A lot of SLRs have a spring loaded stop down button which has to be depressed. Generally speaking, older mechanical models and professional SLRs had a lever that could be engaged, as had some late cameras on the body. Stop down mode began to be seen as a luxury as camera technology developed, even as a push button facility. I can't recall whether the OP's F3 and FM can be locked into stop down. If they can, I'd advise experimenting with it for street shooting.
Originally Posted by AgX
Strongly coloured filters can provide a similar job of 'unifying' the field of view, but that means shooting B&W.