Does focus change with magnification?

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kmd1984

Member
Feb 20, 2016
997
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Utah, United States
HI,

When I look at a target at 10x mag and focus it, then change it to 30x, the image is out of focus? I would think it has to do with,

A. My eyes are not the best anymore
B. Zooming in gives you a higher res so "flaws", or out of focus, is going to be more noticeable. The image is probably out of focus at 10x all ready, I just don't see it? Does that make sense?

Thanks,

Kmd
 
It's not your eyes guys! I've got 20/10 vision after Lasic surgery and the same thing happens to me. 
I think there is much less tolerance at the higher zoom levels. Like you said at 10x things are in focus. You can even move your focus knob around a bit and remain in focus. But, at 30x you have a very small window that has everything in focus. 
If you struggle a lot with finding that sweet spot, double check your ocular adjustment. Maybe it moved or maybe your eyes did change a bit over time and it needs a little tweaking. 
It's the same thing with eye relief. At the higher zoom, you have to get it perfect to see everything bright and clear. At the lowest zoom, you have more wiggle room.
Tom
 
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This will be largely down to Depth of Field / or Depth of Focus - exactly as you describe KMD the scope seems to be out when you increase magnification as the Depth of Field decreases (sometimes quite dramitically) this effect is used to accurately range find via AO or side focus. At very high mag. an object in the field of view can be finely focussed but others a few inches further back or forward can be fuzzy - shallow depth of field.

The aspect Tominco mentions is the effect of changing magnification on the eye relief - although it is rarely talked about (certainly NOT by scope makers) the 'perfect' (for your eye) eye relief changes, forward and backwards, with scope magnification - like all light through lenses it's cone shaped and the apex or point of that cone moves with the magnification - your eye will probably find a point about 4-7mm wide before the point of the cone as the natural place*. If you're lucky your scope maker might mention 'Minimum Eye Relief', they rarely list the full range of Eye Relief.


*There is another 4-7mm 'suitable' slice beyond the apex/point - but it's upside down :)
 
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@ tominco I agree. The more you zoom, the less room for error. Thanks for the input.

@ AirSupply Yeah, I have done that and it works for sure. However, I do not like to zoom back and worth, as my scope has no physical "stop" at the 10x mag spot. So If I zoom in or out and then want to go back to 10x again, I have to take the rifle down, look at it from the top to see where 10x is, and then shoulder it again. Very annoying. 

@BRS Very interesting! Thanks for the input!!!

So,from my understanding, scopes do not change focus when changing magnification. Instead, focus only changes with increased magnification, due to us not having it focused right (at the lower magnification) to begin with!? Correct?

Thanks,

Kmd
 
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"kmd1984"
So,from my understanding, scopes do not change focus when changing magnification. Instead, focus only changes with increased magnification, due to us not having it focused right (at the lower magnification) to begin with!? Correct?

Thanks,

Kmd

Click to expand...
That is almost exactly correct. The scope mechanism does have some play and can exhibit a difference in focus when you zoom out. That is usually such a small amount of error that it can't be noticed. Also as mentioned by others the "depth of field" becomes shorter at higher magnifications unless you also change the aperture. 'Course these issues are analog issues and were all worked out back in the day when digital zoom didn't exist so there is that I guess.

What happens when you zoom and focus is that you are forcing the focus to be better "constrained" for lack of a better term. You are forcing the focus into a narrower depth of field. As you then zoom out the focus remains in that narrow depth of field but the actual depth of field of the device becomes deeper. Since the focus is centered on that narrower depth of field it is literally a "better" focus at the lower magnification. I always use this method with my X-Sight II because the digital zoom on that device is so pixelated that zooming and then focusing at max will leave me with a very crisp focus when I zoom back out to my reference magnification.

Since you don't want to zoom in and focus and then zoom out to your reference magnification (reasonable) maybe you should think about getting a large adjustment wheel for your optic. I am actually just buying fixed magnification glass these days because fixed power optics are less complicated and therefore easier for the vendors to build. That means that it is easier for them to build a mechanically better scope and sell it at a lower price. Absolutely LOVE the quality of fixed mag SWFA glass. If they are good enough for the SEAL teams they are good enough for my old eyes.
 
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"BRS"

*There is another 4-7mm 'suitable' slice beyond the apex/point - but it's upside down :)
Click to expand...

TRUE DAT! and the actual eye relief varies with the magnification (or you may have said that and I didn't catch it ;) ) That is why one should check eye relief at both high and low magnification settings when mounting a variable magnification scope. In particular with a springer where every thing you do different changes your POI one needs to make sure that the scope is mounted so that you don't have to move your head to follow eye relief as magnification changes.
 
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