Exit Pupil vs Twilight Factor: Which Factor Matters More for Binoculars in Low-Light Conditions
When it comes to selecting binoculars for low-light or night viewing, the two most commonly discussed factors are Exit Pupil and Twilight Factor.
While many people emphasize the importance of a larger Exit Pupil for brighter images in low-light conditions, higher magnifications with larger objective lenses can result in a higher Twilight Factor rating.
However, as the Twilight Factor increases, the Exit Pupil size may decrease. So, which factor is more important when choosing binoculars for low-light conditions?
Let’s see the exit pupil and twilight factor in detail and know the answer
Exit Pupil
In simple terms, Exit Pupil refers to the diameter of the beam of light that exits the eyepiece and enters the eye. It is an important factor because it determines how much light is reaching the eye and, as a result, how bright and clear the image appears.
Calculating Exit Pupil size is a simple process that involves dividing the diameter of the objective lens by the magnification factor. For example, if you have a binocular with a 50mm objective lens and a magnification factor of 10x, the Exit Pupil size would be 5mm (50mm/10x).
The relationship between Exit Pupil size and image brightness is straightforward. A larger Exit Pupil size means that more light is entering the eye, resulting in a brighter image. In low-light conditions, a larger Exit Pupil size can make a significant difference in the clarity and visibility of the image.
The optimal Exit Pupil size depends on the specific use case and personal preferences.
As a general rule, a larger Exit Pupil size is better for low-light conditions because it allows more light to reach the eye.
However, a larger Exit Pupil size may result in a larger and heavier binocular or riflescope, which may not be desirable for all users.
Here are some examples of Exit Pupil sizes for different magnifications and objective lens diameters:
- Binoculars with a 7x magnification and a 50mm objective lens have an Exit Pupil size of 7.14mm (50mm/7x).
- Binoculars with a 10x magnification and a 42mm objective lens have an Exit Pupil size of 4.2mm (42mm/10x).
- Riflescopes with a 4x magnification and a 32mm objective lens have an Exit Pupil size of 8mm (32mm/4x).
- Riflescopes with a 12x magnification and a 50mm objective lens have an Exit Pupil size of 4.17mm (50mm/12x).
It’s important to keep in mind that Exit Pupil is just one of several factors that affect the low-light performance of binoculars or riflescopes. Other factors such as Twilight Factor, lens quality, coatings, and prism type also play a role in the overall performance of the optics. Therefore, when selecting optics for low-light conditions, it’s important to consider all these factors and choose the best optics for your specific needs.
Twilight Factor
Another important factor that affects the low-light performance of binoculars or riflescopes is Twilight Factor.
Twilight Factor is a rating that takes into account both the Exit Pupil size and the magnification factor of the optics.
Twilight Factor is calculated by taking the square root of the product of the Exit Pupil size and the magnification factor.
For example, if you have a binocular with a 50mm objective lens and a magnification factor of 10x, the Exit Pupil size would be 5mm. To calculate the Twilight Factor rating, you would take the square root of the product of 5mm and 10x, which gives a Twilight Factor rating of 22.4.
The relationship between Twilight Factor rating and low-light performance is straightforward. A higher Twilight Factor rating means that the optics are capable of gathering more light and producing a clear and detailed image (not necessarily bright) in low-light conditions. In general, optics with a higher Twilight Factor rating will perform better in low-light conditions than those with a lower rating.
Here are some examples of Twilight Factor ratings for different magnifications and objective lens diameters:
- Binoculars with a 7x magnification and a 50mm objective lens have a Twilight Factor rating of 18.7.
- Binoculars with a 10x magnification and a 42mm objective lens have a Twilight Factor rating of 20.5.
- Riflescopes with a 4x magnification and a 32mm objective lens have a Twilight Factor rating of 16.
- Riflescopes with a 12x magnification and a 50mm objective lens have a Twilight Factor rating of 25.9.
It’s important to keep in mind that Twilight Factor is just one of several factors that affect the low-light performance of binoculars or riflescopes.
Other factors such as Exit Pupil, lens quality, coatings, and prism type also play a role in the overall performance of the optics.
Therefore, when selecting optics for low-light conditions, it’s important to consider all these factors and choose the best optics for your specific needs.
