Written by Mark Greer
What I want to accomplish today is to help shooters understand what is happening when you adjust power settings on a variable power scope. To begin with we need to realize that there is a marked difference in a variable power first focal plane scope and second focal plane scope. Not only a marked difference in operation but usually price as well. Each has it’s pro’s and con’s. I will refer to both scopes as having the same low to high power settings as well as reticles. In this case lets use a Vortex 6 x 24 with an EBR 1 reticle that is in MOA subtensions in both First Focal Plane (FFP) and Second Focal Plane ( SFP).
Let’s begin with the first focal plane (ffp) scope and discuss some functions. How do I know if my scope is a ffp or sfp? Look through the optic in question and while watching the reticle adjust the power setting from 24x to 6x. If the reticle changes in size ( lines get closer together) when you decrease the power, you have a ffp optic. With the Vortex ffp the reticle will change size from large to small as you vary the power setting. The reason is as you adjust power settings for your shooting conditions ( low light, mirage, or distance) the reticle should subtend the same distance between subtension lines (plex’s, bars, moa lines…) no matter which power you set your scope at. In other words, with this scope ( remember the Vortex Viper PST) each line in the reticle away from the crosshair represents 2 minutes of angle ( moa) at 24x ( which is 2″@ 100 yards using smoa). Now as we move power down to 6x we notice the reticle gets smaller in our scope tube. Well guess what, that same line we referenced at 24x still subtends 2moa at 6x. The great part about this type of scope is no matter what power setting we are at the reticle subtensions subtend the same distance across the entire power settings. Great for ranging and using the bars for holdovers or even for wind.
There is a down side ( there is always good and bad isn’t there ) to this type of scope. I am not trying to create any argument(s), however, let’s consider shooting on a bright sunny day in Oklahoma say mid July. You would typically have an unbelievable amount of mirage to contend with. Also as you increase distance to target where you would normally enjoy a higher scope power setting you may not even be able to see your target. How do we compensate? Decrease power setting! Well guess what, you cleared up your mirage but your image (target) size decreased as well and so did your reticle. No problem you say, however, at some distances ( say 1500 yards ) some shooters ( I for one ) enjoy a full size reticle for distances like that.
Now for a second focal plane ( sfp ) optic. Let’s use the same brand scope and variable power settings but the HST model that is a second focal plane scope. Set your power on 24x then while looking through the optic adjust the power down to 6x. You should not see any change in reticle size as power is changing. These optics can and do function very well in close as well as distance shooting, however, there are some things you need to clearly understand when changing power settings on a sfp optic. First they work great at long distance when using the teticle subtensions for holdover and wind correction even when you need to reduce power due to mirage or low light levels. Usually when we take time to read the scope manufacturers directions on use ( rarely for me ) we will see a statement that says something like ” the ranging reticle works on high power”. Well we don’t have to take that literally as it will work on other power settings. What happens in SFP optics is when you change power settings from, say 24x to 6x what your are doing is changing the size of your target in the scope from large to small. Remember now the reticle size in a SFP does not change in relation to power setting ( reticle will stay the same size in your scope tube) so each line/bar/moa subtension will subtend a different distance on your target on different power settings. A simple exercise to see exactly what I am talking about is to set up a target with 1″ squares on it at exactly 100 yards. Now look through your optics set at 24x and each line away from the crosshairs should cover two squares or be 2″ apart. Now put your power setting at 12x. That same line will subtend 4″. Change power to 6x and that same line should subtend 8″s. Pretty cool as long as you understand that function. You can even check each power setting to see what the reticle subtends but I personally stick with High, middle and low settings to avoid confusion between multiple optics.
I use a lot of second focal plane optics that have a simple plex system instead of some form of full blown Mil-dot or Minute of angle reticle in this same manner. I definitely suggest which ever type you have you stop long enough to setup the above mentioned 100 yard exercise and check to make sure your optics do match the area of subtension the manufacturer says it is supposed to. I love to show folks that you can connect on a target at 700 yards using ( for example) a Burris Fullfield II 4.5 x 14 Ballistic Plex reticle by simply changing the power setting on your scope so the plex’s subtend a different amount of inches per hundred yards ( not gonna jump into that fray right now ). You might want a ballistic program for those but the principle is the same.
I certainly want to emphasize I am not advocating one type of scope over another. I just want to help people understand what they have and how to use them correctly. I use both and depending on what I am trying to accomplish that day will depend on what system I am using. I do believe in using the right tools to accomplish the goal(s) set for that particular day or shoot.