From: David J Taylor on
"DanP" <dan.petre(a)gmail.com> wrote in message
news:d603312e-320c-4c88-b6c4-f0f448226c9d(a)j8g2000yqd.googlegroups.com...
[]
> I only have 3 lenses:
> EF-S 18-55mm f/3.5-5.6 42mm diameter
> EF 50mm f/1.8 18mm diameter
> EF-S 55-250mm f/4-5.6 (did not measure lens diameter but close to
> 42mm)
[]
> DanP

A 50mm lens with a claimed f/number of 1.8 must have an entrance pupil
larger than 18mm diameter. A typo?

Cheers,
David

From: DanP on
On Jun 11, 11:27 am, "David J Taylor" <david-
tay...(a)blueyonder.co.uk.invalid> wrote:

> A 50mm lens with a claimed f/number of 1.8 must have an entrance pupil
> larger than 18mm diameter.  A typo?

Well spotted, is a mistake.

I have double checked the diameters:
EF-S 18-55mm f/3.5-5.6 42mm diameter
EF 50mm f/1.8 32mm diameter
EF-S 55-250mm f/4-5.6 45mm diameter

DanP
From: Peter on
"DanP" <dan.petre(a)gmail.com> wrote in message
news:cc4df53e-0fbe-4cdf-93b6-40953f397a40(a)j8g2000yqd.googlegroups.com...
On Jun 11, 11:27 am, "David J Taylor" <david-
tay...(a)blueyonder.co.uk.invalid> wrote:

> A 50mm lens with a claimed f/number of 1.8 must have an entrance pupil
> larger than 18mm diameter. A typo?

Well spotted, is a mistake.

I have double checked the diameters:
EF-S 18-55mm f/3.5-5.6 42mm diameter
EF 50mm f/1.8 32mm diameter
EF-S 55-250mm f/4-5.6 45mm diameter


Last time I checked my math tables, 32mm is larger than 18mm. <G>

--
Peter

From: DanP on
On 11 June, 16:15, "Peter" <peter...(a)nospamoptonline.net> wrote:
> "DanP" <dan.pe...(a)gmail.com> wrote in message
>
> news:cc4df53e-0fbe-4cdf-93b6-40953f397a40(a)j8g2000yqd.googlegroups.com...
> On Jun 11, 11:27 am, "David J Taylor" <david-
>
> tay...(a)blueyonder.co.uk.invalid> wrote:
> > A 50mm lens with a claimed f/number of 1.8 must have an entrance pupil
> > larger than 18mm diameter. A typo?
>
> Well spotted, is a mistake.
>
> I have double checked the diameters:
> EF-S 18-55mm f/3.5-5.6 42mm diameter
> EF 50mm f/1.8 32mm diameter
> EF-S 55-250mm f/4-5.6 45mm diameter
>
> Last time I checked my math tables, 32mm is larger than 18mm. <G>
>
> --
> Peter

True, but I am comparing 32mm with 42mm, so the prime lenses are still
smaller.


DanP
From: Wolfgang Weisselberg on
DanP <dan.petre(a)gmail.com> wrote:
> On Jun 9, 11:46 pm, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>
> wrote:
>> DanP <dan.pe...(a)gmail.com> wrote:
>> > Big lens: 42mm diameter, 50mm focal length, f/1.2 aperture.
>> > Small lens: 18mm diameter, 50mm focal length, f/2.8 aperture.
>> > Set them both to f/2.8 or smaller and you get the same light.

>> So use a 200mm as the big lens and a 50mm as the small lens
>> and answer again.

> 200mm?
> I am talking about lens diameter not focal length.

And I am trying to point out that a 200mm f/2.8 lens at f/2.8 will
capture more light from a star than a 50mm f/2.8 lens at
f/2.8.

It's patently obvious that a 200mm f/2.8 lens has a larger
diameter than a 50mm f/2.8 lens.

-Wolfgang
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