Domes and Teleconverters: Entrance Pupil ( Nodal Point )

[Adventurer]

I will be utilizing 2.0x and 1.4x Teleconverters with a fisheye zoom lens behind a dome.

Currently doing the math for optimum dome positioning and sizing.

 

I am under the assumption that the entrance pupil will move by adding a TC.

And I mean by that, not equivalent to the 31mm physicial space that the teleconverter adds between camera and lens.

 

Can you share your thoughts and maybe even the math for this endeavor?

[RomiK]

13 hours ago, Adventurer said:

I will be utilizing 2.0x and 1.4x Teleconverters with a fisheye zoom lens behind a dome.

I'd say it's either-or scenario. You'd  need different port extension for each of them

 

13 hours ago, Adventurer said:

Currently doing the math for optimum dome positioning and sizing.

 

I am under the assumption that the entrance pupil will move by adding a TC.

And I mean by that, not equivalent to the 31mm physicial space that the teleconverter adds between camera and lens.

 

Can you share your thoughts and maybe even the math for this endeavor?

 

I'd say the approach might be less scientific - basically push the lens back as far as not to get vignette at widest setting. The dome size is far more important than some millimeters in positioning. Down below left is 140mm glass dome with TC2x and lens at 8mm (=16mm) and correct position and right is 180mm dome with lens at 15mm (no TC) so far back that it already vignetted. Still the CA (see the blue lines) is so much more pronounced even though the right image scenario is sharper even in corners (but there is another variable like the lens was at 15mm and not 8mm and it didn't have TC on it). 

 

I have no real life samples as to what effect this extreme CA would have on the real image underwater. 

Edited February 3 by RomiK

[Adventurer]

Thanks @RomiK for posting the examples. For correct optical performance you have to get the dome size and positioning exactly right on a millimeter level.

 

You are introducing quite a bunch of variables by changing dome size and TC at once. 
 

Knowing that Nauticam and Marelux 180mm domes are not full spheres your test was destined to fail. The 180mm dome with conical rear part was simply not made for that lens. The Canon 8-15 FE without the TC is very likely to perform better behind the 140mm dome, which is (almost) full sphere and suitable for fisheye lenses.

[Adventurer]

Coming back to the initial question and topic, the theory and some formulas suggest that by using the TC the entrance pupil (common name nodal point) will move into the direction of the cameras sensor plane.

 

So if the Kenko Teleplus HD 2x DGX for Canon has 35mm length, an additional extension ring of just 15mm might be enough; if you keep using the same dome and it was absolute precisely positioned before you added the teleconverter.

 

In this example we assume the entrance pupil moved 19.78mm towards the sensor. This is just an example to give you a general idea on how it works and what might be going on, with the real 15mm port extension not computed nor tested / verified.

[John E]

My understanding is that compromises are involved, and with the Canon 8-15mm the perfect theoretical positioning does not necessarily line up with the best extension, depending on what type of photos you want and what dome you use.

 

For close up wide angle shots, because the maximum effect is possible with a small fisheye dome, taking advantage of the minimum focus distance, and placing the dome closer to the lens, makes a big difference to the effect, creating a larger main subject and a better photo than having a longer extension which theoretically improves image quality but reduces the size of the central subject. For a larger dome, which is not ideal for close-up wide angle anyway, this doesn't apply.

 

 

 

[RomiK]

11 hours ago, Adventurer said:

Thanks @RomiK for posting the examples. For correct optical performance you have to get the dome size and positioning exactly right on a millimeter level.

 

You are introducing quite a bunch of variables by changing dome size and TC at once. 
 

Knowing that Nauticam and Marelux 180mm domes are not full spheres your test was destined to fail. The 180mm dome with conical rear part was simply not made for that lens. The Canon 8-15 FE without the TC is very likely to perform better behind the 140mm dome, which is (almost) full sphere and suitable for fisheye lenses.

