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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