Two glasses frames can have the exact same measurements. One fits perfectly while the other feels like it’s sliding off your face. The reason for this usually comes down to one thing.
Bridge fit.
If you’ve ever noticed the series of small numbers on your glasses frames like; 50 □ 18 145, you might’ve wondered what they actually mean.
It’s that second number, 18, that refers to the distance between the lenses, known as DBL.
While it’s a useful reference point, DBL isn’t a reliable indicator of how a frame will fit your nose. In fact, that measurement has more to do with your lenses than how the frame feels.
So what’s going on?
DBL is the horizontal measurement in millimetres between the closest inner edges of the lenses. It’s always the middle number in the three number sequence (mentioned above).
Bridge fit, on the other hand, describes how the frame actually interacts with your nose. It’s influenced by shape, nose pad angle and material thickness, not simply the distance between the lenses.
To understand what you should look for in a new pair of glasses, let's take a closer look at the factors that can make a frame more comfortable for your nose.
Where do I find the DBL on my glasses?
On almost any pair of glasses or sunglasses, you’ll usually find the DBL as part of a three number sequence printed or etched onto the inside of the temples (arms). This is the standard way of how glasses frames are measured.
For example:
50 □ 18 145
50 = lens width in mm
18 = DBL in mm
145 = temple length in mm
So in this case, the DBL is 18mm.
Why DBL is helpful (but limited)
DBL is useful as a starting point. If you already own a pair of glasses that fit well, comparing the DBL can help you narrow down similar options. It gives you a rough idea of how closely the lenses sit together.
But here’s the catch.
Two frames with the exact same DBL can feel completely different on your face. This measurement doesn’t account for how a frame actually interacts with your nose.
That’s because bridge fit is influenced by things like:
- Bridge style
- Bridge location
- Frontal angle
- Bridge splay
- Material thickness
What is bridge fit?
Bridge fit refers to how a frame fits your your nose and the (maxilla) bones on either side. Unlike DBL, it’s not a fixed measurement. Instead, it’s influenced by the design and construction of the frame itself.
Two frames with identical DBL measurements can fit completely differently depending on how their bridges are designed.
To understand why, it helps to break bridge fit down into the key factors that influence it.
#1 Bridge style (Keyhole vs Saddle)
As we don't make metal-rimmed frames here at Banton, this simplifies the two types of bridge style amongst full-rim acetates frames. (The kind we handmake.)
A keyhole bridge has a distinctive shape with a small oval gap above the nose, just like the name suggests. This design tends to sit slightly higher on the face, with the contact point positioned lower within the frame. Depending on the angles, it can feel more open and less restrictive.
A saddle bridge has a smooth continuous curve that follows the contour of your nose more closely - a bit like an archway. There’s usually little to no gap between the frame and your nose, which often creates a more “hugged” aesthetic and interaction on your nose
#2 Bridge location
Bridge location refers to where the bridge sits within the frame.
High bridge placement, closer to the top of the lenses, typically creates a tighter fit.
Low bridge placement, positioned lower within the frame, usually creates a wider fit.
Even with the same DBL, this alone can dramatically change how a frame feels.
#3 Frontal angle
Frontal angle describes how the lenses converge towards the bridge.
A gently narrowing angle creates a closer, more compact fit.
A slacker angle opens things up, giving a wider feel.
It’s subtle, but it plays a big role in how a frame sits across your nose.
#4 Bridge splay
Bridge splay is the angle at which the frame rests against your nose. A useful way to think about this is like a valley.
A steeper valley creates a tighter fit.
A shallower valley creates a wider, more relaxed fit.
This is one of the biggest contributors to whether a frame feels snug or roomy.
#5 Material thickness
Material thickness is often overlooked, but it matters more than you might expect.
A chunky acetate frame might have a DBL of 20mm, but a significant portion of that space is taken up by the material itself. Think of this like wall thickness.
In practice, that reduces the usable space for your nose, making the frame feel tighter than the number suggests.
Thinner frames, on the other hand, can feel more accommodating even with a smaller DBL.
Why DBL and bridge fit don’t always match
A good example of this mismatch is aviator sunglasses. Some large aviators have a DBL as small as 13mm, which is smaller than many children’s frames.
But because of their large lenses, high bridge placement and wide frontal angles, they can still offer a wide fit.
So how do I choose new glasses?
