Question about manual exposure

Another former student of mine, John Hamshaw, emailed me with the following question about my article on using Manual Mode exposure settings...

May I ask a question about your M is magic blog article no 1? You give the example of the two basket pics, where you say that you manually set the aperture to 5.6 and manually set the speed to 1/30th sec. This gives a reading on the exposure scale that the pic will be underexposed (obviously by the camera's built in meter), and you therefore correct it manually to 1/8th sec to set the scale reading to '0'. Isn't this the same as setting the camera to Aperture priority, set at 5.6, and when half pressing the shutter release, the same camera reading/meter will set the camera automatically to 1/8th sec?

(No doubt you will think this is a dumb question and what on earth has JH learnt from me in two courses??...but if it isn't and you want to use this question on the blog please do so)

Best wishes, John

Thanks John,
No, those courses were not wasted - I know what beautiful photographs you take.
The only dumb question is the one that is never asked.
Yes, in effect, what you say is correct. BUT... when you are using an automatic exposure mode such as Aperture Priority, unless you use the Auto Exposure Lock (AEL) function, every time you make an adjustment to the your framing of your composition the chances are that the exposure will change as the metrering system reads from a different area of the subject.

This is precisely why I cannot be bothered with auto settings.

I want to take my reading from the area of the subject I choose and for that exposure not to change when I re-frame the picture. Yes, I can do that by locking the exposure - but that seems to defeat the object for me.