How to Use Marine Monocular
Marine Monocular: How to Use It
Your monocular does three jobs:
- It makes far things look close.
- It measures how big something is and how far away it is.
- It shows you which way you are looking.
This guide explains all three, one step at a time. No experience needed.
What 8x42 means
Two numbers, and they tell you almost everything.
8x is the magnification. Things look eight times bigger. A boat that is 800 metres away looks like it is only 100 metres away.
42 is the size of the front lens, in millimetres. A bigger front lens lets in more light. That is why the picture stays bright early in the morning and late in the evening, when a small pair of binoculars would look dim.
Divide one by the other and you get the exit pupil: 42 ÷ 8 = 5.25 mm. That is the little circle of light that leaves the eyepiece and enters your eye. In dim light your own pupil opens to about 5 to 7 mm, so almost all of that light gets in. This is the reason an 8x42 feels comfortable and bright rather than tiring.
Why 8x and not 20x? The more you magnify, the more your hand shake is magnified too, and the smaller the piece of the world you can see. On a moving boat, 8x is about as much as you can hold steady. That is why marine optics almost always stop around 7x or 8x.
1. Adjust the focus
Look, then turn
Hold the monocular up to your eye and look at what you want to see. Turn the focus wheel slowly, one way and then the other, until the picture looks sharp and clear. Stop there.
Turn the wheel again every time you look at something at a new distance. Something close needs a different setting from something far away.
Wear glasses? Keep them on. If you are short sighted and take your glasses off, the monocular may not be able to focus properly on things far away. Leave your glasses on and fold the eyecup down, as explained below.
The closest you can focus on is about 12 metres. Anything nearer than that will stay blurry no matter how you turn the wheel. That is normal for a monocular of this size.
2. Set the rubber eyecup
Around the eyepiece there is a soft rubber ring. It folds two ways, and setting it correctly is the difference between seeing the whole picture and seeing only the middle of it.
- Not wearing glasses: keep the eyecup up. It holds your eye at the right distance from the lens.
- Wearing glasses: fold the eyecup down. Your glasses now sit where the eyecup was.
If you see a dark ring around the edge of the picture, or you can only see part of the view, the eyecup is set the wrong way. Flip it and look again.
This works because the monocular has 18 mm of eye relief. Eye relief is how far your eye can sit behind the eyepiece and still see the whole picture. 18 mm is generous, and it is what makes this monocular comfortable for people who wear glasses.
3. The reticle: a ruler inside the view
This is the part that turns a monocular into a measuring tool.
Look through the monocular and you will see a cross of fine lines with small numbers along them. That is the reticle. One scale runs side to side, the other runs up and down.
The numbers are marked in mils. Each number on the scale is 10 mils.
So what is a mil?
A mil is a very small angle. You do not need to understand angles to use it. You only need to remember one sentence:
That is the whole idea. The reticle does not measure metres directly, because how big something looks depends on how far away it is. It measures the angle that the object takes up in your view. Once you know that angle, and you know either the real size or the real distance, you can work out the other one.
Measuring how wide something is
Line up one edge of the object with a line on the side to side scale. Read the number where the other edge falls. Add the two numbers together.
Example: a sailboat that stretches across 2 numbers is 20 mils wide.
Measuring how tall something is
Put the bottom of the up and down scale on the bottom of the object. Read the number that lines up with the top of the object.
Example: a lighthouse that reaches 6 on the scale is 60 mils tall.
Too big for the scale? Measure it in two or three steps and add the numbers together. Measure from the bottom to a window, then from the window to the roof, then add.
4. Work out the distance
If you know roughly how tall something really is, the reticle tells you how far away it is.
Left: what you see through the monocular. Right: what it means out on the water. H is the real height. L is the distance you are looking for.
The same formula works with width instead of height. Use the real width of the object and the number of mils it covers side to side.
More worked examples
| What you are looking at | Real size | Mils you read | The sum | Distance |
|---|---|---|---|---|
| Lighthouse | 30 m tall | 60 | 30 ÷ 60 | 500 m |
| Mast on a small yacht | 12 m tall | 20 | 12 ÷ 20 | 600 m |
| A person standing up | 1.7 m tall | 10 | 1.7 ÷ 10 | 170 m |
| Motorboat, side on | 10 m long | 5 | 10 ÷ 5 | 2 km |
| Channel buoy | 2 m tall | 4 | 2 ÷ 4 | 500 m |
Rough sizes to help you guess
The formula only works if you know the real size. You will not always know it exactly, and that is fine. A good guess gives a useful answer. Here are some starting points.
| Object | Rough real size |
|---|---|
| An adult standing | 1.7 m tall |
| An adult in a dinghy, sitting | 1.0 m tall |
| Mast of a 10 m yacht | 12 to 15 m tall |
| Channel or racing buoy | 1 to 3 m tall |
| House, per floor | about 3 m |
| Shipping container | 2.4 m tall, 12 m long |
These are typical sizes, not exact ones. Lighthouses in particular vary enormously, from a few metres to over 60. If accuracy matters, look the real height up on a chart.
How accurate is this? It is a good estimate, not a laser measurement. Two things affect it.
First, your reading. If you read 10 mils when it was really 11, your distance is out by about 10 percent. The smaller the object looks, the more a small misread matters.
Second, your size guess. If you guess a mast is 12 m when it is really 15, your answer is out by a quarter.
