How does the Ikelite dome port work?

[DazedAndConfuzed]

Since my flat port is in a stage where it might go kaput in my process of clearing whatever crap that is on it, I was looking at the option of getting a dome port. I had a few questions about how it works.

1. I heard it creates a virtual image 1 or 1.5ft in front of the lens (or is it film plane?) Does that mean everything from 1 or 1.5ft (lens, port or film plane) to infinity is in focus?

2. How do I take picture that is closer than 1 or 1.5ft? My 24mm could focus down to 10".

3. If I decrease the f-stop to f16 or f22, would that increase the depth of field of the dome image down to 9in or 1.2ft (depending whether it is 1 or 1.5ft focus.)

4. I heard a 24mm lens in a dome port preserves its above water coverage angle. I presume this is true.

Thanks for any information that you could provide.

 

[scorpionfish]

Hi, again, Lwang!

Let's start with a confession: I do not and have never owned an Ikelite Dome Port or Housing. However, I have a Subal and my guess is that the physics/optics do not change (much).

A dome is really a lens because it is curved and refracts light. Due to the shape of the dome, it will create an apparent image that is in front of the lens and makes the subject appear closer. For my Subal, the image is also smaller. So, I add a +4 or +3 diopter to the filter threads on the front of the lens. This addition is needed whether I am using a zoom or a wide angle lens. For some reason that is not clear to me Ikelite only requieres a diopter when a zoom is used. I do not have an Ikelite Dome Port to compare to my Subal, but I suspect the design is somewhat different. I am going find out (someday).

I do not have DOF charts for my lenses when housed, but it seems logical that the DOF changes because of the refraction. I just cannot quantify it at this time. However, for most U/W photography, focus at infinity or long distances is not that important because there is insuffient light. Of course, if you are Jim Church, you can afford to have a crew with all sorts of slave strobes, etc. to provide that light. Last time I looked, I was not Jim Church!


I have a 20mm lens which I use with my Subal and it works "fine" on land: so, I presume your 28mm will, too. However, the problem is if you try to split your screen and take one of those "trendy" half in-half out of the water photos, it will not look right. The refraction in water and air are different and, so, things underwater will look closer, as usual. To make one of the 1/2-1/2 photos you use a diopter that only covers half the lens and you put the dividing line at the water line.

Hope this helps,

Scorpionfish

 

[DazedAndConfuzed]

Thanks for the info.

The depth of field is the part that confuses me. I presume objects closer than the dome's minimum focus gets refracted to a point somewhere so that if I close down on my lens, those points would become in focus. But then the fact that the dome manufacturers say that everything from 1ft to infinity is in focus (on a 6" port) when one only has to focus the lens to 1ft makes it sound like the image's depth of field will not be determined by my lens, but by the dome. As if I am focusing on a TV screen, where things out of focus cannot be brought back into focus.

So if my subject is 6" away, and I focus my lens down to 6", will the subject be in focus?

 

[vicky]

The virtual image that the dome port creates depends on the port's radius. Ikelite's dome ports create a virtual image at 12" in front of the lens. most zooms, even those that manufacturers call them "macro" cannot focus at so close distances. Therefore you need a +4 close up lens and you will be able to focus underwater with a dome port.
According to Ikelite regular lenses (not Zooms) can usually focus at closer ranges thus no +4 diopters is needed. BUT, many lenses are not able to focus at 12", so you'll need a +4 close up lens anyway. For example, my Canon 50mm f/1.8 lens nearest focusing distance is about 40cm (16inches) and I must use a +4 close up lens on it too. How can you determine if you need one? Look at your lens and see how closer it can focus. Most lenses have the number written somewhere..If it less than 12" you better add one.

BTW, Adding more that +4 diopters may be problematic since you may find yourself that you can focus only from very close to the port and therefor not so practical. Most photogrpahy subjects will run away if you try to put the camera 1 inch from them... And the strobe is difficult to position and most of the time will act as side lighting..

 

[vicky]

The virtual image created by the dome port is actually like compressing (or re-scaling) the real world into a smaller range. It applies to depth of field etc, so depth of field is still determined by your focal distance and aperture and not by the dome. The dome only determines the new "location" of the image. Just hold the port in your hands (without camera or anything else) and look at things through it. You'll get the feeling of an hologram floating in the air. That's your virtual image, and it's pretty cool. You may want to reach your fingers and touch the objects . It looks real but small, it is a virtual image (deceives your mind), not a projected image like the ones on your TV..

 

[DazedAndConfuzed]

vicky,

thanks for the info. What I am not clear about is that I think they said with the 6" dome, and my camera focused to 1', eveyrthing from 1' to infinity is in focus.

If I focused the lens down to 8" (as a 20mm lens could do,) will things 1" in front of the dome (since 8" focus would be to film plane, film plane to dome would probably take up the remaining 7") be in focus?

