Shorter port = sharper pics?

I'm shooting a Canon 5D in an Ikelite housing. I had been using a Sigma 105mm macro in a 5505.58 port. I recently switched to a Canon 100mm macro and really like the lens, but the photos are not as sharp as I'd like them to be. There's quite a bit of distance from the lens to the edge of the port with the new lens, about 1 to 2". Ikelite recommends the 5508.45 for this lens (which also adds a manual focus knob). Would this port make my marco photos sharper?

Thanks.

Jeff, I've been wondering the same thing, I shoot my nikon 60mm in a S&S port made for a 105mm VR lens... so I have about 1.6 inches of extra space in there.

I've researched it a little and haven't found any definite answers, but I'll be borrowing my friend's 60mm port in a week or so to do some tests...

I'd like to see what other people think.

I've gotten some fairly sharp photos with my setup, but who knows, maybe they can be even sharper?

Who knows, maybe the extra inch and a half of air could even be a good thing, since water between your lens and subject reduces contrast and color...but maybe this is just wishful thinking

examples: (both are about 1400 pixels wide, significant crops from the original 3872 pixel wide photo)

http://scottpenny.smugmug.com/gallery/3710...882_ZP8Qq-XL-LB

http://scottpenny.smugmug.com/gallery/3710...4696_KpSkA-L-LB

Its an interesting question.

I usually correct macro shots for colour fringing in the corners (this is pretty well correctable in ACR) and the amount of correction varies depending on lens I'm using and the focus distance (which itself is not constant). Colour fringing is caused by refraction when using a flat port and as I use various methods to create the macro ports that I use (single fixed port or a shorter port combined with various port extensions) so the distance from lens to port 'glass' will vary, and certainly I've not been able to come up with a default setting in ACR to apply generally.

Which all means that the distance between lens and port glass almost certainly does have some effect, BUT...

I can't say that I've noticed any variance in actual sharpness as a result of this variation and I do find the Canon 100 macro to be an extemely sharp lens for underwater use!

Since housing manufacturers suggest ports that are just long enough for the lens, I assume that a shorter port is optimal.

I just did a three-day on the Vision and the macro shots look a bit soft to me. I'm going to try to find the money for a new port before my Bonaire trip in a month. Ikelite makes a port specifically for my Canon 100mm Macro that also has a manual focus ring. Apparently you can leave it in autofocus then tweak the focus manualy, which would be super-sweet.

Scott, it's tough to judge the sharpness of a photo unless you post the raw image. I assume that Photoshop added sharpness, if not in Unsharp Mask then at least in the RAW converter. (And if you didn't add sharpness, why not?)

I'm no optical expert but there are some other disadvantages to longer macro ports.

If its too long it will start to vigenette your photos.

Also, if the point of macro working distance is to get closer to critters and have room for lighting, then too long a port will interefere with this.

I like to have at least enough room inside to accomidate a diopter, no more.

For older lenses don't forget about lens extension.

Well, I just ordered the shorter port from a member who PMed me about his used one. I'll let you know if it improves things. It has the added bonus of a manual focus control which should be fun.