From: J�rgen Exner on
Leon(a)nospam.com wrote:
>I often see lenses that have "special glass" elements, and some that don't. They
>claim better transparency.

Well, not so much "better transparency" as rather a different refractive
index which allows, when used in the right combination with ordinary
glass, to significantly reduce chromatic abberation.

>My question is, if you set up a certain shot with certain parameters, and then
>switch to a better lens, would you have to change the f stop or speed or
>something to match the better glass?

In theory maybe yes, but in real life no. The differences are just too
small to make any impact. If you reduce the light loss from let's say 5%
per element to 3% per element (which is a lot), then with e.g. 10
elements (which is a lot) the total light passing is about 60% versus
74%. That's less than 1/4 f-stop difference.

>I'm just wondering about things like "rule of sun on grass" - would that be
>wrong with a better lens?
>
>It just seems odd to me that two, 50mm lenses, set at f8 for example, would give
>different amounts of light. Is this considered when the manufacturer makes the
>lens? Do they compensate somehow?

The difference is just not significant enough to make a big deal out of
it.

jue
From: ColinD on
Leon(a)nospam.com wrote:
> I often see lenses that have "special glass" elements, and some that don't. They
> claim better transparency.
>
> My question is, if you set up a certain shot with certain parameters, and then
> switch to a better lens, would you have to change the f stop or speed or
> something to match the better glass?
>
> I'm just wondering about things like "rule of sun on grass" - would that be
> wrong with a better lens?
>
> It just seems odd to me that two, 50mm lenses, set at f8 for example, would give
> different amounts of light. Is this considered when the manufacturer makes the
> lens? Do they compensate somehow?
>
> Curious !
>

Since the meter reads the light coming through the lens, variations in
light transmission are automatically taken care of by the meter. It is
only of consequence if you use an external meter and set shutter and
aperture manually.

Colin D.
From: jaf on

<Leon(a)nospam.com> wrote in message news:cni3a5trn10q5lcterucmmkep3d764396k(a)4ax.com...
>
> I often see lenses that have "special glass" elements, and some that don't. They
> claim better transparency.
>
> My question is, if you set up a certain shot with certain parameters, and then
> switch to a better lens, would you have to change the f stop or speed or
> something to match the better glass?
>
> I'm just wondering about things like "rule of sun on grass" - would that be
> wrong with a better lens?
>
> It just seems odd to me that two, 50mm lenses, set at f8 for example, would give
> different amounts of light. Is this considered when the manufacturer makes the
> lens? Do they compensate somehow?
>
> Curious !
>

Two 50mm lenses set at f8 are two 50mm lenses set at f8.
Same focal length.
Same aperture.
Same amount of light.


John
From: David J Taylor on

"jaf" <me(a)here.com> wrote in message
news:VamdnW5SiuBUwz_XnZ2dnUVZ_sOdnZ2d(a)giganews.com...
[]
> Two 50mm lenses set at f8 are two 50mm lenses set at f8.
> Same focal length.
> Same aperture.
> Same amount of light.
>
>
> John

Not so. f/8 is a mechanical measurement relating to aperture and focal
length. It actually says nothing about the actual amount of light, simply
the /maximum/ light possible. The less than 100% transmission of the
glass will reduce the actual amount of light (albeit, perhaps only by a
small amount).

Two lenses with the same T-stop would pass the same amount of light. See:
http://photonotes.org/cgi-bin/photo-entry.pl?id=Tstop

Also: consider a standard and a mirror lens both at f/8. The mirror lens
typically has a central obstruction which reduces the light....

Cheers,
David

From: jaf on

"David J Taylor" <david-taylor(a)blueyonder.not-this-part.nor-this.co.uk.invalid> wrote in message
news:4Auom.75842$OO7.73399(a)text.news.virginmedia.com...
>
> "jaf" <me(a)here.com> wrote in message news:VamdnW5SiuBUwz_XnZ2dnUVZ_sOdnZ2d(a)giganews.com...
> []
>> Two 50mm lenses set at f8 are two 50mm lenses set at f8.
>> Same focal length.
>> Same aperture.
>> Same amount of light.
>>
>>
>> John
>
> Not so. f/8 is a mechanical measurement relating to aperture and focal length. It actually says nothing about the actual amount
> of light, simply the /maximum/ light possible. The less than 100% transmission of the glass will reduce the actual amount of
> light (albeit, perhaps only by a small amount).
>
> Two lenses with the same T-stop would pass the same amount of light. See:
> http://photonotes.org/cgi-bin/photo-entry.pl?id=Tstop
>
> Also: consider a standard and a mirror lens both at f/8. The mirror lens typically has a central obstruction which reduces the
> light....
>
> Cheers,
> David

Post some data that proves you would need a different exposure time for 2 otherwise identical lenses.


John