Hi there! Newbie underwater photographer here!

Recently got an underwater housing for my Canon R8 and have already tried it on out my dive trip. I used it with the RF 15-30 lens. The photos are great, but I have noticed a slight lack of sharpness. When I first got the housing, I tried it with my RF 24-105 lens and photos were way too blury in comparison to what I shoot on land. After some research how to position the lens in relation to the port I have learned a lot of underwater photography science, so I decided to buy RF 15-30 lens for the start. I'm using Sea Frogs housing with acrylic 6" dome port (it's real outer diameter is actually 160 mm). Not the top choices, but very affordable and good value for money.

Some aspects are not fully clear to me and after my dive trip I have some questons. Please, share your knowledge on the topics below:

1. Lens position. I have seen on this forum that right position is when the entrance pupil (I-P according to photonstophotos.net) is exactly in the middle of the dome. So I have chosen spacer of 37 mm to get the minimal misalignment and for RF15-30 its maximum value is 9 mm and the lens always sits in front of the dome center. MFD of RF15-30 lens is big enough, but not a problem at all. I ruined some shots of nudibranches, but all the rest was in good focus. It seems 14-35 is the better lens option for close focusing.

There always be misalignment, but what is better: to have lens positioned slightly in front of the dome center or slightly behind?

Also I have seen formula: Minimum dome radius = MFD-(I-P), therefore:

• for RF 15-30 R=280-98=102, so D should be 204 mm

• for RF 14-35 R=200-112,5=87,5, so D should be 175 mm

Is it ok to have 160 mm dome port for both lenses? How does port size affect quality?

Are my calculations of lens position correct?

2. Virtual image. How to calculate real MFD underwater for this two lenses? I put MFD plane line on pictures above just for info. I know something about virtual image but don't really understand it.

3. Over/under shots. How to make over/under shots look sharp on the both sides? I saw in some videos that it is possible with wide angle lenses at the f-stops of f/4, f/5.6 on a full-frame cameras, but even at f/16 I had one side blurry and the magnification was different. Is it because of position of the lens inside a dome?

4. Hyperfocal distance underwater. How to calculate it? On some web pages I found formula: 2 dome diameter - 4 thickness of the dome, but no clue from where it should be measured. Does it make any sence?

Here are some of my photos that I like to share:

Edited August 24Aug 24 by KPV

Hi KPV

I'll leave it to others more technically qualified than me to advise on lens positioning. I just goo with what the housing/port manufacturer recommends.

Some thoughts though: wide-angle lenses on an FF camera are notoriously hard to get sharp edges. The camera is effectively trying to focus on a curve and the depth of field is usually insufficient even if stopped down to say f16. This is especially the case with a relatively small dome port. The bigger the domeport the better the chance of even-ish focussing and sharpness but even with a 230 domeport, folks can have problems.

Over/Unders: yep, stop down the aperture and focus the camera at a mid-point. It's very hard to get both elements sharp if, especially, the topside element is distant.

Hyper focal distances and virtual images are interesting subjects if you are into technical details. But, frankly, knowing what they are and how to calculate them won't make much difference to taking images. Composition and lighting is what it's all about.

As Tim states Full Frame sensor cameras behind any dome will "challenge" sharpness out the edges.

Larger 230mm domes at 15-30mm will be a bit sharper at f8-16 but still I wouldn't count on a huge difference.

My advice with gear you have:

Shoot with the dome and extension you have if that's what Seafrogs recommends for your particular lens.

At 15mm for divers, wide scenics, etc. stop down to maybe f8 - 11......

You didn't say if those were shot only ambient light or with a strobe(s).

For over / under shots a larger dome is preferred. You can "cheat" with your smaller dome port. Use f16-22 focussing on the underwater portion then bringing the dome up to get above and below in reasonable focus but all larger dome ports will work better.....

Zoomed to 30mm I'd shoot only critters down to maybe 5" long or so.

Macro tiny critters will be easier with a longer focal length preferably a dedicated macro lens and flat port. Or shoot what you have an crop and sharpen in post for online sharing.

Here's a couple photos of the kiddos in a pool.

Canon R100 APS-C sensor 24 MP camera and RF-S 10-18mm lens (equal to 16-28.8mm on my camera.) Used in the Ikelite DLM housing and compact dome port with pair of the new Ikelite Ecko strobes on TTL using the housing's Fiber Optic TT5 transmitter.

