This is a post I copied off of Optics talk forum and a direct quote from Chris Farris .. He is the owner of SWFA and knows more about optics than anyone I can think of . From manufacturing to retail . Here's what Chris has to say about the different positions that the Adjustments for the objective focus on a scope can be located . Quote Chris Farris
1. Front
2. Side
3. Rear
1. Front
This method moves the objective lens in and out hence the term "adjustable objective". Its the first way manufacturers figured out how to adjust parallax and it is probably still the best way to do it. The scope maker can display more yardage markings because of the increased circumference of the adjustment and they can use slow or fast pitched threads.
2. Side
This method is a side mounted third knob on the center saddle section of a scope. Easily viewed and adjusted (for right hand shooters).
This method uses very fast threads to achieve major adjustments in a short movement because of the limited circumference, making it difficult to fine tune. This method also adds to the cost of the scope because it is very expensive to manufacturer. Susceptible to damage because it is the farthest protruding object on the left side of the rifle.
3. Rear
Located directly in front of the eye piece where you would normally change the power on a variable scope. Not widely used because it can only be implemented on a fixed power scope. Easy to access and read for right or left hand shooters. Quite a bit less expensive to make when compared to front or side.
1. Front
2. Side
3. Rear
1. Front
This method moves the objective lens in and out hence the term "adjustable objective". Its the first way manufacturers figured out how to adjust parallax and it is probably still the best way to do it. The scope maker can display more yardage markings because of the increased circumference of the adjustment and they can use slow or fast pitched threads.
2. Side
This method is a side mounted third knob on the center saddle section of a scope. Easily viewed and adjusted (for right hand shooters).
This method uses very fast threads to achieve major adjustments in a short movement because of the limited circumference, making it difficult to fine tune. This method also adds to the cost of the scope because it is very expensive to manufacturer. Susceptible to damage because it is the farthest protruding object on the left side of the rifle.
3. Rear
Located directly in front of the eye piece where you would normally change the power on a variable scope. Not widely used because it can only be implemented on a fixed power scope. Easy to access and read for right or left hand shooters. Quite a bit less expensive to make when compared to front or side.