If you’re researching our handmade eyewear, DBL is a good place to start. But it shouldn’t be your only reference. That’s why we use a simple bridge sizing system alongside the measurements:
Regular bridges are the middle ground. If you’ve never had issues with glasses sitting too high or pinching your nose, this is usually a safe place to start.
Wide bridges are a little larger. Not a vast chasm, just a little roomier if you have prominent bones on the sides of your nose.
Extra Wide bridges are a very generous fit that prevents the frame resting too high or pinching your nose.
To help find the right frame for your required bridge fit, you can use the filters on the left hand side of our collection pages. See the screenshot below.
How do I know what size to choose?
If you’re new to wearing glasses, bridge fit is something you tend to learn through experience. That said, there are a few clear signs to look out for.
If a frame sits too high on your face or pinches your nose, the bridge is likely too small. Conversely, if it sits too low or slides down easily, the bridge may be too wide.
But a big nose doesn’t mean you need a large bridge.
I personally need a wide bridge fit, not because my nose is large, but because my nasal bones sit slightly wider than average. Regular bridges tend to sit too high on me.
Lucy, on the other hand, has lower nasal bones, so regular bridges tend to sit just right. If she wore a wide or extra wide bridge, the frame would likely drop too low.
Is DBL irrelevant?
No, DBL still gives you an idea of a frame’s proportions, but in some ways, it’s more of an indication of lens coverage than how it will fit your nose. Children’s glasses will naturally have smaller DBL measurements, typically 14-17mm. But with adults’ glasses, a small DBL simply means the lenses are closer together.
For example, very large Aviator sunglasses can have a DBL as small as 13mm! This is to maximise lens coverage and give greater protection from the sun.
FAQs
What DBL should I choose for my face?
There isn’t a universal “correct” DBL, as it depends on how a frame is designed as a whole. The most reliable approach is to use a pair of glasses you already own that fits well. Check the DBL printed inside the frame and use that as a reference point when comparing new options. From there, consider the shape and construction of the new frame, as a similar DBL in a different style can still fit quite differently.
Can two frames with the same DBL fit differently?
Yes, and this is where a lot of confusion comes from. Two frames can share the exact same DBL measurement but feel completely different on your nose. This is because DBL only measures the distance between the lenses, not how the bridge is shaped or angled. Factors like bridge style, where the bridge sits within the frame, and the thickness of the material all influence how that space is actually used.
Is a bigger DBL always more comfortable?
Not necessarily. A larger DBL might suggest more space, but that doesn’t always translate to a better fit. If the bridge has a steep angle or sits high in the frame, it can still feel tight. Likewise, a smaller DBL with a more open shape or lower bridge placement can feel surprisingly roomy. Comfort comes from how the frame interacts with your nose, not just the number itself.
Why do my glasses sit too high on my nose?
Glasses that sit too high are usually a sign that the bridge is too small or too tight for your nose shape. This can be caused by a steep bridge angle, a high bridge position, or thicker material reducing the available space. In some cases, even a frame with a moderate DBL can feel restrictive if the design closes in around the nose.
Why do my glasses keep sliding down?
If your glasses slide down, it often means the bridge is too wide or too open for your nose. A shallow bridge angle or a low bridge placement can reduce grip, especially on smoother acetate frames. In this case, a slightly tighter bridge fit usually helps the frame sit higher and feel more secure.
Does frame thickness affect fit?
Yes, more than most people expect. Thicker frames, especially in acetate, take up physical space at the bridge. This reduces the usable room for your nose, which can make a frame feel tighter than the DBL suggests. Thinner frames leave more open space, which can make the same measurement feel more accommodating.
Are keyhole bridges better than saddle bridges?
Neither is better, they simply behave differently. A keyhole bridge tends to sit slightly higher and can feel more open, depending on its shape. A saddle bridge usually follows the contour of the nose more closely and can feel more evenly supported. The right choice comes down to how each design interacts with your individual nose shape.
Should I prioritise DBL or bridge fit when buying online?
DBL is a useful reference point, especially if you are comparing against a frame you already own. However, bridge fit is what ultimately determines comfort and positioning on your face. When buying online, it’s best to use DBL as a starting guide, then rely on descriptions of bridge fit and frame design to make a more informed decision.
Hopefully you found this article helpful. Please check out our other eyecare blogs. Thanks for stopping by.