Good luck!

David Haas

Edited August 24Aug 24 by dhaas

Thanks for the reply!
I don't care much about corner sharpness, but would like to get it within reasonable linits. On this trip I was shooting in natural light during the day and I also have two 5000K video ligths that I used earlier with my action camera. But I don't want to use them for photos because the bright light scares creatures I want to shoot. I also not ready for strobes because they are wery expensive and bulky and I dont want to overcomplicate things from the start.

All of the images I attached was taken with shutter speed around 1/160 - 1/250. Anemones and clowns are not in sharp focus because of their movement and mooray eel is very sharp because of good ligthtning conditions and steady shot.

I think that the lack of sharpness that I'm refering to is caused by the wrong lens position like described in https://oceanity.com.au/articles/view/understanding-flat-port-and-dome-port-theory. So I wander is that the case, how much does this affects and what is better to have entrance pupil in front or at the back of the dome center?

I don't have much opportunity to test all this underwater so I want to find out as much as I can on the shore.

You’ve got a SeaFrogs Canon R8 housing? I was trying to see what SF recommend as a port and EXR for your lens but they don’t seem to list an R8 housing.

TimG, Sea Frogs recommended 6" dome port for EF 16-35 f/4 and f/2.8 lenses. Both lenses have MFD of 28 cm. But they have an 8" dome port and I haven't seen any recommendations if it from Sea Frogs. And I also don't want to buy it for now because it is too bulky.

Just now, TimG said:

You’ve got a SeaFrogs Canon R8 housing? I was trying to see what SF recommend as a port and EXR for your lens but they don’t seem to list an R8 housing.

Actually RP housing. Sea Frogs don't have separate R8 housing. RP and R8 shares the same body so you can use it for both cameras. Picture from their official site.

7 minutes ago, KPV said:

TimG, Sea Frogs recommended 6" dome port for EF 16-35 f/4 and f/2.8 lenses. Both lenses have MFD of 28 cm. But they have an 8" dome port and I haven't seen any recommendations if it from Sea Frogs. And I also don't want to buy it for now because it is too bulky.

But you’re not using a 16-35, right? But a 15-30? Why not ask SF what they recommend? It’s not necessarily going to be the same. I don’t think the MFD is relevant. It’s how the lens is positioned in the dome which is key. This is determined by the size of the dome and the length of the extension ring.

33 minutes ago, TimG said:

But you’re not using a 16-35, right? But a 15-30? Why not ask SF what they recommend? It’s not necessarily going to be the same. I don’t think the MFD is relevant. It’s how the lens is positioned in the dome which is key. This is determined by the size of the dome and the length of the extension ring.

They said they haven't tested other lesnes. Positions of lenses specified in that chars are as follows:

Almost like those RF lenses I mentioned earlier.

Hmmmm, might be worth seeing if a comparison on the Nauticam website is any help. They have recommendations for the 15-30 albeit with an 8” dome.

There are some members who are techno wizzes when it comes to the data on that schematic. One of them may well chip-in. I’m afraid they mean precious little to me. Sorry!

I hope they will be interested in this topic :)

Not sure what you mean by "dome centre" but as a starting point for calculating extensions, the lens entrance pupil (EP) should be aligned with the optical centre of the dome.

Almost no domes are full hemispheres, so the optical centre is somewhere behind (camera side of) the port mounting flange. To find exactly where means calculating the actual radius of your dome, if the manufacturer won't tell you.

In terms of dome thickness, I don't think it matters much whether you measure the radius from inside or outside the dome...

With a properly aligned dome and wide angle rectilinear FF lenses up to say 105 degrees FOV, you should be able to get sharpness across the frame to the far edges of the FF format at f11-13, leaving only a little corner softness. That's with Nauticam 140 and 180 domes.

Wider lenses will need larger domes for decent results, OR go fisheye and you can have your small domes with good results.

With narrower normal focal length macro lenses behind domes, you can get sharpness right into the extreme corners from f8; something that is impossible with flat ports at any aperture.

A lot of discussion already, the simple fact is that a 6"dome is really too small for a 15mm rectilinear lens. I would also mention that Seafrogs will get your camera underwater but they use generic extensions and often don't have the optimum spacings in their port chart, they will fit and take photos and generally won't vignette but are not always in the optimum position. For example the R5 plus 8"dome is said to work with 16-35 and 24 105 lenses(with EF adapter) and lenses. Nauticam uses 50mm extension for the 16-35 and 70mm for the 24-105.

