As you know, I’ve been thinking about prism optics a lot lately. As I’m gearing up to do a review and comparison of some of the options on the market, there were a lot of questions I didn’t know the answer to. So I thought it made sense to ask an actual expert.
This week I brought Ilya Koshkin, the Dark Lord of Optics, back on to the live stream to answer my questions (and those of the audience). We covered the basics of how a rifle scope works, what makes a prism scope slightly different, and some of the trade offs that we have to consider when making the switch.
We covered a lot of ground in this session, more than I can effectively summarize on short notice. But here are a few key points.
Rifle scope magnification works based on the relationship between an objective lens and an eye piece lens. While modern scopes have far more than two lenses, this is a helpful visualization.
The distance from the objective lens to where the refracted light comes to a point is called the focal length. In general, the longer this focal length, the better the opportunity to magnify the image. In order to be more compact, prism scopes work by folding this light path amongst several different angles before letting it pass through to the eye piece lens.
Eye Relief and Field of View
There are several engineering decisions that go into balancing magnification, eye relief, field of view, and the overall size of an optic. You could make a prism scope with long and forgiving eye relief, but you will sacrifice field of view if you don’t also increase the size and weight of the optic along with it.
Since prism scopes tend to go on low-recoiling AR-15 platforms, engineers have been willing to sacrifice that long eye relief in favor of compactness. This is best seen with the new generation of microprism optics like those from Primary Arms.
The Trijicon TA33, which I have on my desk for review, remains very compact, but sacrifices field of view in order to have a forgiving eye box. The Swamp Fox Trihawk, also on my desk, maintains the same 3x magnification, but offers a massive field of view and relatively forgiving eye box for the tradeoff of size and weight.
LPVO vs Prism Optics
Ilya made an interesting point while we were talking. He mentioned that the inexpensive prism optics have gotten so good that he would suggest a $300 prism paired with a offset mini dot long before he would recommend any LPVO optic below $1000.
While the flexibility of LPVOs cannot be denied, the optical quality and durability available with prism scopes even at lower price points should not be ignored.