28 replies to this topic

[Bobu]

Thanks Paul and John!

 

It seems that I have to do my own testing when I get my housing. Today I ordered the Sigma 15mm and plan to compare it with both Zeiss lenses.

 

But I have some more questions:

 

1. Is the effect of soft corners (at not fully stopped down appertures) with a dome port as visible over water as under water or is there a difference?

 

2. Will I get better corner sharpness with the 21mm lens vs. the 15mm? According to the above linked Seacam article less field of view is better for corner sharpness as is more DOF? The question is, which is the dominating effect (for underwater shots, forgetting the special DOF situation for half-half-shots)?

 

3. Can the 21mm Zeiss lens also be used with a flat port? Assuming there is no vignetting, is there any disadvantage underwater of a flat port besides the ca. 30% more magnifaction and therefore 30% less field of view? For half-half-shots the advantage of the bigger area of the dome port is clear.

 

4. I would guess that corner sharpness is (with a flat field lens like the Zeiss 21) always better with a simple flat port compared to a dome port, over water and underwater. Is this true?

 

And one additional comment regarding the 14-24 lens. This lens has a very strong field curvature, at least if focused at close distances. I've posted an example of this lens behavior on Fred Miranda some years :

http://www.fredmiran...um/topic/917814

Lloyd Chambers (digilloyd.com) has also written about the strong field curvature of the 14-24 lens. I've therefore no idea why this lens is so difficult to use with domeports. Maybe the field curvature is just not a spherical curvature as nearly all dome ports have.

 

Boris

[Paul Kay]

Hi Boris

 

In answer:

1. The effect is when they are used underwater - above shouldn't result in much degredation.

2. 21mm - a 90 degree field of view has long been accepted as the rough limit for which a simple concentric dome port can provide a reasonable image. Beyond this, there has always been a problem and this is why specialist underwater optics have been produced for ultrawide (weitwinkel) lenses (like Fathom lenses for videgraphy - unfortunately none have been produced for full frame 35mm format lenses. I have looked into this and having talked to optical designers it appears uneconomic to do so - and probably each lens would require its own underwater lens too - way too expensive to be viable).

3. It will fit behind a flat port no doubt, but the underwater image will be curtailed by severe image degredation due to refration - as a rule of thumb a focal length of no greater than 35mm (on full-frame) should be used with a flat port. Again search the web for examples - there are bound to be some though I've not tried this myself yet.

4. No it will not! - its like shooting through a very thick filter which is ok for longer focal lengths but not for wides.

I forgot to comment last post that both domes and flat ports will introduce distortion too.

[Bobu]

3. It will fit behind a flat port no doubt, but the underwater image will be curtailed by severe image degredation due to refration - as a rule of thumb a focal length of no greater than 35mm (on full-frame) should be used with a flat port. Again search the web for examples - there are bound to be some though I've not tried this myself yet.

4. No it will not! - its like shooting through a very thick filter which is ok for longer focal lengths but not for wides.

I forgot to comment last post that both domes and flat ports will introduce distortion too.

 

But will I have the same problems when I use the flat port (with the 21mm lens) over water / in the air for example for surf shots? Distortion is not a big problem for my kind of images but loss off corner sharpness is a big problem.

 

Boris

[Tom_Kline]

Hi Boris,

If you shoot behind a flat port with a 21mm focal length lens on FF 35mm format you will get a significant amount of pincusion distortion. The refraction of light by the flat port that causes the pincusion distortion varies by wavength of light, hence you will also get quite a bit of chromatic aberation. It is worse in the corners.

An issue for over-under shots is that the part under water will be magnified relative to the part not under water. This will be an issue for any objects passing through the surface within your photo. One purpose of dome ports is to negate this magnification effect.

Tom

 

But will I have the same problems when I use the flat port (with the 21mm lens) over water / in the air for example for surf shots? Distortion is not a big problem for my kind of images but loss off corner sharpness is a big problem.

 

Boris

Edited by Tom_Kline, 24 March 2013 - 11:12 AM.

[Paul Kay]

Hi Boris

 

My thanks to Tom has explained the problems.

 

To sum up, if corner sharpness is really important then don't use a flat port for focal lengths of less than ~35mm (on full frame) and using lenses wider than ~20mm (on full frame) behind a dome port will need to only be used at a small aperture (f/11-f/22).

[Bobu]

Thanks Tom and Paul, but I'm affraid I didn't fully understand the problem. The effect of different magnifications for half-half-shots is clear.

But if I shoot with a 21mm lens and a flat port in the air shouldn't it be the same as using 2 or 3 stacked glas filters with the same lens. If I do this with high quality filters I get nearly no loss of IQ and still perfectly sharp corners. I have for example used a combination of ND filters and a polfilter (which consists of two glasses surfaces glued together) both on my Zeiss 21mm lens and on my Leica 21mm lens with perfect sharpness.

Why is this completely different with a flat port? Is the glas of the flat port much thicker than 2-3 filters?

 

Sorry for the many questions from my side, but before making a significant investment in this underwater equipment I try to gather as much information as possible to avoid being dissapointed afterwards.

 

Boris

[Tom_Kline]

The pincusion distortion related discusion above relates to shooting under water or the portion that is under water in the case of over-under shots.

Thanks Tom and Paul, but I'm affraid I didn't fully understand the problem. The effect of different magnifications for half-half-shots is clear.

But if I shoot with a 21mm lens and a flat port in the air shouldn't it be the same as using 2 or 3 stacked glas filters with the same lens. If I do this with high quality filters I get nearly no loss of IQ and still perfectly sharp corners. I have for example used a combination of ND filters and a polfilter (which consists of two glasses surfaces glued together) both on my Zeiss 21mm lens and on my Leica 21mm lens with perfect sharpness.

Why is this completely different with a flat port? Is the glas of the flat port much thicker than 2-3 filters?

 

Sorry for the many questions from my side, but before making a significant investment in this underwater equipment I try to gather as much information as possible to avoid being dissapointed afterwards.

 

Boris

[Bobu]

The pincusion distortion related discusion above relates to shooting under water or the portion that is under water in the case of over-under shots.

 

Ok, thanks. So for images of waves, shot above the water, a flat port in adition to a dome port for underwater makes sense, right?

 

Boris

[Paul Kay]

Ok, thanks. So for images of waves, shot above the water, a flat port in adition to a dome port for underwater makes sense, right?

 

Boris

Both will work ok above water - flat ports are thick (~5mm) and do degrade corners marginally (as do polas and NDs if stacked and according to som, any filters depending on how critical you are).