Let’s now try to find the answer to the original question Which Factor Matters More for Binoculars in Low-Light Conditions
Exit Pupil and Twilight Factor
Twilight Factor is a rating that is used to determine the low light performance of binoculars and riflescopes. The higher the twilight factor, the smaller details we can see at dusk.
However, the mathematical calculation of the twilight factor can be misleading because it cannot be considered a simple index suitable for all lighting conditions and all kinds of objects.
While magnification plays a crucial role in determining how much detail we can see in low light conditions, in some situations the twilight factor overrates the role of magnification, and in other situations, magnification is much more important than the twilight factor suggests.
Now coming to the exit pupil;
Exit Pupil, on the other hand, determines the overall brightness of the image. It is not related to the number of details seen in the image. For instance, a 7×50 gives a brighter image than a 12×50, but the latter allows us to see more details, even in very low light conditions.
Now, Based on the information presented, it seems that a high twilight factor may be more advantageous than a large exit pupil.
But, This is not always the case;
The advantage of a larger exit pupil is that: The larger the exit pupil is, the quicker you can see the details provided by the binocular i.e the time taken to focus and get clarity on the images is less.
Let me elaborate more;
A high Twilight Factor is preferable for seeing small details, whereas a large Exit Pupil may be advantageous for quickly seeing the object in question.
Also, If the object needs to be identified in a fraction of a second, a large exit pupil may be interesting. On the other hand, if the object can be studied with ease, high magnification is preferable.
Therefore, It is a matter of compromise between the two factors and depends on the situation at hand.
Note: it’s important to note that the human eye has a limited ability to dilate, or open up, to allow more light in. This means that even if the Exit Pupil size or Twilight Factor rating is large, the eye may not be able to take advantage of all that light. For example, the human eye typically has a maximum dilation of around 7mm, so an Exit Pupil size larger than 7mm would not provide any additional benefit.
So, how do Exit Pupil and Twilight Factor work together in different situations? Let’s take a look at a few examples:
Birdwatching:
If you are birdwatching in low-light conditions, you want to be able to see the colors and details of the bird’s feathers.
A larger Exit Pupil size will allow more light to enter the eye, resulting in a brighter image.
However, if you are using binoculars with a high magnification, such as 15x or 20x, a large Exit Pupil size may result in a shaky image. In this case, a lower magnification with a higher Twilight Factor rating may be more effective.
Hunting:
When hunting in low-light conditions, you need to be able to see your target clearly.
A higher Twilight Factor rating will allow more light to be gathered by the optics, resulting in a brighter image.
However, if you are using a high magnification, such as 10x or higher, a larger Exit Pupil size may be necessary to provide enough light to the eye.
Choosing the Right Binoculars or Riflescope
Twilight factor and geometric light gathering figure (exit pupil diameter squared) are simply mathematical equations used to calculate the low-light performance of binoculars or riflescopes.
While they can be helpful in comparing different models, they do not provide information on the overall optical quality of the device.
A poorly made 8×42 binocular will have the same twilight factor and exit pupil size as a top-of-the-range 8×42.
Let’s see other factors to consider
Factors to Consider
Here are some other factors to consider when selecting binoculars or riflescopes for low-light conditions:
- Lens quality: The quality of the lenses used in the optics can affect the brightness, clarity, and color accuracy of the image.
- Coatings: Coatings on the lenses can reduce glare, increase light transmission, and improve contrast.
- Prism type: Different prism types (roof prism or porro prism) can affect the overall image quality and brightness.
- Magnification: Higher magnification can make the image appear larger but can also reduce brightness and field of view.
- Objective lens diameter: Larger objective lens diameters can gather more light and produce a brighter image, but can also make the optics heavier and bulkier.
See: How To Choose Binoculars: A Complete Guide Of Choosing Binoculars For Different Applications
An Interesting Video
Conclusion
In conclusion, when choosing binoculars or riflescopes for low light conditions, it is important to understand both the Twilight Factor and Exit Pupil. A higher Twilight Factor is better for seeing small details, while a larger Exit Pupil can provide quick and bright views of the object.
Also, keep in mind that even a low-quality binocular with the same magnification and objective diameter can have the same twilight factor and exit pupil as a high-end one. So these are not the only factors you should see to choose your binoculars.
By taking into account other factors, one can select the right optics for optimal performance in low-light conditions.
