03/2013 20:05, Eric Stevens wrote:
On Sun, 24 Mar 2013 06:11:32 -0700 (PDT), RichA
wrote:

On Mar 23, 10:47 pm, Eric Stevens wrote:
On Sat, 23 Mar 2013 01:28:39 -0700 (PDT), RichA
wrote:

IMO, if a lens wide open exhibits more than 0.7 stops of vignetting at
the corners, it should not be allowed to call itself what it claims
its f-stop to be. It's like a lens that can only resolve to a claim
in the very centre saying it is capable of "X" resolution.

It's got nothing to with speed, vignetting, image quality or anything
else. It's merely the diameter of the aperture expressed as a fraction
of the focal length. The expectation that this says something about
usable speed is a figment of your own expectation.

Vignetting IS a reduction in lens speed at the edge, which is why you
end up with it being darker than the rest of the image. The way to
combat it is with better lens design (more expensive).

It still doesn't affect the ratio of lens diameter to focal length.
The f number remains the same.

But it is only accurate near the optic axis. Vignetting is in essence
the change in geometric illumination factor as a function of angle off
the optical axis. If you think about it then it is clear that it is very
difficult to maintain uniform field illumination at large angles.

--
Regards,
Martin Brown

On Sun, 24 Mar 2013 21:28:15 +0000, Martin Brown
wrote:

On 24/03/2013 20:05, Eric Stevens wrote:
On Sun, 24 Mar 2013 06:11:32 -0700 (PDT), RichA
wrote:

On Mar 23, 10:47 pm, Eric Stevens wrote:
On Sat, 23 Mar 2013 01:28:39 -0700 (PDT), RichA
wrote:

IMO, if a lens wide open exhibits more than 0.7 stops of vignetting at
the corners, it should not be allowed to call itself what it claims
its f-stop to be. It's like a lens that can only resolve to a claim
in the very centre saying it is capable of "X" resolution.

It's got nothing to with speed, vignetting, image quality or anything
else. It's merely the diameter of the aperture expressed as a fraction
of the focal length. The expectation that this says something about
usable speed is a figment of your own expectation.

Vignetting IS a reduction in lens speed at the edge, which is why you
end up with it being darker than the rest of the image. The way to
combat it is with better lens design (more expensive).

It still doesn't affect the ratio of lens diameter to focal length.
The f number remains the same.

But it is only accurate near the optic axis. Vignetting is in essence
the change in geometric illumination factor as a function of angle off
the optical axis. If you think about it then it is clear that it is very
difficult to maintain uniform field illumination at large angles.

While I agree with you, field illumination has nothing to do with f
number. 'f' number is no more (and no less than) lens aperture divided
into focal length. This is not altered by the desire of RichA (or
anyone else) to read something else into it.
--

Regards,

Eric Stevens

Alfred Molon wrote:
In article , Eric Stevens
says...
On Sun, 24 Mar 2013 21:28:15 +0000, Martin Brown
wrote:
But it is only accurate near the optic axis. Vignetting is in essence
the change in geometric illumination factor as a function of angle off
the optical axis. If you think about it then it is clear that it is very
difficult to maintain uniform field illumination at large angles.

While I agree with you, field illumination has nothing to do with f
number. 'f' number is no more (and no less than) lens aperture divided
into focal length. This is not altered by the desire of RichA (or
anyone else) to read something else into it.

The two above paragraphs are quite correct.

But people usually expect a certain level of illmúmination with a
certain aperture. And some lenses simply do not give it.

People may expect that, but at some point in the
learning curve about photography they necessarily will
learn not to expect that, because it is virtually never
true.

Look up the term "t-stops". And follow up on what
Martin Brown has stated about what vignetting is, as
opposed to what aperture is.

Perhaps the analogy of a lens with and without a Neutral
Density filter can show that how much light lands on the
sensor is not uniquely a matter of aperture. Less light
does not necessarily mean a different aperture.

--
Floyd L. Davidson http://www.apaflo.com/
Ukpeagvik (Barrow, Alaska)

In article , Eric Stevens
says...
On Sun, 24 Mar 2013 21:28:15 +0000, Martin Brown
wrote:

On 24/03/2013 20:05, Eric Stevens wrote:
On Sun, 24 Mar 2013 06:11:32 -0700 (PDT), RichA
wrote:

On Mar 23, 10:47 pm, Eric Stevens wrote:
On Sat, 23 Mar 2013 01:28:39 -0700 (PDT), RichA
wrote:

IMO, if a lens wide open exhibits more than 0.7 stops of vignetting at
the corners, it should not be allowed to call itself what it claims
its f-stop to be. It's like a lens that can only resolve to a claim
in the very centre saying it is capable of "X" resolution.

It's got nothing to with speed, vignetting, image quality or anything
else. It's merely the diameter of the aperture expressed as a fraction
of the focal length. The expectation that this says something about
usable speed is a figment of your own expectation.

Vignetting IS a reduction in lens speed at the edge, which is why you
end up with it being darker than the rest of the image. The way to
combat it is with better lens design (more expensive).

It still doesn't affect the ratio of lens diameter to focal length.
The f number remains the same.

But it is only accurate near the optic axis. Vignetting is in essence
the change in geometric illumination factor as a function of angle off
the optical axis. If you think about it then it is clear that it is very
difficult to maintain uniform field illumination at large angles.

While I agree with you, field illumination has nothing to do with f
number. 'f' number is no more (and no less than) lens aperture divided
into focal length. This is not altered by the desire of RichA (or
anyone else) to read something else into it.

But people usually expect a certain level of illmúmination with a
certain aperture. And some lenses simply do not give it.
--

Alfred Molon
------------------------------
Olympus E-series DSLRs and micro 4/3 forum at
http://tech.groups.yahoo.com/group/MyOlympus/
http://myolympus.org/ photo sharing site

Alfred Molon wrote:
In article , Eric Stevens

While I agree with you, field illumination has nothing to do with f
number. 'f' number is no more (and no less than) lens aperture divided
into focal length. This is not altered by the desire of RichA (or
anyone else) to read something else into it.

But people usually expect a certain level of illmúmination with a
certain aperture. And some lenses simply do not give it.

http://store.sony.com/webapp/wcs/sto...mber=SAL135F28
aka http://preview.tinyurl.com/bsa5l57

Do you consider the fact that f-stop != t-stop a defect in
this lens?

-Wolfgang