Pentax FA 100/2.8 macro lens

Dear friends!
I would like to ask about Pentax AF 100/2.8 lens.
Does this lens change the aperture when focusing?
For example Tamron 90/2.8 SP when focused at 29cm
is about f5.6

thanks

Are you sure you are right? Lenses are not meant to change aperture when focussing, and I certainly wouldn't expect a change of the sort you mention on the Tamron lens.

Some zoom lenses are designed with different maximum apertures at different ends of the zoom range.

I think what is being referred to is the change in illumination of a lens as it is focused closer. Normally this effect is not important, but it becomes noticeable around a magnification ratio of 1:10 and greater. That is why 50mm lenses usually focus down to 50cm or so, and 100mm lenses say down to 1 metre.

The effect increases as we go closer. It is the same effect as using extension tubes - the further the lens is racked out from the film plane, the more light is lost.

am sure about Tamron - it is written in the manual, and I have also checked this, but what about the Pentax lens? has it inner focusing?

The Tamron manual sounds like a bad translation from Japanese. I have a Pentax FA 2.8 Macro and f2.8 stays f2.8 and F32 stays F32. However like any lens extention system that gives a 1:1 life size image on the film be it 35mm or 5" x 4" it will loose approx. two stops of light in the process. With a built in metering system this is not a problem but one will have to compensate two stops when using a hand held meter or a flash meter which I do a lot. I have in fact fixed a strip of paper on the lens barrel so as the lens extends I have marked the paper in 1/2 stops sections to show the compensation required.

regards David

David: yes - that´s it, I´m speaking about. The Tamron SP 90/2.8 is loosing
about 2.7 EV of light when focused to 1:1 lifesize macro, so Pentax FA 100/2.8 is about the same?