Manual Focus - Very strange setting used

Wondering if anyone has some thoughts on this one:

On my Cocos trip the water condidtions are ok (40-60 feet vis) but the usuall plankton and fishy debris floating everywhere. Murky I would say. To compound matters it was usually raining (low light). Finally we were often deepish around 100 feet.

I almost always shoot on manual focus for wide shots to aviod the dredded "focus popping". In asia and caribbean/bahamas, infinity setting usually works fine. However, using infinity in cocos produced consistently blurry and infocused footage almost without exception.

I am using the Bluefin HD housing with 80degree wide lens. I fiddled for awhile on the trip and finally found a manual distance that worked. Guess? .3 meters with cc filter on! In better light and no cc filter up to .5 meters. This created a situation where sharks passing close were in focus and those in the distance still "relatively" sharp outlines. In these conditions, "sharp" was always "relative".

Other specs: shutter usually at 60, iris manual, gain auto.

Thoughts?

Shawn

Hi Shawn,

Do you not get "grainier" footage with the gain set to auto? The camera can easily hike up the gain to 18dB which would cause more noise in the picture. I used to do that but have switched to manual and 0dB...

Anders

What Iris do you typically shoot in the Carib. v. the Iris you shot in Cocos?

Mr H

Another method you may want to try is instant focus on the subject (depending on line of travel of course).

The problem with using infinity and low light is of course the iris is more wide open thus killing your DoF. With good viz and light, high f stops would increase DoF. Vid cameras have great DoF but opening up the iris can reduce it dramatically.

An instant focus button is very useful sometimes.

Drew,

I used the 1 touch focus and that is where the .3 meters came from. I focused on divers near by and couldn't get a instant focus number higher than .3 meters that worked. What I am wondering is 'why/how' .3Meters managed to work? I understand why infinity did not work. What have you found to work? I expect a UWA lens will increase the manual distance with more light as well.

Anders,

I have my gain button set at 0db. On my dives i haven't noticed much of a dif when gain is locked or not except on late afternoon/evening dives. Perhaps you are doing more deep wreck diving which is lower vis and much deeper (hence driving gain up). For good form I should probably lock it at 0db most always I suppose.

Ryan,

Re: Iris, it is hard to say. For action shots I run Iris on auto b/c panning with a subject your light varies dramatically. Locking Iris in such circumstances I find that brief portions of the footage are better and the rest is either under or over exposed (when I manual iris I always lock shutter b/c I don't want my shutter speed jumping above/below 60 in a single sequence). In more stationary situations I like to run an iris somewhere in the middle to maximize my range (Iris of 5 - 7). Into the sun I like lower, sun at my back higher (higher number).

The caribbean typically alows for a higher Iris (7 plus) and the pacific (west coast americas) a lower range (5 or less).

Any other thoughts?

A UWA increases your DoF, thus 0.3m is probably the hyperfocal distance of the camera + UWA in those conditions. I think the equation is H = (L x L) / (f x d). L= focal length, f=F stop and d = circle of confusion diameter.

Mr H

Another method you may want to try is instant focus on the subject (depending on line of travel of course).

The problem with using infinity and low light is of course the iris is more wide open thus killing your DoF. With good viz and light, high f stops would increase DoF. Vid cameras have great DoF but opening up the iris can reduce it dramatically.

An instant focus button is very useful sometimes.

<{POST_SNAPBACK}>

Drew is correct . With the Phenom and the fstop near f11 and on full wide, focus will be from the outside of the port itslef to infinity but as soon as the iris opens up to very low numbers things get out of focus and one has to re focus. db gain helps but I would not go any higher that 6. If you are on a low fstop, kick in the db gain and it will allow you to go to a higer fstop hence giving you more DOF.

This is good stuff folks. I am quite familiar with wider Iris/low DoF. What I don't understand is how .3 Meter fixed focus is allowing me to get images in focus from my lens to infinity...doesn't seem right. Any takers.

O I see..yes that is strange. Could be the camera focus is out of wack. On Betta SP cameras the focus can get out of wack, but you can adjust it again in the field.

Like I can remember I was zoomed in focused and then pulled back only to find the image was unsharp. It's something you check each day before you start filming.

I also had a MINI DV camera that the focus would go out of wack after I had zoomed and had to refocus again. I think it got zapped by the boats 240 inverter or something as I use to get a zap when I would plug the video lead in to watch on the TV on the boat.

I would test the camera outside on things to see if it's the camera or not.

Wags,

Good ideas. I tested extensively on land between and after the dives and the manual infinity worked flawlessly...in fact 30 feet vs infinity was distinguishable with expand focus. It seems that in the conditions I was diving, that was the setting that worked. In other locations in the world, infinity has worked great. Perhaps with very wide iris and some gain, the close setting works because the DoF is really flat to start with from a viewers perspective...i.e. your eye won't discern DoF either in those conditions i suppose.

Hmmm I never tested my Z1 but maybe it's not a parfocal lens.

Anyhow the hyperfocal distance of your 80° WA on the ol FX1 is probably 0.3m. Remember the 1/3front 2/3 back rule. Video CCDs are cropped sensors and using a WA would increase your DoF. Also that WA is actually pushing the focal point the camera is focusing on, not the actual subject since it is an external lens. Thus the camera thinks it's focusing at 0.3 (due to the lens on the camera) but in effect it is focusing on a subject that is further away. I could do the math to explain why but I use to get paid for that crap and you still owe me a dinner.

I put it down to self-induction in the marmalade circuit.

Like Drew says, you can't think of those complex 3-element wide angle lenses in the same terms as a dome port. I'm sure Barrett could chip in with some theory.

One of these days I'm going to get my head properly around lens theory. Here's some suggestions for starters:

http://en.wikipedia.org/wiki/Lens_%28optics%29 (Wikipedia rules!)

http://www.camerahobby.com/EBook-Lenses_Chapter6.htm

http://members.tripod.com/~Camera_Dave/lens-theory-2.htm

Nick

I put it down to self-induction in the marmalade circuit.

Lady Marmalade, is that you?!