Thoughts on Mini Domes

by Alex Mustard Wednesday, March 31st, 2010

Testing Mini-Domes

I go into some detail below, so for those who don’t want to get too techie, the take home message is mini-domes are best for CFWA and WAM. The most suitable lenses are close focusing fisheye lenses (e.g. Tokina 10-17mm, Sigma 15mm), rectilinear wide angle zooms will not work well. A mini-dome and fisheye can be used for more standard wide angle, but the lens needs to be stopped down to at least F13 (approx) for decent corner sharpness with DX/APS-C cameras (more on FX/FF). They are unsuitable for split-level images. You can skip to the concluding comments if you want now!

The main negatives associated with mini-domes relate to how the optics affects the virtual image. Every dome port acts as a negative lens underwater, creating a virtual image of the subject (much closer to the camera than the subject really is) that the lens must focus on. A small, highly curved dome will produce a virtual image that is closer to the camera and more steeply curved than a large dome.

Comparison of the virtual images created by an mini-dome and standard dome of a subject at infinity in water. Note that a mini-dome creates a virtual image that is both closer and more curved than is created by a standard dome.

Optical theory tells how close the virtual image will be for a hemispherical dome. Underwater, the virtual image of a subject at infinity will be 4 times the (spherical) radius of the dome (from the centre of the dome), in other words 3 times the radius from the front of the dome.

So for a hemispherical mini-dome, with a diameter of 100mm (4”) and a radius of 50mm (2”), a subject at infinity will be seen as a virtual image (distance = 3 x radius) just 150mm (6”) from the dome. Of course, our underwater subjects are always closer than infinity, so will require the lens to focus closer still. Standard sized fisheye domes will produce a virtual image that is between 350mm and 450mm from the dome (14”-18”).

So mini-domes require lenses that can focus close. Most modern fisheyes fit the bill perfectly, but the Nikon 16mm (that I use on my FX sensor D700), for example, does not focus close enough to be useful with a mini-dome. It provides a valuable lesson about the restrictions of a mini-dome (see below). The Nikon 16mm has a stated minimum focus (note this is sensor to subject) of 250mm, compared with 140mm for a Tokina 10-17mm or Nikon 10.5mm. This translates to about 110mm from the front of the lens for the 16mm. Most rectilinear zoom lenses from Nikon and Canon have minimum focus distances between 280mm and 300mm, which is one limitation of using them with small domes.

You need a lens that focuses close to make full use of a mini-dome. Tthe 16mm FE, although considered a close focusing lens, cannot focus close enough and achieves a larger subject size in the frame with a standard dome, than a mini-dome. These two images were taken with the Nikon 16mm on my D700 behind a standard dome (left) and mini-dome (right). The lens was set on minimum focus and backing slowly away from the subject, I took the picture when the subject was in focus. This lens cannot focus that close (to about 100mm from the dome) and is unable to focus on the virtual image of closer subjects created by the mini-dome.

However, things are very different with the right lens. The Tokina 10-17mm, which can focus very close achieves much larger subject magnification with the mini-dome. I was not able to get any closer to the rubber duck with the standard dome because the duck (on the left, below) was already pressed against the glass. The mini-dome does not stick out as far, I can therefore get the lens closer to the subject and achieve a larger magnification (right).

You need a lens that focuses close to make full use of a mini-dome. The 10-17mm (here on my D2X) can focus right to the dome, which means with the standard dome the subject magnification is limited when glass hits the subject! The smaller mini-dome allows the lens to get closer to the subject and to almost fill the frame with the subject. Note that the lens used for these images has the same angle of view as the previous images.

The other optical consequence of a mini-dome is that the virtual image is steeply curved. Lenses are built to focus on a flat plane and when we focus in the centre of the virtual image the corners are closer to the lens and therefore out of focus. The closer to the camera and the more curved the virtual image (the smaller and more curved the dome) the more of a problem this is. At any given aperture, the corners of our image will be less sharp with a mini-dome than with a large FE dome. Rectilinear wide angle zooms often struggle for corner sharpness behind standard domes, a problem made a lot worse with a mini-dome.

We can overcome corner sharpness by increasing depth of field, stopping the lens down (when possible). Using an APS-C or DX camera will also give us more depth of field for the same angle of view and aperture than on FF/FX, which will also improve the corners. We can also help matters by making sure we have open water in all four corners of the frame.

