written in October 2017
I was talking to a friend of mine about reticles and the discussion, as is often the case, turned to reticle design. Reticle design is important and, overall, has improved dramatically in recent years. However, I still see some spectacularly boneheaded reticle design decisions on a regular basis (see a recent post on Styrka scopes). One of the key factors that determines reticle design is whether the reticle is located in the Front Focal Plane (FFP) of the scope or in the Second Focal Plane (SFP) of the scope. In this post, I will refrain from addressing specific reticle designs neyond an example or two, but rather focus on which is preferable for which application.
If I only need the reticle as an aiming point, I want some sort of a simple and visible design, likely in the Second Focal Plane. If the reticle is intended to be used for rangefinding and holdover in addition to aiming, than I definitely prefer more complicated patterns in an FFP design. When the reticle is placed in the Second Focal Plane, it looks the same regardless of which magnification the scope is set on. When your turn the zoom ring, the image will either shrink or magnify; however, the reticle will appear to be the same size (although its size changes with respect to the image). It makes for a consistent aiming point (most of the time), but reticle dimensions (compared to the image) are different at every magnification setting. When the reticle is in FFP, it shrinks and magnifies together with the image. Hence, reticle subtensions cover the same portion of the image regardless of magnification setting (very helpful for ranging among other things). Also, if there are holdover points in the reticle, they will represent the same holdover values at ALL magnifications. Holdover reticles are very popular these days in SFP scopes. The way they work is as follows: since relative reticle dimensions change with magnification, you tweak your scope’s zoom ring until the reticle subtensions work for holdover with your cartridge at the ranges you are interested in. Once that is determined, you are pretty much stuck using that magnification if you need to make a long shot, forcing you to use a variable power scope as a fixed power one. I am not a big fan of that scenario. I think that magnification setting should be chosen based on the conditions (like lighting) and holdover points should work at all magnifications. However, opinions differ. That having been said, I am sort of “on the record” that I think holdover reticles in SFP scopes, in most cases, is a sub-optimal situation.
The are a couple of exceptions to that, of course. The most obvious is a situation where you are only likely to use various reticle features at a magnification setting defined by a hard stop. Excellent examples of that are various low range variable scopes commonly used on ARs: 1-4×24 and 1-6×24. Most of these are equipped with reticles that offer some manner of holdover points of either BDC style or mrad-delineated variety. One of the better examples out of the scopes I have seen lately is the Tactical Dot reticle in Hawke Frontier 1-6×24 scope (picture below is shamelessly copied from Hawke website):
This reticle is comparatively straightforward and the intended use for it is as follows: Use the prominent center-dot aiming point when you need to go fast (i.e. when you are not shooting very far away). When you need to make a longer shot, grab the magnification ring and spin it up until you get to a hard stop (which is 6x in this case). At that magnification setting you have a mrad-based reticle that you can use for holdover, POA correction, etc.
Another exception is target shooting of all sorts where you are shooting at targets at known distances (or in case of a lot of airgun use, at distances that you will be measuring using the parallax ring of your scope; i.e. close range by the standards of a precision/tactical shooter). Target shooters tend to shoot at very small targets and use very high magnification. Because of that, they really benefit from SFP reticles that can be made very thin.
If you have a complex reticle in a scope where you may called upon to use holdover points at setting where there is no hard stop (and frequently under time pressure), things get a bit more complicated. Now, it is perfectly doable and people have been using SFP scopes for years. However, that does not mean it is the best way to go. A fundamental problem with using SFP scopes between the maximum and minimum magnifications is that to use the reticle effectively, you have to know exactly what magnification you are at. To know exactly what magnification you are at, you have to stop looking through the scope and look at the magnification ring. Sometimes, that means you break your shooting position.
When using an FFP scope, the reticle subtensions mean exact same thing at all magnifications, so ultimately, I neither know no care which magnification I am at. If I have to shoot at a target at an unknown distance that I have to find first, I will start at a comparatively low magnification where I have a wide enough field of view to locate this target. Then, I can increase magnification a bit and make some educated guesses on how far the target is and what the conditions look like. Then, if needed I can further tweak the magnification until I get the look I want prior to pulling the trigger. Or might not have the time for any of that and with the target being sufficiently close, I will bump the magnification up a bit after locating the target and fire. Or, if I am spotting for a shooting partner, I might keep the magnification as high as the conditions allow for helping him spot his shots and call out corrections. With a FFP reticle, I do not need to know what magnification any of that is happening at. I never have to take my eye from the target, and I never have to change my shooting position.
