F stop cant open to F2.8 for Sigma 105mm f2.8 mounted on Nikon D600 body

[rayman6640]

Hi All
Can anyone help me with the issue.
Sigma micro 105mm f2.8 (nikon mount) mounted on D600 body, aperture can only open till f3.5 all setting at manual. Can't open any further why?
Thanks

 

[catchlights]

if you are shooting very very close up, the aperture number will only show the effective value of f3.5, not just sigma, it is the same for all the micro lenses including the Nikon lenses when focus in very close range.

noting wrong with your camera or lenses.

 

[rayman6640]

Thanks, my ignorance