Canon 17-55 f/2.8 in Ikelite 8" dome?

Hi all, I have a Canon 17-55 f/2.8 lens which I would really like to use with my underwater rig because it is quick and sharp. Ikelite's web site is of no use because it simply states that the lens diameter is too large to use in their port system. I have done some successful mods to Ikelite's ports and focus/zoom rings in the past, when they said it wouldn't work, so I thought I could probably make this work as well. (For example, I made a very simple mod to a zoom ring that allows me to zoom my Sigma 10-20 in the 8" dome.) I have already fabricated an extended zoom ring for the 17-55 from a couple of standard zoom rings which I had no use for. It operates the zoom ring very smoothly in my current 5510.16 port body (without the dome attached). So that takes care of the zooming part, but what about the optics?

I have done some simple estimates using the dome and the standard port body, and it appears that the longest port body that ikelite makes (5510.28, for the Nikon 17-135) would give me enough room at the 55mm focal length and put the front of the lens at 17mm in roughly the same position as the Sigma 10-20 lens is with the standard port body.

I have a reasonable grasp of the optics of a dome port underwater and I understand that the best IQ is obtained when the iris of the lens is at the centre of the sphere defined by the inside of the port (so 4" from the inside of the dome in this case). In this case, it appears that the iris is in approximately the correct position at the 17mm focal length, but it is very difficult to determine the position of the lens iris when it is zoomed to 55mm.

My questions are,

1) has anyone tried this combination lens and port?

2) can anyone with a better understanding of the optics tell me if the IQ be lousy at the long focal length (55mm) when the front of the lens is very close to the inside of the dome (even if the lens iris is approximately where it should be)?

Thanks for any and all feedback.

Cheers!

Bruce

Bruce,

As an exclusive Ikelite housing and Canon shooter I hear you on wanting to use the 17-55mm f2.8 lens. If you are comfortable making up some zoom clamp that can accommodate the large diameter barrel, so be it. I had one guy force a zoom ring on his Sigma 10-20mm and man, is it tight! I worry about him flooding the damn port doing this....

As far as worrying about the nodal point distance, etc. I think like many shooters who want to use "in between" lens will spend a whole lot of time and trouble with extensions, with or without diopters and still never be happy at some focal lengths with edge sharpness

Rectilinear lenses with multiple moving elements cannot possibly be tack sharp edge to edge at all focal lengths. Yes, the 8" dome port will be a little more forgiving due to the large radius, blah, blah, blah........But will it ever be perfect at all focal lengths? I doubt it....Many Ikelite shooters I know use their mid-range zoom lenses behind a 6" extended port, or even a flat port and get great shots.

Don't take my word, do a search on the forums for this exact same subject.

When I use a "tweener" lens I accept it will be sharper at certain focal lengths and f-stops than others. And try and find subjects to maximize the frame filling at those settings.

Just one guy's opinion

dhaas

www.haasimages.com

Thanks Dave,

As you have pointed out, I'm trying to use a 'tweener' lens to try and get something that will give me moderate wide angle (I'm using a 40D) through to something for closer fish portraits etc.

Your post seems to suggest that I may have better luck with a extended 6" dome. I can understand why I might have better edge to edge sharpness with a flat port, but wouldn't a dome with a smaller radius cause even more loss of edge sharpness?

The 17-55 lens barrel extends 27mm when zoomed to 55, which would bring it quite close to the inside of the port. If I understand the situation correctly, that would mean that at 55mm there would be a smaller curve through which the lens is shooting, which should really make a rectinear lens sharper.

The drawback of the flat port is the distinct possibility of vignetting at the short end.

So it looks like I may not be satisfied no matter which way I go...

Thanks for your help. I will take your advice and also search for others who have tried to get this type of lens to work behind a port.

Cheers!

Bruce

Sorry Dave, I've misunderstood your post. On re-reading it, I understand that you think that I could use a 6" dome or a flat port and get acceptable results if I choose the focal lengths and f-stops carefully...and that the 8" dome might be a bit more forgiving than either of those, but that not all f-stops or focal lengths.

Thanks

Bruce