Angle of coverage vs. focal length?

Is there a simple way of converting a lens' focal length to angle of coverage? And how do I account for flat vs dome ports in all this?

I'm just about to buy a Sony HVR-A1E; what is not yet decided is the housing. Looking seriously at the LMI, Sea and Sea, and very seriously at the EVO and the manual AquaVideo.

One of the key areas I need to understand better is angle of capture for wide angle domes, converters etc. I believe I'll need a minimum of 80degrees, but the wide angle converters I'm looking at don't mention angle, just x0.7 or x0.45 etc (see examples below). Someone help me here with my math, and dome vs flat decision. Logic tells me a dome will give wider angle of acceptance, as well as less distortion, but how to calculate? Also does a dome perform better under low light (more 'eyeball' area to accept light?)

The A1 has a Carl Zeiss Lens/ Optical Zoom 10x/ Focal Length (equiv 35mm) 40 - 400 mm/ actual FL 5.1mm to 51mm /Filter Diameter 37 mm

The converters around include:

VCL-HG0737X HG Wide-conversion lens x 0.7 37mm type

VCL-SW04 ENS .45 FITS MOST 37MM DIA LENS

VCL-HG2037X HG Wide-conversion lens x 2.0 37mm type

VCL-ST30 Tele conversion lens for 30mm & 37mm lens camcorders

I am assuming that converter X focal length = 'new' focal length. ie 5.1mm x .45 = 2.295mm, BUT how do I convert this into 80deg, 100deg, flat or dome or what?

Cheers, Nick

OK so I am assuming that the fact no-one has offered an answer yet on this means one of a three things:

1) Realy dumb question

2) No one actually knows the answer

3) No one is really interested in the question as we just take the manufacturer's word for it

Sorry to be persistent but any guidance here on choosing dome/flat/wide angle converter etc would be really useful. I'm trying to understand what limitations/features I can afford/need etc in a new housing.

Any input really useful!

I am just a newbie, but here are some observations.

The converters you are looking at attach to the camcorder. That means your housing will need to have a lot of room between the camcorder and the port.

I'm not positive, but I don't think L&M, Amphibico, Sea & Sea will have enough room for an internal converter. Best bet for information is to go to their websites.

Thanks for giving it a try Ron. I have been using a W/A converter with my TRV950 on my USVH housing for a couple of years. I still don't know what angle of coverage I get - although I know I'd like more. Also this big chunk of glass stuck on the front of the camera definitely seems to impair my low light capability, as I found in the Galapagos last year - dark water, lots of hammerheads, grainy clips ...

I'm trying to understand whether I should fork out on an expensive dome, and I don't really understand if I can use a converter with a dome, or whether the dome IS the converter, and which of either gives me the least problems in low light, and which is the most cost effective way of getting better wide angle coverage - I don't have an unlimited budget ....

I'm a bit confused <_<

Have you spoken to Warren ? I think your USVH will work with the A1. Speaking of the A1, most get the A1 because of the 1 push MWB, which means you want look for a housing that has mechanical MWB like Gates, Ikelite, Amphibico pro. Or you could stick with your USVH for now and buy later.

I've spoken at great length to Warren, he's not yet worked out the manual WB control yet. He's looking at a servo that is electronically enabled so he won't have to drill a hole in the housing but he is very stretched with staff at the moment so he's unable to give me a time scale.

The A1 does fit in the USVH 950 housing fine - I can even use it today in v basic mode (no monitor, no w/a converter, no WB - other than auto) and I also could get a new PCB from Warren for extended LANC controls for the A1 - there is some incompatibility between the 950 and A1 LANC access - although even with the current PCB with no mods I can switch on and off, zoom, etc. Unfortunately the internal LCD monitor needs a new cable as the A1 AV-out plug is not the standard jack plug.

I am toying with the idea of getting our local SCUBACAM machine shop to drill a hole in the USVH case opposite the A1 manual assign and fitting an Ikelite or similar rod. If I do that I'll have to reinforce the plasic housing somehow, but it looks possible (I'll also need another cam mounting plate to use one of my w/a converters as the tripod tap in the bottom of the A1 is too far back, and pushes the converter out of the housing in the front)

This may be the route for a really cheap A1 housing to kick off with! It doesn't help me with more wide angle/dome coverage (or a better CC Filter arrangement - one area I think the USVH housings are weak in) but it could get me started until I decide to fork out the $$$s for a Gates/Amphibico/AquaVideo or other housing.

Angle-of-view:

The angle-of-view is a function of the focal length related to the diagonal of the actual picture frame. Thus a lens for a 10x8 film camera of 165mm (Super Angulon) is very wide-angle whereas it would be very long-focus on an APS frame camera.

Check lens specifications as per the manufacturer.

Lenses refract light as it passes from thin air through thick glass and out through thin air again.

A flat port reduces the angle-of-view by about a third underwater due to the refraction of the light as it passes from a dense medium (the water) to a less dense medium (the air in the port). Dome ports maintain the angle-of-view as it is air-to-air by always keeping the 'glass' at right-angles to the node of the lens throughout the viewing angle.

Does that help?

Thanks John. I understand the principle of flat ports vs domes and angle-of-view and I fear I may be getting unnecessarily complex here, but do domes come in 'plain domes', and also dome lenses? In other words could I use a w/a converter behind a plain dome? Or are the domes available in commercial housings basically to replace the need for a w/a converter by giving super wide angle optics themselves?

Second - I also think I undertand that depending on the distance between the film/ccd and the lens, then the angle of coverage will change dramatically - hence many manufacturers quoting '35mm equvalent' etc

So given the range of different video cam sizes is there anyway at all to calculate the angle of coverage? I have not seen 'angle-of-view' mentioned in any cam tech spec sheet - only f=xxx numbers depending on capture mode (full scan etc).

