Binos and Aging Eyes

The advantages of a big objective lens begin to diminish as we grow older.


A dockside expert told me that larger objectives made binoculars brighter, and that his optical view was actually brighter than ambient conditions. The problem with that statement is that it violates the first law of thermodynamics:

It is impossible for a self-acting machine, unaided by any external agency, to convey heat from one body to another at a higher temperature.

Lord Kelvin, 1848

You can’t make something brighter with just lenses; you can only make it bigger. Making the image brighter would require increasing its temperature, and you can’t do that without adding electricity as do night vision devices (see PS April 2011, “New-gen Night Vision for Mariners”).

In fact, dimmed by lens absorption and reflection, the brightness of the magnified image will always be slightly less than the original, according to the efficiency of the optics. However, the larger the objective lens does allow for a larger exit pupil, allowing more light to reach the eye’s pupil. The image is not, however, brighter.


Why does this matter? As it turns out, a larger exit pupil benefits some people more than it does others. In short, the notion that “bigger” binoculars are brighter than smaller ones, simply doesn’t hold true.

For example, my bird watching binoculars are compact 10 x 25s, which means the magnification is 10x and the objective lens’ diameter is 25 mm— roughly half the diameter of the 50 mm objective size recommended for sailing binoculars. When I take my 10 x 25s to the local marsh at mid-day, they are just as bright as my 7x50s. Both binoculars are Nikon brand, with optics of similar efficiency rating.

There are notable differences. The field of view of the 10×25 field glasses is narrow, and the greater magnification amplifies the jitters of an unsteady hand. They are fine for peering at stationary wildlife with my elbows propped on my knees, but not for searching for a day mark on a rolling boat.

The other difference is the exit pupil size (the size of the focused beam leaving the eye piece). You might not think exit pupil matters much, but it does.

In bright conditions our pupils contract to 2-3 mm. The exit pupil of the bird watching binoculars is 2.5 mm, so nothing is lost so long as you position them accurately (exit pupil = objective/magnification, or 25/10 in this example). When it gets dark our pupils dilate, making the 7×50’s larger exit pupil more useful.

But when you consider age, and how that impacts pupil diameter in low light conditions, not all eyes are the same.


So what does this mean? I’m 60. Assuming I’m near the average, my exit pupil is about 4-5 mm at dusk and 5.6 mm only when fully dark. The exit pupil on a 7×35 binocular is 5 mm and the exit on a 7×50 is 7 mm. When the sun gets low, the 10×25 binoculars become noticeably dim, as though your pupils have not adjusted. However, well into dusk, I can perceive no difference in brightness between a 7×35 and a 7×50 binocular, because the exit pupil is still as large as my pupils. Additionally, night vision is logarithmic; our day/ night adaptation range is about 4,000 times, not the mere five times increase in sensitivity that pupil dilation accounts for (see PS October 2020, “Are Chart Lights Steering Us Wrong?”). If a larger objective really helped, 7×100 binoculars would be commonplace, and they aren’t.

The exit pupil of my 7×50 Nikons (7.1mm) is more than as large as the exit pupil on my birding binoculars (2.5 mm). If you have young eye that adapt well, twice as much light reaches the retina with the 7x50s. But, if you have mature eyes and the sun has set, they are only 25 percent better. The “better fit” makes it easier to keep your pupil in the binoculars focused beam (exit pupil). This is particularly helpful if you wear glasses when using binoculars.

Additionally, there are downsides to larger objectives. The weight of 7×50 is nearly twice that of 7×35, making them harder to stabilize in chop. They are also longer, increasing the leverage and making them bounce. They take more space to store and they cost nearly twice as much for the same quality optics. The field of view in a 7×50 and a 7×35 is the same, about 7 degrees.

Eye relief. Binoculars are designed to afford certain measure eye relief, the specified gap between the lens and the eye for ideal viewing. A person who wears glasses will need at least 15 mm of eye relief. A person without glasses will only need a few millimeters. Most binoculars come with rubber cups that provide the specified eye relief for people who don’t wear glasses.

Unless you have an astigmatism— which can be corrected only with glasses—you won’t have to wear your glasses for best viewing. Virtually all binoculars have a compensation dial to compensate for differences in magnification required in each eye. If you have two or more users with different eye requirements, they can each mark “their spot” with nail polish on the diopter compensation dial, so they can quickly preset the lenses so they are instantly focused.

Autofocus. Many binoculars advertise autofocus, when in fact they are simply pre-focused to be sharp from about 60 feet to infinity. If you wear glasses for astigmatism, and keep them on when using binoculars, they will work just fine. I prefer to take my glasses off. It’s easier to keep the binoculars steady, and there is less stray light—so autofocus is not helpful to me.


The popular 7×50 binoculars are going to remain the standard, and for good reasons. They suit the needs of the average cruising sailor. I like them on my cruising cat. But for daysailing, I’m increasingly happy with my 7x35s. They are lighter, smaller, cheaper, just as bright, and easier to hold still on a smaller boat.

Ultimate sharpness doesn’t matter as much for most uses at sea, and reviews based on reading an eye chart with the binoculars stabilized on a tripod certainly don’t apply to a boat. What matters is whether I can hold them still in a seaway and if they offer the best advantage for my own eye to pick up channel markers and day beacons. If you have both a 7×35 and 7×50 at your disposal, try using each at night. The difference—or apparent lack of one— may surprise you, too.


