+4 close-up lens question

[Marty!]

I have a question about the use/recommendation of +4 close-up lens' in Ikelite's lens port guide, as I'm a little new to SLR photography and optics.

I have the Nikon 18-105mm VR lens, and the recommended +4 close-up lens. I know these allow the lens to focus closer than without it, and allow it to focus in front of the 8" dome (which I have), but when using the +4 on dry land the lens can't focus at infinity at full 105mm zoom; is this a drawback of using this lens/+4 diopter combo, or is this somehow solved when the lens is brought underwater? Thanks for the help.

[Ikelite Staff]

Marty,

The optics change underwater, so you should remove the +4 diopter for land use.

The purpose of the +4 diopter underwater is to allow lenses to focus down to 12" in order to create an apparent image of your subject on the dome. Lenses that normally focus to less than 12" do not need the diopter in order to obtain focus behind the dome underwater. The minimum focusing distance on your lens is 1.48', so a +4 diopter must be used underwater behind your dome.

Best Regards,

Brett Foster - Ikelite

[Marty!]

Marty,

The optics change underwater, so you should remove the +4 diopter for land use.

The purpose of the +4 diopter underwater is to allow lenses to focus down to 12" in order to create an apparent image of your subject on the dome. Lenses that normally focus to less than 12" do not need the diopter in order to obtain focus behind the dome underwater. The minimum focusing distance on your lens is 1.48', so a +4 diopter must be used underwater behind your dome.

Best Regards,

Brett Foster - Ikelite

Thanks very much, that clears it up somewhat. So you're saying that because of this "apparent image" behind the dome, the lens is able to focus to infinity with the +4 at full zoom underwater?

[VicScubaVentures]

What I think Ike Staff is trying to say is .... without the +4 the lens will only focus on something 1.48 FEET from the image sensor and therefore can not focus on the image created on the dome glass (in reality, the camera is taking a picture of the back side of the dome, not of the subject itself) at a distance considerably closer to the image sensor than 1.48 feet. Using the +4 allows the camera to focus on that image created on the dome glass and therefore gives you a clear in-focus image. Out of the water and out from behind that marvelous dome, you have to remove the +4 to allow the camera to focus on the actual subject, not its artificial image) at its normal focusing distance. Trust me .... it works!! Hopefully I've been able to address your questions ...??

Craig.

[Ikelite Staff]

Well stated Craig. I will add that in some cases you can lose the telephoto end of your lens underwater behind the dome. This occurs because when you zoom out, the minimum focusing distance moves away from the image sensor slightly and beyond the point where a virtual image can be created on the dome. If this happens, the camera will simply struggle to get a focus lock.

Best Regards,

Brett Foster - Ikelite

[Marty!]

Well stated Craig. I will add that in some cases you can lose the telephoto end of your lens underwater behind the dome. This occurs because when you zoom out, the minimum focusing distance moves away from the image sensor slightly and beyond the point where a virtual image can be created on the dome. If this happens, the camera will simply struggle to get a focus lock.

Best Regards,

Brett Foster - Ikelite

Thanks very much to you both, this helps a lot.