Full Frame SLR wide angle corrector port tests

[MJvC 9]

Hi Alex

This is looking very promising and exciting. Do you have any plans to sell them commercially and for other housings.

[scuba_d 6]

I love the motion blur picture.

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Thanks for the update.

 

 

Pete

[Alex_Mustard 0]

I am not planning to make these commercially, but I hope these tests prove and promote that the concept, and it is something that our community wants. Given the interest, I would hope that one or more of the existing manufacturers would take this on.

 

Here is the setup by the pool the other day, this is with Nikon 20mm f1/8 inside.

 

 

Alex

[Larry C 24]

Looks like everything above f5.6 is brilliant. Love the Gorgonian with the sunburst. Not fond of the background blur with the 20mm, but I imagine that's the lens.

[olegam 0]

Alex,

In my opinion, this is the most exciting underwater photography news in many, many years. Ever since I sold my Nikonos cameras, including the 15mm and started using SLRs in 1990 I have struggled with sharpness when using wide angle lenses.

 

Many years ago I read an article written by one of the original members of the BSoUP. Back in the 60's, they had to make a lot of the kit themselves, because there was not much that could be bought. The corrector port was one of the things that was popular to make. They made them of acrylic material that was shaped (in a lathe, I presume) and then polished up. But the corrector was soon taken over by the dome ports as it was much easier to make.

 

Seing that the corrector is close to impossible to find, the only alternative appears to make one. The design is described well in many books. And its not new. Introduced by Gallileo Gallilei. The corrector uses the principle in the reversed way. Also used for optical viewfinders for rangefinder cameras.

 

From what I have been able to find out the formula for a Reverse Galileo telescope is Fp + Fn = d, where Fp is the focal lenght of the convex lens, Fn the focal lenght for the plano-concave lens, and d is the distance between them. The Rebikoff corrector will correct the magnification of 1.33 caused by the front port by adding a 0.75 magnification. Fn = 0.75 x Fp. I have attempted to calculate, using a +4 close-up lens (the Fp) as a starting point. The positive lens (a given) focal lenght is 25 cm. The plano-concave lens is 18.75 cm. The distance between the two is 62.5mm.

Do you know the focal lenghts for the two lenses involved on your corrector? And the distance between them? Does my figures make sense when compared to the real thing?

[oskar 6]

Great work!

 

I have also been musing on that corrective elements should be brought back into UW photography, á la Nikonos, but a general afocal solution is great! I'm very interested in what the practical / theoretical limits of FOV could be. Also convex front elements of ultra widangles probably cause problems with fixing the lens. I suppose the internal and external corrective lens distance and co-axial pracements are very very important.

 

Keep us posted, and I hope some manufacturers catch on!

 

Cheers

/O

[Alex_Mustard 0]

I am slowly building up a collection of shots of different subjects shot with this lens. Many of the subjects here in Cayman are more suited to the fisheye, so I have not been using the set up that much. Took it to the stingrays this morning.

 

 

Shot stills and video and will try and upload the video when I find a CF card reader!

 

Alex

[waso 0]

Very interesting thread - I'll keep following...

I also had the chance for a more rigorous pool test between dome and corrector port, this evening. I tested if there was any lost in angle of coverage compared with dome - as there was not.

 

Dome:

CAY15_am-101984.jpg

 

 

Corrector Port:

CAY15_am-101993.jpg

 

 

Also corrector consistently seems between 2 and 3 stops better in terms of corner sharpness. I.E. f/4 on corrector port had comparable corner sharpness to f/8 or f/11 on my best dome.

 

Alex

What happend to the very corners?

They seem to have a much stronger vignetting.

 

Wahrmut

Edited January 22, 2015 by waso

[Alex_Mustard 0]

Here is a video taken with the UW corrector port today, which is just a quick clip, but shows the lack of distortion with the lens and corner of the frame performance.

 

[adamhanlon 0]

This is great info Alex-many thanks for sharing it.

 

Optics seems to be the "next frontier" in the industry at the moment. The good news is that I know of one manufacturer who is currently developing a commercial product that is similar, in performance at least, to Alex's corrector port (although it will probably use the Nikkor 50mm lens). The project is still in the prototype phase, but I am very confident that there will be a product for sale in the near future. I should point out that it is likely to be expensive! Once it is out, Wetpixel will be testing and using it!