18 TO 19 YEARS 6.85 mm (range: 5.6 to 7.5 mm) 
20 TO 29 YEARS 7.33 mm (range: 5.7 to 8.8 mm) 
30 TO 39 YEARS 6.64 mm (range: 5.3 to 8.7 mm) 
40 TO 49 YEARS 6.15 mm (range: 4.5 to 8.2 mm) 
50 TO 59 YEARS 5.77 mm (range: 4.4 to 7.2 mm) 
60 TO 69 YEARS 5.58 mm (range: 3.5 to 7.5 mm) 
70 TO 79 YEARS 5.17 mm (range: 4.6 to 6.0 mm) 
* National Institutes of Health


MANUFACTURER Steiner Steiner Nikon Nikon Nikon 
EXIT PUPIL 7.1 4.3 7.1 2.5 
NOTES Pre-focused, exit pupil matches younger twilight vision Pre-focused, exit pupil matches older twilight vision Exit pupil matches younger twilight vision Exit pupil matches older twilight vision Exit pupil matches daylight vision; for land use only 
RELATIVE BRIGHTNESS 50.4 18.5 49 25 6.3 
WEIGHT (OUNCES) 27 18 32 24 
PRICE $689 $499 $110 $80 $195 
Making Sense of Binocular Terminology

The seemingly simple binocular is actually loaded with features that many new users don’t know about. If you are in the market for new binoculars, be sure to first check the PS website for our many previous tests (

Armor – Plastic or rubber coating that protects the binocular from dents and dings, provides a sure grip, and makes them easy to clean. A must for binoculars used in the marine environment.

Binocular – A hand-held tool used to magnify distant objects by passing the image through two side-by-side lenses joined by a hinge. Creating an image for both eyes simultaneously provides a sense of depth perception. Binoculars are available in a great variety of sizes, magnifying powers, and features to suit any purpose or preference.

Coated Optics – Microscopically thin lens coatings designed to reduce light loss and glare while increasing both sharpness and contrast. Coatings can range from a single layer on one glass surface to multiple coatings on all air-toglass surfaces, and depending on color can affect the color temperature of the light passing through the binoculars.

Diopter Adjustment – A focusing ring on the eyepiece allowing the user to adjust the focus for any differences in vision between eyes. Binoculars with a main center focus will have a single diopter adjustment while binoculars lacking a center focus will usually have a diopter adjustment for each eye. In high-quality binoculars, the diopter adjustment is marked and accurate enough to be set to an eyeglass wearer’s prescription.

Exit Pupil – The diameter, in millimeters, of the circle of light leaving the binocular eyepiece. A larger exit pupil provides a brighter image and makes the binoculars easier to use in low light and at night. The size of the exit pupil is calculated by dividing the diameter of the objective lens by the magnification. All the binos we tested have a 50-millimeter objective lens and a 7x power rating yielding a hefty 7-millimeter exit pupil. In the dark, the average 50-year-old will only have a pupil aperture of 5 to 6 millimeters. This means the binoculars are providing more light than most eyes can use.

Eye Relief – The distance, in millimeters, the binocular eyepiece lens can be held away from the eye with the user still able to see the full field of view. Longer eye relief generally provides more comfortable viewing and is essential for eyeglass wearers.

Field of View – The width or height of the circular viewing area seen with the binoculars. It is measured in feet at 1,000 yards. The manufacturers provided the data in our table.

Fog Proof – Sealed binoculars that are nitrogen purged or filled to prevent fogging due to rapid temperature or humidity changes.

Interpupillary Distance – The distance, in millimeters, between the centers of the pupils of the eyes. The average interpupillary distance in an adult is between 58 and 72 millimeters. The hinge between the lens tubes allows each binocular to be adjusted for various interpupillary distances.

Magnification – The power of the binoculars. All the binos in our test are 7x, meaning they make an object appear 7 times closer than it would with the naked eye. This is considered the maximum usable magnification in the marine environment.

Objective Lens Size – The diameter of the front lens measured in millimeters. A larger objective lens allows more light to enter the binocular.

Drew Frye
Drew Frye, Practical Sailor’s technical editor, has used his background in chemistry and engineering to help guide Practical Sailor toward some of the most important topics covered during the past 10 years. His in-depth reporting on everything from anchors to safety tethers to fuel additives have netted multiple awards from Boating Writers International. With more than three decades of experience as a refinery engineer and a sailor, he has a knack for discovering money-saving “home-brew” products or “hacks” that make boating affordable for almost anyone. He has conducted dozens of tests for Practical Sailor and published over 200 articles on sailing equipment. His rigorous testing has prompted the improvement and introduction of several marine products that might not exist without his input. His book “Rigging Modern Anchors” has won wide praise for introducing the use of modern materials and novel techniques to solve an array of anchoring challenges. 


  1. Once you go to a good image stabilized binocular, you will never go back. You can easily hand hold a 12 power. Canon and Nikon both make reasonably priced ones. The heavier models are easier to hold steady. The gyro stabilized ones are really heavy and really steady but not worth the price and power draw. Try them; you will be astonished.

  2. I am not sure about binoculars BUT I do know rifle scopes benefit from larger lenses. And the DO allow you to see better in low light conditions. A scope is not a single lens (and nether are binoculars) I am not sure about the science BUT the lenses DO give the practical effect of make the most of the available light to allow you to see what you can not with your naked eyes. (Also a lens can capture light and focus that light to a point strong enough to cause a fire so that is practically the same as increasing the heat as it is directing the available light’s heat to a smaller point this same principle is what allows you to see a brighter image projected to you eye then when Not viewed through the lens. As it is taking the dimmer larger image in the first larger lens to a smaller optic But the image seems brighter. Just like light from a larger lens protected to a smaller point is brighter . I could see as much as 20-30 min longer at dusk vs the naked eye so yes optics can effectively produce a brighter image to the eye.