Moose,
I meant the "front element opening" being the filter thread size diameter.
You are correct in stating that Olympus calculated those maximum
f-stop aperture values based on their knowledge of the actual lens
characteristics.
I intentionally did not want to use the term 'filter thread size', but
wanted to
loosely point out the relationship between the max f-stop versus the
physical diameter of the front element lens openings. The 180/2.0, 250/2.0,
and 350/2.8 have much larger front element diameter compared to the rest,
and therefore, no stepping down of the max f-stop.
Sorry to confuse you or mislead other folks on the list. I thought I was
serving the List members when I first saw Skip Williams' post on the
subject and decided to translate and share the information during off-work
hours. If you think I do lousy translation, then I will shut up.
Jim Terazawa
-----Original Message-----
From: Moose <olymoose@xxxxxxxxxxxx>
To: olympus@xxxxxxxxxx <olympus@xxxxxxxxxx>
Date: Tuesday, January 20, 2004 8:38 PM
Subject: [OM] Re: Free OM-to-E-System adapter - Update2
>
>I don't know what the column "Front Element opening diameter" is
>aupposed to mean, but it isn't what it is labeled. It is the filter
>thread size, which bears only a vague relationship to the actual clear
>diameter of the front element. For a simple example, the clear diameter
>for the 85/2 is about 42.5mm while that for the 50/1.8 is about 28mm,
>although they both use 49m filters. This is simple math (85/2 = 42.5,
>50/1.8 = 27.8) confirmed by measurement.
>
>I suspect there is no logic that will work from this data. Oly likely
>calculated the angle of incidence of light at the corners of the 35mm
>frame from their knowledge of the actual lens characteristics. Thus the
>200/5 doesn't require stopping down at all, while the 200/4 requires
>stopping down to f8 (2 stops, rather than the 1 noted in the table.).
>This indicates that the 2 lenses, although of the same focal length and
>less than 1 stop different in speed, are optically quite different.
>
>Moose
>
>jimt@xxxxxxxxxxxxxxxx wrote:
>
>>--=_Listar-UnMime-Boundary
>>Content-Type: text
>>
>>Accroding to the table, if I understand the conversion correctly,
>>the focal lengths and the max aperature will be as follows for
>>some lenses.
>> Front element
>> Max f delta opening diameter
>>21mm/f2.0 becomes 42mm/f2.8 - One f-stop : 55mm
>>21mm/f3.5 becomes 42mm/f4.0 - Half f-stop ? : 49mm
>>24mm/f2.0 becomes 48mm/f2.8 - One f-stop : 55mm
>>24mm/f2.8 becomes 48mm/f4.0 - One f-stop : 49mm
>>50mm/f1.2 becomes 100mm/f4.0 - >3 f-stops : 49mm
>>50mm/f1.4 becomes 100mm/f2.8 - Two f-stops : 49mm
>>55mm/f1.2 becomes 110mm/f2.8 - >Two f-stops : 55mm
>>100mm/f2.0 becomes 200mm/f2.8 - One f-stop : 55mm
>>100mm/f2.8 becomes 200mm/f5.6 - Two f-stops : 49mm
>>200mm/f4.0 becomes 400mm/f8 - One f-stop : 55mm
>>200mm/f5.0 becomes 400mm/f5 - No change : 49mm
>>
>>180mm/f2.8 becomes 360mm/f4.0 - One f-stop : 72mm
>>180mm/f2.0 becomes 360mm/f2.0 - No change :100mm
>>250mm/f2.0 becomes 500mm/f2.8 - No change :130mm*
>>350mm/f2.8 becomes 700mm/f2.8 - No change :130mm*
>>
>>* estimate since no front filter used.
>>
>>As stated in the hyperlinked chart notes, the maximum openings are
>>the recommended f-stops based on calculations done by Olympus
>>spec. I assume they are such as considering physical openings
>>at the front element openings and vignettings, etc. I think some
>>max aperatures could be used differently per user..
>>
>
>
>
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