Similarly, if I miss and see the impact, I can immediately determine how far away from each other POI and POA are using the reticle subtensions and adjust for the next shot.
All of that is doable with SFP scopes, but unless you are setting the magnification ring at a hard stop of some sort, it is a bit more involved.
Now, onto the downsides:
-FFP reticle that looks appropriately thin at high magnification will look very thin at low magnification.
-From a manufacturing standpoint, the physical size of the reticle cell is smaller for FFP vs SFP, so it is usually more expensive to make.
-Because of that smaller size, it is often harder to make a brightly illuminated FFP reticle
In my opinion, all of these issues can be overcome with good design; however, all of that costs money. For example, with FFP scopes, especially ones with broad magnification range, I insist on having reticle illumination. It really helps in low light at low magnification. Another thing that many reticle designers do it incorporate some fairly thick and visible features into the reticle that are outside the FOV when you are at high magnification. If you need a brightly illuminated aiming point, you need sophisticated design where you may end up with an additional bright dot in the SFP combined with a ranging reticle if FFP (think Minox ZP8).
Lastly, it is worth considering which scope configurations benefit the most from having a FFP or SFP reticle.
As I mentioned above, if you have a 1-4×24 or 1-6×24 AR scope, a good SFP reticle design will save you some money and you will be using the scope at either 1x or whatever the top end magnification is 99% of the time. However, once we get to 1-8x or 1-10x designs, I start leaning toward FFP, simply because the exit pupil gets pretty small at 8x or 10x, so I will be spending a fair amount of time using the scope at some intermediary magnification setting especially if the lighting conditions are questionable.
If you are looking at a hunting scope where you are unlikely to ever take a shot beyond MPBR of your rifle, SFP will work fine for you, although FFP in most cases will too. If you go with a SFP design, make sure your point of aim does not change significantly with magnification. Every SFP scope changes POA slightly with magnification, so that is something you want to check. On quality scopes, this shift should be sufficiently small that you can not easily find it by shooting (I can virtually always see it when testing a scope in my lab). FFP scope, by definition, do not have that issue. These scopes are along the lines of 3-9×42, 2-10×42, 2-12×42, etc. Now, these scopes can be extended to longer distances if need be, but for that, I will dial it up to top magnification.
If you are planning to take longer shots at the range, but the distances are always known, both FFP and SFP will work fine. Choose what looks better to you. I lean toward FFP, but that is mostly because I am lazy and I do not want to worry what magnification I am on. Where I live, it gets pretty hot so mirage is a problem. Because of that, I am seldom at top magnification of a scope.
For example, I’ve got a Leica Magnus 1.8-12×50 on my hunting rifle. The reticle is absolutely day bright and on 1.8x it is exceptionally fast to acquire. It is not quite like a red dot, but not far off. However, at 12x, it is a mrad-based reticle and, for practice purposes, I have shot that rifle with that scope out to 600 yards with good results. I will take it to 1000yards whenever an opportunity presents itself. Of course, it does not hurt that the optomechanical quality of this scope is just superb (and it better be that good considering how much it costs). However, even with warm conditions, I can almost always use 12x. If this scope extended to higher magnification, this would not be so easy and I would lean toward FFP reticle designs.
The most popular configuration for PRS-style shooting is something along 5-25×56 (that includes similar configurations like 6-24×56, 4.7-28×56, 4.5-27×50, 5-30×56, etc). This type of scopes is really well suited for FFP reticles and for tactical/precision shooting. Personally, I use the magnification range along the lines of 3-15×50 the most and prefer it with the FFP. The reason for that is simple: right now around 30% of my shooting practice is done either entirely unsupported or from various screwy shooting positions. For those situation, I really prefer to have 3x available. At low magnifications, the reticle looks steadier and I am less tempted to jerk the trigger.
Once you get into very high magnification: 10-50x, 15-60x, etc, you are dealing with an entirely different shooting discipline. Here SFP reticle rule.