on 18/06/2003 05:53, jason.gidden@xxxxxxxxxxxxxx at
jason.gidden@xxxxxxxxxxxxxx wrote:
> Hi Chris,
[snip]
> However I decided to conduct a small experiment last night comparing
> readings from different lenses to see if that 'shed any light on the
> problem' (sorry, awful pun ;-)). The problem has been mainly when I have
> been using my Tamron 135mm f2.8 lens (I know I'm inviting a lot of "serves
> you right for using 3rd party lenses" etc). I took a reading (all wide
> open BTW) from a light source using a 50mm 1.4 Zuiko (about 1/500 sec) and
> then compared the reading from the same light source using the Tamron. The
> resulting reading was 1/8 sec, quite a difference!! Subsequent readings
> from a couple of other lenses concurred with the 1/500 sec measurement.
> Now the view to the naked eye is considerably darker when using the Tamron,
> even wide open, but that's a bigger difference than might be expected is it
> not???
>
> Now I'm even more confused.
>
> Jason.
>
[snipped]
> <|_:-)_|>
>
> C M I Barker
> Cambridgeshire, Great Britain.
>
Wow !!:
> ?±¨ k zX¯z·3⁄4&-…n°?–)?-?²بžЦj)m£?¢\||?褣??=¨ ?f褣??¢\||??†5
Hi Jason,
In addition to Piers explanation, about your little experiment i will quote
below what I was explained here about the very same experiment. Please read
my question at the end pf this post. Thanks all.
---
on 6/05/2003 12:17, C.H.Ling at chling@xxxxxxxxxxxxx wrote:
> During the test I found that 200/5 is darker than F5 and the darkest one
> even among the F5 zooms. On the other hand my resolution test with all long
> lenses at 200mm shown that 200/5 is the sharpest one even wide open.
Some 18 years ago I think, using my new T32, I found that the recently
launched Zuiko 28mm F2.8 was darker @2.8 than my older 50mm F1.4 SC @2.8.
I remember having sent the Ektachromes to Olympus Japan, who returned them
together with some explanation I don't recall. Does this happen often?
I didn't test my other two lenses. Now I remember that I implemented this
test because of different speed lectures @2.8 on both Zuikos.
---
on 6/05/2003 13:39, Dizel at webmaster@xxxxxxxx wrote:
>> Some 18 years ago I think, using my new T32, I found that the recently
>> launched Zuiko 28mm F2.8 was darker @2.8 than my older 50mm F1.4 SC @2.8.
>> I remember having sent the Ektachromes to Olympus Japan, who returned them
>> together with some explanation I don't recall. Does this happen often?~
>
> Always ;) f/stops are different than real stops, called t/stops.
>
> The f-stop is a geometric relationship between focal length and effective
> aperture and does not take into account light loss within a lens.
> Each air to glass surface reflects some light. So real 'opening', called
> t/stop is different than f/stop. This difference depends of lens construction
> (number of elements, coatings used..) and usually is NOT given.
> Only some companies, most notably Kiron, callibrated their
> diaphragm scale in t/stops.
---
on 6/05/2003 18:49, Moose at olymoose@xxxxxxxxxxxxx wrote:
> Another exception is mirror lenses. I did some measuring a while ago and
> a lengthy post. the quick summary is that all 4 mirror lenses (350, 500,
> 600 & 1000mm) I checked (and the Zuiko, according to Gary's test) were
> marked noticeably slower than the f-stop calculated from the actual
> clear aperture with central mirror mount subtracted. It appears they are
> generally marked with t-stop.
>
> A lot of lens tests note the difference between f and t stop.
>
> Dizel wrote:
>
>> Always ;) f/stops are different than real stops, called t/stops.
>>
>> The f-stop is a geometric relationship between focal length and
>> effective aperture and does not take into account light loss within a
>> lens. Each air to glass surface reflects some light. So real
>> 'opening', called t/stop is different than f/stop. This difference
>> depends of lens construction (number of elements, coatings used..)
>> and usually is NOT given. Only some companies, most notably Kiron,
>> callibrated their diaphragm scale in t/stops.
---
So, what´s the use of Guide Number when using flash in Manual Mode???????
OTOH, it works !!!
Another question that has puzzled me for years.....
Rgds,
Fernando.
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