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longbord1

Dome port with non-fisheye lens

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Just a question about optics.

I have a dome port I'd like to use with a 24mm. I have tried it with a 40mm lens, and the image is very magnified. Just wondering if the dome port imparts any effect on non-fisheye lenses, and will prevent underwater magnification, or does a non-wide angle, non-fisheye lens focus through the dome port and the dome port just acts as a flat port?

 

Thanks,

 

Michael

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58 minutes ago, longbord1 said:

Just a question about optics.

I have a dome port I'd like to use with a 24mm. I have tried it with a 40mm lens, and the image is very magnified. Just wondering if the dome port imparts any effect on non-fisheye lenses, and will prevent underwater magnification, or does a non-wide angle, non-fisheye lens focus through the dome port and the dome port just acts as a flat port?

 

Thanks,

 

Michael

The angle of a given lens, fisheye or not, should be the same behind a dome port as shooting the lens normally (in air). To work properly the dome must be properly configured to the lens and the lens must be able to focus on the virtual image generated by the dome. This is NOT like a flat port.

See: https://www.scubageek.com/articles/wwwdome.html

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So what if it is not configured properly? I am getting nice sharp photos, but honestly, the photos seem much more zoomed in than a 40mm on land.

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If the dome is to close to the lens you get barrel distortion if it is too far pincushion distortion (if it doesn’t vignette)
Both case the lens looses the land field of view and you get magnification
On top of distortion you get secondary issues of the lens magnified typically chromatic aberration


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Can you share a bit more information about your camera/lens/dome set up?

It is hard to diagnose the issue you describe without knowing this.

Ideally, a couple of images showing the issues you are describing would be good too.

A dome port cannot "magnify" or "zoom in," so it strikes me that something else is at work...

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Just now, adamhanlon said:

Can you share a bit more information about your camera/lens/dome set up?

It is hard to diagnose the issue you describe without knowing this.

Ideally, a couple of images showing the issues you are describing would be good too.

A dome port cannot "magnify" or "zoom in," so it strikes me that something else is at work...

Dome ports do not magnify ONLY if they are correctly positioned with regards to the entrance pupil of the lens. In cases of lens too close or too far to the dome the loss of field effectively becomes magnification

I spare you the hundreds of test shots of a ruler underwater I have taken changing dome extensions and keeping the camera in the same point...

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8 hours ago, Interceptor121 said:

Dome ports do not magnify ONLY if they are correctly positioned with regards to the entrance pupil of the lens. In cases of lens too close or too far to the dome the loss of field effectively becomes magnification

I spare you the hundreds of test shots of a ruler underwater I have taken changing dome extensions and keeping the camera in the same point...

Thank you, 

The reason I ask, is I am interested in getting a 24mm or 35mm prime for the same set up to get more of a cinematic feel to some of my shots. My concern was would they be magnified, so I should go with the wider prime. 

I am using an Ikelite housing, Canon SL2 and 15mm canon prime currently.

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The dome will restore the air field of view if the entrance pupil of the lens is in the centre of curvature of the dome
If the combination is not recommended by the manufacturer or you want to check it you need to know the curvature radius of the dome the position of the entrance pupil so you can work out the correct extension assuming you can use one
I know the Nauticam system and it has an error margin of 5mm no idea about ikelite


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Posted (edited)
3 hours ago, longbord1 said:

Thank you, 

The reason I ask, is I am interested in getting a 24mm or 35mm prime for the same set up to get more of a cinematic feel to some of my shots. My concern was would they be magnified, so I should go with the wider prime. 

I am using an Ikelite housing, Canon SL2 and 15mm canon prime currently.

Since you are using APS-C a flat port will work with these focal lengths and so be less challenging to set up. I have a 7D2 and have used the Canon 24 and 40mm pancake lenses with it when traveling but I have not used them for underwater photography. Is your query about these lenses? Since these lenses are so stubby it might be an issue properly adapting a dome port to them. Their entrance pupils might be closer to the camera compared to your 15mm lens which I am guessing is a fisheye, the Canon model? You might want to ask Ikelite about what you want and if they have specific recommendations for your lenses. You need to be more specific about your gear when you communicate with them. Other 24 and 40mm lenses are much larger than the two Canon pancakes so would take different port setups.

