Diopter and image quality on wide angle zoom?

I've read through the archives and I'm a bit confused on the use of diopters with wide angle zooms. Specifically, I'm thinking of using one on a Canon 16-35 with an Ikelite housing. My reason for wanting to do so is to reduce the minimum focus distance. Here are my questions:

1) According to the formula on the wetpixel dome theory page, I should be using a 4x diopter with the Ike 6" dome. My question is: will using a high quality diopter significantly influence image quality, and will it make it better or worse? Also, would a 4x be better/worse than, say, a 2x?

2) My second question is more technical, and less important. Does using a diopter change the properties of the lens in such a way that a different amount of extension should be used on the dome port?

Thanks for the help!

Hi!

A diopter always reduces the image quality in such a case. A good diopter degrades a bit less than a single lense one. The optimum would be a large dome or a lens that can focus close enough.

There's no need in changing the position of the dome glass (no need for step rings) as long as the dome is at the correct position in the first place.

Helge ;-)=)

You will not need a diopter with the 16-35. That lens can focus at 11", which allows it to focus on the virtual image inside a 6" dome.

The 16-35 in a #5503.55 6" dome should work well on a cropped sensor camera such as a 20D or digital Rebel.

I have found the combination has problems the full frame 5D. There is a significant problem with refraction at the edges. Hopefully the new biggah 8" dome will prove to be a better optical match on FF.

I know I don't *need* the diopter as I've been using it fine without for a while. However, I would like to be able to focus a little closer than I can currently (while still maintaining the ability to focus at infinity) and was thinking of trying out the diopter. I may not like it but I thought it was worth a try.

I have thought about how to get closer WA shots as well, and figured that I would first try cracking that nut with a FE lens... another lens that needs Ike's biggah dome on a FF camera.

I am a Nikon user so my comments may not be relevant here. But while I was in Cayman I did a lot of shots with the Nikon 12-24mm behind a Subal FE2 dome with a dioptre and the corner sharpness was much better than any shots I have seen posted by anyone, with any UW wide angle lens who was NOT using a dioptre.

In my opinion, and I know not all share it, when it comes to wide angle zooms "no dioptre good, use a dioptre better"

Alex

I think Helge is right as long as you evaluate lens/dioptre combination only. Every additional glass element has a potential to decrease lens’ image quality but this can be of minor importance as soon as the lens/dioptre combo is used behind a dome glass. The dome glass has a strong optical influence and focus range is shifted. I agree with Alex that dioptres can increase corner sharpness. And they have proven to do so in several cases.

Having that said, I don’t use dioptres at the moment because I like split shots and I am too lazy to buy some for the time being.

Julian