The example below compares corner sharpness at two apertures (2 stops apart) taken with 10-17mm @ 11mm on DX camera using the Zen mini-dome (since all the mini-domes are the same shape you can expect close to identical performance on this issue from other brands). Note that both look fine at screen resolution, but at 100% the difference in the corners is clear. Also note, that if you mainly show your images projected or online this shortcoming of the mini-dome will be less important to you.

Corner sharpness issues associated with a small dome. These two images of the same subject are taken with D2X + 10-17mm @ 11mm using the Zen mini-dome. The left at f/7.1, right at f/14 (this is a 2 stop difference). At screen res, both look acceptable.

These images are 100% crops of the corners of the images above. At f/7.1 corner sharpness is not acceptable to me. But shut down two stops (f/14) and it is much improved, and while still not perfect is acceptable IMO. As long as you can shoot at f/13-f/14 or more you should get acceptable results on DX/APS-C with the a mini-dome. These are screen grabs from Lightroom.

I also did pool tests, where I was able to compare the mini and stand domes. From these I concluded that the corner sharpness difference between a big dome and small dome is about 1.5-2 stops (i.e. for the same corner sharpness as f/8 on a large dome, you need to be at f/14-f/16 on the small dome). I have deleted these files so I can’t show them. But Ryan Canon said on the Wetpixel forums that they found the same in their tests of the Zen mini-dome against the standard Zen dome.

An obvious limitation of the small size of a mini-dome is that it is unsuitable for split-level images, because the meniscus of the water takes up a much larger proportion of the frame and is harder to control.

Finally, mini-domes, because the dome is so close to the lens, can cause problems housing a variety of lenses. Fisheyes, in particular, with their wide view can be troublesome and the dome really needs to be set up for one particular lens. If the lens is too far forward the hood/shade will hit the inside of the glass. If the lens is too far back it will see the sides of the port/shade. The difference is very small and too small to be solves with port extension rings.

For a real world example, the Zen mini-dome I tested was setup for the Tokina 10-17mm on a DX Nikon in a Subal housing, and worked perfectly. However, when I tried to use this port with a Nikon 16mm and Sigma 15mm on a FX Nikon in a Subal housing I had problems. The Nikon 16mm, a slightly shorter lens than the Tokina 10-17mm would see the shade of the port, causing cut off in the image (see first duck images). (This should not be called vignetting, BTW, which is an optical effect of a lens causing darker corners of the frame. This is just blockage or cut off, no need for fancy words that mean something else!). The Sigma 15mm was even more of a problem. Its longer hood meant that it could only fit in the port focused on infinity, when it tried to focus closer its longer hood hit the glass. While as the same time also suffering cut off from the port shade. The Sigma 15mm requires either some of its shade or that of the port being machined away to work.

In all this techno chat it is easy to loose sight of the fact that mini-domes allow us to create images that we could not with a big dome: taking wide angle views with a relatively small subject dominating the foreground. And because of the small size of the dome we can achieve a high quality of lighting (with big domes these photos always have the light behind the subject and the face of the fish in the semi-shade). This is exciting news for the underwater photographer.

So, should you get one?

If you are a keen underwater photographer, who already has a large dome port then a mini-dome should be high on your shopping list. Like the best accessories it will expand your portfolio allowing you to take photos that you cannot at present (such as WAM). The case will be even stronger if you shoot with an APS-C or DX camera. A mini-dome will also provide a light-weight travel alternative to your main dome, such as when travelling to macro focused destinations such as Lembeh.

If you are new to SLRs and looking for a cheaper alternative to a standard sized fisheye dome then the mini-dome does fit the bill. With a very close focusing fisheye lens (such as the Tokina 10-17mm), it will allow you to take a wide range of images from WAM to CFWA to standard wide angle. Just remember to keep the lens stopped down whenever possible. A mini-dome will also lessen the inevitable baggage expansion that comes hand in hand with the upgrade to SLR.

But there are a few warnings. First, as discussed above, the mini-dome has some optical shortfalls. If it is your only dome you will not be able to shoot quality split level images and you will struggle to get good results from lenses other than your fisheye. If you already own or consider buying a rectilinear wide angle zoom, then you will be buying a standard sized dome in the future too. Buying both and mini-dome and standard dome is always more expensive than just buying a standard dome in the first place!

Many of my most popular images were produced using mini-domes and I will always remain a big fan. I consider it an essential piece of kit. Just less essential than a standard sized dome.

