Hi Chris
I can attempt to answer your questions: I am an Ikelite dealer here in NZ, and although I haven’t used the housing for the 350D, I feel I know my way around Ikelite housings reasonably well.
First up, the EV controller.
Lars is correct in that it allows you to manually control Ikelite strobes. It gives you 10 power settings, and because the EV controller is usually mounted near the top of the housing handles, the control is provided close to hand, rather than having to go to the strobe to control the power (note that the strobes themselves offer fewer power settings: usually full, half, quarter and eighth).
However, the EV controller is an optional item with the housing for the 350D - you don’t really need it. The housing has eTTL circuitry built in, enabling you to plug your strobe directly to the housing using a synch chord. On the back of the housing is a row of buttons that allow you to set either TTL mode (where the camera’s TTL control is used to manage the strobe output automatically) with the ability to adjust the flash output by up to 2 stops either way, in half stop increments, or to set the strobe power manually. In this manual mode, you can control the power level of the strobe using 8 half stop increments, so you get nearly the same range as the EV controller would give you.
I would suggest you try the direct connection option (where the strobe is connected to the housing with the sync chord) first, and if you find you are really hankering for those two extra levels of power control, then add an EV controller. You are correct in that if you use the EV controller, you would not be able to use the camera’s TTL control to automatically manage the strobe.
Opinion varies on the value of TTL underwater, however, some people find it works OK for macro shots. General consensus seems to be that TTL is not much use for wide angle shots underwater.
Second - domes
Yes, you need a dome. Like most housing manufacturers, Ikelite supply a range of domes for different lenses. For your Sigma fish eye you will have a choice of either the #5503 6” dome port, or the 8” port system. Ikelite sell the 8” port system as a combination of the “dome assembly” which is the dome itself, and a “port body” which is like a tube, or ring, that goes between the dome assembly and the housing. The port body is a specific length to accommodate your lens. If you were using a different lens, you could use a different port body. This has the benefit that you only need one dome assembly (the expensive bit) to accommodate different lenses by using different port bodies (the cheaper bit). Your Sigma 15mm FE would need the 5510.10 port body, and of course the 5510.45 dome assembly.
The 8” dome probably gives you better results optically than the 6” dome, e.g. edge sharpness. If you search this website, or wetpixel (http://www.wetpixel.com) you will probably find comparisons between 6” and 8” domes. Depending on your sensitivity to the minutiae of image quality, you may find the 6” dome is fine. The other difference between the 6” and 8” is compatibility with other lenses. If you think you are going to get other lenses to use underwater, you may need to buy completely different ports if you are using the 6” domes, which in the long run may mean you spend more than if you had got the 8” system. To assess this though, you really need to know what lenses you might want to use.
Third - where to buy
I must admit, I didn’t realise Amazon.com sold Ikelite housings. Whether they are cheaper or more expensive than other places, I’m not sure. Ikelite’s website has a dealer list that can tell you where the nearest main dealers to you would be: http://www.ikelite.com/dealers/dealer_hm.html.
I would suggest you talk to a dealer rather than Amazon because of the product support and after sales backup you will get.
Finally - strobes
Opinion varies on the ‘best’ strobe. Ikelite’s have a generally good reputation, but are physically larger than some others out there (e.g. Inon). As you are looking at an Ikelite housing, going with Ikelite strobes gives you immediate compatibility benefits, in that all the strobe circuitry in the housing is designed to work with Ikelite strobes. You can connect other manufacturer’s strobes, but generally you can only use manual control, and will probably have to use the power settings on the strobe itself, rather than the ones on the back of the housing.
Where you get strobes from depends on what manufacturer you choose. Again, I would recommend you talk to an appropriate dealer near you. Such dealers are usually listed on the manufacturer’s website. If you post a question here stating your location, and what manufacturer’s gear you are interested in, people may well recommend some dealers to you.
If you are in NZ, I’d be happy to assist you!
Hope that helps.