40mm, 42mm, 44mm & 50mm Objective - Optical Question of the Year



I am a bit worn out this evening; however I have THE hypothetical question of the year to post. You may or may not be aware of this, however as the light enters into the first lens of the scope and continues on through approximately 10 more, the light rays bend, just like they do when light rays hit a prism or water. This principle affect of light causes bullet impact to shift throughout the day. Hence, the wandering Zero. When the sun is behind you, the bullet will impact a little low; to the right and it will impact a little left, left and the poi will be a little right. This is because of the bending of the light rays as it passes through the lenses of the scope. The question? Does a larger objective and erector tube (larger lenses) magnify or reduce this affect?

That is a strange proposition. If there is any truth in it, I would love to find out more.

Here is my understanding of the situation.

Glass lenses bend light. This is how they form focussed images. Light beams from the target fall on the objective lens of the scope. Consider that light beams from all parts of the target may fall on all parts of the front of lens. The lens and subsequent lenses bend each light beam from each part of the target so that they coinside to form a focused image at some point. In doing this the lenses act as an analog optical computer.

As the scope is processing light beams reflected from the target, the position of the sun is irrivelant in this matter.

The position of the sun during the day should have no optical effect on how the scope holds zero.

Could your observations be due to parallex error, wind, mirage or athmospheric conditions changing as the sun changes position.

Mirage could be a factor. This is where the air between the target and scope bend the light. I have no experience with shooting through mirage as we do not get much here in Ireland.

Does anybody have any info on this.


700: I am not being a bully in saying this, however, it is true; the position of the sun does affect the bullets poi. In addition to this being true, what happens when you add a pair of shooting or sun glasses to the equation?

But, to STAY ON THE TOPIC, Does a larger objective and erector tube (larger lenses) magnify or reduce this affect?
I will throw in my 2 cents and agree with 700.

The image we see through the scope is a straightline reflection from the target (assume no atmosphere). How the target is lit will affect shadows and the air between the target and the obj of the scope. The target doesn't move.

The obj is still in a straight line however, the image reflection may be distorted/bent by the changes in atmosphere conditions.

So the change in POI is a function of the distortion of the reflection due to the changes in air - temp, wind, etc. If we shot in a vacuum, the POI would not change regardless of the position of the light source (assume that amount of light emitted is the same).

As to the amount of distortion vs obj size, the effect is very small and would contradict due to the differences in lense quality, number of lenses, coatings, internal finish, size and thickness of lenses.

You will not be able to qualify that one obj diameter is better then another. Too many other components play a part.

However, if you assume that our eye can only absorb so much light and the exit diameter of light from the scope is equal or larger then the pupil, then the quality of coatings will play a much larger effect then size of objective.

I have "small" obj scopes that work better in dim light then "big" obj scopes. Same diameter tube so let's assume same diameter erector lenses. The number, quality and coatings of the better "small" scope plays a more important role then size of objective lense.


Peter: Especially those little tiny cups of coffee... How are you? Hope all is well in Europe...

Jerry... The poi shift from sunlight is for real. Contact David Tubb, he will concur... Try to accept this point as fact, because it is.

I have sent an email to the one person who can probably answer this and am hoping that he will jump on this string. This same person insists that building scopes with the parallax adjustment on the front ocular bell instead of on the side is the best build for similar reasons. Let’s see what comes of it.

[ 06-11-2004: Message edited by: W ]
I just had a conversation with a friend about scopes after he returned from a sworovski? school. His response was very similar to yours. Dont worry so much about objective size, go for better coated lenses and 30 mil tube. Light gathering is more BS It is actually how much light makes it from one end of the scope to the other that is important and that is where larger tube and better lenses come in.
Rimfire, ah the truth is out there.

If you think obj size matters above all else, go buy a low end Chinese scope with a monstrous front bell. Bet it will not work worth a hoot in low light. Probably will give you a nice headache in any light no matter where it is.

Internals and coatings are what make a scope. "What you see is what you get"

No disrespect to Mr. Tubbs, the laws of physics don't really care what we think. Light travels in a straight line, at least at rifle ranges without black holes near by. Since you and the target are static, any distortion of that image is due to the air between you and the target. The atmosphere is the only thing that can affect a "shift" in that image. Period.

W. would that be the law office? and oh yeah,, those small coffee's,, but is stick 2 or 3 in 1big cup..Europe is looking up this week, had the Euro elections and many anti EU MP's where voted in in many EU countries, i guess you could say that the people are beginning to voice thier discontent.
I don't think this l;ight bending or not is an issue, just bey good optics, and that means usualy European, and use em..
Allow me if you will to throw in a little about the things I know about scopes.

1. Light does indeed travel in straight lines, however incidental light hitting a lens can cause images to be affected because of internal reflections with in the lenses. I usually therefore add a 4" or longer mirage tube to the scope as shown below.

However, I believe the most pronounced effect on POI is caused by incorrectly set parallax adjustments. 2 to 4 moa variations can result if parallax is set improperly.

2. Increased magnification does result in a reduction of lens transmission efficiency. It has been my practical experience that increasing the objective size can compensate for this to some extent. Most noticeable is the effect of a 58mm objective on my 6-30x58 SN3. This scope by the way has a 35mm tube which increases allowable erector tube travel. However, for lower powers (and the function appears to be non-linear) say 12x to 15x and below don't require larger objectives unless shooting in lowlight conditions, again in my opinion.

3. Coatings tend to enhance color fidelity and may therefore increase resolution indirectly, by making color boundaries on the target more apparent.

4. Lens centration, the alignment of their axis is important in maximizing lens performance. This is a direct result of accuracy of tube construction and lens mounting system.

5. I find that with premium scopes that mirage becomes (and this is a subjective judgment) incremental rather than continuous, allowing it to be seen and therefore shot through. The mirage distortion appears to be in waves rather than a continuous blur.


[ 06-14-2004: Message edited by: DMCI ]
Pete: No law office... I hve been hearing that the EU "citizens" are not happy with the decisions that have been made for them. We are in Very interesting times for sure. As far as the light issue is concerned, perhaps it isn't That big of a deal, although it does hold true.

DCMI: From the looks of things, you are really enjoying yourself. Nothing bad to say about USO; scopes or their "help." Holy Mackerel...
Well I don't know about all that light stuff, but I do know one thing. You can slap my face and wipe the drool off my chin, That is one fine looking rifle there. That boy is in tall cotton.

Thank you for all the complements, my rifle is very appreciative.

Donald, pardon me, but it ain't the tall cotton, as much as it is the short grass, and those damned elusive PDs down on the farm at 700 plus yards.



[ 06-17-2004: Message edited by: DMCI ]
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