How does the Ikelite dome port work?
- Thread starter DazedAndConfuzed
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"1. I heard it creates a virtual image 1 or 1.5ft in front of the lens (or is it film plane?)"
It creates a virtual image in front of the dome, not at a specified distance from lens or film plane.
"Does that mean everything from 1 or 1.5ft (lens, port or film plane) to infinity is in focus?"
No, what is in focus is defined by the normal parameters of your lens.
"2. How do I take picture that is closer than 1 or 1.5ft? My 24mm could focus down to 10"."
The rule of thumb is the lens must be able to focus at 2x the radius of the dome. So, with the Ike dome you must be able to focus down to 12". Since the 24mm can focus below 12", then you have no problems. You can shoot from as close as you can get with no problem with this lens.
"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.)"
You are confusing DOF with minimum focus distance. Again, the DOF works under the same parameters of the lens above water.
"4. I heard a 24mm lens in a dome port preserves its above water coverage angle. I presume this is true."
Absolutely. The angle of coverage of any lens behind a dome port will maintain the same angle of coverage as above water.
Vicky's first post is right on the money.
Note 1: Minimum focus specs are distances from film plane to subject. Not end of lens to subject.
Note 2: The addition of a diopter will reduce minimum focus distance. It will also eliminate the ability to focus to infinity. This doesn't come into play underwater.
Note 3: The same camera and the same lens may require a diopter for one brand of dome versus no diopter for another. This has to do with the physical positioning of the camera inside the housing/dome.
Note 4: Just as on land, the DOF of a wide angle lens (i.e. short focal length) is far greater than a standard or long lens, often as much as 25 times at the same f-stop. Since you are shooting scenes that are within, say a 50' distance or less, DOF doesn't really come into play with wide angle. You will find yourself shooting from f4 to f11 depending on ambient light for your background, most often f5.6 and f8.
I meter my background, often blue water or that which is beyond the ability of my strobes to light. I'm shooting at 1/60, which is where my camera goes with normal flash sync. Adjust strobe power to match shutter speed, aperture and strobe to subject distance.
It creates a virtual image in front of the dome, not at a specified distance from lens or film plane.
"Does that mean everything from 1 or 1.5ft (lens, port or film plane) to infinity is in focus?"
No, what is in focus is defined by the normal parameters of your lens.
"2. How do I take picture that is closer than 1 or 1.5ft? My 24mm could focus down to 10"."
The rule of thumb is the lens must be able to focus at 2x the radius of the dome. So, with the Ike dome you must be able to focus down to 12". Since the 24mm can focus below 12", then you have no problems. You can shoot from as close as you can get with no problem with this lens.
"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.)"
You are confusing DOF with minimum focus distance. Again, the DOF works under the same parameters of the lens above water.
"4. I heard a 24mm lens in a dome port preserves its above water coverage angle. I presume this is true."
Absolutely. The angle of coverage of any lens behind a dome port will maintain the same angle of coverage as above water.
Vicky's first post is right on the money.
Note 1: Minimum focus specs are distances from film plane to subject. Not end of lens to subject.
Note 2: The addition of a diopter will reduce minimum focus distance. It will also eliminate the ability to focus to infinity. This doesn't come into play underwater.
Note 3: The same camera and the same lens may require a diopter for one brand of dome versus no diopter for another. This has to do with the physical positioning of the camera inside the housing/dome.
Note 4: Just as on land, the DOF of a wide angle lens (i.e. short focal length) is far greater than a standard or long lens, often as much as 25 times at the same f-stop. Since you are shooting scenes that are within, say a 50' distance or less, DOF doesn't really come into play with wide angle. You will find yourself shooting from f4 to f11 depending on ambient light for your background, most often f5.6 and f8.
I meter my background, often blue water or that which is beyond the ability of my strobes to light. I'm shooting at 1/60, which is where my camera goes with normal flash sync. Adjust strobe power to match shutter speed, aperture and strobe to subject distance.
caymaniac
Contributor
With my lens, it does focus at a closer distance with a dome port.
Caymaniac
Caymaniac
OP
DazedAndConfuzed
Contributor
vicky,
Thanks for the link. Very interesting reading, although since I wasn't planning on using the diopter with my lens, reading about its application sort of threw me off because it is not transpose what they said w/diopter to a non-diopter situaion.
They did mention that the image of infinity would be at 2x diameter, in which case for the Ikelite 6" dome would be 12". Focusing any closer would be up to my lens' capability. And with my lens' having a focusing range of 10" (its minimum film plane to subject distance) to ~21" (9" film plane to tip of dome, plus the 12" of virtual image I have to focus on.) It sounds like I cannot focus that close, having to rely on the depth of field of the wide angle lens for things to stay in focus.
I guess that is where the diopter would come in, somehow having infinity setting on the lens become the point where the virtual image is, and having the whole focus range of the lens to adjust based on my subject's distance.
So I presume the mfg that said that all I have to do is put the lens focus at 12" and everything from 12" to infinity would be in focus is not really a true statement.
I don't have a Ikelite dome, but I've seen reflections/reflactions on other domes and have noticed the floating holographic image that seem to be playing tricks on my eyes.
scorpiofish,
I am trying to first set at minimum focus distance, then set f-stop to f22, this way, subject from 7" to 13" would be sharp. I might have interchanged some terms unintentionally, so when I say minimum focus when stepped down, it means subject still sharp.
Thanks for the link. Very interesting reading, although since I wasn't planning on using the diopter with my lens, reading about its application sort of threw me off because it is not transpose what they said w/diopter to a non-diopter situaion.
They did mention that the image of infinity would be at 2x diameter, in which case for the Ikelite 6" dome would be 12". Focusing any closer would be up to my lens' capability. And with my lens' having a focusing range of 10" (its minimum film plane to subject distance) to ~21" (9" film plane to tip of dome, plus the 12" of virtual image I have to focus on.) It sounds like I cannot focus that close, having to rely on the depth of field of the wide angle lens for things to stay in focus.
I guess that is where the diopter would come in, somehow having infinity setting on the lens become the point where the virtual image is, and having the whole focus range of the lens to adjust based on my subject's distance.
So I presume the mfg that said that all I have to do is put the lens focus at 12" and everything from 12" to infinity would be in focus is not really a true statement.
I don't have a Ikelite dome, but I've seen reflections/reflactions on other domes and have noticed the floating holographic image that seem to be playing tricks on my eyes.
scorpiofish,
I am trying to first set at minimum focus distance, then set f-stop to f22, this way, subject from 7" to 13" would be sharp. I might have interchanged some terms unintentionally, so when I say minimum focus when stepped down, it means subject still sharp.
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