Appropriate objective diameter

[DisplacedHusky]

I will be purchasing a new scope soon and I’m interested in the Trijicon Credo HX line. I’m trying to decide which objective diameter would be the best for me. I primarily hunt big game in Eastern Washington in open country. The options are:
2.5 - 15 x 42
4 - 16 x 50
2.5 - 15 x 56

Weight and cost play a factor and I was leaning toward the 2.5 - 15 x 42, but is that going to hinder me in regards to light transmission at first and last light? I don’t have enough practice experience with different scopes in this range so I will put this to the committee.

 

[willfrye027]

I personally do not like 50mm or larger objectives because you have to mount them higher and it messes with cheek weld. Have been a part of depredation hunts for pigs at night and 40mm scope has been plenty beyond legal shooting light, if that answers your question.

 

[Reed104R]

I agree with Will. The closer your sight is to the bore axis, the better. There is very little benefit to large objectives in my view. Back the power down a little to increase your exit pupil and draw in a little more light at dusk and dawn. Use good binoculars to identify your game. There's a trend toward giant scopes with mammoth tubes and objectives. This is not for me. Maybe I'm old fashioned, but in almost 50 years of hunting, I have never needed an objective over 42mm. You will also find a high quality scope with a 40-42mm objective will draw in more light than a budget quality 50mm. This principle holds true for binoculars as well.

 

[JohnJohnson]

The closer your sight is to the bore axis, the better.

It doesn't really have much of an effect on anything how high your scope is mounted as long as you can get a good cheek weld. You should mount for comfort.

 

[DisplacedHusky]

This is a great place to hang out and hook a brother up with some advice if you need a break from the Rokstock or What cause the Rokslide shift to smaller cartridges threads.

 

[sbaker0029]

This is a great place to hang out and hook a brother up with some advice if you need a break from the Rokstock or What cause the Rokslide shift to smaller cartridges threads.

I'll throw my two cents in, factor in inflation and that I'm just some dude on the internet so take it for what value you find it.

I recently acquired a credo 4-16x50 and have a couple nf shv 3-10x42. I feel they are comparable in glass quality.
So going off those two particular scopes, I don't even notice a real significant difference at twilight hours.

You're eyes can only take in so much light so exit pupil size for each scope at a given magnification is going to matter more than front objective size. I'm not sure how exactly it works enough to explain it but some scopes with a 40 or 42 objective might give off more exit pupil light at full magnification than a 50 or 56.
Just depends on scope manufacturer and what kind of voodoo they use I guess.

That's my semi educated enough in the matter to be dangerous opinion of an answer. Hope it helps.

To answer your question more direct:
Go with the 2.5-15x42 if that's what you're leaning toward. You're not going to see a significant difference by going to a larger objective to really matter enough to change your mind. You won't be missing out.

 

[DisplacedHusky]

For those that rock a big objective, what are the reasons? More light transmission and larger FOV?

 

[Terrapin]

The larger the front objective the better the theoretical light transmission, but parallax causes more problems and the higher you have to mount the scope.

On the other hand, if you go too small it’s a pain to get snow and pine needles out.

I like ~42mm front lenses.


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[JohnJohnson]

For that rock a big objective, what are the reasons? More light transmission and larger FOV?

My two remaining (centerfire) scopes are 50mm and 56mm objectives. I just wanted scopes with certain features/mag ranges and they happened to have bigger objectives. I didn't specifically target a certain objective diameter.

 

[Reddish]

The human eye can’t tell the difference in light transmission between a 50mm and 42mm objective. Quality of glass, polishing and coatings are what determine light transmission. Lens coatings are not equal either…in quality or quantity. Companies boast multi-coated lenses, but of the 9 lenses in an adjusted power scope only the outside two may have more than one coating…it’s become a marketing game.

Scopes designed for low light will have 56mm-60mm objectives and high quality lens coatings.

For normal legal shooting hours 40mm or 42mm objective is sufficient.

 

[Fartrell Cluggins]

This is a pretty good read on light transmission and objective lens diameter. https://www.outdoorlife.com/story/hunting/understanding-riflescope-brightness-misconceptions/

I don't really worry about a 50mm objective scope being mounted 5mm higher than a 40. That's less than 1/4 inch. Hold your fingers that far apart and ask if it's going to make a difference to you. It doesn't to me on any of my rifles.

 

[wind gypsy]

Bigger objectives just make it easier to have a higher optical quality. Think 80+mm spotting scope vs 65 vs 50 - they tend to be much more forgiving of eye placement and offer better light transmission. The more magnification you have, the more having a large objective is beneficial.

But in regard to the OP, any of the options you listed should be more than adequate for hunting. While I don't mind mounting my optics a little higher, all of my hunting scopes are still 42-44 mm objectives just because that what comes on the models that fit my needs in other aspects. If the 3-9 SS or the LRHS/LRTS lines had 50mm objectives and were otherwise the same, i'd use em just the same.

 

[dirtytough]

I have scopes from 40mm-56mm objectives. I specifically bought scope #3 on your list for coyote hunting. The main scopes I have for big game have 50mm objectives and that’s just what they come with.

Idk exact numbers and this could all be 100% incorrect. I’ve read in the past that 4.6mm exit pupil is max usable. So if you have 10x mag you need a 46mm objective for max light transmission.

My thoughts on the 2.5-15x56. Best stands for coyotes are usually dawn/dusk. If it’s out there a ways I don’t mind cranking the zoom. In theory I can crank it to 12x and still have max light transmission.

For big game I don’t feel it’s as big of a deal.

 

[squid-freshprints]

The human eye can’t tell the difference in light transmission between a 50mm and 42mm objective. Quality of glass, polishing and coatings are what determine light transmission. Lens coatings are not equal either…in quality or quantity. Companies boast multi-coated lenses, but of the 13 lenses in an adjusted power scope only the outside two may have more than one coating…it’s become a marketing game.

Scopes designed for low light will have 56mm-60mm objectives and high quality lens coatings.

For normal legal shooting hours 40mm or 42mm objective is sufficient.

It is about exit pupil. This is not nothing.

 

[DisplacedHusky]

This is a pretty good read on light transmission and objective lens diameter. https://www.outdoorlife.com/story/hunting/understanding-riflescope-brightness-misconceptions/

I don't really worry about a 50mm objective scope being mounted 5mm higher than a 40. That's less than 1/4 inch. Hold your fingers that far apart and ask if it's going to make a difference to you. It doesn't to me on any of my rifles.

Thanks for the article link. That helped a lot.

 

[peaceful_ruler]

Don't larger objective diameters create a smaller depth of field? This means that for quick acquisition applications a smaller objective would be advantageous.

 

[cqh1111]

Weight and size are important to me. I’ve never had any problems shooting at big game during legal shooting hours with 42mm objective scope. Could always dial the zoom down a little, if needed.

 

[Reed104R]

It doesn't really have much of an effect on anything how high your scope is mounted as long as you can get a good cheek weld. You should mount for comfort.

A higher scope increases point blank range a little, but not as much as some might think. Rimfire hunters are more apt to miss at close ranges with higher mounted scopes because of the steeper angle of trajectory. A higher scope is more prone to canting errors and easier to knock off zero, which seems to be a problem with many shooters on this forum. The higher a scope is mounted, the more torque exerted through the mounts during recoil. Mammoth scopes mounted high seems to be the trend these days. Maybe I am just old fashioned, but it's not for me!

 

[JohnJohnson]

A higher scope is more prone to canting errors and easier to knock off zero

I don't know about knocking off zero but the "more prone to canting errors" thing is essentially a myth. Mount for comfort with non-trash rings and you'll be fine.