One of the key features of underwater photography is the
dome port. Despite it's ubiquity, it's effects are largely misunderstood
or at the very least, poorly explained. In this post I'm going to talk
about why we use dome ports, how they work and what's the best way to
use them. Most importantly of all, I'm going to try and explain all of
this in a way that makes it simple to understand. There is a bit of
physics involved but I've tried very hard to make it as easy to
understand as possible.
if you search the interwebs for dome port theory, you may be extremely
discouraged to learn that the information that is out there is not only
confusing but in some cases downright contradictory. The most accurate
explanations of how dome ports work are highly technical, but still
leave out important concepts of dome port theory. The right information
is out there, but it's buried under a lot of noise that doesn't make a
whole lot of sense. My main motivation in writing this post is to try to
clear it up.
A lot of what
I'm talking about in this post is to do with the physics of light and
refraction so a quick refresher might come in handy. Physics may or may
not be your strong suit, but it's worthwhile trying to understand. I
will constantly make references to these concepts throughout this post.
rules of refraction (Snell's law) means that light changing medium
(e.g. from air to glass) causes the light to bend and thus changes
direction. In fact this is basis of all photographic lenses. Without
refraction, photography would be very difficult indeed. Refraction
happens in different amounts for different colours. So red light will
bend less than blue light and cause the colours of the light to split
up. This is called chromatic aberration. The more refraction, the more
chromatic aberration. You don't need to understand why light bends when
it changes medium. You just need to know that it does.
The colours that make up white light refract different amounts. Red
light refracts less than blue light. This is what chromatic aberration
is. The shallower the angle of incidence, the more refraction and the
more chromatic aberration there is.
As you probably already know, the angle that light hits the new medium
affects how much refraction there is. Light that hits the medium dead-on
(at a right angle) will travel straight through. The shallower the
angle, the more refraction there is.
Light that hits a new medium at a right angle, doesn't bend or split up.
In fact no refraction occurs and the light continues straight through.
Here is another important concept. Light travels in
packets that we call photons. Think of them like water drops in a stream
of water. When a
photon of light hits an atom in the real world, that atom absorbs the
light, and then re-emits it in a random direction. Because there is so
many photons hitting it, it means that the atom can be 'seen' from every
all sounds very complex but it basically just means that you can see an
object no matter where you're standing. More scientifically, it means
that a point source of light (an atom) emits light in all directions.
In this little section I want to talk about the two main ways of thinking about the path of light. As
mentioned above, an atom will emit light in every possible direction.
That's great but we also need to consider that real-world objects are
made up of billions of atoms, each of which do the same.
this means for us in the context of this subject is that there are two
ways to show the path of light. These are both shown in the diagram
The two ways of thinking about light. We will need to think about both to understand all the concepts involved.
two sides to the image above almost look like opposites, but they are
not. In fact they are just showing different parts of the same thing.
The image on the left shows the light that is being emitted from a
single point (i.e. in all directions). The image on the right only shows
a single ray from each point.
The reason this is done is
so that the image doesn't get too cluttered. Imagine drawing the image
on the right, but with 3 rays coming from each point. It would get busy
very fast and very confusing.
In most articles on the
subject of underwater photography you will only see diagrams similar to
the one on the right (Type B). I believe this makes dome port theory
harder to understand. In fact, I actually think that diagrams like the
one on the left (Type A) are crucial to proper understanding.
underwater camera housings use some kind of port. In this context, a
port is not a place for a ship to dock. In fact, it's analogous to a
porthole on a ship. It's a small see-through window which you can look
On a camera housing, ports are often modular which means
they can be switched out for ones that are different sizes or shapes.
There are two main types of ports, flat ports and dome ports. The
differences are obvious when viewed side-by-side. One is a flat window,
and the other is shaped like a dome.
A flat port on the left. A dome port on the right.
ports are cheaper, simpler and more durable than dome ports but they
are limited. In order to understand why we need dome ports, we must
first understand how flat ports work, and where they fall down.
flat port is simple. It is a port with a flat piece of acrylic or glass
placed in front of the camera lens. Above water, the effect of this
port is unnoticeable (to most people), but if you take this port below
water, well that's when things start to get interesting.