 

As it happens, I was recently able to shoot pool test shots of the 20mm Nikkor with a 9" dome in the Bahama and was very impressed with its corner sharpness too.

 

Adam

[Alex_Mustard 0]

What happend to the very corners?

They seem to have a much stronger vignetting.

 

Wahrmut

 

 

Hi Wahrmut,

 

I don't yet have a proper mount for the internal lens of the corrector port. So I have to stick it to my camera lens with blue-tac. When I do this carelessly (for the pool tests) I get some vignetting in the extreme corners. I have commissioned two replacement lenses which are bigger and won't have this problem. The simple solution was ready for this trip, but does not work that well. So I am still using the original lens, which is a bit small and therefore vignettes if I fit it carelessly.

 

I am mainly using the 20mm for now, but have tested the Nikon 24-70mm, 16-35mm and 17-35mm with the port too. The 24-70mm is troublesome because of the barrel-length changes during zooming.

 

I am only posting uncropped photos in this thread - as posting cropped ones (even if the crop improves the photo) would be misleading with regards to port/lens performance.

 

Here is a photo taken with the port and 20mm, of an eagle ray feeding, that I have already shared on Facebook. Again, uncropped.

 

 

Nikon D4 and 20mm lens. Subal ND4S housing, Alex corrector port,2 x Seacam 150 strobes, 1/250th @ f/11, ISO 320.

 

Alex

[Photec 5]

Hi Alex,

 

Did this lens come from a movie camera housing or a stills camera and which housing was it designed for?.....if stills what film format? I am just fascinated to know where the adaptor originated from and how old it might be? If the design truly can be used with any lens (excluding fish eye) then it is a very interesting trial indeed!

 

Mark

[waso 0]

 

 

Hi Wahrmut,

 

I don't yet have a proper mount for the internal lens of the corrector port. So I have to stick it to my camera lens with blue-tac. When I do this carelessly (for the pool tests) I get some vignetting in the extreme corners. I have commissioned two replacement lenses which are bigger and won't have this problem. The simple solution was ready for this trip, but does not work that well. So I am still using the original lens, which is a bit small and therefore vignettes if I fit it carelessly.

 

Thanks for the plausible explanation, Alex!

 

Cheers,

 

Wahrmut

[Alex_Mustard 0]

Optics seems to be the "next frontier" in the industry at the moment. The good news is that I know of one manufacturer who is currently developing a commercial product that is similar, in performance at least, to Alex's corrector port (although it will probably use the Nikkor 50mm lens). The project is still in the prototype phase, but I am very confident that there will be a product for sale in the near future. I should point out that it is likely to be expensive! Once it is out, Wetpixel will be testing and using it!

 

Seems strange choice of lens. Sure a wide angle would be better? Not convinced about stretching a 50mm to a wide angle for stills (with presumably a water contact front element) will be that great for high resolution shooting. Probably OK for sub 10MP applications, like 4K video.

 

Did this lens come from a movie camera housing or a stills camera and which housing was it designed for?.....if stills what film format? I am just fascinated to know where the adaptor originated from and how old it might be? If the design truly can be used with any lens (excluding fish eye) then it is a very interesting trial indeed!

 

My glass came from an old medium format, scientific photogrammetry survey camera. But all my tests so far show it should work on a very wide range of lenses.

 

I have probably done my last dives with the system now. So will pull together a selection of images.

 

Alex

[loftus 42]

This is really exciting stuff Alex. Great job. Can you spill the beans a little more about the exact construction of the lens, setup, optimum lens to port distances you have found etc. Is this essentially a flat port with a dioptre type lens element rather than a flat glass. Got thrown a bit when you discussed the lens threads; is there a component that screws to the lens as well?

Edited January 27, 2015 by loftus

[Alex_Mustard 0]

This is really exciting stuff Alex. Great job. Can you spill the beans a little more about the exact construction of the lens, setup, optimum lens to port distances you have found etc. Is this essentially a flat port with a dioptre type lens element rather than a flat glass. Got thrown a bit when you discussed the lens threads; is there a component that screws to the lens as well?