Perhaps sometimes you can get too close. The author photographed with Zen mini-dome. D700 + 15

Alex Mustard – March 2010.
Many thanks to Adam Hanlon for lending me his Zen 100 port and for help and support from Richard and Ryan and Reef Photo and Video.
www.amustard.com

Alex, with the Minidome and a Nikon 10.5 on a FX body would you recommend to shoot in DX mode (and hence loose resolution) or use it in FX mode and crop the image later in post? Also, what might be the effects of circular or rectangular if in FX or DX mode, can you elaborate on this (I still do not yet have the minidome neither the lens so I cannot test it myself yet). Thanks Juerg
Posted by buddy on 2011-06-28 11:10:01 Bill, If you look at the drawings of the virtual images for the dome ports, you will see that they are curves, parallel to the curve of the dome. Camera lenses have a subject plane that is a straight line, parallel to the film plane. The distance to the edge of the subject plane is increasingly farther away than the center of the subject plane as the lens gets wider and wider. Because of this, trying to impose the curved subject plane of a dome port onto the flat subject plane of the lens, the wider the lens is, the more out of focus the edges of the image are compared to the center. NO MATTER HOW PERFECTLY THE LENS IS CENTERED IN THE DOME. AND NO MATTER HOW BIG THE DOME IS. A bigger dome simply places the curved virtual image farther away and makes the focus discrepancy easier to "cover" with depth-of-field by stopping down the lens aperture. Fred
Posted by divegypsy on 2011-06-20 20:22:08 Fred, I believe that if you get the nodal point properly placed (locate the dome at the proper distance) that the edges will be in focus. I agree that the larger dome with the greater depth of field will allow for more error in this calculation and placement as you have stated as opposed to the mini-dome. That is why the nodal point placement is so critical with these smaller domes and the associated lenses. As Alex stated "and the dome really needs to be set up for one particular lens" is very true. Alex, Thanks for providing all the great information. One other comment on the mini-dome is that they are complete 1/2 hemisphere domes and the area captured is greater than a lot of the larger domes that are less than 1/2 hemisphere domes. Bill Libecap UwCameraStuff.com
Posted by blibecap on 2011-06-16 12:33:00 Alex, If the nodal point of the lens is properly placed, at or near the nodal point of the dome, the virtual image formed by the dome is parallel to and 3x radiu beyond the dome. The virtual image in each case is a portion of a hemisphere and has exactly the same curvature relative to the nodal point of the lens. The major factor, I believe, in less corner sharpness is that the lens is focused much more closely and therefore has less depth-of-field at any give aperture. The same is true with a regular rectilinear lens in a dome. The bigger dome moves the virtual image farther away and therefore less depth-of-field is necessary to bring the corners into sharper focus. Fred
Posted by divegypsy on 2010-05-26 06:28:37 @ Barry. Just what I talked about and demonstrated in the article. Because of the shape of the dome, all mini-domes will produce a virtual image that is closer to the camera and more curved than a standard-sized dome. The problems are mainly seen the corners because lenses are designed to focus on a flat plane. The virtual image of a mini-dome forces the camera to have to focus closer, which (importantly) limits depth of field making it harder to get those bendy corners sharp. Camera to subject distance (or in the case of domes, camera to virtual image distance) is an important control over depth of field and therefore corner sharpness. Empirically, in my tests, I found that the same lens had to be stopped down about a 1.5-2 stop more to get equivalent corners on a mini-dome and large dome. Hope that this is clearer. Alex
Posted by Alex Mustard on 2010-04-02 10:47:27 Nice article Alex and thanks for the plug. You stated the mini domes do not perform as well as larger domes, which is just the opposite of what I found with my dome. Would you please explain what you mean by your statement? I must admit that I am no optical engineer, but I have a page showing the calculations I used to design my dome. If a dome is not calibrated to a particular housing/lens combination, I can see there could be problems as explained here http://www.creativeillusionsphoto.com/virtual.htm. Remember, I am NOT an optical engineer, but the reasons I chose my dimensions seem logical to me and most of all they give me great results. Barry
Posted by Dupsbear on 2010-04-02 09:46:41 just ordered mine from Ryan at Reef :-)