 

These were just a few of tests of different configurations I made... gave you those which I thought would be the most polarized in order to deliver the  point - millimeters really don't matter, there are other things in play and we only need to be in a ballpark figure. Like 30mm vs 35mm extension? No real difference. Like 180mm sharper (minus the CA) than 140mm with Sony TC in ideal position? OMG the 180mm was supposed to be awful wasn't it?  And Kenko was even worse🙈 than Sony. But you would have to spend time shooting charts underwater to understand this. Unfortunately the Internet is full of theories with "demonstrative" pictures from Raja Ampat 😂 which tend to muddy the waters somewhat. So I had to make my own tests. Good luck and I am looking forward to your UW test charts and conclusions 🙂

Edited February 4 by RomiK

[Adventurer]

3 hours ago, RomiK said:

 

Like 180mm sharper (minus the CA) than 140mm with Sony TC in ideal position? OMG the 180mm was supposed to be awful wasn't it?  And Kenko was even worse🙈 than Sony. 

 

Well, it depends what you define as "awful". 😉 This is usually a very subjective expression of the situation. 
More objective would be:

• miimeter distance of the entrance pupil
• milimeter distance of the port (required vs available extension rings)
• stronger image compression
• able to shoot the lens more oben (F4 or F8 instead of mandatory F11 or F16)
• priority on zoom flexibility
• priority on corner sharpness
• priority on avoiding chromatic aberrations
• priority on travel weight lightness

In physics and underwater photography there is usually no free lunch.

Meaning there is one death you have to die (as we say in German).

 

My approach will be to verify the theoretical moved position of the entrance pupil, with a paralax test on a nodal rail.

For that I do not need to go diving or get the system wet.

 

Again, thanks for sharing your results @RomiK !

 

However, I think there are combinations that will perform much better considering IQ in your rig.

I hope you will also be able to benefit from the findings in this thread.

[RomiK]

20 hours ago, Adventurer said:

 

Well, it depends what you define as "awful". 😉 This is usually a very subjective expression of the situation. 
More objective would be:

• miimeter distance of the entrance pupil
• milimeter distance of the port (required vs available extension rings)
• stronger image compression
• able to shoot the lens more oben (F4 or F8 instead of mandatory F11 or F16)
• priority on zoom flexibility
• priority on corner sharpness
• priority on avoiding chromatic aberrations
• priority on travel weight lightness

In physics and underwater photography there is usually no free lunch.

Meaning there is one death you have to die (as we say in German).

 

My approach will be to verify the theoretical moved position of the entrance pupil, with a paralax test on a nodal rail.

For that I do not need to go diving or get the system wet.

 

Again, thanks for sharing your results @RomiK !

 

However, I think there are combinations that will perform much better considering IQ in your rig.

I hope you will also be able to benefit from the findings in this thread.

 

All good points save for one I would disagree with. You absolutely have to get the system wet (not yourself necessarily 🙂 - I shoot my pictures laying on edge of my home pool shooting down 😉) in order to test as there are things coming to play like refraction and virtual image. The cutting mat is the best water resistant test bed I have found :-). I did try to play with Petzval surface phenomenon but didn't have that much patience to have meaningful result in case you'd like to really dive in 😁. Cheers 

[Adventurer]

On 2/2/2025 at 10:46 PM, Adventurer said:

I am under the assumption that the entrance pupil will move by adding a TC.

And I mean by that, not equivalent to the 31mm physicial space that the teleconverter adds between camera and lens.

 

So I did my parallax testing on a nodal rail to verify / de-myth the whole thing.

 

The result: the above stuff I read on the internet and which seemed to be confirmed by two reasoning AI models is complete bullshit.

 

The NPP / rotation point / entrance pupil will stay exactly where it was!

 

Looking from the frontglas side at the Canon 8-15mm Fisheye F4 L it is stays at 17.98mm (@15mm zoom position) or at  19.12mm (@8mm zoom position).

 

No change if you add a 1.4x TC or 2.0x TC in between.

 

So assuming your dome position was already perfect before adding the TCs you will have to add:

 

18mm port Extension for the 1.4x Kenko

and

35mm port extension for the 2.0x Kenko

 

But as you may very likely have used a too short port extension to be able to use the full zoom range of the native 8-15mm without vignetting, adding a 50mm port extension for Marelux Housings is the way to go for the 2.0x TC. It is very likely to be the same for Nauticam when using their 140mm dome.