Regarding dome port theory this article is very good and includes a calculator to see where dome port virtual image is located.

Understanding Flat Port and Dome Port Theory

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 ta

You asked about hyperfocal distance, this really isn't a thing underwater with dome ports. The lens is focusing on the virtual image very close to the port and is close to its minimum focus distance. To calculate minimum focus distance, UW you can make use of the calculator linked above, as follows:

• calculate how far the minimum focus distance is beyond the dome

• Set the dome size and aperture. The aperture is the size of the entrance pupil so set to minimum

• Change the object distance until the virtual distance = the minimum focus distance from the dome surface you calculated.

You asked if it's better to have too much or too little extension. probably doesn't matter much, one way gives barrel distortion and the other pincushion. 'However too much extension causes vignetting as the dome shades start to show in picture.

Regarding the 14-35 lens and your calculations, the critical things to estimate are the radius of your dome and the entrance pupil location, you didn't mention how you came up with those figures. You can calculate the radius from dome diameter and height with circular segment formula, but generally you need to measure the location of the entrance pupil. You want to use the entrance pupil location at the widest setting. I think at this stage and especially in a 6"dome you won't make a material difference in your photos by investing in another lens. Lots of things contribute to sharpness underwater and it's not all about the lens used though that can make a difference.

On the subject of unsharpness, there are a few things to consider, what aperture were you using? I would think you should use f11-16, tending more towards f16 due to the small dome. How close were you? The first rule of UW photography is get close and if you think you are close enough, get closer still. Secondly you are using natural light it seems and while you can take nice shots in natural light, UW shots lack generally lack contrast. The third shot with divers for example looks very milky - this is because they are too far away and you are shooting into the light. If you were using strobes you could drop the shutter speed to darken the background water and illuminate the subject mainly with strobe light - of course strobe range is very limited and you want all your main subjects at a similar distance so you can illuminate them. Getting this right gives the impression of sharpness from contrast.

In the meantime when shooting natural light, be aware of where the light is coming from, having it behind you tends to work best , UW the light doesn't get under overhangs etc and they come out black and the shadow side of fish for example tends to be very dark.

On the topic of split shots, without strobes the biggest challenge is the wide brightness range. To focus in this situation remember the virtual image is very close and depth of field extends behind the focal plane more than the front. So focus on the UW part and have it quite close to you and use f22, you really need to be in relatively shallow water for it to work well. It is particularly challenging with a small dome as the virtual image is very close and optically the water line is very thick with small domes, not to mention that water movement makes it challenging to hold the split line where you want it.

On the question of your calculations, it is nice to be able to do them to confirm things, however practically speaking are you able to obtain extension tubes close to what you have calculated? Sea Frogs only has a rather limited range available.

I can only say that the change from the 6,5" to the 9" dome (Isotta) with the Rectilinear RF 10-20 is a big quality change.

I have now sharp corner. and i'm a happy diver 😍

It looks like on the Seafrogs website that the extension that they recommend for the 14-35 F4L is the 61mm extension (for the R5 which is really what I think you should be looking at chart wise) - which is inline with what Nauticam and Marelux also have for that lens (40-55mm depending on dome - but I would think that the Seafrogs is more like the 50). For those brands, the 15-30 uses between a 10 and 20mm shorter extension, and mostly 20mm shorter.

I shoot underwater with both of these lenses and the 15-30 is the more forgiving lens and really a great underwater lens (however I often just use it for video so can't comment much on it for stills). I wouldn't 'upgrade' to the 14-35 unless you already own it above water and can use the proper extension unlike the 15-30 (although the 37mm MAY work well with the 15-30 underwater?)

I will also echo what everyone has said about f-stop. I shoot F/11 to F/14 with these lenses and often zoom in just a bit as that sharpens things up a bit (more so with the 14-35 - the 15-30 I tend to shoot at 15). I can shoot F/8 if I am not going to look at the corners.

Also - your photos are nice! If you are producing those quality images, then I don't think you need to focus on fixing gear but more experiment with lighting and camera settings to get more shots that you like. (and adding a strobe - even a cheap one - is the game changer).