For my first blog in a while I thought it would be good to draw together answers to some of the questions I’ve seen asked on Facebook both on our own group and other diving and underwater photography groups. I’ll start this series off with some questions relating to dome ports and domed wide angle wet lenses.
If you want to learn more, please visit our website optec.
This is a somewhat misunderstood thing, in that people often think that a dome widens the camera’s angle of view. This isn’t completely true. Instead fitting a dome containing an air space in front of a camera lens or a wide angle wet lens actually restores the lens’ original field of view. We do this because the physics of light passing from water through a flat clear surface into an airspace results in things appearing about 25% closer than they actually are. This isn’t an issue for macro photography where we want small subjects to be magnified but for wide angle photography where we want to fit as much as possible in the shot from the shortest distance it’s better to correct this effect. That’s what a dome does and this is why dome ports or add-on air domes such as the Fantasea BigEyes don’t make your camera’s field of view wider if used on land, only when underwater.
There can be a number of reasons why a camera won’t focus when used underwater. But if it only happens when you are using it with a dome port it’s most likely because of an issue involving how domes work in underwater photography. This is the bit that people often struggle with, but to be honest it isn't essential that you understand it to use a dome port or lens. Essentially, domes use a quirk of optical physics that causes a virtual image to form as light passes from water through the dome material to the air in the dome. Your camera then focuses on the virtual image which to it will appear to be a short distance in front of the dome.
Issues occur with lenses that don’t have a close enough minmum focusing distance for the camera to focus on the image. Users of interchangeable lens cameras may find they have to add diopters on to their camera to be able to use some lenses behind a dome. Diopters in this case are add-on lenses that shorten the focusing distance. The larger the dome you are using the further away the image will appear to the camera and the less likely you are to need a diopter. In circumstances where one might have a dome port or domed wet lens on a compact camera housing it may be neccessary to put the camera in macro mode to get it to focus.
Always bear in mind that domes need to be a specific distance from the camera lens to give the best results. To ensure that you get the appropriate dome and extension rings to suit a lens you will need to consult the relevant port table from your housing’s manufacturer. A better option to avoid risk of a purchasing error is to consult a dealer before buying, they will likely know their way around the port tables and be able to advise you on whether your lens is the best option for underwater. Also if you give them the details of your camera, lens and housing it will fall at their door if they don’t sell you the correct port.
Having the centre of your picture in focus but the edges distorted or blurred is a very common issue when using a dome port or domed wet lens. It stems from using a wide aperture (small f number setting) on your camera. I’ll be honest my optical physics knowledge isn’t up to explaining why this happens but it means that you are limited as to how wide you can set your cameras aperture.
This issue is another reason why having manual control of your camera’s settings is so important for underwater photography. If you keep your camera set in a mode where aperture is controlled automatically while using a dome port or domed wet lens you may find that you are getting soft edges to a lot of your images.
If you do notice distortion at the edges of your pictures try reducing the size of aperture you use by increasing the f number it is set to in Aperture priority or full Manual mode. This issue is more pronounced with larger sensor cameras so you will likely need a smaller aperture when using a full frame camera with a dome port than you may do when using say a Micro Four-Thirds sensor camera.
Have a read of my previous blog ‘Automatic for the People?’ to understand more of the issues surrounding using your camera in Auto while underwater.
This is a question I’ve seen a few times and I try and answer it as quickly as possible before any harm can be done.
Firstly, if you can’t see the scratches in your pictures then leave things alone. Damage too small to show in a picture is unlikely to affect the structural integrity of your lens.
If it does show up in your pictures then you have an issue.
If you are looking for more details, kindly visit Optical lens domes.
While dome ports for housings are made of acrylic or glass many of the domes used on wide angle wet lenses are made from coated polycarbonate. Unfortunately if you try and use a polishing system such as Novus (which works fairly well on acrylic dome ports) to remove scratches from these coated domes you’ll damage the coating and likely make whatever is showing up in your pictures much worse. It is possible to remove the coating entirely and polish out scratches but it is difficult and time consuming. The alternatives are to get the dome replaced which can be costly or edit the scratches out of your images afterwards.
So take care when using these lenses to avoid getting a scratch in the first place. Use neoprene covers or pouches and keep in mind how close you are to things when taking pictures to avoid coming into contact with something.
