8x42 vs 8.5x45 Binoculars

The generally “accepted standard” and probably the most commonly made and sold configuration of all full sized binoculars is 8X42 – that is binoculars with an 8x magnification and with 42mm diameter objective lenses. There are many good reasons for this, but is this set-up right for everyone or would something a little different be better for your specific needs?

Having recently reviewed a a couple of binoculars breaking the “standard” mould like the excellent Vanguard 8.5x45 Endeavor ED, I thought it was time to look a little more closely and compare the 8x42 configuration with that of 8.5x45 and see just what difference it makes and see which configuration is better suited to your specific needs.

Why are 8x42 Binoculars So Popular?

First lets take a look at the reasons why the 8x42 configuration is so widely used, especially by birdwatchers:

8x Magnification
An Eight-x magnification is a popular choice, with many people preferring it (especially birders) to more powerful magnifications like 10x. The main reasons for this include:

  • Wider field of view. You spot a bird in the tree canopy and quickly raise your binoculars to your eyes. The wider the field of view, the more likely that the bird will be in it. This is especially important for small, fast moving objects like birds that don’t stay still. Lower magnification in binoculars generally means a wider field of view, this is why so many birders prefer a lower magnification like 8x to higher ones like 10x
  • Better image stability. More power isn’t always a plus in a handheld optic. A 10x image is slightly shakier because any hand movement is magnified as much as the image is. Because of its steadier image, some people find they can actually see more detail hand-holding 8x than 10x binoculars.
  • Often Greater Eye-Relief. This is mostly important to those wearing eyeglasses as your eyes are positioned farther away from the eyepieces. How far back your eyes can be and still see the whole picture is called eye relief. The calculation of eye relief is complex, though generally, the higher the magnification and the larger the intended field-of-view, the shorter the eye relief. So it’s usually easier to find 8x binoculars with a wide field of view and with enough eye relief to work with your glasses than it is to find 10x ones.

42mm Objective Lens
Ok, so we have seen the advantages of using an 8x magnification, especially when compared to 10x. But why is a 42mm objective lens so popular?

  • All other things being equal, the larger the objective lens, the better the resolution in the image. But you pay, with increased weight and size. How much resolution you need depends on how much you’re going to magnify an image. For 8x or 10x magnification, a 42mm objective lens provides plenty of resolution. There’s no need to exceed your eyes’ ability to see. The reason people might choose larger objectives would not be for increased resolution but for more brightness in dim light or when night viewing or star gazing.
  • Brighter sometimes. All 8x42 binoculars create a 5.25mm exit pupil (42mm/8 = 5.25mm). The exit pupil is the diameter of the column of light coming out of the eyepieces. A larger objective lens provides a wider column and allows more light to enter your eyes when your pupils dilate at night.
    The average youthful pupil dilates only to about 7mm even in total darkness, but as you age, your eyes’ ability to dilate gradually diminishes, so a 5.25mm exit pupil may deliver almost all the light your eye can use, even in fairly dim-light conditions. Therefore, a 42mm objective is a good, practical compromise between brightness and weight. In daylight, when your pupils contract to about 3mm, most of the light coming out of the binoculars will fall outside the pupil and never enter your eyes at all. Making the exit pupil larger in this situation won’t make the image look any brighter.
  • Easier to use. Even though it won’t make the image brighter, an exit pupil that is bigger than the pupil of your eyes is sometimes useful. This is because it’s easier to position and keep your eyes in the column of light and prevent vignetting – which is a helpful feature if you have shaky hands or on a unsteady platform like a boat!

So as you can see and to sum up an 8x42 binocular generally offers a great compromise between the size and weight of your optics and their field of view, magnification, exit pupil and eye-relief. However a lot of these assumptions are based on the extremes (comparing them to vastly different configurations), but what if we just tweaked the 8x42 configuration a little could there perhaps be an even better compromise for some people?

8x42 vs 8.5x45 Binoculars

To decide which set-up has the advantage in a particular area and to decide who these advantages would suit, I have decided to take a look at a number of key areas that are affected when you change the magnification and the size of the objective lenses:

Aspects to consider:

  1. Exit Pupil & Image Brightness
  2. Twilight Factor – Ability to resolve details in low light
  3. Magnification
  4. Field of View
  5. Size & Weight

First lets take a few of the constants:

1) Exit Pupil & Image Brightness

  • 8x42 binoculars have an Exit Pupil of 5.25mm (42mm/8 = 5.25mm)
  • 8.5x45 binoculars have an Exit Pupil of 5.3mm (45mm/8.5 = 5.3mm)

So as you can see all 8.5x45 binoculars have an Exit Pupil of 5.3mm – If you compare that to 8x42 binoculars, which have an Exit Pupil of 5.25mm, it shows that the column of light exiting the eyepiece is very slightly larger on the 8.5x45.

