18 replies to this topic

[Painted Frogfish]

Hi all,
Does anyone know if a dioptre is needed to improve close focus shots using Nikkor 12-24mm lens behind a Seacam 9 inch superdome port? Or is the dome large enough?

Thanks.

[acroporas]

No you do not need one.

The general rule is a Dipoter is required when the closest focusing distance is greater than 2x the dome diameter.

Thus the 12-24 which foucses at 11.8 inches only needs a 6 inch dome. So a 9 inch dome is definatly big enough!

[Painted Frogfish]

Thanks for clearing that up!

[Alex_Mustard]

This is my current pet peeve.

Wide angle lenses used behind a dome will work without a dioptre, but will work better with one. Also dioptres are not just about enabling close focusing, but also help overcome the curved plane of focus caused by a domeport. Meaning you get sharper corners.

I know Seacam don't reccomend one. But I am not convinced by their arguments or the results I have seen. For example the winning shot in the UW Category of the WPOTY is a stunning shot of a leopard seal. If you look at the corners of a print of that picture (unfortunately you can't see it on the web thumbnail) it looks like the detail has been smudged by someones thumb.

Now I know it has been taken with the dodgy Canon 17-40mm, on a full frame 1Ds Mk2, known for having corner issues, but I believe that this lack of sharpness has little to do with Canon (it was shot at F6.3) and more to do with the lack of dioptre. I think a Nikon with the 12-24mm without a dioptre would have had similar problems. And I know with a dioptre it wouldn't have.

I chose this image as an example because it allows me to counter my argument by saying that if you win such an award, why worry about perfect optics. And that is a fair point!

I am a Subal user. And they also do not recommend a dioptre for this lens. But I found out at the Antibes festival why this is - and I had steam coming out of my ears! It was based on the recommendation of one (well known) photographer. For the record I always use a dioptre and I am very happy with my corners even at open apertures with the 12-24mm

Sorry for the rant. It is just that this topic is reall winding me up at the moment!

Alex

[echeng]

Also, the second you put a diopter on, you're going to see an awesome split shot on the surface, and the entire upper half is going to be blurry.

[Paul Kay]

OK Alex

Rant number two!

If you use a lens which has a field of view of greater than 90 degrees (ie 20mm on the 35mm format) then the curvature induced into the image by a dome port is unlikely to be compensated for by depth of field if you are working at anything other than 'long' distances (ie several meters) and anything wider than f/11ish. The exception being fisheye lenses. A diopter MAY help compensate for some of the field curvature but this is not its prime function (which is to recorrect the focus for the much closer virtual image).

The paradox is that using a 'poor' diopter lens (one with a flat rear surface) is said to induce some degree of opposite curvature to the dome, thus compensating the curved image to a small degree. BUT using yet another single element piece of glass into the optical system, and a poor one at that, probably causes as many problems (albeit different ones) as it solves. The worst case scenarion is to shoot at a relatively wide aperture on a wide-angle zoom and at close focus distances. As yet there are no really effective optical solutions for this problem and it is why the 15mm UW-Nikkor was always rated as being so good (although it is now a very old design).

The 12~24 will have issues like the 17~40 but because it is a smaller format lens the depth of field will 'appear to take up more of the slack'. The 17~40 is not a poor lens (even on FF digital) and behind Seacam's Superdome will produce as good results as possible on the 1D series cameras. But corners may still be soft.

As I've said elsewhere on wetpixel, the final image is what counts and a shot like George's is stunning whatever the optical shortcomings of current underwater optics.

[cor]

I am a Subal user. And they also do not recommend a dioptre for this lens. But I found out at the Antibes festival why this is - and I had steam coming out of my ears! It was based on the recommendation of one (well known) photographer. For the record I always use a dioptre and I am very happy with my corners even at open apertures with the 12-24mm

<{POST_SNAPBACK}>

Which diopter do you use? When the 12-24 first came out, Subal did recommend a diopter, and I used one. Then they did a 180, and said you shouldnt use one. I do agree that the images without the diopter look better to me than with the diopter, but maybe I was using the wrong diopter (+4 I think).

Cor

[james]

FWIW, I'll take part of the blame for manufacturers not recommending the use of a diopter with the 12-24. I was the first uw photographer in the US to own this lens and I tested it out behind an Aquatica 8" dome using various extension rings. I did five dives, using various extension rings, and I found that when using a 15mm extension ring, the lens yielded the best results.

Because Lee Peterson observed that the lens has "problems" when mated with a diopter - even topside - I just didn't mess with them. I've never done close focus WA comparison tests of the lens when using a diopter vs leaving it off.

Cheers
James

[davephdv]

Alex, what diopter power? I use a +2. Would a +4 be better?

Also does anyone know of a brand of "slim" diopters in 77 mm?

[Tom_Kline]

This has come up in the on-going book thread:
http://wetpixel.com/...837
See my post #8 with quote from Mertens.
Tom B)

[John Bantin]

I am not an expert but I am a witness.