And if I get a 24mm lens, which focuses down to 10", but with a f-stop of 22, its depth of field makes things 7" in front of the lens sharp. If I had those settings, would something stay in focus right to the tip of the dome port?

 

[scorpionfish]

Lwang once bubbled...
vicky,

What I am not clear about is that I think they said with the 6" dome, and my camera focused to 1', eveyrthing from 1' to infinity is in focus.

If I focused the lens down to 8" (as a 20mm lens could do,) will things 1" in front of the dome (since 8" focus would be to film plane, film plane to dome would probably take up the remaining 7") be in focus?

Hi, Lwang!

Well, it has been too many years since university physics and I am too lazy to try to find a book on advance optics. So, I did the experiment with my Subal Dome and my Nikkor 20mm f/2.8 lens. Here is what I "discovered":

(1) Without the dome or the +4 diopter the lens focused down to ca. 10" at f/22. The DOF was ca. 10" to 12".

(2) When I put the +4 diopter back on along with the dome, I could get to within ca. 1" off the subject, but I could create the same image AND the seemingly was no increase, as far as my eyes could tell, in the DOF. As I indicated in my first post and as Vicky indicated, too, if I left the diopter off, the image size was much reduced.

(3) Now I took off the diopter and dome again and focused at around 3'. The DOF was ca. 1.5'-infinity.

(4) And, for the final experiment, I put the diopter and dome back on and focused at 3' again. Here, too, I could move the dome closer than 3' to the object, but when I got the same image in the view finder that I had in (3), the DOF appeared to be the same, i.e., 1.5'-infinity.


Well, I am not certain where all this leads us, or, in fact, what difference it all makes! I have been using my housed camera for over a year now and these issues never seemed to have arisen. Of more import is the field of vision of the lenses I use. As I indicated earlier, the DOF is mostly moot because of light limitations. With one or two strobes, subjects beyond about 10' will not be well lighted; however, you will be able to see whether they are in focus, an important artistic issue.

Also, when you are focusing on something close, normally you want to highlight that subject and not have "background" subjects detracting from the "foreground" subject. In fact, the use of selective focus in macro photography frequently leads to wider aperatures to focus attention on the macro subject.

I hope this helps--somewhat--

Scorpionfish

 

[caymaniac]

I currently have an ikelite dome port, I usually use a sigma 28 wide angle close focusing lens. There is a virtual image created in front of the lens. My lens will focus down to about two inches at max. Everything past one foot will not be in focus in a close sitauation. When the virtual image is created you must still focus on the virtual area that the subject is in. At macro type shots, everything will not be in focus, but on wide angle the depth of field is pretty good. I think the trick is to buy a lens that is a close focus lens, who wants to monkey with diopters if you dont have to.
I hope this answers your question

Dive Safe,
Caymaniac

 

[scorpionfish]

caymaniac once bubbled...
who wants to monkey with diopters if you dont have to

Hi, Caymaniac!

Actually, there is no monkeying! You just screw on the correct diopter for a given lens and away you go. Even Ikelite requires a diopter when using a zoom lens with their domes.

I am not certain why my Subal requires a +4 or +3 diopter with my 20mm Nikkor lens and Ikelite does not require one. I am going to do a little research and see if I can discover the reason.

Scorpionfish

 

[DazedAndConfuzed]

Thanks scorpionfish and caymaniac.

scorpionfish,

with the dome port off, but +4 diopter on, is the minimum focus the same as with the dome port on?



I guess I mostly wanted to pre-focus for a close range and forget about it in most close up shots. I have a 24mm f2 which focuses down to 10" (film to plane. I don't know why they constantly use subject to lens distance for UW photography,) in which case I won't have to use the diopter with the Ikelite dome.

I know that if I use the flat port, and I set the 24mm lens to 10" focus (lens tip to film plane) and f22, the depth of field would be from 7" to 13" (to film plane) would be in focus. So I could point at anything from the tip of the port's housing (roughly 7" from film plane) to 6" away.


Just don't know if it applies to the dome port, since all I heard is that they tell you to set focus at 1 feet and everything from 1' to infinity would be in focus (I assume this even applies when the lens is at f2.)

There is nothing about if my subject is closer to 1 feet (don't know if it is subject to lens or subject to film plane,) can I turn I still get the subject in focus by turning the focus ring to less than 1 feet? Or if I close down the aperture from f8 to f22 and keep the same focus, will that subject suddenly snap into the camera's DOF range?

There is alot of f22 talked here because with those closeup shots, my strobe specifies a f22 aperture when strobe to subject is ~2ft, which with extened arm strobe, would stay steady, whether subject to port is 1 inch or 8 inch.)

I guess w/o a dome port in front of me, I cannot experiment on my own, and with scorpionfish's diopter, it is a little more like apples and orange since I don't think I will be using it.