If you stick a flat port underwater, it has the curious effect of enlarging everything. At least, that's what everyone says it does.
sure looks like it, although it's really not a very good description of
what's going on. For instance, many people equate it to having your
lens increase in focal length (by say 25%). That's really not what's
happening at all and in fact that kind of thinking can actually get you
in trouble when you start thinking about domes.
popular belief, a flat port does actually create a virtual image. It's
just nobody talks about it. As far as I can tell not many people know
that's the case. In my searching I couldn't find a single article that
discusses the virtual image created by a flat port. Many articles
actually incorrectly state that flat ports don't produce a virtual
As you can see in the diagram below, when light travels
through water and is refracted by a flat port it's angle is changed thus
creating a virtual image. In this case the virtual image appears closer
to the lens then where the actual object is. Put simply, it makes
objects look like they are closer to the lens than what they actually
Type A diagram showing how a flat port makes the red
object appear closer to the lens. The white lines are the actual path of
the light. The faded line is where the light appears to be coming from.
This is known as the virtual image and is represented here as a blue
Objects appear to be closer, therefore they appear to be bigger as well. That's just common sense.
might ask what the difference is between this and simply zooming in
with a lens. The answer is focus. When you zoom in with a lens, objects
are magnified but they do NOT appear closer. Focus still occurs at the
In this case the virtual image of objects are
actually appearing closer which makes them look bigger. This means in
order to focus on them, you must focus closer than the actual distance
to the object. In the diagram above, this means focussing on the blue
dot, not the red.
Check out the interactiveFlat Port Virtual Image Calculatorto see how this virtual image works.
large number of underwater photographers are aware that a flat port
will 'magnify' objects (even if they are unaware that it's caused by a
virtual image). This 'magnification' means that wide angle lenses (every
lens actually) won't be as wide underwater. This is alright for
macro photography but not so great for taking wide-angle shots
underwater. This is the first problem with flat ports but it's not the
Have you ever stuck
your face right up to the glass of an aquarium? You can see fine when
you look straight ahead, but not so great when you try to look through
it at an angle. There's too much reflection and distortion to see
clearly. Well it's actually exactly the same thing with a flat port.
to the limitations of Snell's law, light won't enter a medium if it
hits it at too shallow an angle. Instead it bounces straight off. It is
reflected. This is known as total internal reflection and it's how fibre
optic cable is able to transmit light around tight curves over
extremely large distances.
Light that hits the medium (glass) at too shallow an angle will simply reflect straight off. This is a limitation of flat ports.
does this mean for photography? Any lens with a field of view greater
than about 90° will suffer from extreme vignetting. That's another
problem but it's still not the whole story.
when I talked about how more refraction means more distortion and more
chromatic aberration? Well yeah. This is the flat port's biggest
problem. Light from objects towards the edge of frame will hit the port
at a much shallower angle than those in the center. As a result, objects
suffer from more and more distortion as they move towards the edge of
frame. This effect is worsened by both wider lenses, and wider
Have a look at this Type B diagram of a flat port.
See how the slight variations in angle up to top (in the water) end up
getting stretched and exaggerated down the bottom (in the air). This
manifests as distortion in photos.
This creates another limiting
effect on field-of-view. Most underwater photographer's won't put a lens
in a flat port that has a field-of-view greater than 60°. For most
people the distortion and loss of quality becomes too great after that
To fix this, we need a way for the light to travel through
the port with as little refraction as possible. The solution to this
problem is the dome port.
a dome port, it doesn't matter what angle the light is coming from, the
curvature of the dome means that it can still hit the port at a right
angle. In other words, it can hit it dead on.
Type B diagram of a dome port. With a dome port, it
doesn't matter where an object is, light from that object can go through
the dome port unrefracted. This is not possible with a flat port.
the light can hit the port at a right angle, it also means that that
ray of light travels through the port unaffected by refraction. That's
It's important to remember though that the diagram above is only half the story. While it's true that the light
hit the port at a right angle, that doesn't mean that it always does.
Objects emit light in every direction. Only one of them can go straight
through. Check out the diagram below.
Type A diagram of a dome port. The greater the angle from the object, the more refraction there is.
the above diagram, there is only one ray of light that goes through
unaffected by refraction. The rest are affected to various degrees. The
further away the light is from the unaffected ('perfect') ray, the more
refracted it is.
Looking at this you might wonder how the dome
port is any good at all. It's not like you can separate out the perfect
ray from all the others. Or can you?
