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:huh: Hi everyone, I'm hoping some of you might be able to help me out. I recently upgraded to a 5D mkiii Nauticam system. I'm shooting with the Canon EF 17-40 1:4 L USM Lens, 50mm extension ring, and a Nauticam N120 8.5" acrylic dome port, as is the suggested setup, but every image has severe vignetting, regardless of the focal length.

At first I thought it might just be because I was shooting primarily at the wide end, but after experimenting I realised that it doesnt matter.

The wierd thing is that when I first used it i was shooting split shots in mangroves and there was no issues. But while diving and snorkelling, all images have vignetting.

This is the first time I have used a rectilinear lens underwater, having only ever used the Tokina 10-17mm previously, so I'm pretty confused. Is it just the lens? Could it be my Autofocus settings? Could it be that the second hand dome has been polished too much so that now it has thicker edges thus creating the blur?

I'm really at a loss here, and after investing so much money to upgrade, I'm really disappointed. So I would really appreciate some feedback guys!!!

Thanks in advance.

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post some images

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It sounds like you are describing image blur around the edges/corners not darkening (vignetting). That’s normal for underwater wide angle and can be improved by stopping down and/or possibly with the Sea & Sea Internal Correction Lens (not sure for that particular lens/dome combo). 

Edited by mackman
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Need to see images, settings. The 8.5 acrylic port is not the best for that lens; a 9" glass dome is much preferred, along with a longer 70mm extension.

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On 9/21/2019 at 5:55 PM, Chris Kippax said:

post some images

Will do soon.

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23 hours ago, mackman said:

It sounds like you are describing image blur around the edges/corners not darkening (vignetting). That’s normal for underwater wide angle and can be improved by stopping down and/or possibly with the Sea & Sea Internal Correction Lens (not sure for that particular lens/dome combo). 

Your right, that's wht I meant. Lens blur. I have gone as low as f5.6, but I haven't actually compared the images. I will take a look. Unfortunately for me, if this is the case, I prefer darker backgrounds/more contrast that smaller apertures give.

I will look through some recent images and post some comparisons.

Cheers mate.

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18 hours ago, JackConnick said:

Need to see images, settings. The 8.5 acrylic port is not the best for that lens; a 9" glass dome is much preferred, along with a longer 70mm extension.

When I can afford a glass dome I will buy one. Lol I bought the camera with the lens, and I'm not even sure if it's the lens I want to stick with. But if I can improve this lens blur problem I may well stick with it.

I never had this issue with my APS-C and Tokina 10-17mm setup, so it's really disappointing after spending so much money to upgrade!

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With the APS-C you’re cropping out the blurry edges. Try stopping down to f8, f11, or even f16 if you have enough light. The correction lens is about $200 if I remember correctly and is said to give a 2 stop improvement. So if your corners look OK at f11 then with the lens they should look ok at f5.6 and you can shoot at lower ISO. 

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3 hours ago, flamebouyant said:

When I can afford a glass dome I will buy one. Lol I bought the camera with the lens, and I'm not even sure if it's the lens I want to stick with. But if I can improve this lens blur problem I may well stick with it.

I never had this issue with my APS-C and Tokina 10-17mm setup, so it's really disappointing after spending so much money to upgrade!

Tokina 10-17mm is a fisheye lens. Fisheye lenses have huge depth of field at the cost of distortion. Your new 17-40mm is a rectilinear lens - it does correct the distortion, keeping straight lines straight, but this comes at a cost of depth of field limitations. Rectilinear lenses correct for field curvature - imagine that you're shooting a flat plane, and draw lines from your lens to the center of the subject and to the corners; depending on your distance to subject and angle of view, the difference in these distances can range from minor to very significant. Many lenses have aspherical elements in them that correct for this - while your nominal focal distance may be set to, say, 50cm, the effective focal distance at the edge will be much longer - this is how you can take a shot of something flat and have the entire image in focus.

However, underwater, we're not shooting a flat subject - the refraction of light as it passes from water, into glass or acrylic and then into air works as an additional lens element that produces a curved image, where edges are closer to your lens than the center even if the subject is actually flat. Fisheye lenses have no problems with that, but rectilinear lenses need to be stopped down, so that they have sufficient depth of field to fit both the center and the edges/corners into it. Sea & Sea Internal Correction Lens is what is called a field flattener, counteracting this effect to a limited extent, but even with that add-on, shooting wide-open in domes only works if you don't care about corners (for example, if you have only water in the background and the subject fits in the center of the frame). I use a Sony 10-18mm rectilinear lens on an A6300 camera behind an 8" dome, and I keep it at f/11 most of the time.

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2 hours ago, Barmaglot said:

Sea & Sea Internal Correction Lens is what is called a field flattener,

This filter really only works well with a 9" dome and improves the corners by about 1-2 stops. It wouldn't do much for a 8.5" dome and it costs $399.

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9 hours ago, mackman said:

With the APS-C you’re cropping out the blurry edges. Try stopping down to f8, f11, or even f16 if you have enough light. The correction lens is about $200 if I remember correctly and is said to give a 2 stop improvement. So if your corners look OK at f11 then with the lens they should look ok at f5.6 and you can shoot at lower ISO. 

I will take a good look through my recent images and take note of setting so as to judge the differences.

It's my first full frame camera, and first time with a rectilinear lens underwater, so I guess theres just more variables to account for then expected....

I guess I will have to start shooting at much higher ISO.

