IIRC, the 650/620/630/600/RT series and EOS 10s were equipped with a dedicated button for DOF preview . All models with ECF included an eyeball sensor for DOF preview, but may not have had the button - I looked over the online users' manuals for the A2 and A2e and saw no reference to one on either.
I found the eye control focussing of the EOS 5 useless, I wear glasses and could never calibrate it well. The EOS 5 (A2e?) also has a very weak mode dial click-stop mechanism which usually breaks. Mine broke very early (the camera still works but the dial spins freely).
It has a lot of good features, can shoot at 5fps using only a 2CR5 battery and the vertical grip available for it is useful.
I found the eye control focussing of the EOS 5 useless, I wear glasses and could never calibrate it well. The EOS 5 (A2e?) also has a very weak mode dial click-stop mechanism which usually breaks. Mine broke very early (the camera still works but the dial spins freely).
It has a lot of good features, can shoot at 5fps using only a 2CR5 battery and the vertical grip available for it is useful.
I recall reading back in the day about glass wearers having issues with that control being hard to use.
I actually found this an early AI gimmick (and there are tons now) that while working in principle, it was not all that useful. For marketing it did make a lot of sense, just look and it switches, wow! But in real use it actually took your eye away from the whole scene, even if for a short moment. Like everything else, with use and practice, it can be "mastered" but like voice activated controls, it never came together for me. So for the premium price at the time, it was not worth it outside of its novelty.
Eye control on the 5 was apparently useless (no personal experience). On the 30E I found it actually pretty reliably once calibrated 3-4 times in both orientations. On the EOS 3 I had to limit the AF to 11 points vs the 45 for it to work effectively, otherwsise there's too many points too close for the system to pick up. Glasses are a no-no though.
On the eos5 it didn't work in portrait mode as I recall. And yeah I second the crappy lock on the mode dial. I had to quasi-fix mine as well (never got it quite back to original).
Re: the DOF preview button: I think they left it out on the eos50 but a hardware dof preview button is present on the eos30 - very convenient. There are many reasons this is still my favorite eos and this is one of them
I had a 630; just re-sold it. Very solid camera, I loved it, but I kept the A2 instead. I also have a Rebel G, which are going for cheap. They are SO light, it's the one my girlfriend likes to use. But it is noisy and slow, compared to the A2.
If my mode dial click stop breaks, I'll just throw the camera away and switch to the Rebel or get another 630 for $10 on ebay.