To get the best result: rest your elbows on something solid, hold the monocular level, and take the reading two or three times. For anything that matters for safety, use a chart, a GPS or a radar.
5. Use the compass
It tells you the direction you are facing, in degrees.
At the bottom of your view there is a compass scale, lined up directly under the up and down measuring scale. Whatever you centre the reticle on, the number at the bottom is the bearing to it: the direction you are looking in.
The numbers count up as you turn to your right, the way a clock goes. Every mark on the scale is one degree. North can be written as either 0° or 360°, they are the same direction.
Getting a reading you can trust
- Hold it level. A tilted compass card sticks or reads wrong. Keep the monocular flat and straight.
- Centre the object. Put what you are taking a bearing on in the middle of the reticle, directly above the compass.
- Keep the small white window clear. Do not rest a finger or a hand over it. It lets daylight in to light up the compass numbers, and covering it makes them go dark.
- Stand away from metal. Engines, rigging, winches, steel railings, phones and speakers all pull a magnetic compass off course. Take your bearing from an open part of the deck.
Magnetic north is not true north
Every magnetic compass points to magnetic north. The north on your chart is true north, the top of the world. They are not in the same place, and the gap between them is called magnetic declination, or variation on a nautical chart.
How big the gap is depends on where you are on the planet, and it slowly changes over the years as the Earth's magnetic field moves. In some places it is almost nothing. In others it is many degrees. Your nautical chart prints the local value and the year it was measured.
So if you are plotting a bearing on a chart, take the value from the compass and correct it by the declination for your area. For everyday use, such as pointing a friend at a buoy or remembering which way you came, the reading straight off the monocular is fine as it is.
6. Care and cleaning
The monocular is waterproof, so rain and spray are not a problem. Salt and grit are.
- Rinse off salt. After a day on salt water, wipe the body with a cloth damp with fresh water, then dry it.
- Blow or brush dust off the glass first. Wiping a dusty lens drags the grit across the coating and scratches it. Use a blower or a soft lens brush before any cloth touches it.
- Then clean gently. A microfibre lens cloth, or a lens cleaning pen, in small circles from the centre outwards.
- Try not to touch the glass. Skin leaves behind a slightly acidic film that can mark the lens coating over time. If you do leave a fingerprint, clean it off before long.
- Store it in the soft case when you are not using it, somewhere dry.
- Never open it. There is nothing inside for you to clean or adjust, and opening it will let in moisture and let out the dry gas that stops it fogging.
7. Safety
Never look at the sun
Never point your monocular at the sun, and never look at the sun through it. The lens gathers a great deal of light and focuses it onto your retina. It can burn your eye and the damage can be permanent.
The most dangerous part is that it does not hurt while it is happening. Your retina has no pain sensors, so there is no warning. The symptoms show up hours later, when the damage is already done.
This applies at sunrise and sunset too, and during an eclipse. Do not risk it, not even for a second.
This monocular is designed for people aged 13 and over.
Specifications
| Specification | Value |
|---|---|
| Magnification and lens | 8x42 |
| Design | Roof prism |
| Prism glass | BaK-4 |
| Angular field of view | 7.0° |
| Field of view at 1000 yds | 369 ft |
| Field of view at 1000 m | about 123 m |
| Exit pupil | 5.25 mm |
| Eye relief | 18 mm |
| Closest focus | 40 ft, about 12 m |
| Lens coating | Fully multi coated |
| Weather protection | Waterproof |
| Weight | 12 oz, about 340 g |
| Reticle | Mil scale, 10 mils per number |
| Compass | Analog, 0 to 360°, 1° marks |
| Age | 13 and over |
Warranty
Your Windie Marine Monocular is covered for 2 years from the day you buy it.
If something is not working the way it should, get in touch and we will sort it out. Keep your order number handy, and tell us what you are seeing.
Common questions
Everything looks blurry and the focus wheel does not help.
You are probably too close. This monocular cannot focus on anything nearer than about 12 metres. Step back and try again. If it is still blurry far away and you wear glasses for short sight, put them on.
I can only see a small circle, with black all around it.
Your eye is not in the right place. If you wear glasses, fold the eyecup down. If you do not, keep it up. Then bring your eye closer or further until the picture opens out.
The compass numbers are too dark to read.
Something is covering the small white window on the side. That window is what lets daylight in to light the compass scale. Move your hand off it. In very low light the compass will be hard to read, because it has no electric backlight.
My distance answer looks wrong.
Check your units. Height goes in metres, and the answer comes out in kilometres. A 30 m lighthouse at 60 mils gives 0.5, and that 0.5 is kilometres, so 500 metres. Then check your size guess, because that is usually the bigger source of error.
Can I use it in the rain?
Yes. It is waterproof. Dry the eyepiece and the front lens before you look through it, or you will just be looking at water drops.
Does it work in the dark?
It gathers more light than your eye does, so it helps at dusk and dawn and on a moonlit night. It is not a night vision device and it cannot show you anything in true darkness.
Can I use it with both eyes, like binoculars?
No, a monocular is for one eye. Use whichever eye you see better with. Many people find it easiest to keep the other eye open rather than screwing it shut, and it gets more comfortable with practice.
About the numbers on this page. The distance formula works because one milliradian covers one metre at one thousand metres, which is what makes the sum as simple as one division. Treat the answer as a close estimate rather than an exact figure: your reading of the scale and your guess at the object's real size both feed straight into it.
Sources used for the safety and navigation information on this page: