Focus Lock with Wide Angle Lens

All right, so I have dived with my Sony CX550V in a L&M Stingray+ housing with Fathom WA90 lens enough to realize that it is time for me to discipline myself and learn how to lock the focus (although the CX550V does an amazing job at autofocusing even in murky water).

If I understand correctly, the depth of field of this combo (camera + optics) is sufficient for most subjects to be in focus from infinity to a reasonably close distance. The catch is "using what focusing distance and down to what distance?".

In murky water in particular, I have the feeling that it would be dangerous to let the camera autofocus on a scenery with no close subjects (in other words, with only distant background objects), lock the focus and potentially realize later that it had frozen the focus on a floating particle. The rear LCD is nice to compose a shot, but of insufficient resolution (and I suppose so is the OLED version of the Bluefin) to figure out a slight out-of-focus situation.

In this particular situation where there is no nearby subjects, it is also difficult if not impossible to use the trick of zooming, focusing and locking the focus, then unzoom (it is difficult to zoom to infinity in dirty water...).

My naive idea would be to use manual focus (which in the Stingray+ involves using the camcorder menu, since there is no manual control knob, unlike in the Bluefin) and set the distance to infinity. I suppose that infinity means infinity, whether the camcorder is used topside or underwater with a wide-angle lens (the case I am discussing). So this approach would "fix the problem" and answer my question.

In this case, what would be the minimum safe "in-focus" distance? Should I just practice and figure it out by trial and error?

If my assumption that there is a rather large depth of field is incorrect, what's the bag of tricks I should start to learn?

Thanks for chipping in.

X.

The methodology you suggested has been in use for decades. Just set your focus distance to 0.5-1m and everything within 30m should be in focus @f1.8 with a WA adapter like the WP90. Remember wiith about 1/3" sensor, your DOF is about 7-8 times that of a 35mm sensor.

The methodology you suggested has been in use for decades. Just set your focus distance to 0.5-1m and everything within 30m should be in focus @f1.8 with a WA adapter like the WP90. Remember wiith about 1/3" sensor, your DOF is about 7-8 times that of a 35mm sensor.

Just to clarify: are you saying to set the focus such that the camcorder display reads 0.5-1m? I thought that the camcorder indication was unreliable underwater (and with added wide-angle optics at that!)? If you mean 0.5-1 actual meter, that sounds very much like the trick I have learned to focus on your fins and lock the focus. I was under the impression (wrongly it seems) that this applied to dome ports, not wide-angle optics (it did not work at all with my previous Bonica housing, which had a cheap wide-angle lens (0.5 x) behind a flat port).

Thanks for the clarification!

X.

I meant that literally... as in focus on a subject 1m (eg your fins from your chest if you're short ) away from the lens. Unless you are in superb 100m viz, everything will be in focus.

I meant that literally... as in focus on a subject 1m (eg your fins from your chest if you're short ) away from the lens. Unless you are in superb 100m viz, everything will be in focus.

OK, pool session then...

I actually meant a 0.5 x lens IN FRONT (that is outside) a flat port, when talking about my previous housing...

Thanks,

X.

Just to clarify: are you saying to set the focus such that the camcorder display reads 0.5-1m? I thought that the camcorder indication was unreliable underwater (and with added wide-angle optics at that!)? If you mean 0.5-1 actual meter, that sounds very much like the trick I have learned to focus on your fins and lock the focus. I was under the impression (wrongly it seems) that this applied to dome ports, not wide-angle optics (it did not work at all with my previous Bonica housing, which had a cheap wide-angle lens (0.5 x) behind a flat port).

Thanks for the clarification!

X.

Don't use auto focus underwater, except for very clear, high contrast subjects.. Just when you least want it to, Murphy's law will start it "hunting" in and out of focus.

Ignore what the camera is saying for the focus distance. It's focusing on a virtual image projected by the WA lens.

Lock focus on your fins during descent, or a clear-contrast subject about a meter away and leave it at that for most shots. Relock the focus on the actual subject, if you want to shoot something small close-up. Reset back to 1 meter when you've finished the CU.

Regards

Peter

Lock focus on your fins during descent, or a clear-contrast subject about a meter away and leave it at that for most shots. Relock the focus on the actual subject, if you want to shoot something small close-up. Reset back to 1 meter when you've finished the CU.

Point taken. Pool session practice scheduled next week-end when my sinuses get a bit less stuffed. We Los Angelinos are not cut to withstand a full week of rain...

Point taken. Pool session practice scheduled next week-end when my sinuses get a bit less stuffed. We Los Angelinos are not cut to withstand a full week of rain...

Don't know whether you've got kids but I always borrow some of my son's lego when I need to do some swimming pool tests. Good for testing both focus and color. Sits nicely on the bottom like a piece of coral. For bigger subjects, a plastic chair works well.

Don't know whether you've got kids but I always borrow some of my son's lego when I need to do some swimming pool tests. Good for testing both focus and color. Sits nicely on the bottom like a piece of coral. For bigger subjects, a plastic chair works well.

I don't, but I have Legos nonetheless !!! At least the Lego Mindstorm version from the time when they were released and I could not resist the fun of checking them out... You would have to build the stuff underwater, right? Otherwise it's going to be mostly air... I'll think of some props that won't look too suspicious.

I'll think of some props that won't look too suspicious.

I once tested the macro function of my camera on the gundge (hair, band-aids, dead bugs) on the bottom of the pool.