It diaphragm actuator may be linear in the K-M lenses in accordance with
diameter, but it doesn't have to be, for the K specification only the
simulator has to be accurate since the diaphragm actuator moves it's
entire travel distance with each exposure. As long as the aperture
stops closing at the stop specified on the aperture ring.
Mark Roberts wrote:
>Cory Papenfuss wrote:
>
>
>
>>On Mon, 9 Oct 2006, Mark Roberts wrote:
>>
>>
>>
>>>Cory Papenfuss wrote:
>>>
>>>
>>>
>>>>I have followed Mark Robert's "matrix metering modification" on
>>>>an old K-mount Sears 135mm f/2.8. The camera doesn't care
>>>>that the identification insulators are
>>>>there... still stuck in CW metering.
>>>>
>>>>
>>>That's why my web page has the following note on the lens mod page:
>>>
>>>
>"It
>
>
>>>will not have any effect on any of the Pentax digital cameras"
>>>
>>>
>>>
>> I was too excited to read the fine print... either that or it
>>wasn't there 18 months ago when I did it.
>>
>> Just figured I'd clarify in the post here. Didn't want people to
>>think it was doable with the digital ones. Again... no GOOD reason for
>>it. Either marketing or engineering oversight.... probably the former.
>>
>>
>
>No, there's a good reason for it: The matrix metering algorithms need
>to know not only the maximum aperture of the lens (which it gets from
>the ID Insulators, but also the actual shooting aperture, which it can
>determine from the non-linear movement of the diaphragm actuator arm on
>"A Series" and later lenses.
>
>(Well, the movement is linear with respect to aperture *area* on "A"
>lenses. On "K" and "M" lenses it's linear with respect to diameter but
>not to area. I think I've got that right....)
>
>
>
>
--
Things should be made as simple as possible -- but no simpler.
--Albert Einstein
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