Posted by stewsmith on 2010-04-01 14:10:44 Thaat´s a good point. The info with the 0,17m is from the Subspace website so I thought it´s correct. Actually I never used the rear filter for underwater photography since it´s better that way. I have measured the focusing distance by myself a long time ago and it was 3cm shorter so You´re right with the 0,216m. Doesn't matter anyway, since my housing was flooded last year at greater depth (not my fault), this year I´m eagerly awaiting to test the Sigma 15mm.
Posted by Andrej Belic on 2010-04-01 13:04:01 Good info Andrej. These photos were all taken without the rear filter on the 16mm (mine never has the rear filter on unless I am using filters!). But as you can see in the tests this lens still does not focus close enough. If your quoted numbers are from Bjorn then he has made a mistake. They are not measured in the same way. The 16mm has a minimum focus (with filter of 0.17m) only if you measure the minimum focus from the subject to the back of the lens. Nikon's figure of 0.25m is measured over the longer distance: the subject to the film plane/sensor. This adds another 46mm to the distance. So adding this to the number for the 16mm without its filter, it actually focuses to 0.216m (216mm). A saving of about 33mm - which is valuable - but still considerably inferior to the 10.5mm or 10-17mm available on DX, which focuses to 0.14 (140mm) measured from subject to sensor/film. I have just doubled checked these measurements for the 16mm. 16mm with filter @ minimum focus: subject to back of lens: 203mm (measured now) subject to film plane/sensor (stated): 250mm 16mm filter removed @ minimum focus: subject to back of lens: 170mm (measured now) subject to film plane/sensor (inferred): 216mm I feel I need a diagram! Alex p.s. It is also possible to fit a small +0.5 dioptre into one of the filter mounts to increase the close filter ability. Peter Rowlands made a number of these for use with Mini-domes back in the 1980s. He actually has a pair of glassed with a pair mounted on them too!
Posted by Alex Mustard on 2010-04-01 08:56:43 Hello to all! Regarding the Nikkor 16mm there is a trick for shorter focusing distance which I discovered a long time ago from a Norwegian nature photgrapher at www.naturfotograf.com. This site prvides a lot of features about alomst all lenses useable for Nikon cameras. Actually the FE Nikkor 16mm focuses to 0,17m instead of 0,25m. All You have to do is take the rear (neutral) filter out. Then You can not focus to infinity which doesn't matter for underwater work anyway. Here´s another link describing how to do it: http://subspace.ch/English/Trick.htm
Posted by Andrej Belic on 2010-04-01 08:11:37 Are there any pointers you could share with regards to strobe positioning Alex.
Posted by stewsmith on 2010-04-01 00:47:40 Nicely done, Alex. Very generous sharing of good insight.
Posted by StephenFrink on 2010-03-31 19:42:38 Thanks Alex, very timely. My new Zen arrives today. Now I have to get a damn duck too? It never ends.
Posted by Steve Williams on 2010-03-31 15:15:29 I wanted to try CFWA and WAM last year with a Tokina 10-17, but did not have a mini-dome to get close to subjects. So, I used the lens behind a regular flat port with no extension ring. Worked well, not too much distortion, sharp focus, and could get very close to subjects. There were two main disadvantages. One was that the port would show when zoomed to wider than about 13 mm. And, could not engage the zoom ring. Despite the problems, it was so versatile it became my favorite lens/port combo. Will definitely get a mini-dome for my next housing. Rocky
Posted by Rock on 2010-03-31 14:55:17 Great article!
Posted by coloradodawn on 2010-03-31 14:23:48 Your own duck! Now that is either extreme dedication or a cunning plan to establish a breeding program. Paul C
Posted by PRC on 2010-03-31 13:56:47 Thanks guys. Re centre sharpness. Just checked the files and no diffraction issues at f/14 versus f/7.1 in the centre of the frame. Pictures with my 12MP D2X, maybe the latest 16MP APS-C SLRs might be more sensitive? Clarification on the duck. This is my own duck I brought over from the UK for the shoot. The Compass Point ducks were there too - you can see one in the Selective lighting thread in the forums! Alex
Posted by Alex Mustard on 2010-03-31 13:31:40 Thanks Alex - seems that Compass Point have an updated rubber duck supply - I must get one ( and a duck ). Paul C
Posted by PRC on 2010-03-31 13:06:53 Best article ever. OK, maybe I'm biased because the whole issue is foremost in my mind at this very moment in time. I wonder if stopping down from 7.1 to 14 had an impact on the quality of the center of the image due to diffraction.
Posted by scorpio_fish on 2010-03-31 12:50:17 could someone please nudge my wife and remind her that my birthday is coming up !!! I want one for Lembeh !!! Stew
Posted by stewsmith on 2010-03-31 12:36:18 Very helpful Alex! Thank you!!
Posted by Nicholas S. on 2010-03-31 12:20:28 Good work Alex
Posted by Alsky72 on 2010-03-31 11:34:32