 

Edited February 6 by Adventurer

[Architeuthis]

41 minutes ago, Adventurer said:

 

 

But as you may very likely have used a too short port extension to be able to use the full zoom range of the native 8-15mm without vignetting, adding a 50mm port extension for Marelux Housings is the way to go for the 2.0x TC. It is very likely to be the same for Nauticam when using their 140mm dome.

 

 

Thank you for doing the measurement, Adventurer...👍

 

I do not understand what you mean with your last sentence: when a hemispherical dome as Nauticam 140mm is used and the entrance pupil is placed right at the center of curvature, there should be no vignetting - right?

 

=> When I use my Canon 8-15, Sony A7R5, Nauticam housing and 140mm Nauticam domeport with the 30mm extension recommended in the port chards, I cannot see any vignetting, neither with the 8mm circular fisheye image nor with the 15mm 180°diagonal image...

In accordance no vignetting with Kenko 1.4x/2x or Sony 2x TC, when I correct for the length of the TCs by appropriate extensions...

 

As a sidenote, there is a tread made by Dreifish, where he tested different extensions (in 5mm increments) UW. His outcome was that 35mm extension is optically even better than 30mm for the Canon 8-15, based on perspective UW/OW in split shots: 

I tested both 30mm and 35mm, but since I did not see a difference in IQ, I personally still prefer 30mm as the shorter extension is more convenient (with 35mm sometimes I could see part of the shade in the image, probably due to sensor IS and camera movement during photographing)...

 

 

 

Wolfgang

 

[Adventurer]

1 hour ago, Architeuthis said:

I do not understand what you mean with your last sentence: when a hemispherical dome as Nauticam 140mm is used and the entrance pupil is placed right at the center of curvature, there should be no vignetting - right?

 

Well,... yeah I wish it would be like that, but in reality even the Nauticam and Marelux full sphere 140mm domes are missing a few degrees curvature, where their glas is mounted into the aluminum frame.

The most practical thing on the Canon 8-15mm is to keep an eye on the entrance pupil: It's located approximately directly under the red dot when you have the (original Canon) land sunshade mounted. You can keep an eye on it, if it sticks out sufficiently out of the black port tunnel of your underwater housing.

 

So the native lens without teleconverters would need the 140mm Marelux and Nauticam domes with 47,9 mm Extension to be perfectly positioned, but as these two domes finish their sphere a little too early a 35mm extension is the absolute maximum you can use, before dark corners (vignetting) kicks in with these ports. However the fact that we might all be approx 13mm to short with the extensions explains the less optimal performance @RomiK illustrated in chromatic abberation test-shot when he used the lens natively. 

 

Hence the teleconverters shrink the FOV a little, so we can live with non-full sphere domes such as the 180mm with conical portside. However if the TCs decrease the optical quality or the slightly offset dome is to be judged.

 

 

[Chris Ross]

Nauticam already has a port recommendation for a Canon 8-15 plus 1.4x in the port chart, they just add 20mm of extension and they have a zoom ring for that combination as well.

 

From what I recall and various evidence of what various manufacturers offer on the internet for this lens it seems that fisheye lenses are not particularly sensitive to positioning the dome port correctly.  A prime example is ikelite 8 inch compact dome which is quite a small segment of a sphere and the Centre of curvature is well behind the dome.  It works and the image samples provided look quite OK at first glance.  I'm not saying that you can't or shouldn't try to do better, but this port is a long way out from ideal placement and still provides quite usable images.  Here is a post with examples.

 

GET the Canon 8-15mm Fisheye Before It’s GONE! // Underwater Review &

 

 

 

 

[Adventurer]

On 2/6/2025 at 10:05 PM, Chris Ross said:

I'm not saying that you can't or shouldn't try to do better, but this port is a long way out from ideal placement and still provides quite usable images.  Here is a post with examples.

