dome port and zoom lens problem

Help! I have just purchased an Ikelite housing for a Canon 20D, with a six inch dome port for my 16-35 L lens and a flat port for my 100 mm macro lens, both Canon. I love the TTL flash, which gives wonderful exposures. But I have a real problem with the port. It gives lousy, blurry pictures with the zoom lens when I focus close. Does the eight-inch port solve this problem? Ikelite was coy about this when I asked them. If I want sharp pictures I may have to go with a flat port and a 24 mm fixed-focus lens, which I hate to do because it will reduce my flexibility. Any info will be greatly appreciated, since I am leaving for the Solomon Islands in six weeks.

Do you have a +4 diopter on your lens? Ike's port chart suggest you use one. I use the 16-35 behind a 6" Subal dome with a B+W +4 diopter and can focus on objects touching the dome.

s.

As I read the Ikelite port chart, it has a ?* that means I don't need to add a 4 x diopter. And the lens is awfully close to the inside of the port, so there may not be much room anyway. But it is intriguing that you can get so close with such a diopter. Does it seem to affect image quality? I am trying to find out if the 8 inch dome may solve my problems, but the possibility of focusing really close intrigues me. I tend to be a closeup sort of person, and also have a port for the 100 mm macro EF lens that works superbly.

It may apply to the 8" port, but if you scroll down this post : http://wetpixel.com/forums/index.php?showtopic=13674 you can see images I took with my 16-35 with a 6" Subal Dome.

Stu

cwills,

The Canon 16-35mm lens focuses to 11 inches and as such can "see" the apparent image ANY dome port creates underwater. But having moving elements it will likely be sharpest at one specific zoom setting. The 8" dome port will have a bit better corner sharpness, especially at the widest settings of most wide angle zoom lenses.

Many zoom users try combinations of diopters to sharpen their corners, but Ikelite ports, especially the smaller diameter 6" dome ports are sized for the lens alone, not even allowing room for a skylight filter. So beware...

Adding a diopter also will limit your ability to simply shoot through the port above water. When I shot Nikon dSLR cameras and the 12-24mm I found zooming ever so slightly, to like 14mm improved my edge sharpness immensely with the "standard" 6" Ikelite extended dome port. This was still a very wide view and my personal solution versus adding a diopter.

Or you can simply buy the 8" dome port. Just make sure you get the correct extension for the lens you want to use. #5510.81

Hope this helps!

dhaas

Thanks for the insights! If it is really true that the eight inch dome won't improve sharpness throughout the zoom range, then maybe I am stuck with only using the zoom range from 16 to 22 mm. The entire zoom range works fine above water, though, so I continue to wonder whether the lens is simply unable to focus on the virtual image in the range from 22 to 35 mm. In that case, having a virtual image a bit further away, like in the eight inch port, might help.

My experience is in the "standard" 6" dome ports the widest end, 16mm on your lens, shows the most softness in the corners. The 8" improves this slightly, but it is also dependent on what f-stop (based on strobe to subject distance, ISO and of course strobe power output) you are set at.

My best advice is to zoom ever so slightly IN, then shoot at f5.6 - 11 or smaller and see if your image edges improve before you do anything else.

Also,the 16-35mm Canon lens, while a great film lens doesn't seem to be the best for cropped sensor cameras compared to the newer Canon 17-40L.

The reality is the 16-35mm is going to still end up as a digital file. When exposed properly it should be more than enough data to result in great images.

Good luck!

dhaas

Thanks to all for the advice! I am embarrassed to say that a more careful analysis shows that the problem is primarily with the lens and not with the dome. I have now taken a long series of pictures using the dome, above and under water, and find that the 16-35 L lens is surprisingly soft at short focus distances in the range from 25 to 35 mm focal length. I hadn't picked that up before because I have primarily been using the lens above water for wide angle shots.

So I will simply have to be cautious in zooming in under water.

The problem may also lie in where your closest focus point is. If you are focussing on your subject in centre frame and there are still foreground subjects, these will definitely look soft because they won't be in the plane of focus of the virtual image. Your DOF scale on the lens won't work anymore because of the virtual image. When shooting CFWA with any lens, focus on the closest thing in your image, not the primary subject, then let the DOF do the rest....

I've had my 16-35 since it was first launched, I shot it on film, cropped and now full frame digital. It will produce 20x30 images from my 10D and will be sharp corner to corner.

Check your technique before blaming the technology too.

s.