Minidome for FX

Hi,

I am interested in a minidome. Curruntly I have a DX camera but my next will be an FX camera. For what I understood 4''is the right minidome for a DX camera und for FX you need a 6'' minidome for good DOF. Is this right? where can I get a 6'' minidome for a Seacam housing??

Rob

4" mini dome can work on FX (the Seacam mini dome is actually smaller than 4"). But you cannot use a Nikon 16mm as it does not focus close enough. You must use Sigma 15mm. And then you much shave the lens shades - so it fits.

Is the Seacam fisheye dome not a 6" one?

Alex

The Seacam Fisheye Port has a 160mm diameter (http://www.seacam.com/en/products/front-ports/dome-ports/fisheyeport-fp). The Seacam Wideport is 140mm (http://www.seacam.com/en/products/front-ports/dome-ports/wideport-wp-) - near enough 6" - I've been using this with a Canon 15mm fisheye for years. Seacam's Fisheye Macro Port (http://www.seacam.com/en/products/front-ports/special-ports/fisheye-macro-port) is a smaller diameter and will work with the 15mm Canon but not close up unfortunately - the WidePort is better.

Hi,

@Alex

So I have to buy the Sigma 15mm FE for close focus wide angle?? For what scenes do you use your nikon 16 mm FE or don't you use it anymore??

@Paul

So you use the Wideport for close focus wide angle and normal FE shots??

And the Superdome for other wide angle lenses??

Rob

4" mini dome can work on FX (the Seacam mini dome is actually smaller than 4"). But you cannot use a Nikon 16mm as it does not focus close enough. You must use Sigma 15mm. And then you much shave the lens shades - so it fits.

 

Is the Seacam fisheye dome not a 6" one?

 

Alex

Hello Alex and all,

I just read about a trick to get the Nikon 16mm lens to focus closer by removing the rear removable filter. Removing the filter shifts the focus from the lens' standard focus range of .25 meters - infinity to .17 meter - 1.2 meters. A quick mm to inches conversion tells us the lens would then focus to a short distance of .17 m = 6.69291, which is close enough to use behind a 4" diameter port. Since he would be shooting behind a dome and focusing on a virtual image, the 1.2 meters far distance of the lens without a filter is not an issue.

I have not tried the above trick yet, but writing this has given me the idea to test it out today. I found this article at http://www.subspace.ch/English/Trick.htm.

Now addressing the first post in this thread, it is my findings that the dome radius from a 6" dome to a 4" dome will not affect the DOF for your 16mm lens; the 16mm Nikon has incredible DOF. Your selected F-stop will have much more to do with your DOF. Dome shape, size and distance from the lens' nodal point will have much more affect on the sharpness of the image, especially in the corners.

We designed the Bare Dome Port with a 5" diameter and our port housing design to give the best sharpness possible, especially in the corners. I found that using the 5" dome in our configuration gives much sharper images than what I was getitng with a 16mm Nikon lens and the Nexus 170mm port.

Thanks,

Barry

www.bareports.com

www.keywordworkflow.com

Hi Barry,

You are right that DOF depends on distance to subject and F-stop. But for FX you need a higher F-stop than for DX if distance to subject is the same to get the same corner sharpness.

As I understand it ( by reading Alex articles) for smaller macrodome you need a higher F-stop to get the same sharpness in the corners compared to a bigger Dome.

Rob

In the middle of work - so just a quick reply.

Removing the rear filter on the 16mm (mine is always removed, unless I am shooting filters) makes only a small difference to the close focus. I have never tried the 16mm with a mini port with the filter in place - always without. But as such it is still pretty useless.

There was some erroneous data on the 16mm floating around on the web somewhere - where the guy got the decimal place in the wrong place - and I think that might be what you are quoting Barry. Removing the rear filter from the 16mm bring the focus closer by just a few cms. From memory to about 22cm (guesstimate).

Like this it will focus behind a mini dome (underwater). But it will not focus on anything close (because the virtual image of infinity might be 4r away, but the virtual image of close subjects is closer. As a result you can only shoot things further away than more than a foot (30cm) with this setup underwater - with rather negates the benefits of using a mini dome (which is most powerful when used just a couple of cm away).

As a result with the 16mm behind a mini dome - the limits on focus mean that you can actually fill the frame more with subject when using a big dome than a small one (which goes against the reason for having a small dome)! You can see a comparative test with the 16mm (with filter removed) in my talk here: http://wetpixel.com/i.php/full/domes-without-the-drone/ (start at 16:20).

Alex

Hi Barry,

 

You are right that DOF depends on distance to subject and F-stop. But for FX you need a higher F-stop than for DX if distance to subject is the same to get the same corner sharpness.

As I understand it ( by reading Alex articles) for smaller macrodome you need a higher F-stop to get the same sharpness in the corners compared to a bigger Dome.

 

Rob

Hi Rob,

Interesting concept you relate about needing a higher F-stop to get better corner to corner sharpness, without regard to port and dome design. Not saying you are wrong, but my findings differ with my port design.

I tests and use my ports almost daily, since I am a working pro, and have over 200 dives with the Bare 5" dome. Agreed, a 5" dome is really not what people are calling a "mini-dome".

I do not have ANY problems with corner sharpness at any of the f-stops I use.

There are some popular dome ports being sold that have a reputation for blurry corners; one could be amiss by a blanket statement saying all domes have this same problem. Maybe what we should say is an "improperly designed mini-dome" or "a mismatch lens/dome configuration" might need the higher f-stops to produce better corner to corner sharpness.

Barry

www.bareports.com

www.keywordworkflow.com

Interesting concept you relate about needing a higher F-stop to get better corner to corner sharpness, without regard to port and dome design. Not saying you are wrong, but my findings differ with my port design.

What full frame camera / lens / subject distance combinations have you tested with the dome you produce that yield acceptable sharpness?

There are some popular dome ports being sold that have a reputation for blurry corners; one could be amiss by a blanket statement saying all domes have this same problem. Maybe what we should say is an "improperly designed mini-dome" or "a mismatch lens/dome configuration" might need the higher f-stops to produce better corner to corner sharpness.

As far as I am aware, concentric (spherical) dome ports all behave in a similar way because they all have similar optical properties and the only differences are in diameter, thickness and refractive index - which whilst variable are not extremely so - and of course they need accurate positioning. I am unsure how one can improperly design a concentric dome port, and I would be extremely interested to see results from any port which can overcome the fundamental problem of the curved virtual image.......