One of the reasons I’m a fan of the latest Inon UWL-95 wide angle wet lenses is that the domes are a separate part to the lens. They also come in acrylic versions that are a lot less expensive to replace than glass or coated polycarbonate ones.
That concludes my first little compilation of answers to underwater photography questions. I hope you found it useful. If you have questions you’d like answering why not join the Alphamarine Q and A group.
Check out our events page for dates of our courses if you’d like some more in depth education in underwater photography or photo editing. to get in touch for any other information including booking us to do a club talk, one to one tuition or if you are interested in buying equipment from us.
Cheers
If you found this blog useful you can support what I do by donating via Buy Me A Coffee.
We are now affiliated to Wex Photo and Video. By clicking on the banner below you can go to their main page and buying something will help to fund this blog and the other things we do. For more information about affiliate programmes go to our Terms and Conditions.
There are a lot of decisions when it comes to choosing the right dome port for your camera and lenses. Is a glass dome better than acrylic? What’s the proper size to use with a particular lens? These are just a few of the questions we will cover in this article as well as explain what a dome is and advantage of a dome lens.
1. Why use Dome ports for Underwater Photography?
• Domes make it better – Domes help correct aberrations that occur when light travels thru water as opposed to air. Domes can deliver better results compared to flat ports, and also make it easier or possible some results that flat ports would not capture.
• Size matters – Bigger domes are better than smaller domes, for a number of reasons. The tradeoffs are; heavier, bulkier, pricier than smaller domes.
• Glass is optically superior – Glass domes are optically superior to plastic or acrylic, and they do not scratch as easily. VY Optoelectronics Co., Ltd. manufacture both material of dome lenses.
• Customized specially for you – VY manufacture and design optical dome lens to fit any or all lenses thru adaptors and clamps. This design protects your investment and optimizes your usability and upgradeability over time.
• Domes are ideal for split-level and underwater use – While dome ports will not take anything away from your work above water, their impact or difference above the surface is irrelevant. Domes are ideal for split-level and underwater work.
2. GLASS VS. ACRYLIC
• Image quality: Both glass and acrylic perform very well and optical are very similar underwater.
• Price: The decisions between the two is often made on price. Acrylic domes tend to be about half the price or less of a similar sized glass dome.
• Flare: Glass handles flares and shooting into the sun much better than acrylic. If your shooting style includes the sun in many of your wide angle shots than try to stick with glass. Acrylic domes often show reflections in these type of images.
• Depth: Acrylic domes have deeper depth ratings compared to glass domes. Most acrylic domes are capable of 150m compared to 100m for glass. You should always check the actual depth rating of your dome as some, especially the larger domes, can have significantly shallower maximum depths.
• Durability: Glass is much harder to nick or scratch but minor damage to acrylic domes can usually be repaired with polishing kits.
• Balance: Glass domes tend to be better balanced and weighted underwater. Acrylic domes, especially the larger size ones, tend to tilt port up due to the buoyancy of the port.
• Splits: Glass tends to shed water better for split images but you can still receive good results with acrylic.
3. DOME SIZE
All domes are sharp in the center, so when we are comparing the image quality of domes we are concerned more with the corners and edges. In general the larger the dome the better the corner sharpness. Knowing this, it seems like the solution is to use the largest dome possible but this is not always necessary, practical or the best solution. Smaller sensor cameras can use smaller domes and still retain good image quality. This is also more practical as most users of smaller sensor cameras are concerned with size. Fisheye lenses can also use smaller domes and still retain acceptable corners. In fact, in most cases a small dome is preferred over a large dome for a fisheye. This is because fisheye lenses are able to focus extremely close and the smaller dome allows us to get physically closer to our subject. This makes them better suited for close focus wide angle and the smaller size has less drag and is easier to use in tight spaces. Rectilinear lenses will benefit from a larger size dome but you want to use what is appropriate for your sensor size and focal length. Split images also benefit from a larger size dome, creating a thinner meniscus or water line. Whenever your image contains important information in the corners it’s a good idea to stop down to retain good detail.
For more Custom Cylindrical Lensesinformation, please contact us. We will provide professional answers.