I have known that the zero parallax setting changed with temperature for at least 30 years, but it didn't have any impact on range estimation since I never used the parallax setting position on a riflescope to estimate range. I am sure there was some POI change due to temps but I wasn't a good enough shot to tell the difference.
This explains why the yardage markings on a parallax dial are just "ballpark" distance markings and also why some makers don't put actual yardage numbers on the dial, just larger to smaller markings indicating longer to closer distances, since the actual zero parallax setting is constantly changing with various temperatures.
- Which March model do you have? The March High Master glass system is said to have less temperature shift than their regular ED glass. I have not tested it myself.
- Did you ever find that you could not focus at 10 yards at some temperatures and could at others? Some of my scopes travel closer than the 10 yard marking, some do not.
- How much off were the settings on your dial compared to the marking from indoors to outdoors at higher temps?
- Was there more of a distance on the dial shift at closer or longer ranges?
- If a parallax dial stops abruptly at 10 yards, have there been instances where you could not dial out the parallax completely at 10 yards because of that? Some of my scopes have movement past 10 yards in the closer range, some do not.
- For the highest precision, it sounds like you would need to calibrate your dial for several different temperature ranges.
- It would be interesting to see how much of a change on the dial there is with every 10 to 20 degrees in temperature. I don't have an oversized dial yet, so can't experiment with it myself.
Feel free to ignore the rest of this as it has nothing to do with the original thread.
For long distance ranging I had a 300 to 30,000 meter Wild optical rangefinder made by Deutsche Optik and some Russian made Newcon rangefinder binoculars that ranged to 2500 yards IIRC which was the longest ranging electronic device under $2000 at the time - in my early years of 1000 to 2500+ yard shooting back in the 80s, I remember reading in an owner's manual for some high dollar scope where it explained why the parallax focus mechanism markings traveled beyond the "Infinity" symbol, because there is no exact yardage associated with the dial position for a zero parallax point - always shifting due to temperature changes. They explained that as the temperature shifts, so does the parallax free set point - thus the extra movement past infinity. I can see how that might mess things up when ranging for Field Target based on calibrated markings.
View attachment 365331 My Wild looks just like this one. Quite accurate for a 60 year old mechanical / optical device. It weighs more than quite a few of my long range rifles and is not fun to carry. If it was deployed at some shooting ranges, I am sure the rangemaster would be running over to stop us from shooting that RPG or Bazooka!
The bar with the squares on either end at the bottom of the photo were used for calibration, which I assume likely reduced errors caused by the current temperature, though I didn't calibrate it in the field very often. In use it is perpendicular to the line of sight and you look down into the eyepiece to take a reading.
This video on YouTube, is on a 1942 WW2 vintage Barr and Stroud brand, but the function is similar to the Wild.
Another video on Youtube that talks about the mechanics a bit used in this type of rangefinder. The Wild uses a split image type of display like what is found in older cameras.
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