But empirical data is what you are talking about below.
Tube diameter has nothing to do with "light gathering ability," that is all in proportion with the size of the objective lens.
How can that be? The Z6 has a 24mm objective so its EP will be 24mm at 1X, not 9mm. So at 1x, the Z6 and IOR have almost the same EP, 24 vs 26.
At 6X, the Z6 does have a 4mm EP (24/6). The IOR has a 4.33mm EP, not 11mm (26/6), a 9% increase over the Z6. I'm not sure where the 11mm comes from as that would be closer to the EP at 2X.
The IOR would have a 2.6mm EP at 10x, which is only 65% (close to half) of the 4mm of the Z6 at its highest magnification. Also the magnification is not double, it's 1.67 times greater.
I doubt very much you eye could detect a difference on 20-25% more light gathering, especially when the gathering capability difference 24 Vs 26mm is less than 10%.
You jumped from comparing light gathering between the two scopes to comparing light transmission in one sentence. How can you even measure the difference with your eye; the differences are incredibly small. You would need specialized instruments to measure the differences.
I have no clue what you just said here. You are mixing in too many concepts that obscure your point.
I did indeed mean empirical. Good catch there. Here's the best way I can come up with to answer thoroughly answer your questions and provide some clarity to the OPs question. I was hoping that using a practical example rather than diving in to the science aspects would be easier, but it doesn't seem so. Ive made an effort to not include too many aspects, but frankly, it isn't a simple question to answer and need to explain other related components to clearly articulate how main tube diameter can improve the light transmission of an optic. My apologies if it is confusing to you. Before I get going, I should mention that I do understand that many scope manufacturers imply that measuring exit pupil (EP) diameter is obj dia/mag. Sometimes it is pretty accurate, many times it is not. It was essentially that simple 20 years ago, but with advances in engineering, optical design, lens technology, etc. you can be WAY off when using that formula. You'll get a more accurate reading by using ocular diameter, f ratio, main tube length, obj dia, and ocular focal length which would give you effective objective diameter and confirm that the Swaro's EP at 1x is 9.6mm.....
Tube diameter alone has no affect on "gathering" of light or light "transmittance". But a larger main tube certainly opens the door to an array of means with which one can improve upon one of more of any parameter of an optics performance - including light TRANSMITTANCE. Ambient light is "gathered" by any optic at least three times and uses different forms of transmitting that light along the optical path. It's gathered at the objective, collimated (one way light can be gathered) at the focal crossover point inside the erector tube somewhere near the center of the main tube (can vary depending on front or second focal plane designs), and again within the ocular as it is transmitted through the lens(es) of the eyepiece. The image radiates away from the center in opposite directions (the incoming and outgoing image). As the cone diameter increases (both to the front and to the rear), the likelihood of blocking some of the edge image and reducing FOV is greatly increased with a smaller diameter main tube. That, along with a greater ocular diameter, various lenses focal lengths, angle-of-incidence of light to lens, f ratio values, and a shorter distance the gathered light must travel through an optic - could have significant effect on the exit pupil diameter and an array of performance aspects of an optic.
Considering 1x variable optics require internal lenses to be of a certain size to satisfy it's intended purpose, the smaller tubes must incorporate a smaller diameter, longer focal length collimating lens(es) that reduce the angle-of-incidence of gathered light to lenses along the optical path thereby decreasing the diameter of the path of collimated light being projected into the ocular. The larger diameter main tube allows an optical engineer to use larger diameter lenses with a shorter focal length that keep the angle-of-incidence of the light closer to 90 degrees - and up to 95 degrees - to the lens surface. This results in the light transmitting along the optical path to the ocular to be larger in diameter. Larger lenses also allow a scope to be designed at a shorter OAL which could improve EP as well. At the same time, depending on multiple variable related to the lens selection, it could also hurt EP. If the advantages of a larger diameter main tube are utilized in the context of the design intent of the optic in can translate into more light transmission, less chromatic aberation and better depth of focus in a shorter package and larger EP across the variable magnification range. Many of these aspects, among others, are part of calculating "effective objective diameter" (which is what you see on the Swaro website when looking at the 1-6x24) and allows one to correctly calculate EP diameter based off of the entire optical system rather than one arbitrary component.
Alternatively, an optic with a larger main tube design that is not being designed for best in class light transmission could push the angles-of-incidence to the usual small-tube values and leverage other design dimensional advantages of the larger main tube to improve performance of some other parameter, such as magnification adjustability range, added illumination features, different illumination facility altogether, etc. Hence, the reason I feel the OPs question is not one that you can technically answer with a simple 'yes' or 'no'. As those with significant time in this industry are very accustomed to hearing, I would suggest that "it depends" is the best answer until qualifying the granular details of the question.
Main tube diameters are a mix of trade-offs. The bigger tube just makes everything easier to get superior performance, should a manufacturer choose to do so. As said, a highly optimized, well engineered 34/35mm main tube could have better performance in one or more parameters compared to a 30 mm tube scope. Depends on the initial design intent. If any given scope is:
1. Designed and engineered equally
2. All associated performance KPIs are held constant
3. All advantages of a larger main tubes are utilized per design intent
..... the performance of a scope with a 34/35mm tube should always exceed the scope with a 30mm tube - as long as you are comparing the same performance aspects in each optic. It would be equally possible to achieve mid-level perfomance from a 34/35mm tube scope using cheaper components or by employing an inferior design. With a larger main tube the engineer just gets more choices as it relates to multiple performance aspects of the optic as a whole.