Well kinda, but we don't
really have to anyway. This is where the aperture in the lens comes into
play. Imagine if we place a small aperture (aka. stop) in the center of
The aperture in the lens allows us to capture just
the good light. Bad light is blocked by the aperture. This also shows
why many underwater photographers will choose to use a small aperture.
There is however one caveat. The lens has to be positioned in exactly
the right spot within the dome in order to capture the best light from
every angle. This is the why dome port positioning is so important.
is only one correct place to put a lens inside a dome port. It's
actually really simple and it's demonstrated in the diagram below.
Type A diagram showing correct dome port position.
There is only one place that will capture all of the 'perfect' rays.
This lens has no glass for simplicity. It consists of just an aperture
on the end of a tube.
The only way for light to get into a lens is
if it goes through the entrance pupil. That is the only thing that
matters. If it doesn't get through there, it cannot be captured by the
If you think of a dome port as half of a sphere then it's
clear that all those 'perfect rays' have to go through the center of
curvature. They are all radii of the dome.
There is a problem
though (yes another one). Most dome ports in the real world aren't
perfect hemispheres. They are only part of a hemisphere. See the diagram
Notice how the common dome has a bit missing. The center of curvature now exists outside the entire dome.
common dome port is missing a part. This makes the port seem flatter
(read: more compact), makes it contain less air and also makes it
cheaper to produce. It's worth noting that many dome port sizes are
available and generally the size they sell it as, isn't the diameter of
the dome's sphere. It's actually the length of the widest part of the
dome. This means that an 8 inch dome port isn't actually part of an 8
inch diameter sphere. It's 8 inches across but it's actually part of a
much larger sphere. This can make it harder to correctly place a lens.
we know that the light has to go through the entrance pupil but what
exactly is that? The entrance pupil is just the apparent position of the
aperture when looking through the front of the lens. It is the virtual
image of the aperture caused by the glass in front of the aperture.
how do you find the position of the entrance pupil? Easy just look at
the lens and measure how far back the aperture appears to be. Remember
the entrance pupil is where the aperture to be, NOT where it would actually be if you cut open the lens to find it.
The entrance pupil is equivalent to the parallax point and the center of perspective. The entrance pupil is NOT a nodal point. This is a common misconception.
The secret to dome port positioning is simple. Align the entrance pupil with the center of curvature of the dome port. That
way all that good light is pointed directly at the entrance pupil,
which will therefore travel through the aperture and thus be captured by
As with the
flat port, the dome port also produces a virtual image. Again, as with
the flat port, in order to achieve focus, a camera must focus on the
virtual image and not on the actual object.
A dome port produces a virtual image that makes the red dot (the object) appear closer to the port. This is the blue dot.
Have a look at the Dome Port Virtual Image Calculator for
a better understanding of how dome port attributes affect the position
of the virtual image. Even better compare them to the Flat Port Virtual Image Calculator for
a comparison of the difference between a flat port virtual image and
that of a dome port. If you do that then you'll see that a dome port's
virtual image has a stronger effect. This means that you need a lens
that can focus a lot closer to the lens.
immediately obvious about the dome port's virtual image from this
diagram is that no magnification occurs. The dome port makes things look
closer but it also shrinks them. This exactly cancels out the
'magnification' effect so everything appears the correct size. Cool huh?
Dome ports solve a lot of the problems that flat ports have but they also introduce some pretty quirky behaviour.
ports have severe field curvature. Basically this means that flat
things appear curved (like the shape of the dome). This isn't a huge
problem because there aren't many perfectly straight, flat things
Dome ports are also bulky, easy to scratch and are full of air (which can be a problem underwater).
that's pretty much all the important dome port theory you need to know.
Hopefully I've helped to dispel a few myths about flat ports and dome
ports and I hope you've found this to be an interesting and new way of
looking at dome ports and dome port theory. I've tried to write this so
that is easy to understand with as few logical jumps as possible. If
you're confused by something, or otherwise feel I didn't explain
something properly, please feel free to leave a comment below. I'll try
my very best to fix anything like that.
Happy underwater shooting!
#article #news #dome #domeport #ports #tutorials #cameras #equipment #underwater #flatport #light #refraction #entrancepupil #lens #distortion #virtualimage