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donw

Over-under focus technique?

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Posted (edited)

I'm just starting to do over-under photos, and have read a lot, but no one has described a trusted technique for focussing.  I'm using a 16-35mm lens at f/16 (Sony a7R2) with an 8" dome, and normal UW subjects focus fine.  I've developed an elaborate scheme focussing on the UW subject, and locking that before raising the camera to include the land half of the composition.  It usually works, but is clumsy and non-spontaneous.

 

Is there a better way?

 

Thanks,

Don

Edited by donw

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the underwater portion you need to have the virtual image fall within the depth of field and the virtual image is 3 dome radii from the dome.  for an 8" dome that might be as small as 300mm away from the dome, while the above water portion you want infinity focus.  so what you are after is hyperfocal distance which is at 0.55m on a 16mm lens.  If you plug those numbers into a depth of field calculator you will see that the near point barely reaches 300mm when at the far point is infinity and would require you to focus rather precisely on the hyperfocal distance.  Whether you can make this work will depend on the exact dome geometry.  300mm from the dome will be close to 500mm focus distance from the sensor

at f22 you will see that at 0.4m or more focus distance you are in focus from 0.2m to infinity.  This gives you a bit more breathing space.  exactly what you need to do will depend upon your dome radius a bit.  8" might be a little small to do splits reliably, depending on the exact radius, some 8" domes might have 120mm radius.  That's the theory part.

Practically I'd suggest stopping to f22 using a focus point low in the frame and focusing on a far point in your UW part of the frame as a starting point to see how you go.  So you would focus with rear button focusing then re-compose your shot.  If that works you can try the same technique and  progressively lower your aperture to see where the limit for your system lies.

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Thank you very much Chris.  I really appreciate the combined technical and real-world explanations.  I tried it all out this morning on some coral heads (about 2m away), and got much better results.  I imagine that is mostly due to the switch to f/22, and I'm an old large-format guy, so I should have known.  Infinity is still not in perfectly sharp focus, but I actually like that better._DSC6519.thumb.jpg.071dc1dbc2ebd2b7d33ba64519b1941a.jpg

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I took a class in over/under shots in June.   The advice was to focus on the most-distant underwater feature of interest - and take many, many shots.  It's a volume game.

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Here's another technique: select your aperture; I use f16 on crop sensor but 22 might be better on FF. Take the camera in the garden with a tripod. Use manual focus. Focus on infinity, take a picture, move focus ring incrementally closer, recording the focus position for each frame. Repeat until you are focussing close. Review the pictures on a monitor. How close can you focus and still get acceptable sharpness in your background at your selected aperture? Tape the lens at this closest position. Try it underwater and see how close things are in focus. Depending on the lens it may be between 0.4m and 0.8m on the lens, or something like that. Find phuketpools on Facebook to see how that works out.

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Posted (edited)
On 8/7/2022 at 1:48 AM, donw said:

I'm just starting to do over-under photos, and have read a lot, but no one has described a trusted technique for focussing.  I'm using a 16-35mm lens at f/16 (Sony a7R2) with an 8" dome, and normal UW subjects focus fine.  I've developed an elaborate scheme focussing on the UW subject, and locking that before raising the camera to include the land half of the composition.  It usually works, but is clumsy and non-spontaneous.

 

Is there a better way?

 

Thanks,

Don

On full frame the magic number for a 16mm lens is f/20 especially as your dome is not that big

This is one of my recent shots due to waves I could not keep the dome totally free of water so the part over is not super clean but it is sufficient

The single most important factor is to evaluate the aperture that covers the over and under adequately

If the aperture is too wide even if you are hitting infinity behind the dome the distant land part may be blurred

layers-20220804.jpg

Edited by Interceptor121
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Posted (edited)
On 8/20/2022 at 2:59 PM, Interceptor121 said:

On full frame the magic number for a 16mm lens is f/20 especially as your dome is not that big

This is one of my recent shots due to waves I could not keep the dome totally free of water so the part over is not super clean but it is sufficient

The single most important factor is to evaluate the aperture that covers the over and under adequately

If the aperture is too wide even if you are hitting infinity behind the dome the distant land part may be blurred

layers-20220804.jpg

There is always a guy that hits confused on my answers. If you are that guy and don't understand the why I suggest you study the physics of underwater imaging

The op has a relatively small dome of 8" this means the infinity focus underwater is aroynd 12" from the port. Some of those wide angle lenses only focus at 28-30 cm from the focal plane which means the useable depth of field underwater is within 28 to 50 cm this is not enough to focus the lens on land as at f/16 and 50cm you may not get decent focus at distance. A larger dome say 10" would push that limit 4" away which means the lens will reach satisfactory focus at distance at lower f/stop

WIth a 10" dome port f/16 on a 16-35mm lens should achieve decent results for the land part but 8" dome would likely fall short even if the underwater part is very distant

Edited by Interceptor121

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7 hours ago, Interceptor121 said:

There is always a guy that hits confused on my answers. If you are that guy and don't understand the why I suggest you study the physics of underwater imaging

Doesn't seem very helpful! 

I use back button focus for (almost) everything. This means that I just move the my focus point underwater, focus underwater using the button and then release it. This will then hold the focus point. I use depth of field to get the surface elements of the image in focus. 

Typically therefore, I tend to focus on a subject that is quite close to me underwater. Typically, my strobes' lights doesn't travel that far, and as I am shooting at a big aperture, depth of field is not an issue. 

Big domes help, as they make it easier to deal with any ripples etc. in the water's surface. Corner sharpness is related to the size of the  aperture. At the kind of open apertures that are required for adequate DOF,  it is not normally an issue with bigger domes. Of course, this is relevant to your sensor size, but a larger dome will always make it easier to get a good air/water line in the image.