 

GET the Canon 8-15mm Fisheye Before It’s GONE! // Underwater Review &

 

I would not draw any conclusions from the cited article as there is not a single photo in it which wasn't cropped and heavily processed. 😁

[Chris Ross]

15 hours ago, Adventurer said:

 

I would not draw any conclusions from the cited article as there is not a single photo in it which wasn't cropped and heavily processed. 😁

I think you are somewhat missing the point, this lens performs acceptably behind a dome where the centre of curvature is around 80mm behind the entrance pupil.  Sure the pics in the link I posted are relatively small, but they are enough to show that they produce  pics a lot of people would be delighted with.  It basically shows that the lens is relatively tolerant of misplacement of the centre of curvature.    If you look at the size of this port, the dome section is only about 20mm high so the lens focuses right down on the port allowing you to get very close to your subject - which makes a significant difference for CFWA, the subject becomes all the more prominent in the frame and small differences in distance makes a big impact with a fisheye lens.  But this of course involves a compromise, with a small deterioration in corner performance.

[Adventurer]

On 2/8/2025 at 4:57 PM, Chris Ross said:

I think you are somewhat missing the point

 

@Chris Ross it depresses me how you communicate here in public, obviously just for writing the 180 degree counter argument to what I contributed here. How should I have missed the point? Can we please leave personal grudge aside? I am obviously going to loose this against a person with moderator status who seems to own this forum.

 

My point and remark was, that in the promotional write-up from the ikelite blog, there are nice images, yes. But the  underwater photographer Steve Miller has obviously cropped and processed his shots. Just a few are still in 3:2 - so he may have cropped where the sharpness is splendid, to make the images look nice. A lot of the photos have also extremely darkened corners. It makes the images look interesting but gives full frame users no clue, how many pixels they have to throw away of each shot to get a sharp piece.

 

On 2/8/2025 at 4:57 PM, Chris Ross said:

this lens performs acceptably behind a dome where the centre of curvature is around 80mm behind the entrance pupil

 

And the curvature IS NOT 80mm behind the entrance pupil on the 8" ikelite dome. An extension of approx 43mm will get you to the NPP of the Canon EF 8-15 fisheye lens.

 

As I have done some more chessfield testing in the bathtub yesterday, I can tell you that also the Fisheye is not invincible when it comes to dome abuse and mis-positioning.

 

Have a read here:

 

Edited Tuesday at 11:33 AM by Adventurer
minor spelling

[Chris Ross]

14 minutes ago, Adventurer said:

 

@Chris Ross it depresses me how you communicate here in public, obviously just for writing the 180 degree counter argument to what I contributed here. How should I have missed the point? Can we please leave personal grudge aside? I am obviously going to loose this against a person with moderator status who seems to own this forum.

 

My point and remark was, that in the promotional write-up from the ikelite blog, there are nice images, yes. But the  underwater photographer Steve Miller has obviously cropped and processed his shots. Just a few are still in 3:2 - so he may have cropped where the sharpness is splendid, to make the images look nice. A lot of the photos have also extremely darkened corners. It makes the images look interesting but gives full frame users no clue, how many pixels they have to throw away of each shot to get a sharp piece.

 

 

And the curvature IS NOT 80mm behind the entrance pupil on the 8" ikelite dome. An extension of approx 43mm will get you to the NPP of the Canon EF 8-15 fisheye lens.

 

As I have done some more chessfield testing in the bathtub yesterday, I can tell you that also the Fisheye is not invincible when it comes to dome abuse and mis-positioning

 

Hi Adventurer,   i'm sorry if you feel it's a personal grudge, believe me there's nothing personal about it.  It's addressing the subject not the poster.  If I believe a post is incorrect or misleading in someway I'll respond to it.

 

I do believe you are missing the point, I am in no way claiming these are perfect to the corner, it's merely to point out these images come from a dome positioned a long way from the optimal point.  Some people could care less about corners others want to be sharp to the corner at 100%.  Myself I'm kind of in the middle, I want decent corners but recognise that there are diminishing returns. 

 

On the subject of the Ikelite port I am referring to the compact dome port.  This one:

 

The  dome is said to be a small segment of an 8" dome and is about 158 dia x 46 high by the specs The dome only rises about 20mm above the edge of the plastic port.  The 8-15 would need to sit right up forward in this dome to avoid vignetting with the entrance pupil almost at the edge of the plastic part of the port to avoid vignetting.  The radius would be 100mm  and this would place the centre of curvature somewhere around 70 -80mm behind the entrance pupil of the lens.  I could see the regular 8" dome having the entrance pupil around 43mm out as you said as that dome is also not a full hemisphere.

