Thoughts on Mini Domes 
Alex Mustard discusses the latest must have accessory
Last Update: 31 March 2010 05:28 PM
22 comment(s) | discuss in forums
Author: Alex Mustard ( Alex_Mustard )
THOUGHTS ON MINI DOMES
Alex Mustard
A mini-dome is small dome port usually around 100mm or 4” in diameter and typically hemispherical in shape. They offer significant advantages to the underwater photographer and I have long considered them an essential piece of kit for certain images. They also have significant drawbacks compared with standard sized domes (200-220mm, 8”-9”). It is important to see both sides of the argument to understand if you should buy one, when you should use it, and how to produce images that you cannot make with a standard dome.
This is not a review of any particular brand of mini-dome (although I used the new Zen 100 dome for the test shots). Instead this is a discussion of the advantages and disadvantages that come with using a dome port of this size and shape (irrespective of the quality of construction, glass etc).
Mini-domes have always had a small, but loyal following in underwater photography. Several of my friends in BSoUP have been using them almost exclusively for the last 20 years or more. Which is how I was introduced to them. Up to now, most have been self-made and Barry Guimbellot recently shared some excellent tips on how to do it.
However, that has changed. Right now, commercial mini-domes seem like London buses. You wait ages for one to turn up and then three come along at once. They are now available in a range of flavours from Aquatica to Zen!
Mini domes offer some significant advantages. They are smaller, lighter and (usually) cheaper than standard domes. All music the ears of almost every underwater photographer!
Photographically their real advantage is that their small size makes it much easier to position (small) strobes close to the port. This greatly improves the quality of lighting we can achieve in true close focus wide angle (CFWA) and wide angle macro (WAM) shots. However, this important advantage only becomes significant when camera to subject distance is less than about 100mm or 4”. Further away it is not significant.
The other advantage for this type of photography is that the small physical size of the port allows us get the lens physically closer to the subject, therefore making it even larger in the frame. For example, I took the photo of the sea urchin (below), which was smaller than half a tennis ball, with the Tokina 10-17mm on its widest setting at 10mm, yet I was able to fill the frame with it. However, this advantage is only realised with a very close focusing lens (like the Tokina 10-17mm or Nikon 10.5mm). Most wide angle lenses will actually give you a large subject magnification with a standard dome than a mini-dome (this is quite a surprise to many people, see examples on page 2).
Small domes can also be used for more typical wide angle photography, but will not perform as well as a standard sized dome. But before we get into the optics we should heed a historical lesson: all the early dome ports were mini-domes. Big domes were too expensive and difficult to make. And they weren’t called mini-domes back then, simply domed portholes (port is an abbreviation of porthole, by the way) as they were the only size available. The innovation photographers wanted back then was actually to make them bigger, to overcome some of their optical issues. Photographers getting over-excited about mini-domes as the latest invention should take note!
Continued on page 2…
Good work Alex
Posted by Alsky72 on 03/31 at 07:34 AMVery helpful Alex! Thank you!!
Posted by Nicholas S. on 03/31 at 08:20 AMcould someone please nudge my wife and remind her that my birthday is coming up !!!
I want one for Lembeh !!!
Stew
Posted by stewsmith on 03/31 at 08:36 AMBest 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 03/31 at 08:50 AMThanks Alex - seems that Compass Point have an updated rubber duck supply - I must get one ( and a duck ).
Paul C
Posted by PRC on 03/31 at 09:06 AMThanks 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 03/31 at 09:31 AMYour own duck!
Now that is either extreme dedication or a cunning plan to establish a breeding program.
Paul C
Posted by PRC on 03/31 at 09:56 AMGreat article!
Posted by coloradodawn on 03/31 at 10:23 AMI 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 03/31 at 10:55 AMThanks 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 03/31 at 11:15 AMNicely done, Alex. Very generous sharing of good insight.
Posted by StephenFrink on 03/31 at 03:42 PMAre there any pointers you could share with regards to strobe positioning Alex.
Posted by stewsmith on 03/31 at 08:47 PMHello 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 http://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 04/01 at 04:11 AMGood 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): 250mm16mm 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 04/01 at 04:56 AMThaat´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 04/01 at 09:04 AMjust ordered mine from Ryan at Reef :-)
Posted by stewsmith on 04/01 at 10:10 AMNice 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.
BarryPosted by Dupsbear on 04/02 at 05:46 AM@ 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.
AlexPosted by Alex Mustard on 04/02 at 06:47 AMAlex, 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 05/26 at 02:28 AMFred, 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.comPosted by blibecap on 06/16 at 08:33 AMBill,
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 06/20 at 04:22 PMAlex,
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). ThanksJuerg
Posted by buddy on 06/28 at 07:10 AM
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