Edited by Tom_Kline

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PS. If you use a flat port it will be like a 1.33x increase in focal length so your field will be narrowed compared to a dome.

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I'm still not sure exactly what you are trying to achieve with the alternate lenses.  With APS_C  and a 24mm lens you are near 40mm equivalent focal length which is in the realm of macro lenses for focal length underwater, which is the sort of lens used for fish portraits etc.    What exactly do you mean by a cinematic look to your shots?  If you are using a 15mm currently (a fisheye lens) then stepping up to a 24mm lens is a pretty dramatic reduction in the field of view.  What sort of subjects are you shooting?

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If this camera is 1.6X crop currently are you shooting 24mm equivalent that is good for reefscapes and has limited perspective distortion

A lens at 24mm would be 38.4mm equivalent fov and will be generally only good for fish portraits once in a flat port this would become around 50mm equivalent this is not an extremely popular choice underwater and I guess it goes back to what cinematic means?

In general terms underwater filming has historically been documentary style and this has a 16:9 aspect ratio not scope, I shoot in cinelike styles but only because it has headroom for grading

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When I say cinematic. I mean non-fisheye/distorted images. 
 
 

Avoid rectilinear lenses underwater for video terrible idea and n 1 cause of blurry videos


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12 hours ago, longbord1 said:

When I say cinematic. I mean non-fisheye/distorted images. 

 

 

It's huge step between a 15mm fisheye to a 24mm lens on APS_C the fisheye is 180° on the diagonal on full frame and 108° on APS-C and the 24mm is going to be about 60°(APS-C)  on the diagonal, which is a significant reduction.    Note I'm assuming your 15mm prime is the 15mm full frame fisheye.  Note you can't equate focal lengths between fisheye lenses and rectilinears the field of view is completely different.

You may be better served with something like a 16-35mm f4 which zooms from 80° to 44° or if you want wider without distortion something like the 10-18 or 10-22 zooms which zoom from about 109° to 76 or 65° on the diagonal.  This would give you a rectilinear non-fisheye distorted frame with similar coverage to what you have now and you could zoom into something as tight or near as tight as your 24mm prime.  You would need something like an 8" dome port to accommodate this type of lens, it may be a little soft in the corners at 10mm, which is a common issue with rectilinear wides in dome ports, but zooming in a little would fix that. 

Going wider means you can get closer with less water between you and your subject.

 

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Here is an example of a photo I captured with the 40mm 2.8 prime on my set up: 

47403804612_6c27b07889_b.jpg

 

 

I was really pleased with the tight crop and sharpness of the photo that gives me more of a cinematic feel. However, this lens has a hard time focusing even remotely close, and is just a bit too tight for me. This was shot through a dome port. So this is why I ask about a 24mm with a dome port. Will it be 24mm with the dome port, or will I be focusing through the dome port, and thus the 24mm will be magnified. The 40mm seemed quiet magnified when I was taking photos underwater with it.

 

 

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First thing please be very specific in answering  questions, there are a lot of nuances to UW photography and knowing exactly which model lens you are talking about and which port and housing is important to give the right answer.

Your 40mm will seem incredibly magnified compared to a 15mm fisheye shot(assuming it's your your current lens) .  The dome port does not magnify when at the right position with respect to the lens, it will change somewhat if not setup in the dome correctly, but I don't think it will jump out at you.  Judging distance and actual size of the subject underwater is an issue as well.

I'm assuming your 40mm prime is the pancake lens - the AF on that lens may not be fantastic.  40mm on an APS-C is quite long for UW usage, if you put it in the same dome as your 15mm the positioning is sure to be out which will impact image quality on the edges.  The min focus distance is 30cm which may struggle a little to focus on the virtual image formed by the dome - AND this depends specifically on which dome we are talking about - we are just guessing.

