5d mkIII manual mode setting and aperture

[wrk4no1]

I had a strange thing happen this morning while doing sunrise pictures. I set my camera up as always shutter, aperture, iso but I notice that when I changed the shutter speed that the aperture was adjust by itself? I dont recall this happening before. I tried turning off the camera, different mode and back to manual but it still did this. I tried to recreate this at home but couldnt get it to happen. Isnt manual mode just that? I setup the settings?

[ebiggs1]

Manual mode means just that, manual. You set everything. Of course there is a new twist to this, Auto ISO.  In my mind setting Auto ISO, removes manual mode.  But that isn't the way some look at it.

Are you sure you didn't mistakenly set it to Tv?  It kinda looks like manual mode but does automatically change the aperture.

[wrk4no1]

I am sure that was the first thing I checked. Its strange because I normally shoot low light so I always looks at the aperture. I'll just have to see wait til next time and see if it does it again. Thanks for the reply.

[Waddizzle]

The aperture can adjust itself when you have a zoom lens set for minimum focal length, and set the aperture at its' widest setting. 

As you zoom out to maximum focal length, the aperture value will change, unless your lens has a constant aperture value over the entire zoom range.  As you zoom back from maximum focal length back to minimum, the aperture should change back to the widest setting that you originally started with before you changed focal lengths.

[wrk4no1]

I was using a 24-70 f2.8 lens.