Bad sharpness in the corner, when I use Inon UWL 100 close to subject.
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That's called vignetting and it's common to some wide angle lenses. It gets worse as you zoom out.
Which lens, housing and camera are you using?
Which lens, housing and camera are you using?
ChrisM
Contributor
Inon lenses are known for that vignetting. You have to zoom in a but to get rid of it
James connell
Contributor
there is very little vignetting in the shot ( i'd say > 1/2 stop and that may be caused by the strobe!). vignette is a fall off of light in the outer parts of the frame caused by the inverse square law of light - @ 2x the distance only 1/4 the light reaches the image. stopping down the aperature helps vignetting alot. also as a general rule the wider angle the lens the more vignetting.
this is a depth of field prob. the closer a lenses is focused the shallower the DoF. the fishes tail (and the rock on the left) is inside the DoF range ( the background is behind it) ,which extends from the focus distance 1/3 in front and 2/3 behind, in other words if the DoF is 3" then the subject will be in focus from 1" in front to 2" behind the point of focus.
since the eye's are in focus this is about as good as this is going to get for this camera setting - no lenses with the same DoF is going to do a much better job. all you can do is stop down the lens (since this is a strobed shot shutter speed will make no dif) and increase the strobe power. the only other thing to do is change the camera angle to put the fish parrall with the film of course this changes the compostion - photography is a trade off!!!
this is a depth of field prob. the closer a lenses is focused the shallower the DoF. the fishes tail (and the rock on the left) is inside the DoF range ( the background is behind it) ,which extends from the focus distance 1/3 in front and 2/3 behind, in other words if the DoF is 3" then the subject will be in focus from 1" in front to 2" behind the point of focus.
since the eye's are in focus this is about as good as this is going to get for this camera setting - no lenses with the same DoF is going to do a much better job. all you can do is stop down the lens (since this is a strobed shot shutter speed will make no dif) and increase the strobe power. the only other thing to do is change the camera angle to put the fish parrall with the film of course this changes the compostion - photography is a trade off!!!
James connell
Contributor
i guess i should also say that 'depth of field' is Very much a matter of personal taste!
as you move away from the point of best focus, the quality of focus gets worse ( the circle of confusion gets larger), there is no "magic" distance at which every thing was in perfect focus and then suddenly isn't .
the speced DoF for a given lens is the manufactures Opinion of what is acceptable - Your opionion will likely be different!
makers of cheaper lenses also tend to have a "broader" opinion of whats 'acceptable' than the makers of high quality glass.
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i went back and look at the pix again - i changed my mind as to the focus - i'd have to say it's a little short. it seems to be focused just in front of the pectal fin. i have the impresion that the fish is about 6" long so i'd say the focus is off by about 1" short. of course that assumes the intended point of focus to be the eye ( and as a general rule that should be the point of focus).
but that would just make the tail even more out of focus so this is a fair trade off.
it's very hard to say " you should have moved the camera to the left and up " because there is no way of knowing if that was even possible.
as you move away from the point of best focus, the quality of focus gets worse ( the circle of confusion gets larger), there is no "magic" distance at which every thing was in perfect focus and then suddenly isn't .
the speced DoF for a given lens is the manufactures Opinion of what is acceptable - Your opionion will likely be different!
makers of cheaper lenses also tend to have a "broader" opinion of whats 'acceptable' than the makers of high quality glass.
-----
i went back and look at the pix again - i changed my mind as to the focus - i'd have to say it's a little short. it seems to be focused just in front of the pectal fin. i have the impresion that the fish is about 6" long so i'd say the focus is off by about 1" short. of course that assumes the intended point of focus to be the eye ( and as a general rule that should be the point of focus).
but that would just make the tail even more out of focus so this is a fair trade off.
it's very hard to say " you should have moved the camera to the left and up " because there is no way of knowing if that was even possible.
blacknet
Contributor
Hello,
Strobes do not cause vignette! This is NOT an example of vignette or a DOF issue.
What we are seeing here is coma. Since this is an addon lens we may have barrel distortion, wrong curvature of field or a number of other nasties.
If you stop down to the midpoint on the lens these problems would be reduced but may not be eliminated, depending on the lens. The question really becomes how much are you happy with? This is why we spend thousands of dollars on quality lens.