 

I'm not saying I would want this situation with a badly mis-positioned dome just that people use this lens/port and are happy with it.  I don't doubt for a minute your findings with the chessboard test, I'm just suggesting that the results in the images taken are not that terrible if they are out by 5-10mm.  If you want to fine tune the positioning of your dome, more power to you.  In the examples you referred to, my eyes I couldn't see a huge improvement in changing the dome position.

 

[Adventurer]

16 minutes ago, Chris Ross said:

The  dome is said to be a small segment of an 8" dome and is about 158 dia x 46 high by the specs The dome only rises about 20mm above the edge of the plastic port

 

OK. Small misunderstand here. Gotcha. The problem with that small dome being not a full sphere is that with correct position you will need to run approx 28mm FullFrame focal lens in it or tighter, to get near the optimum position. There is just a very small part in the center which will give you non-soft IQ. If you operate it on APS-C oder MFT cameras that's driving you towards that solution of your IQ problem. Also the 16:9 crop in video very often let's you use the center of that lens. All the above is not specific to the EF 8-15mm and also applies to other lenses. 

 

The get the FF image IQ you paid for on the superb EF 8-15,  it's recommended to get at least the Marelux or Nauticam 140mm (almost) full sphere dome.

Edited Tuesday at 12:22 PM by Adventurer

[Chris Ross]

1 minute ago, Adventurer said:

 

OK. Small misunderstand here. Gotcha. The problem with that small dome being not a full sphere is that with correct position you will need to run approx 28mm FullFrame focal lens in it or tighter, to get near the optimum position. There is just a very small part in the center which will give you non-soft IQ. If you operate it on APS-C oder MFT cameras that's driving your towards that solution of your IQ problem. Also the 16:9 crop in video very often let's you use the center of that lens. All the above is not specific to the EF 8-15mm and also applies to other lenses. 

 

The get the FF image IQ you paid for on the superb EF 8-15,  it's recommended to get at least the Marelux or Nauticam 140mm (almost) full sphere dome.

Yes I know that, but not really an option if you have a camera in an Ikelite housing, the larger 8" dome they offer is not ideal either, a bit too big to get in close.  The compact dome under discussion is really only for fisheyes for CFWA work where it lets you get in really close, It's a compromise to get that in your face CFWA perspective at the price of less than perfect corners.  The fact it does as well as it does is quite surprising really.

[Adventurer]

On 2/4/2025 at 8:04 AM, RomiK said:

OMG the 180mm was supposed to be awful wasn't it?  And Kenko was even worse🙈 than

 

Actually No. @RomiK

My recommendation for you, after testing it all out.

If you want the 180mm (non full sphere dome), add the 1.4x TC or the 2.0x TC

Add more Extension than just the recommended 20mm for the 1.4x TC and ditch the wide zoom parts IQ/Vignette.

Nauticam says the Radius of the 180mm dome is 110mm. So put a pen inside the glas and measue 11cm backwards.

Buy the extension ring, that will get that radius on the red dot on the EF 8-15mm sunshade.

 

Menu ready cooked! 

 

Voila, enjoy great IQ zoomed in and even make decent split shots.

 

PS: just realized @Chris Ross you hi-jacked this thread and totally took it off-topic. 

Please look at the thread title and re-evaluate who is missing a point here.

In fact it's easy to miss the Non Paralax Point (NPP) here 😉

Edited Tuesday at 12:37 PM by Adventurer

[Adventurer]

13 minutes ago, Chris Ross said:

CFWA perspective at the price of less than perfect corners.

Chris, I think you are still not understanding. It's not just about perfect corners. The wrong way will introduce all sort of optical degradation including abberations etc. 

[Adventurer]

18 minutes ago, Chris Ross said:

Yes I know that, but not really an option if you have a camera in an Ikelite housing, the larger 8" dome they offer is not ideal either, a bit too big to get in close.  The compact dome under discussion is really only for fisheyes for CFWA work where it lets you get in really close, It's a compromise to get that in your face CFWA perspective at the price of less than perfect corners.  The fact it does as well as it does is quite surprising really.