 

 

Hey Jeff,

 

Yep. While the port looks flat, it is actually slightly convex on the outside and strongly concave on the inside. I think it is actually made up of two elements. The second part (which mounts on the lens) is convex on the outside and concave on the inside. I have measured the dioptre strength of the internal lens - and tried replacing it with a simple dioptre lens (of the same strength) and it did not work well (reduced the optical performance to that of the dome). I am not having a new internal lens made that is the same dioptre strength and curvature (inside and out) as the current internal lens. This will allow me to use a much wider range of lenses. I am currently limited by the relatively small physical diameter of the internal lens.

 

The thread on the outside of the internal lens is a non-standard size and I don’t have a step ring that will hold it. I will have to have one made. So at present I am holding it on with blu-tac (a putty), which works well, but can cause problems of misalignment through carelessness or vibrating boat journeys.

 

The other modification I need is a lens hood. Both for shading and to protect the optical surface.

 

Alex

[Alex_Mustard 0]

Lots of people have been asking me about the port and the best analogies/descriptions I have come up with so far are:

 

1) Think of it like a D810 - can’t really take any shots that I couldn’t take already, just takes them with a better quality.

 

2) It creates an invisible boundary between the lens and the water. So the lens works exactly as it does on land, with none of the optical problems of a flat port or a dome port at the air to water interface.

 

And here are some more photos from Cayman taken with it:

 

Eagle Ray Nose Dive (f/14):

 

Reef Shark (f/10):

 

Diver Exploring Reef (f/9):

 

It is particularly suited for people photography.

 

Alex

[Alex_Mustard 0]

Here are some more photos taken with the port in Grand Cayman:

 

Lionfish:

 

Stingray:

 

Eagleray:

 

Reef Shark:

 

Diver:

 

Alex

 

 

 

 

 

 

[thetrickster 328]

Photos look ultra sharp and clear.

 

Maybe impossible - but two similar cameras with the same lens but one with and without the corrector port to really show the differences?

 

If I'm honest I have no idea what is going on!! So you saying if this sort of thing works, the days of domes are over?

[Alex_Mustard 0]

If I'm honest I have no idea what is going on!! So you saying if this sort of thing works, the days of domes are over?

 

 

No. Because this solution cannot be used with fisheyes. Can’t be used for split level images. And will always be more expensive to produce than a dome. Furthermore it is a solution that gives you very good optical performance, compared with good (for a dome). And good is perfectly good most of the time.

 

But I do hope my experiments generate interest in this solution - if a manufacturer committed to mass production then a reasonable price could be achieved. I think if it was 2-3x the cost of a dome, there would be a lot of interest from serious shooters to sell 2-500 globally. But I think anyone who wants to shift those units needs to make adaptors for all housings, rather than making it brand specific.

 

Alex

[errbrr 73]

Thanks for the explanation of the components Alex - I studied the housing photos you posted earlier but wasn't able to spot the difference. I am very interested in something similar with my rig based on those results you are posting. I currently shoot the Canon 14mm full frame in the caves behind an 8" dome and rarely dip below f8 because the rocks go all fuzzy around the edges. Even at f8 or f9, close up rocks give an irritatingly fuzzy foreground. A bigger dome is not feasible as it may not fit through the restrictions. Your setup would solve that problem very nicely.

 

The 14mm lens doesn't however take a filter as the front element is not flat - would it still work? Maybe with a little duct tape on top of the blu-tac...

[loftus 42]

For me this is an awesome development particularly for my pool work where I often tend to frame and crop pretty tightly.

[Alex_Mustard 0]

I am continuing to use this port on all my shoots and these give the opportunity to use it in more varied conditions and exploit its advantages.

 

Here is a shot from the Red Sea. The dolphin encounter was very late in the evening - and shooting without strobes it was a big advantage to be able to shoot at f/4.5 with a rectilinear lens on a full frame camera and still achieve good corner sharpness.

 

 

This was taken at ISO 2000, f/4.5, 1/320th. A fast shutter speed is essential to keep fast moving dolphins sharp. Although I would have preferred to be shooting at about 1/500th, the very low light levels meant compromising a bit. It was therefore beneficial to be able to shoot with an open aperture to maintain corners.

 

Alex

[JohnnyQuest 8]

Thanks for all this information. Your shots are amazing as always. Have you heard anything indicating a manufacturer is moving to produce these or is this just to tease us?

[Alex_Mustard 0]

Definitely interest in the concept from major UW manufacturers. Whether they will become a production reality will be confirmed before/at DEMA for sure.

 

Alex