The usual process for picking a lens is to refer to a port chart, if you have an Ikelite SL-2 housing this is the port chart:

http://docs.ikelite.com/reference/port-chart-dlm-c-system.pdf

Pick a lens from the list that seems suitable for your needs and perhaps ask specifically for experience with that lens on another post. 

Not every lens is suitable to use behind a dome even if it fits.  The lens needs to focus quite close to focus on the virtual image which sits 3 dome radii away from the dome when focused on infinity, the lenses listed in the port chart are there because they are popular and work well underwater.  Other lenses may work but we would need to know the specific model/version you are talking about and which specific  dome you want to put it behind.  They are mostly zooms because they are just way more convenient to use underwater.

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Posted (edited)

ChrisRoss,

 

Honestly, the charts are helpful, but at the same time, my 15mm 2.8 prime by canon works very well and is not listed. My main goal was to determine if the 40mm pancake lens is focusing on the virtual image of the dome/being correct, or if it is just focusing through the dome port and being magnified by the water.  In my experience with it, it takes very sharp photos, but it is just a bit too cropped for me. I am wondering if the 24mm 2.8 pancake will do something similar, or will actually utilize the dome virtual image? Underwater photography is very niche and there are so many options that it seems like buying the lens and actually experimenting is sometimes the only way to determine the actual outcome. 

 

Again my goal was, not whether the 24mm prime is compatible, but if the dome port will prevent magnification, or will the lens just focus through the dome port.

 

Thanks,

Michael

Edited by longbord1

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You need to look at the minimum focal distance of the lens in question then work out where the virtual image is with respect to the dome and see if the virtual image is too close or not.  The min focal distance quoted is distance from sensor to subject and can be found online.

This link does the calculation:  https://oceanity.com.au/blog/view/dome-port-virtual-image-visualiser

Input all the variables.  The aperture is the focal length over the f-stop you are using  a 24mm lens at f8 is 3mm so set it to minimum.

For an 8" dome the virtual image for infinity is:  27.9 cm from the port apex and a 0.5m distant object is 16cm.  Add to that the distance from the sensor to the apex of the dome port.  This is how far from the sensor to the virtual image.  The sensor is 44mm behind the lens flange.  If the distance measured is more than the min focus distance you will be OK.  If you have a different dome input it's diameter _ but watch for domes that are not full hemispheres.  The calculator is assuming it is a hemisphere - what it actually wants is the radius of curvature and it gets this by dividing dome diameter by 2. 

Regarding your question the only thing that can happen with a dome is to focus on the virtual image.  I can't find any references that estimate how much a lens will magnify if it is not correctly placed at the centre of curvature of the dome but It is not going to be as much as the 1.3x magnification of a flat port.  It will also certainly have a wider field of view than the 40mm.  If it can focus on the virtual image it will take OK photos, it may suffer from soft edges due to the dome position. 

Your 15mm fisheye is not listed probably as the Sigma is more popular as it focuses slightly closer and the Canon 15mm is discontinued.

 

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Chris,

Thanks for your response. Very thorough and helpful. I emailed ikelite about the sigma vs the canon 15mm and they said they simply hadn't listed it due to discontinuation, but it should be fine for my system.

It's interesting, the 40mm 2.8 prime has a terrible minimal focusing distance. So I'm not sure what is happening with the image.

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I think you are getting further confused as you go along

The size of the dome and the focus distance of the camera lens determine if the combination will ever focus

The position of the dome impacts FOV and distortion so the lens may as well focus but the image will look gross if the position is incorrect

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the 40mm prime is about 30cm minimum focus distance, again I don't know which Ikelite dome you are using but if it's the 8" dome I would guess the virtual image would be about 30cm away for an object 0.5m away so it only just focuses there and won't focus much closer. 

As far as dome position goes it is 23mm long vs 61mm for the Canon 15mm so assuming the fisheye is well placed in the dome the 40mm lens entrance pupil could be positioned about 38mm behind the centre of curvature of the dome based on the difference in lengths.

your 24mm could work but being so short (assuming pancake lens again) could suffer vignetting, but you could test that on land.  It will certainly be wider than your 40mm.

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