Control aperture while exposing in bulb mode?

Is this possible somehow someway with the EOS system without using a manual focus lens? Have a bunch of great Nikkor primes but I want the glare reduction and coatings of my 24-105L, got a subject that requires it.

Control aperture or modify aperture during the exposure?

If it is the former, just set the aperture normally/manuall
y as it were normal manual exposure. The latter I cannot help you with.

Not possible afaik. I curious though why you need to control aperture during exposure, if in bulb mode and you want to reduce the amount of light temporarily while keeping the shutter open just cover the front of the lens with something black.

It's a shot of an oncoming train, stopped down to f16/22 ISO 50 on a 5d.

As the train approaches, the moment the gates drop the shutter is activated, at f16 or 22, last time I did this at f16 and got proper exposure. As the locomotive approaches, it's got a very reflective side, and just before it hits the crossing I would open up to wide-open and flash the side of it with an off-camera flash at full power, freezing the reflective portion while maintaining the time-lapse long exposure of the gate drop and approach.

I've done similar get up's at f8 and the lights are too bright to maintain proper exposure and have the locomotive exposed properly.

a 10 dollar Nikon adapter (already own one) and a Rokinon 28mm f2.8 would suffice, but I'm not sure if the Rokinon will leave me with orbs and other random ****.

The train will be moving at roughly 60mph, so I only have one chance to get the shot, and I have been in touch with a few railfans to find out when one of these locomotives with the reflective side would be leading a train at night. Everything has to come together and when it does I only have one chance to get it.

The other thing I have considered is a rather costly setup since all of my flashes are EZ-series, but linking them all together in a chain and firing them at once. I have 4 at my disposal, two 540ez's and two 420ez's, and if I chained them together somehow or linked them up with triggers I wouldn't need a variable aperture and could just fire full-power off to the side.

Why not just handhold a ND filter when you need to stop down? That way you only have one variable to worry about.

I dont have an ND filter, but I can't believe I didn't think of that.

The only thing I'm worried about is that on this subject in the past filters create internal reflections and it shows up on the image. CPL's, UV's, whatever. I might do a test shot of a gatedrop with an ND and see if there's reflections, but with the other two I usually get them.

This all sounds fascinating I can't wait to see what you get, good or bad.

can't do it

I wonder if you could still make it work doing this. I don't have my camera here right now so I can't test it

1) set lens to the desired aperture f/16\22, hold DOF button and twist the lens just enough to disconnect the contacts
2) set your exposure and open the shutter
3) as the train approaches twist the lens back into position, this should open up the aperture to max automatically
4) stop exposure after the flashes trigger

Usually the lens immediately resets when you twist it back on, not sure if this works with an open shutter or if the camera stops/resets but it might be worth a shot.