The 90mm macro lens is designed to extend its front group largely toward the subject in close-up macro photography. In that situation, the amount of light reaching the film decreases. The aperture displayed stays the same in most cameras, but some SLR cameras are equipped with a feature that shows the effective f-values, and smaller open aperture values are displayed when the lens barrel is extended for close-up photography. This phenomenon is not a lens defect or flaw, but is a camera characteristic.