What’s with these huge objective lenses?

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Larger objective lenses bring in more light for poor lighting situation or at dusk/dawn etc. A 56mm obj lens is the largest I have owned. The scope tube size (1”, 30mm, 34mm, 35mm) allows more windage/elevation adjustment for longer range shooting in the larger sizes. The quality of the glass is important regardless of the objective or tube size. I have used a variable at 25X and I cannot discern pellet holes at 100 yds but are easily visible with a 10X scope.
 
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Only practical reason from a FT point of view is to reduce DOF so parallax can help distinguish the range (turning parallax adjustment into a range finder to distinguish between 10-55 yard distances). For a fixed magnification (16x for HFT), a bigger objective generally reduces the DOF so image comes into focus for a given range, this range is marked on the parallax wheel so now you can use the given holdover for that target. If same setup uses a 30mm objective the DOF is wide so hard to distinguish between different ranges. Offcourse, this is a generalization.
 
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Another point in favor of a larger objective lens is the exit pupil. The exit pupil is calculated by dividing the magnification into the diameter of the objective. So a 16x scope with a 56mm objective will have an exit pupil of about 5.5mm. All of the light gathered by the objective exits the optic in a circle 5.5mm in diameter. At dusk and in low light conditions you want that exit pupil to fully fill the pupil of your eye as it dilates to compensate for darkness. The average pupil when dilated is between 4 and 8 mm in diameter. 
 
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The size of the objective lens improves two things when all else is equal.

It lets in more light, and allows more potential detail in the final image.

As magnification increases, more light is required. You can see this quite perceptibly in most zoom scopes, as you get to their highest magnification, image clarity and brightness decreases. So the more magnification you want, the larger the objective has to be. Again, at the same time, assuming quality of glass remains constant, that larger element is also allowing for more detail. The "sharpest" most detailed lenses in photography world are the multi $K "super telephotos like 600mm f/4 with dinner plate size objective lens out front. These have higher resolving power.

For your 7x fixed or 3-9x zoom, 30-40mm is just peachy for most shooting situations. If you increase the magnification without increasing the objective, the final image will be darker and darker and with less detail.


 
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on a budget scope they do nothing, all benefits are cancelled out ... but on a decent quality scope image clarity and size are increased, gives you a brighter, larger field of view at a given mag when your zeroing in on ole fuzzy nuts ... of course the compromise is more size and weight and at really high mag you need it just to squeak by acceptable .. at higher mag big objective in itself wont fix things without quality design and glass, in other words, a 24x bargain scope with a 56obj is still a cheese stick and anything over 12x or so is practically unusable ...
 
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Then why stop at 56mm? Why not 65mm, 75mm, 90mm? As many of you added, quality of design from the gitgo is the key, and that encompasses all of the rest of the details in the manufacture of a telescopic rifle sight. Furthermore, I’m not sure that the diameter of the objective plays any part in the resolving power of a lens. What little I know(and it is only a little) about lenses is that resolving power without decent contrast is worthless. So, I’m kinda thinking(guessing really), that a larger objective is actually helping increase the contrast of a lens due to its greater brightness-thereby creating the “illusion” of better resolution. I’m also thinking that with larger objectives comes shallower depth of focus. I’ve been an amateur photographer for over 45yrs. I learned a long time ago that the smaller the f-stop of a lens, the greater the depth of focus. And an increase in depth of focus can also be wrongly interpreted as an increase in resolving power. Additionally, quality multicoatings reduce flair, which then increases contrast…which can make a significant increase in image quality. But I’m talking about photography here, not rifle sights. So, like I stated above, I’m very ignorant about the subject. I know there is exact science on the matter, but I’m a terrible mathematician, so I hit a wall of understanding the physics here very quickly. Nikon has a reputation for making ultra high quality lenses, so back in 03 I bought a Nikon scope-it had terrible resolution and I ended up selling it due to my disappointment. I learned that going only on a brand name to decide on a purchase can be a huge mistake. Same can be said about objectives, going by size alone is not a constructive method by which I can discern quality. It’s a deep rabbit whole, and I think I’ve already gotten lost in it. 
 
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The reason most scope don’t go bigger than 56mm is front of the scope will touch the barrel without super high rings while relatively minimum light gathering gain in percentage while gaining weight and size considerably. The most important reason is if the objective lens get too big then the light has to bend more as it reaches the lenses in the main tube which create image quality problems, there is a strain degree of bend scope manufactures trying to stay under. 56mm objective on 34mm main tube configurations is where all the high end scope manufactures decided to stick to for many many reasons. some even go as far as 56mm objective lens with 40mm main tube, while this sounds like the object lens should increase dramatically but the internal lenses in 40mm tubes scopes are really much/any bigger compared to 34mm scopes. 


also size and quality obviously aren't interrelated two completely different subjects. Made in Japan Nikon was cats meow back in the days but Nikon scope was made in Philippines with ok/good quality but not Nikon of yesteryears. 








 
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Is it fashion, or is it actual advancement in technology? Do they really need to be that big? I, personally, am fine with 30mm objectives. Even in low light situations, I’ve never had a problem. Additionally, glass is heavy, so less is lighter. Thoughts and opinions?
Click to expand...

I keep mine small & light for hunting! But for target work or just testing loads I will sometimes place a 6-18x44 on my rifle.
 
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🔹Some time ago, I started to look at scopes with smaller objective lenses for hunting – because the depth of field increases – or as I call it, the Sharpness and Parallax Range (SPR) increases. The benefit being that I don't have to adjust the image sharpness and parallax so finely every time my quarry moves a bit closer or farther – it's all pretty sharp and parallax free.



🔹BUT... 😕

Almost at the same time I got my first scope cam.

And I really like filming at a very high frame rate (240fps) in order to slow down at playback and get detailed pics of the projectile in flight.

And (a) filming with a scope cam, and (b) filming at a high frame rate, guzzle down light like a wino after a 10-day stay stint in custody of the state. 😲

So, I ditched that idea and now I go for the big objective lenses. (Their benefit is also that the eyebox is larger....)





🔹Of course, I keep telling myself that I really should continue to look for smaller objective lenses, and make up for the need of more light by getting better quality scopes that come with glass with a higher light transmission. 

So far, so good. But, once I start telling that same story to my wife.... ...she will finally have PROOF that I have lost my mind, buying $1500 scopes to mount on $500 guns.... 🤣 

Matthias





🔹PS: Yesssss, the argument could be made another way, as in: "First, buy a $2000 gun. Then you have a justification to buy a $1500 scope for it." 

However, I love my wife and don't want to be kicked out. So, I will not make that particular argument....! 🤣 




 
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