Scope Brightness

[EdWalton]

Twilight Factor is a useless number, why can't the rifle scope manufactures use F-stops or something equivalent as camera lens, so while shopping for a scope I could make an informed decision.

 

[bruce_ventura]

Manufacturers love to throw specs at customers. Zeiss introduced the twilight factor decades ago and aspiring manufacturers followed suit. It really is a meaningless metric. Unlike camera lenses, telescopes are a focal - they use the human eye to form and detect the image. The eye has a variable aperture stop called the pupil, so brightness depends a lot on the pupil size of the observer's eye.

Just remember that as long as the exit pupil of the telescope is larger than the eye pupil, the image seen through the telescope will be as bright as the image seen with the naked eye. The larger objective scope produces the same size exit pupil at a higher magnification. Also, eye pupil size in low light is a function of a person's age. A telescope that appears bright to a 70 year old person may appear to have low brightness to a 20-year old.

 

[918v]

Fancy names are a marketing tool.

I see the same problem, but it's more about light transmission than exit pupil. Some manufacturers claim the coatings they use allow 99.5% of light transmission per lens surface, but won't tell you how many lenses the scope has or if all the lenses have the same coatings. From reading the available info it seems only the ocular and the objective get the fancy coatings. Who knows what they do to the internals.

 

[EdWalton]

At beginning or end of day my dad's 50mm Swarovski is brighter than my 56mm Kahles, is brightness only driven by pupil exit.

Thank You MR. Ventura

 

[918v]

At beginning or end of day my dad's 50mm Swarovski is brighter than my 56mm Kahles, is brightness only driven by pupil exit.

No it isn't.

 

[bruce_ventura]

Lens transmission does have an effect on brightness, but it's usually small compared to exit pupil effects. Most scopes have 6-11 lenses or other glass elements, such as an etched reticle. Typical multilayer anti-reflection coatings transmit about 99% per lens for green light, the color for which transmission is usually measured. Very expensive coatings transmit a little more (99.5%), cheaper single layer anti-reflection coatings less (97-97.5%). Most scopes above a $500 price point have fully multi-coated lenses.

With lens coatings, you tend to get what you pay for. Most scopes in the $300-600 price range will have a total transmission of 80-88% and 7 lenses or less. Scopes above about $600 range have fully multi coated optics but more lenses, so the total transmission rises only to 85-93%. A few companies are able to push the limits and get 95% transmission, but those products are usually in the $2,000+ range. There are exceptions, but they are uncommon.

So, the difference in transmission between a decent scope (88%) and a very good scope (95%) is only 7%. However if your eye pupil diameter is 6 mm in low light and you put a 50 mm scope set at 10X in front of it, the brightness will drop by 30% due to the pupil effect alone, which is a much bigger effect than differences in lens transmission from one scope to another in a similar price range. If you back off the magnification from 10X to 8X, however, the brightness of the 50 mm scope goes back up to that of the naked eye (less the loss due to lens transmission).

Can a person see a 7% difference in scope transmission in a side-by-side comparison? Probably, but it won't be dramatic. How about a 3% difference? Unlikely. I can't do it reliably and I've been working in the optics field for over 30 years. A 30% loss of brightness is pretty obvious.

To put it another way, if you set the magnification properly, the light loss through a decent to good scope will be about 10%, give or take a few. If you increase the magnification too far by only 25%, the loss in brightness will be ~40% or more.

 

[Rich Coyle]

Maybe I don't understand brightness. I am seventy years old and can certainly tell the difference in brightness, to me, by how long they last after sun set. The brightest scopes I have used are Nightforces.