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6 hours ago, Barmaglot said:

Tokina 10-17mm is a fisheye lens. Fisheye lenses have huge depth of field at the cost of distortion. Your new 17-40mm is a rectilinear lens - it does correct the distortion, keeping straight lines straight, but this comes at a cost of depth of field limitations. Rectilinear lenses correct for field curvature - imagine that you're shooting a flat plane, and draw lines from your lens to the center of the subject and to the corners; depending on your distance to subject and angle of view, the difference in these distances can range from minor to very significant. Many lenses have aspherical elements in them that correct for this - while your nominal focal distance may be set to, say, 50cm, the effective focal distance at the edge will be much longer - this is how you can take a shot of something flat and have the entire image in focus.

However, underwater, we're not shooting a flat subject - the refraction of light as it passes from water, into glass or acrylic and then into air works as an additional lens element that produces a curved image, where edges are closer to your lens than the center even if the subject is actually flat. Fisheye lenses have no problems with that, but rectilinear lenses need to be stopped down, so that they have sufficient depth of field to fit both the center and the edges/corners into it. Sea & Sea Internal Correction Lens is what is called a field flattener, counteracting this effect to a limited extent, but even with that add-on, shooting wide-open in domes only works if you don't care about corners (for example, if you have only water in the background and the subject fits in the center of the frame). I use a Sony 10-18mm rectilinear lens on an A6300 camera behind an 8" dome, and I keep it at f/11 most of the time.

Thanks for the very detailed response. I think I understand and follow what your saying. I guess I will have to adapt to this and start shooting at higher ISO to compensate for the aperture.

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Are you shooting in natural light? If yes, then a strobe or two would be the best thing you could get for improving image quality.

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23 hours ago, Barmaglot said:

Are you shooting in natural light? If yes, then a strobe or two would be the best thing you could get for improving image quality.

I have twin strobes. But for the split shots which I had no issues with, I was using ambient light.

The change up to full frame and a rectilinear lens is just something I need to get used to I guess. My old 7D performed terrible at high ISO, but I'm yet to get a feel for my 5D mkiii. I guess if I was shooting at f11 and lower and I boosted my ISO as high as 800 it would probably be fine.

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I had a quick look through some of my images last night, and it does seem to improve when I stop down, however in some images it was still there at f13.........

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You may find it's still there at f13 for images where the corner is much closer to the image plane than the centre is assuming you focussed on a subject near the middle of the image plane.    The reason corners are soft is that the edges of the virtual image are significantly closer to the image plane than the centre of field.  Focusing further rout from the centre of field can also help as can using a filed flattener like the Sea & Sea lens mentioned earlier.

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On 9/25/2019 at 1:54 PM, ChrisRoss said:

You may find it's still there at f13 for images where the corner is much closer to the image plane than the centre is assuming you focussed on a subject near the middle of the image plane.    The reason corners are soft is that the edges of the virtual image are significantly closer to the image plane than the centre of field.  Focusing further rout from the centre of field can also help as can using a filed flattener like the Sea & Sea lens mentioned earlier.

So what can one do to avoid this problem chris? Is there a way around it other then wht has already been explained? Do I just avoid taking shots with a longer depth of field? Should I consider a different lens?

As I have stated previously, this is my first Full Frame setup, and my first time using a rectilinear lens underwater. What setup do you use?

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4 hours ago, flamebouyant said:

So what can one do to avoid this problem chris? Is there a way around it other then wht has already been explained? Do I just avoid taking shots with a longer depth of field? Should I consider a different lens?

As I have stated previously, this is my first Full Frame setup, and my first time using a rectilinear lens underwater. What setup do you use?

What I'm pointing out is that the corner unsharpness is at least partly a depth of field issue.  The lens needs to focus on the virtual image at 3 dome radii out from the dome for stuff at infinity.  It's even closer for nearby objects.  

You say some images are still unsharp at f13 and one reason may be that you are not square on to your subject and the stuff in the corner is physically closer to you and may fall outside the depth of field you have available.  This is easy to have happen for example when shooting along a wall, the wall closest to the camera will be physically closer than the subject. unless you turn square on to the wall.

Typically depth of field is distributed 1/3 in closer to your focus plane and 2/3 beyond the focus plane.  If you focus on the closest point of your subject it is more likely you will contain all the important bits within your available depth of field.  You can also try be aware of the layout of your subject and approach it squarer on if that composition will work.  

Regarding the corners, the 17-40mm focuses pretty close at 28cm from the sensor but is not regarded as the sharpest knife in the drawer.  Full frame is quite demanding and works better with bigger domes, if you combine a smaller dome with a lens that is not so sharp in the corners the performance may not be what you hoped for?  The new 16-35 f4 is much sharper in the corners.

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Stephen Frink did some extensive testing of the 17-40 Canon lens as well as other lenses a few years ago. See results here: https://www.seacamusa.com/uncategorized/lens-testing-underwater-use/

and  https://www.seacamusa.com/lenses-ports-domes/canon-16-35mm-ii-lens-test/

The super dome he used has counterparts in most underwater housing systems - the glass part may be made by one company. Note that both the 35-40 and 16-35 lenses that were tested have been replaced by much better lenses. As well, the relatively new Sea and Sea Internal Correction Lens is a game changer for this using type of lens (there are several threads on Wetpixel about this lens). Also on Wetpixel - there are a lot of threads on the 17-40 going way back as this was popular lens because of its cost.

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