Finally *gasp* even if I knew that at full w/a zoom without converter on my cam I got 70 degrees coverage, how woud I estimate the effect of the converter. Would it be 70 divided by say 0.5 i.e. 140 degrees?

Does any of this matter - or do I just buy the most expensive dome lens I can affort and shoot through that

I think you are getting confused between domes and lenses. Dome ports are plane-parallel but come with different radiuses and in water form a virtual image ahead of the port on which the lens must focus. They are not lenses in themselves and do not refract light when used with air on both sides of them.

In water they avoid the water-to-air refraction problems of flat ports. However, the front node of the lens ( a theoretical point) must be in the correct position relative to the centre of the dome's curvature. I fear a w/a adaptor will push the front node too far forward.

Manufacturers have got used to giving 35mm camera lens equivalents because people understand that. 50mm is a normal lens on 35mm film but 80mm is normal on 6x6cm. 150mm is normal on 5x4inch and 360mm is normal on a 10x8 inch camera. If my memory serves me correct, 52 degrees angle-of-view is considerd normal. A 30mm lens would give that approx. angle on a digital Nikon SLR.

You could use a w/a lens behind a flat port but it would lose about a third of its angle-of-view (and also bring in problems of achromatic aberrations too) so it would be pointless. However, I have successfully used a macro lens behind a dome port although the size and bulk of it made things less than convenient!

Where's my mummy

I new all of the above

Dive safe

DeanB

Whoa, I have a headache.

I guess since I never did any photography, all this techno talk is very foreign to me.

OK the mist is clearing John - I am a lot clearer on domes. I now need to go and play with a few. My confusion about a 'dome lens' came about because looking at an Amphibico dome for the EVO last w/e I was struck by how thick it was - looked like a solid piece of acrylic rather than a 'plane parallel' bubble that you describe.

But ... I'm still not too clear on how to estimate the angle of coverage (!) If 52 deg is 'normal' with a 50mm lens on a 35mm film camera (and I think I also recall that from my emulsion days!) what would a wide angle say 24mm give me? 50/42x52 i.e 108?

Underwater I have been using a wide angle converter on my cam with a flat port for years, and as you say you lose a good percent of it's effectiveness because of refraction (although the best converters don't give the edge aberrations so much). I'm trying to work out if I can get a good quality wide angle converter, and calculate the actual angle of capture using a flat port, and compare that with the cost and performamce of a dome.

... it seems there is no simple way to do that calculation, which means I'll have to just try all the housings and ports offered - which is a much more long winded route!

and yes I have a headache too

Oops that should be 50/24x52=108

USVH and Aquavideo are the only mfr 's I can think of that use internal w/a converters. Most of the other housing mfr's use housing mounted lens/ports and their angles are listed on their websites.

I don't think housing lens/ports are interchangeable between housing mfr's with the possible exception being INON which have been used by Gates and Ikelite.

I currently have a Gates housing for my TRV900. When I decide to upgrade to an A1, I'll be going with a Gates since I can re-use all my lens/ports including my INON 130 degree wide angle dome lens.

This has been a really useful thread for me - even though I may have confused everyone in the process.

It's amazing how experience dictates perception. I have only been using w/a converters and flat ports - for maybe 10 years now (Sony housings first, then USVH - before that for 20 yrs I was a Nikonos bigot) the idea of domes is relatively new to me, but everyone serious seems to swear by them - and I can see why now. I'm sure I have downgraded my low-light capability over the years by sticking a big chunk of glass on the lens AND suffering from lowering my angle of acceptance through refraction! Ah well - I'm just an amateur ....

Re the A1, looks like I'll probably retro-fit my USVH 950 housing for a few $100 in the short term, and then save up for a dome-enabled housing like the Gates or Amphibico EVO in due course.

BTW I was playing with my new A1 over the w/e - it's a lovely piece of engineering. I'm certain the FX1 edges ahead in some important areas (eg low light, colour saturation and total picture quality), but in each of these potentially show stopping areas, the A1 HDV quality is such a leap up from DV that I'm very happy with my decision. This is exactly the small footprint, low(er) cost entry I needed to start playing with HD and HD editing. Highly recommended!

The great thing about video is that you can build the information over time, thereby using a series of shots cut together. You do not need the wide-angle to be so wide.

The poor stills photographer has to tell the whole story in one moment so the extreme wide-angle shot is more important. I would love to show you a film I directed of a (supposedly) wedding reception for a BrookeBond tea ad. I shot it in a garden of a house in Pasadena. There is not one wide shot. It is a series of quick cut close-ups that give the impression of a big party over the time scale of 30 seconds.

V good point John. But having an extra wide angle facility u/w really helps you control conditions - especially when the viz is rather poor. If you are from the UK you'll know what I mean! We used to be based outside Twickenham in St Margarets and all our early diving was around the UK and Ireland.

BWT loved those early BrookBond ads - "Do y' know the piano's on ma foot?" "Naa, but you 'um it an' I'll play it"

... but I don't suppose your wedding party was filmed with chimps

Not PG Tips. Brooke Bond D - the tea with the 3D taste! (J Walter Thompson - Creative Team Tom Moult/Terry Howard. Tom Moult now has his own agency in Sidney NSW. Me? I am just wasting my time going divin'.)

Arrgh! The brain is going soft - too much Nitrox (or is that age?) Of course PG Tips! Not nearly as sophisticated as yer BrookeBond D - I even forgot the 'e' in Brooke! Tom has the right idea, Sydney is where I'll end up too (and that's Sydney with a 'y')

Cheers - got to put the kettle on ......