During daylight hours and in bright conditions, when your eye pupil size will be only about 2 to 3mm, both 8x42 and 8.5x45 binoculars have an exit pupil larger than your pupil and so will provide your eyes with more light than they can take (some of the light will be blocked by your iris and will not reach the retina) and so you will perceive the image as being equally bright.

However in darker conditions and with your eye pupil dilated to 6 to 8mm, the larger exit pupil on an 8.5x45 will mean that you perceive the image to be brighter (all else being equal) because the greater amount of emergent light at the eyepiece fills more of your eye’s pupil. Thus they will produce a slightly brighter image and with better resolution in low light conditions.

The larger exit pupil also means that the 8.5x45 binocular would be very slightly easier to use. This is because an exit pupil that is bigger than the pupil of your eyes is easier to position and keep your eyes in the column of light and prevent vignetting – which is a helpful feature if you have shaky hands or on a unsteady platform like a boat.

2) Twilight Factor

  • 8x42 binoculars have a Twilight Factor of 18.3
  • 8.5x45 binoculars have a Twilight Factor of 19.6

Twilight performance is not a measure of brightness, but a guide to the amount of detail can be resolved in low light. A larger twilight factor, the more detail you can see in low light, so a larger twilight factor indicates a better resolution when viewing in the twilight or the dim light.

It is calculated by first multiplying the magnification by the objective lens diameter and then finding the square root of the result.

This mathematical formula that shows how both the size of the objective lens and the magnification contribute to a binocular’s ability to show detail in dim light but you must remember that it does not take into account the quality of the lenses and prisms or their coatings and so it can only be used to estimate performance and should only be used as a rough guide to compare the performance of different configurations of binoculars in low light or poor light conditions.

The reason it only indicates resolution at twilight is because the actual factor that has the greatest impact on resolution or image detail, is dependent upon the amount of light available during the time of observation. During daylight hours, when your eye pupil size will be only about 2 to 3mm, magnification will be the principal factor in image resolution. At night, with the eye pupil dilated to 6 to 8mm, aperture size is the controlling factor. In twilight conditions both of these factors control resolution effectiveness and thus the twilight factor is the term that compares binocular performance under these conditions.

So assuming all else being equal, we can see the 8.5x45 binocular will provide you with a better image resolution in poorer light conditions than when compared to an 8x42.

3) Magnification

With 8.5x, you get just that fraction bit closer and with the 8x you get just that tiny bit of a steadier image. The preference really is personal, but in reality the perceivable difference between the two is very, very minimal.

4) Field of View

In general, binoculars with a lower magnification tend to have a wider field of view (FOV). The difference in FOV between most 8x and 8.5x binoculars is not huge and far less than when you compare most 8x with 10x. To put it into perspective, take a look at some of binoculars I have reviewed below:



5) Size and Weight

It stands to reason that with “all else” being equal, the larger the objective lens, the bigger and heavier your binoculars will be. But not “all else” is equal, different manufacturers and indeed different models from the same manufacturers use different materials which have different weights and thickness’s.

The difference in size and weight is most obvious when comparing compact or mid sized binoculars with very small objectives with full sized ones, but what if the difference is only 3mm? How much difference do 45mm objective lenses really make to the size and weight of a binocular compared to ones with 42mm objectives?

Using the Vanguard 8.5x45 Endeavor ED Binoculars as a typical example of a high quality binocular with 45mm objectives, you can see they sit mid table when compared to other quality, high end 42mm binoculars. This leads me to the conclusion that in reality, the difference in weight between most 42 and 45mm binoculars is more dependant on all materials used in the whole binoculars construction rather than just the size of the glass.

So what about comparing the dimensions of the same binoculars:

Once again you can see that the 45mm Objective lenses really have not made that much difference to the dimensions of the binocular as a whole and the Vanguard 8.5x45 Endeavor ED compares very well with most other 42mm models.


So as we have seen, compared to an 8x42 binocular, an 8.5x45mm will get you slightly closer to your subject and has the potential to provide you with a brighter and higher resolution image in poor and low light conditions and all without adding any weight or size to your instrument. The difference in good lighting conditions would be almost zero.

The only downside is a slight reduction in the field of view, so unless a very wide field of view is critical to you, an 8.5x45 configuration is well worth considering.

Ideal Uses

  • Bird Watching – especially those who get up early for the dawn chorus or those who continue birding at twilight will benefit from the improved low light performance of the 8.5x45mm configuration
  • Bird Watching – In thick forests or jungles where once again light is poor although the reduced field of view compared to an 8x42 may be something to consider
  • General Wildlife Observation – an 8.5x45mm binocular is ideal, getting you that bit closer to the action and performing better at dawn or at or just after sunset when most animals start to become more active
  • Hunting – for the same reasons above and the fact that you get all the benefits with out any real increase in size or weight
  • General Use – just like an 8x42, most 8.5x45 binoculars will excel in a wide variety of areas, making them perfect for general use
  • Astronomy – whilst I would not recommend these as dedicated binoculars for astronomy, if you occasionally like to look to the stars they will perform better than an 8x42

Further Reading

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