a company called SRB in the UK make the MonoVue viewing filter. It is a small round almost deep olive filter with a handle and an eyecup to block out stray light. It does take some getting used to and I find I can only use it outside in daylight (it's too dark for indoors) but it is very helpfull in studying the tonal values prior to exposure, and informing which, if any filters to use to aid contrast. I certainly use mine a lot.