 

Putting a TC1.4x or TC2.0x in the system + getting a loooong port extension to use the 8-15 @ 15mm zoomed in -> 21mm / 30mm focal length with domes will give you better IQ and build the CFWA system you mention. 

[Chris Ross]

13 minutes ago, Adventurer said:

Chris, I think you are still not understanding. It's not just about perfect corners. The wrong way will introduce all sort of optical degradation including abberations etc. 

Yes no doubt, but IMO not enough to worry about if you are in the ballpark, I'd be happy for you to post shots showing all this harm to your images and I don't mean chessboards, sure it's telling you something but how much impact  does this have on the final image quality? the examples I've seen that were 10mm out vs a lot closer didn't set the world on fire withe differences in quality to my eyes.

[Chris Ross]

37 minutes ago, Adventurer said:

PS: just realized @Chris Ross you hi-jacked this thread and totally took it off-topic. 

Please look at the thread title and re-evaluate who is missing a point here.

In fact it's easy to miss the Non Paralax Point (NPP) here 😉

😂Yes we both went off topic a ways but it's interesting stuff . We disappeared off into the weeds to show that millmeter precision in entrance pupil positioning is nice but not essential to use the 8-15mm lens and that you can produce pleasing images with sub-optimal setups as long as you are not miles off base.

[fruehaufsteher2]

Hi Chris, hi Adventurer, 

I feel almost uncomfortable stepping into this very technical discussion 😁. I have a practical question: while my WACP-C is on its way to Honkong for being repaired, I've had some silly ideas and have been able to buy a Canon 8-15 at a very reasonable price. I already own a 1.4TC from Sony and am now thinking of putting together an alternative setup for my A7IV in Nauticam - just in case I don't get my WACP-C back in time.
Can I ask for your advice? Does the MC11 from Sigma fit the 1.4TC from Sony? Based on your comments above, the 140mm dome port seems to make the most sense? How much extension?

Best

Martin

[Architeuthis]

42 minutes ago, fruehaufsteher2 said:

Hi Chris, hi Adventurer, 

I feel almost uncomfortable stepping into this very technical discussion 😁. I have a practical question: while my WACP-C is on its way to Honkong for being repaired, I've had some silly ideas and have been able to buy a Canon 8-15 at a very reasonable price. I already own a 1.4TC from Sony and am now thinking of putting together an alternative setup for my A7IV in Nauticam - just in case I don't get my WACP-C back in time.
Can I ask for your advice? Does the MC11 from Sigma fit the 1.4TC from Sony? Based on your comments above, the 140mm dome port seems to make the most sense? How much extension?

Best

Martin

 

Hi Martin,

 

I have just bought the the Sony 1.4x TC for use with Canon 8-15mm in Nauticam housing and (mostly) 140mm dome (A7R5; have been using the Sony 2xTC&Canon 8-15mm now for two diving holidays (Croatia and Tansania) and it worked also nice (Image sharpness in center, more or less, comparable to WACP-C with Sony 28-60mm (both are, however, not "excellent"), corners similar. I do expect, however, better IQ with Sony 1.4x TC, therefore the acquisition of 1.4x)...

The length of Sony 1.4x TC is 17mm, therefore an additional extension of 20mm should be perfect (in addition to the 30mm extension recommended by Nauticam; remember that Dreifish, based on the chessboard test, recommends 35mm instead of the "official" 30mm as theoretical optimum for the pure lens, w/o TC). The 3mm more, when the 20mm extension is used with the 1.4x TC, is fully in line with Dreifish's recommendation (alternatively one could use a 25mm (instead of 30mm)&20mm extension and this is probably also o.k.; I will be able to test the different extensions in the Red Sea only in March and can report afterwards (takeoff in 2_1/2 weeks 😋); I will also report my personal opinion, whether the switch to 1.4x TC indeed brings additional IQ and is worth (2xTC offers much better zoom range)...

 

I have the Metabones V adapter and this works perfect with the Sony 1.4x TC and Canon 8-15mm. In case you have already the Sigma MC11 adapter you can test it out easily by yourself. In case you do not have an adapter yet, why not the Metabones V (at least theoretically, it should support AF modes a little better)?

 

Wolfgang

Edited Tuesday at 03:06 PM by Architeuthis