Extending any lens beyond its infinity position begins to reduce its
effective aperture regardless of whether it's done with a bellows or
with the focusing helicoid of the lens. It's just that in the distance
range down to about a meter or so it's small enough the we can ignore
it. When we get to the range of quarter or half life size the loss of
light is getting sufficiently high that we can't ignore it any more.
These days we may not notice because our TTL meters are fixing
everything for us but that don't mean it ain't happening.
Having the meter figure it out is good since focusing with the helicoiod
on lenses with internal focusing movements won't exactly follow the
rules since, although nominally a fixed focal length lens, they may be
changing focal length as well. But when following the marked
magnifications on the lens barrel, if it says 1:1 you're effective
aperture is down by two stops and a 4X exposure time adjustment from
that which is indicated.
Chuck Norcutt
On 12/15/2010 4:59 PM, Andrew Fildes wrote:
> You mean, they're not f2 at full extension?
> How annoying - must check that on my DZ 50/2.
> Bellows I can understand but....
> Andrew Fildes
> afildes@xxxxxxxxxxxxx
>
>
>
> On 16/12/2010, at 8:38 AM, Ken Norton wrote:
>
>> Actually, Andrew, Chuck is correct in this. It's not a crop factor
>> he's talking about, but light-loss due to extension.
>
--
_________________________________________________________________
Options: http://lists.thomasclausen.net/mailman/listinfo/olympus
Archives: http://lists.thomasclausen.net/mailman/private/olympus/
Themed Olympus Photo Exhibition: http://www.tope.nl/
|