We had someone in the cenotes shooting over/unders with a tiny 4.3" dome on one of our workshops earlier this year. As the surface was completely still, it worked fine!

Lastly, I find that fisheye lenses tend to work better for me aesthetically. I find the curve of the water line of the large dome mirrors the distortion somewhat. I appreciate that you are using a 16-35mm, but if you can get hold of an 8-15mm with an adaptor, this may make your over/unders simpler too!

 

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Doesn't seem very helpful! 

I use back button focus for (almost) everything. This means that I just move the my focus point underwater, focus underwater using the button and then release it. This will then hold the focus point. I use depth of field to get the surface elements of the image in focus. 
Typically therefore, I tend to focus on a subject that is quite close to me underwater. Typically, my strobes' lights doesn't travel that far, and as I am shooting at a big aperture, depth of field is not an issue. 
Big domes help, as they make it easier to deal with any ripples etc. in the water's surface. Corner sharpness is related to the size of the  aperture. At the kind of open apertures that are required for adequate DOF,  it is not normally an issue with bigger domes. Of course, this is relevant to your sensor size, but a larger dome will always make it easier to get a good air/water line in the image.

We had someone in the cenotes shooting over/unders with a tiny 4.3" dome on one of our workshops earlier this year. As the surface was completely still, it worked fine!

Lastly, I find that fisheye lenses tend to work better for me aesthetically. I find the curve of the water line of the large dome mirrors the distortion somewhat. I appreciate that you are using a 16-35mm, but if you can get hold of an 8-15mm with an adaptor, this may make your over/unders simpler too!

 

Due to dome port optics something at 3x radius from the dome surface is already st infinity. Normally around 30cm
Depth of field is an issue for the land part not for the underwater part
With a fisheye lens that focusses at 15cm everything is in focus almost immediately not because of depth of field but because the lens simply focuses closer
With regards to split shots with domes a normal person doesn’t like the water line to be distorted. Once you go rectilinear you have challenges of depth of field for the land part


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20 hours ago, Interceptor121 said:

There is always a guy that hits confused on my answers. If you are that guy and don't understand the why I suggest you study the physics of underwater imaging

Actually, you equally confuse women with many if your answers as well... as for the suggestion that they study the physics of u/w imaging - that's a subjective comment. This might/might not be useful for lots of different people..incl. the poster.

@adamhanlon - Yes, this is an equally unhelpful comment. I acknowledge this fully...

 

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Actually, you equally confuse women with many if your answers as well... as for the suggestion that they study the physics of u/w imaging - that's a subjective comment. This might/might not be useful for lots of different people..incl. the poster.

[mention=8943]adamhanlon[/mention] - Yes, this is an equally unhelpful comment. I acknowledge this fully...

 

If you don’t understand something the approach to rectify is to ask for clarification of what you didn’t get
If you hit confused you will remain so
Your post is in fact useless to any purpose as you are just having a go


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Optics is certainly confusing (try explaining why light rays bend when they hit a transparent surface of different index at an angle!). Personally, I find Interceptor's messages, possibly terse, but not impolite. And certainly informative. Enough to make me go looking for a bit more. I appreciate them as much as the posts of many other experienced contributors to this forum.

I found the following website with a good beginner's primer to port optics:

https://oceanity.com.au/articles/view/understanding-flat-port-and-dome-port-theory

and, in case you miss the link towards the end of that page, they have a nice interactive visualization of what happens to the virtual image (that your camera sensor is seeing) as the real object moves closer or farther:
https://oceanity.com.au/tools/dome-port-virtual-image-visualiser

Kind regards

Ajay

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Optics is certainly confusing (try explaining why light rays bend when they hit a transparent surface of different index at an angle!). Personally, I find Interceptor's messages, possibly terse, but not impolite. And certainly informative. Enough to make me go looking for a bit more. I appreciate them as much as the posts of many other experienced contributors to this forum. I found the following website with a good beginner's primer to port optics:https://oceanity.com.au/articles/view/understanding-flat-port-and-dome-port-theory

and, in case you miss the link towards the end of that page, they have a nice interactive visualization of what happens to the virtual image (that your camera sensor is seeing) as the real object moves closer or farther:

https://oceanity.com.au/tools/dome-port-virtual-image-visualiser

Kind regards

Ajay

 

 

Jeremy simulator is very useful and so are sites like scuba geek however what most people miss is that while the infinity focus point is 4x the (dome radius - glass thickness) distance from the dome center the lens working distance is calculated from the focal plane which is behind the dome center. So you don’t really need a lens that focus really close to work at all however to reach the closest point to the dome you do need a lens that focuses at least the distance from the focal plane plus one radius plus the minimum virtual distance. Many decent wide lenses will also be able to do that as long as the dome radius is big enough however this also mean that your actual distance from a subject with a large dome will be quite farFinally the infinity point for the virtual image may be relatively close and this may fall under the hyper focal distance of the lens on land which means the split shot is focussed underwater but the land part is blurred even if your underwater part is far away in relative terms.

Closing the aperture moves the hyper focal distance closer so the land part gets in reasonable focus

Shooting a fisheye lens that has half the working distance of a wide lens and has distorted edges removes almost all issues but gives you a distorted water line and if you use a large super size dome for stability you also add field of curvature of the dome itself to the mix

 

 

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Wow, all very interesting. I've always been reticent to use such a small aperture as f22 when doing over/under shots, because of diffraction. That said I've always used Nikon APS-C cameras rather than full frame. I tend to shoot f11 to f16. However, I've never got a single shot I've been happy with:blush: so I'm not saying I know anything about it, more asking if f16 to f22 is still the right choice?

I should add I'm very happy with my cameras when I take landscape photographs above water :) it is only over/under shots that I'm struggling with.

Edited by MTH
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