I've always used mostly primes but have picked up the odd zoom here and there. Now that I'm retired and sitting on my deck watching the sun set between the oak and maple in my back yard, I have time to contemplate the mysteries of lif, like why is my 200mm prime 75mm longer to the middle of the lens than my 75-210 zoom? Both focused to infinity and the zoom set at 200mm. I know there are people out there that know the reason, but can you put it in terms that an old soldier can understand?