lanierb

In the articles section there is an article by Scheunemann that lists the correct diopter formula as: D = 100cm / 3.03*r where r is the radius of the dome in centimeters. However, searching through the archives the following formula often comes up: D = 100cm / 4*r Which one is strictly correct? (I have a feeling the first one is correct and the second an approximation but I want to verify that.) Lanier

Noone has an answer to this? Question is, basically, should he try Canon 500d (2-element, +2, $150) or B&W (+4 -- assumes 6" dome, $50)? I'm also interested in the answer... Lanier

Yes it does.

If I had to guess I would say that you probably mistakenly left the eye cup on the camera viewfinder and it is not allowing the camera to seat properly. Try taking it off and see if it fits better. The fit on these housings is very precise so something small like that can throw it all off.

"can the sharpness be affected by the focusing distance"? The short answer is yes. Normal lenses are designed to minimize aberrations at infinity focus, whereas macro lenses are designed to minimize aberrations throughout the range (and sometimes only at close focus).

"So if it's just about the image quality (at f/11 or f/16), it seems like a waste to spend money on the 60 mm." There are many situations in which the 60 is vastly sharper than the 18-55, including anything at a wide aperture or close focus. At f/16 on a 1.6 crop camera all lenses are essentially equally sharp (because they are diffraction limited), which is why those shots show no differences. Note also that there are some other issues when you use lenses underwater. Flat ports add substantial chromatic aberration on wide angles so you won't want to use the 18-55 behind a flat port. You sound like you want to try the 18-55 first. Why not just try it? The dome is only $135. My guess is you will quickly see its limitations and want something better.

It depends what you want. If you want an "all around" lens that will let you do a lot of things not very well the 18-55 is fine. For macro it's not very sharp (at close focus and/or wider apertures), and can't focus very close. For wide angle it's not very sharp and not very wide. It might be worth experimenting with for a while, but I think you'll find the 60 to be a big improvement.

Is it an either/or? The Ikelite ports are about $135 so for that much extra you get the 60 and still use the kit lens once in a while. My feeling is that the 60 is much better than the kit lens for macro and fish portraits. The kit lens isn't really great for anything, including wide angle.

I have the 60mm and an Ikelite housing and there is almost no air between the port glass and the lens. Also, the 60mm is IF so does not change size as you focus, and its true focal length increases as you focus closer, extending the subject distance. I haven't experimented much with it yet but given that I already own it I think it's worth a try. Lanier

I thought people here might be interested to know that you can use the EFS 60mm/2.8 on full frame Canon's via the EF12II extension tube with essentially no vignetting (that I can see anyway). I found this thread on another site: http://www.fredmiranda.com/forum/topic/360672/0 and then tried it myself on my EOS3 (film camera) and it worked great. In air this combination limits you to about two feet max focus distance, but perhaps more underwater? If you put it behind a dome you might even be able to focus to infinity, but that's just a guess. Anyone want to give it a try underwater? (I don't have a housing for my EOS3.) Lanier

...except that you can't have it both ways. You can't justify needing a 10+MP camera and then say that in practice diffraction doesn't matter because, with a 1.5x crop sensor, sharpness is so bad at f/22 that you may as well have a 5MP camera. If you think you need a 10+MP camera, then for the same reason you are limited to shooting at f/11 or wider (on 1.5x crop cameras). If anyone is interested in seeing examples, there's a good discussion of diffraction with examples here: http://bobatkins.photo.net/photography/tec...iffraction.html. Lanier

That's a tough one. I'm lusting after a 5D for the larger viewfinder, but the seacam housings are a fortune and there seems like a much better chance you would be able to use the 1DII housing down the line. (On the other hand, the 5D may not become obselete very quickly.)

I agree with chris (exept that I think the 60 is pretty good even for medium size stuff). What kind of diving do you typically do? Absent any information I would suggest the 60, especially as a first lens as I think it is more versatile and easier to learn for a beginner. You can shoot the 60 on any dive. The wide angles won't be much use if the vis is bad or if the dive site doesn't have any good scenery or if there's nothing decent sized swimming around or if you can't get close to anything. Lanier

In my experience 18mm is definitely wide enough to experience problems behind a flat port. The smudging also looks about right for that. Usually you get cyan fringing as well, which I can't see much of, but my guess is a dome port would be a large improvement. Lanier

The main tradeoff seems to be NFL and Howard Stern (Sirius) vs MLB (XM -- they have their own talk shows too but I don't know what they are). The music channels are really similar and the news/weather channels too. Sirius has better NPR coverage if you like that. XM has a bay area only traffic/weather station where Sirius lumps in the bay area with Seattle (I'm pretty sure you're from here, right?). You can sign up for a week of free online listening with both and try them out if you want. I got XM because I like listening to baseball sometimes. One more thing: they both have awful sound quality so if you're picky about that prepare to be disappointed. It sounds like 64-bit MP3 to me. Lanier