Carl,Zeiss Flektogon 35mm f2.8

Yes, it's normal. It's called aperture/distance compensation and Zeiss Jena were one of very few companies to incorporate the feature.

Technically, all unit-focussing lenses (where the whole lens moves to focus) change aperture as the focus is shifted closer. I.e. The aperture number gets larger as the lens is focused away from infinity.

Zeiss Jena indicated this change by having the aperture ring rotate with the focus to indicate the true, effective, maximum aperture of the lens. But strangely the feature was only implemented on a few lenses; the 'zebra stripe' 35mm f/2.8 Flektogon being one of them.

For the technically minded, the mechanism used was very simple: The slide coupling the aperture iris to the control ring was simply slanted at such an angle that as the lens and iris moved forward the coupling was rotated by a few degrees, effectively limiting the indication of the control ring to reflect the true maximum aperture.

Note: Once the aperture ring is turned away from maximum the indication no longer follows the effective aperture properly. So not really a very useful idea. Just a reminder that 'what you see is not what you get' with a close focussed lens, aperture-wise.

Incidentally, the later 35mm f/2.4 Flektogon is a much sharper and better corrected lens, but it doesn't have the aperture compensation.

Yes, it's normal. It's called aperture/distance compensation and Zeiss Jena were one of very few companies to incorporate the feature.

 

Technically, all unit-focussing lenses (where the whole lens moves to focus) change aperture as the focus is shifted closer. I.e. The aperture number gets larger as the lens is focused away from

 

Incidentally, the later 35mm f/2.4 Flektogon is a much sharper and better corrected lens, but it doesn't have the aperture compensation.

Brilliant, thanks. I’ve had a chance to play with it a bit more now and can see that change in aperture gradually as you focus from infinity all the way to 18 cm. The 2.4 would be nice I’m sure but this will do for 30 quid.

The Flektogon is one of the contenders for being the first use of retrofocus to fit a wide-angle into a 35mm camera with a mirror.

1950 Flektogon and 1950 Angénieux 35mm f/2.5 RF R1 retrofocus

Flektogon 35mm f/2.8 - one of the first wide-angle lenses for SLRs

Carl Zeiss Jena and Carl Zeiss Oberkochen were indeed not far behind, if at all. However Angenieux managed to have the name he chose for his lenses (for colour cinematography using beam splitters behind the lens first), Retrofocus, become the generic type name.

That will be, i believe, because he may not have been the first introducing Retrofocus lenses for stills cameras, but his cinema Retrofocus lens preceded all of the contenders for first-in-stills.

So if the effective f-stop is reduced when focussing close, why does it make sense to limit the physical diaphragm setting to less than full open?

No, It's still the same physical aperture, maximum. But, as seen from a point in the focal plane, the aperture "circle" (technically, the exit pupil) is farther (because the lens moved forward), and therefore looks smaller and delivers less light in the focal plane. This feature of the 35/2.8 spares the user the calculation of exposure compensation.

No, It's still the same physical aperture, maximum. But, as seen from a point in the focal plane, the aperture "circle" (technically, the exit pupil) is farther (because the lens moved forward), and therefore looks smaller and delivers less light in the focal plane. This feature of the 35/2.8 spares the user the calculation of exposure compensation.

I know. But the OP said the lens only opens to f4, not to effective f/4.