Rifle Scopes What’s the plus of having a bigger tube?

[Currycure]

there is also better light transmission through the 30mm tube. same concept as objective lenses. the larger the objective lens, the more light it allows to pass. i've found that with a scope with a 40mm objective lens with a 1" tube vs. a scope with a 50mm objective lens and a 30mm tube the latter will allow you to see your target for up to 45 minutes longer during the hours of dawn and dusk. ie, 45 minutes earlier in the morning and 45 minutes later in the evening.

 

[Glassaholic]

Are you just going to keep posting the same thing Curry? I think you need a "cure" from your amnesia.

 

[7WSMShooter]

Not very good at trolling or maybe his boyfriend touched him deep enough to hit the rewind button.....?

 

[Burdy]

In a rifle scope, the equivalent of what you are calling F-stop is the diameter of the objective lens.

ILya

Is this why DOF is much better with larger objectives? Isn't there a relationship to the length as well?

 

[gnochi]

Is this why DOF is much better with larger objectives? Isn't there a relationship to the length as well?

With laser welders, lenses that are larger or have longer focal lengths give you deeper spot tolerance. For example, a 30mm lens with a 100mm focal length might be able to weld material that’s within a quarter mm of that focal length, while a 50mm lens with a 400mm focal length gives you a couple mm margin either direction. (Numbers not exact.)

I imagine that depth of field is similar, since it’s your eye saying stuff in that general area is close enough to the right distance to be in focus.

 

[goosed]

Is this why DOF is much better with larger objectives? Isn't there a relationship to the length as well?

Is a larger objective lens that big of a factor in DOF?

Might be off base, but just thinking through the basics of how a scope works...

A larger objective gives you more light and as a result clarity potential, but I don't believe clarity is a major player in DOF. Instead a longer objective bell allows the light to be focused over a longer distance, which I would venture to guess plays a larger part in creating what appears to be more DOF.

 

[mmlook]

With laser welders, lenses that are larger or have longer focal lengths give you deeper spot tolerance. For example, a 30mm lens with a 100mm focal length might be able to weld material that’s within a quarter mm of that focal length, while a 50mm lens with a 400mm focal length gives you a couple mm margin either direction. (Numbers not exact.)

I imagine that depth of field is similar, since it’s your eye saying stuff in that general area is close enough to the right distance to be in focus.

need to calculate the f# to determine the depth of field or speed of the lens

100mm focal length/30mm objective = f/3.3
400mm focal length/50mm objective = f/8

high the f#, the longer the depth of field

so you can have say:
50mm focal length/50mm objective = f/1.0
- larger objective than the 30mm, but way faster(shorter depth of field)

in relation to scopes, the objective diameter is only once spec in the entire system.
unless you have two scopes that exactly the same,
can't really say a larger/smaller object diameter absolutely means a faster/slower system

 

[koshkin]

need to calculate the f# to determine the depth of field or speed of the lens

100mm focal length/30mm objective = f/3.3
400mm focal length/50mm objective = f/8

high the f#, the longer the depth of field

so you can have say:
50mm focal length/50mm objective = f/1.0
- larger objective than the 30mm, but way faster(shorter depth of field)

in relation to scopes, the objective diameter is only once spec in the entire system.
unless you have two scopes that exactly the same,
can't really say a larger/smaller object diameter absolutely means a faster/slower system

This is absolutely correct.

If you make a scope of the same objective lens length, but with larger diameter, it will have shallower depth of field (and off-axis performance is harder to control).

ILya

 

[gnochi]

Follow-on question, then; is this a big driver for using smaller objective lenses in scopes without parallax adjustment (like most LVPOs)?

Regarding lasers, looks like there are some other factors at play in the weld spot that favor using a larger focus lens - beam divergence as a function of local energy density, maybe? - but that’s enough out of my field that I don’t want to make a definitive statement. Thanks for the correction on focal length vs objective diameter!

 

[koshkin]

Follow-on question, then; is this a big driver for using smaller objective lenses in scopes without parallax adjustment (like most LVPOs)?

Regarding lasers, looks like there are some other factors at play in the weld spot that favor using a larger focus lens - beam divergence as a function of local energy density, maybe? - but that’s enough out of my field that I don’t want to make a definitive statement. Thanks for the correction on focal length vs objective diameter!

F/# is a driver, but mostly for a different reason, not FOV although it is also important.

Available FOV is inversely proportional to focal length. The longer the focal length, the larger of an image you have to use, so the larger tube diameter you will need. Since LPVOs have wide FOV, you have to use fairly short objective focal length. If you use short objective lens focal length, you have to have fairly small objective diameter as well or a bunch of stuff gets complicated.

ILya

 

[beetroot]

MOA, because inches.

 

[Glassaholic]

F/# is a driver, but mostly for a different reason, not FOV although it is also important.

Available FOV is inversely proportional to focal length. The longer the focal length, the larger of an image you have to use, so the larger tube diameter you will need. Since LPVOs have wide FOV, you have to use fairly short objective focal length. If you use short objective lens focal length, you have to have fairly small objective diameter as well or a bunch of stuff gets complicated.

ILya

We really need your whiteboard Seriously, don't let us rush you, hope you get settled soon

 

[davere]

... a bunch of stuff gets complicated.

Which is why fast, wide angle photographic lenses get stupid expensive... just like fast super-telephoto lenses get expensive, but for very different reasons. CA control, distortion control, etc, all come at a serious cost...