It's a quirk of compatibility.
Most of my very old M42 screw-fit lenses, proudly boast to being 'automatic'. They have a pin on the lens flange that when pushed 'in' stops the aperture iris down to that set; whilst 'out' the iris stays wide open at whatever the max aperture setting may be. The pin only gets pressed in to 'stop down' to the 'taking' aperture, when you press the shutter release so that the view-finder stays 'bright'.
On older 'non automatic' lenses, the view-finder dimmed with the aperture setting, and most 'automatic' lenses had an over ride switch, so they worked like a on auto lens, which meant composing wide open, then stopping down to taking aperture if you were using an SLR that has the view-finder looking through the taking lens.
On later bayonet mount 'system' SLR's the feature was incorporated with some other means of interconnect, to stop down to taking aperture when you released the shutter, and lenses usually featured a 'DoF Preview' button to rapidly stop down to taking aperture, and dim the view-finder to see the effect through the periscope.
Slipping ahead a few decades, the Electric-Picture-Maker gets rid of the complicated mechanica inter-connects between camera and lens, and uses electric contacts, to do essentially the same thing... stop down an electric controlled iris when you press the shutter, but leave it wide open for a bright view-finder image the rest of the time. There may or more likely is not a DoF preview feature to stop down to taking aperture.
I'm not entirely sure when Cannon pioneering electric controlled lenses started using such technology, but I am fairly sure it was on the last film cameras, rather tha early digital, so on that brand it probably is very much an age 'thing' as anything.
But, using film era lenses on digital cameras.... I can mount most of my old M42 screw fit lenses onto my modern Nikon Widgetal via adapter... first up, the EPM expects to 'talk' to an electric lens through its brass contacts, on the lens mount... but lens wont talk back! So, first up the camera's internal TTL metering don't work, as it isn't getting aperture settings from the lens; so I have to meter manually with a hand meld meter or other camera, guesstimate by f16-Sunny sort of guide-line or simply chimp-it from a few test shots! AND I have to be sure that the lens is set to 'manual' because there's no mechanical linkage to stop it down to taking aperture when the shutter released.. so I have to be careful about that, or it will take everything I shoot 'wide-open'! And be a bit 'diligent' making sure I manually stop-down to the taking aperture after composing wide open, to get bright view-finder image.
This sort of anomaly, is then par for the course with using adapted lenses, and one of the reasons I suggest horses for courses, and if you wat to get the best from old legacy lenses, use them on the old legacy film camera they were intended or! If you want the convenience of wdgetal, use the electric lenses designed for the widgetal camera; trying to cross pollinate usually brings more compromises to either way around for the few advantages you may be looking for! But if you DO have to mix and match, you really do need to know the ins-and-outs of what's going on with both camera and lens and apply that much more diligence to the hardware, even than you might have had to with an old fully manual film camera, to compensate for this sort of quirk!
Your call, really.... how many rolls of film could you buy for he price of a cheap 'kit' electric lens for the EPM, or how many could you buy for the e-bay price of the body? Or is pursuing the legacy lenses on EPM worth the hassle and learning and compensating for what's causing it?