I have always hated the results got from using a wide-angle lens behind a small dome port and have blamed it on the plus-dioptre needed to get things in focus. Image edges became smudged. Therefore I have always used a big dome port and have them by Sea & Sea Aquatica and Nexus.

Then I went digital and bought a 12-24mm Nikon, a Subal and a large dome port. On my first outing I found that results were very sharp but I could not focus on anything as close as an arms-length away (eg a computer in my hand). I now use that lens behind a big dome but with a plus 2 dioptre. Results? Perfect!

[KenByrne]

I bought a +3 diopter to use with a Nikon 12-24, however on the first dive with it I found that it caused vignetting at the wider settings. I think this is because it is in quite a deep mount and if I can find a way of removing the excess it will probably be OK. If you are in the market for a diopter be aware that it might cause more problems than it solves.

[Paul Kay]

To calculate the dioptric power (P) needed for a dome, the calculation is:

P = 1000/4R where R is the radius of the dome

This corrects the focus range of the lens and operates by utilising the lenses' infinity setting to focus at 4R (ie on the virtual image of a subject which is theoretically at infinity underwater). In essence using a diopter effectively optimises the focusing ability of the lens behind the dome.

But note that this use of the diopter is to correct for focus only. Side effects MAY be to help flatten out the curved virtual image but to do this to any degree requires a specially designed corrector lens which is both camera lens and port specific and it also needs to be positioned correctly. Such lenses have been designed but not for the likes of us - they are used in technical applications and would be very expensive and probably pointless for us to use as they would be part of an optical system which probably has completely different design parameters to ours!

On the question of optical quality, whilst wide-angles and wide-angle zooms are far better when used to close-focus than they used to be (it used to be that manufacturers stated the incorporation of close-focus correction optics in wide-angle primes, let alone zooms), using them to focus very close is asking a lot and they will probably operate better at infinity. This might explain why quality improves when using a diopter as, although a cheap diopter will do the optics no favours, the lens is at least operating as though at a longer focus setting.

As always, the bigger the dome, the better the image (as you are focussing further away and there is a greater depth of field available to take up the curved image)!

[frogfish]

I'm with Alex (and John). It may not be strictly necessary for every shot, but there's no question in my mind that the 12-24 mm lens with the large Subal dome works better with a +2 diopter, particularly for CFWA and also images with significant close detail in the corners and/or sides.

Frogfish

[Tom_Kline]

To calculate the dioptric power (P) needed for a dome, the calculation is:

P = 1000/4R where R is the radius of the dome

snip

<{POST_SNAPBACK}>

Tacit in the above formula is measurement in mm. Mertens (1970) gives the same formula as 1/(4R) with measurement in meters.
Standard diopters are given as the reciprocal of their focal length in meters. For example the focal length of a +4 diopter close-up lens is 1/4 of a meter = 25 cm = 250 mm. Mertens states that when using his formula, R, the port radius, must be in meters.
Tom

[Tom_Kline]

OK Alex
snip. As yet there are no really effective optical solutions for this problem and it is why the 15mm UW-Nikkor was always rated as being so good (although it is now a very old design).

snip

<{POST_SNAPBACK}>

The Nikonos 15mm lens and the other Nikonos lenses with 'UW' in their nomenclature (take a look at the full name inscribed on the lens) including the R-UW lenses for the RS are water-contact lenses, designed to focus in a water medium and not air. Accordingly, they have a levels of optical correction not possible with housed lenses.
Tom

[John Bantin]

Is not the front element of the Nikonos 15mm lens actually a perfectly matched and positioned dome port?

[Paul Kay]

As far as I am aware, yes the 15mm UW Nikkor does incorporate a dome (and so probably has some degree of corrective optics - rather than a simple diopter - built in as it won't focus above water). This said, it may well have been a good design, but as the Nik5 came out over 20 years ago it probably isn't as good as many still seem to rate it to be (compared to modern optics). It would be very interesting to see it used with a high MPixel digital camera as these seem to be pushing all other wide-angle designs.

Formulae in mm - would we use anything else? But this is a guide as you rarely need to focus at infinity underwater (certainly not in the temperate waters around Britain) so a +3 might be a useful compromise.

To get back to the post, I used to use a 12~24 with an S2Pro and tried it with and without a +4 Subal diopter (which I already had). I certainly gave better images with the diopter in that it focused closer and corners were sharper, but I wonder if the image curvature was worst without the diopter simply because the lens was working close to its minimum focus most of the time?

Personally I'm sticking to fixed focals......

[tunc]

I have a similar problem with my Sea&Sea DX-70 and the big Fish eye dome port.
With the 12-24 the images are only sharp in the middle. I tried adding a 40mm extension ring which extended the sharp area a litlle bit but still was really blurry all the way to the corners. I even tried a +3 Dioptre but couldn’t get any good results. Sea &Sea is recommending the fish eye dome port with out any extension ring .Which puts the front of the lens only 2 cm away from the port glass. As far as I know this should be wrong .(I’ve read the article about proper dome positioning and dome size) Is there some one who has the same setup and can help me out here.

Thanks,
Tunc