Reinvested dividends - income or capital gain?

I think I've established that automatically reinvested dividends are treated as a capital gain on sale. Is this correct? If so, at what point is the capital gain evaluated? eg If I've held a share for several years and each year dividends have been reinvested, it would seem that any capital gain depends on the share price at point of aquisition not the price at point of sale. How therefore do I work out and declare the correct capital gain?

Hi,
It will be when you sell the shares.
Thank you.