How does WA wet lens affect zoom/focus?

[highdesert]

Greetings -

A manufacturer of a 20mm WA wet lens says on their website, and confirms via email, that if you zoom with their lens in place, the lens essentially disables the focus capabilities of the camera ... or at least that's what I think he/they are saying. Is this the general case with all brands of similar lenses? I can't think of anything that would make it a particular trait of their brand only, if that's what they're saying.

Comments?

Thanks!

 

[Larry C]

With some cameras, you need to zoom a bit to keep wet WA lenses from vignetting (black corners because the distance from camera lens to WA lens brings the WA lens body into the outer edge of the picture.) This shouldn't affect your cameras autofocus in most cases. You may have to close down your aperture a bit to get good edge to edge focus on some WA lenses. In most cases on a compact camera, you shouldn't have to close it down past f5.6. This may require a slower shutter speed depending on the light available.

 

[highdesert]

I get the part about stopping down for a greater depth of field; maybe that's a part of the explanantion about this lens. I don't think vignetting is entering into his explanation at all. His quote:

"The lens does not affect the zoom, if you zoom with the lens on the housing the focus of the camera doesn't work, that's all."

And that's from a manufacturer's rep. He goes on to say he doesn't know if this is typical with all similar lenses of different brands. Since I don't want to start a brand war, I'm not stating whose lens it is here, because I'd rather know if there's some general theory I'm missing about these lenses, or if someone who is very familiar with them as a group will pop up and say, "Aha, you're referring to XXX brand lens, which has this problem because ...." But to my eye (my UW photo savvy is well behind topside) it doesn't appear much different than other similar 20mm wide angle wet lenses.

 

[Larry C]

I don't think there would be an issue as far as lens preference. I think you need to state both the lens model and camera model to get accurate information. Different cameras have different focus systems, and different lenses work best with different cameras and housings. Many lens suppliers specify that on particular housings, you may have to zoom to avoid vignetting, and I haven't seen any mention of autofocus issues, nor have I experienced any with my Olympus camera and either Sea & Sea or Inon wide angle lenses. They've both autofocused just fine while zoomed a bit. I'm curious which lens and which camera you're having an issue with.

 

[highdesert]

OK, OK ...

I have a Canon S90 in an Ike housing, 67mm threaded port. Nothing I've been told by Dyron (the 15mm or 20mm lens in question, both are available and are threaded 67mm) has been made specific to my setup. The remarks I've been given are in general terms, seeming to apply to either lens on any setup. I should also add that they post photos on their site for both lenses, on Canon's OEM housing, showing no vignetting.

 

[bvanant]

I think there is a translation problem here. In general with a wide angle lens as an external lens you will have reasonable autofocus with your camera zoomed all the way wide (why would you do otherwise). In general also you will be able to zoom in some amount (depending on many of the things that Larry C is suggesting) before AF will not work anymore. Nemrod will be able to give you more particulars, he is the god of wide angle compacts.
Bill

 

[highdesert]

In general with a wide angle lens as an external lens you will have reasonable autofocus with your camera zoomed all the way wide (why would you do otherwise). Bill

Yeah ... I understand that it's totally counterintuitive to want to zoom if you've got a WA lens mounted ... sort of defeats the purpose. I'm just trying to see if there's something I'm missing here ...

 

[Puffer Fish]

Point and Shoot camera's all use contrast detection to focus. Some camera's don't use visible light, but rather something on the IR edge of what we see.

Note: You can tell if that is the case, if the focus aid light is red.

In low light, the camera may have focus issues, because the contrast is not high enough (this varies a lot with cameras).

Add on lens can do three things to mess this up:

1. Their size may block the assist light.

2. The vary nature of them is to reduce the total amount of light reaching the sensor..

3. They may focus IR light a different distance than visual light.

I have an Inon UCL-165 macro lens, which when put on a S90 in a Ike housing, will not focus where it is pointed. It focuses, but alway a bit long. It locks in, but always in the wrong distance.

With a really small sensor, and a really wide angle, focus is actually not an issue.. as pretty much everything will be infocus... but zoom in and now the focus needs to work.

I have several wide angle wet lens, and some work better than others if you zoom in, depending on the camera. If the MFG says it does not work, I would be tempted to believe them.

Hope that makes sense.

 

[highdesert]

Puffer -

A very lucid explanation ...thank you! I have a better understanding now about this lens and similar ones.

 

[Puffer Fish]

Puffer -

A very lucid explanation ...thank you! I have a better understanding now about this lens and similar ones.

Thanks, except I missed one... darn...

4. Lens can make images sharper and softer, many wide angle lens are "soft", and soft can interfere with contrast detection systems. This may be a very small visual effect, but big to the camera.

There, I think I got them all this time.