Also keep in mind we are dealing with flat port optics here, see my article in the tips/techniques thread.
This is also why we rarely see full frame images. Just crop off the nasties.
Ed
Strobes do not cause vignette! This is NOT an example of vignette or a DOF issue.
What we are seeing here is coma. Since this is an addon lens we may have barrel distortion, wrong curvature of field or a number of other nasties.
If you stop down to the midpoint on the lens these problems would be reduced but may not be eliminated, depending on the lens. The question really becomes how much are you happy with? This is why we spend thousands of dollars on quality lens.
Also keep in mind we are dealing with flat port optics here, see my article in the tips/techniques thread.
This is also why we rarely see full frame images. Just crop off the nasties.
Ed
Ed...could you explain 'coma'? I've always seen it referred to as vignetting.
Which also goes a long way to defeat your purpose of a wide angle lens to start with. As was mentioned Inon has a reputation of vignetting in their WA lenses. Look into Sea & Sea and Olympus.
If we knew what camera and housing you had, it would be easier to recommend lenses for it.
Which also goes a long way to defeat your purpose of a wide angle lens to start with. As was mentioned Inon has a reputation of vignetting in their WA lenses. Look into Sea & Sea and Olympus.
If we knew what camera and housing you had, it would be easier to recommend lenses for it.
blacknet
Contributor
I'll answer the second question first. What we are doing is taking a lens, putting it in a housing which add's another element, then adding attachments, yet another optic, then the water. Essentially we have deviated drasticaly from the base unit and added many for points for failure.
As for coma goes check this link
Basically you have blured lines around the edges, like in this photo. In the whole equasion we factor in all the optical additions I mentioned above and the camera/lens settings. This is why no lens addon will ever be opticaly viable as just a single lens.
Hope this helps. Theavatar and I have discussed this intensely on undernet before I made this post. We even ask many great photographers for their input and not a single one could see vignetting.
Ed
TheAvatar
Contributor
Oh boy! Optical physics! Ugh... that's some terrible lens abberation...
I'd like to reinforce and expand on a few points.
That image is an excellent example of coma problems, as blacknet mentioned. I think there might also be spherical abberation as well as curvature of field, which was already mentioned. I can't tell if there is any distortion as I don't see any lines to look at for reference. Blackent gave a great description of coma. For some simple explanations see the following links on coma, distortion and field curvature, and spherical abberation.
I have a suspicion that this lens might be a semi-fisheye adaptor... when you have it on above water do you get a slight fisheye effect? That would explain barrell distortion as accepted in the lens design.
There is NO appreciable vignetting in this image. As Blacknet said, neither he nor I nor any of the other photographers we showed the image to saw vignetting. This lens may be prone to exhibiting appreciable vignetting, but it certainly isn't visible in this image. Vignetting, sometimes known as "uneven field illumination," is light falloff at the edges of a photo. Vignetting is not loss of sharpness, its light falloff at the edges. Lenses are always sharper when stopped down (up to a point usually f/11 or f/16 diffraction overcomes) than when wide open. Vignetting often occurs with wide angles lenses and is also solved by stopping down the lens.
Here are some examples:
Image
Image With vignetting
As to the strobe, the only ways I can possibly imagine as strobe causing vignetting is if under the following conditions: misaligned strobe (not pointing where it should, bigger possibility of this problem at closer range if you arent paying attention) which would cause uneven vignetting on a particular side or corner, or if you happen to hav a flash that illuminates say only 90 degrees and then use a lens with a diagonal angle of view of say 110 degree, causing insufficient illumination on the very edges. This can be less of a problem at closer range I think. These problems are easily avoided if you check what you are doing.
The solution to the softness? Lose the junk lens as suggested is the only total solution. But that may not be an option for you at the moment. In the meanwhile, take the advice already given and stop down the aperture as this will minimize the abberations. I'd recommend 3-5 stops from wide open when you are willing to make it go away as much as possible. The minor losses from diffraction wll probably be outdone by the minimization of hte distracting lens defects. You might also consider not taking pictures at close range as close focus propably makes the abberations manifest themselves more.
I'd like to reinforce and expand on a few points.
That image is an excellent example of coma problems, as blacknet mentioned. I think there might also be spherical abberation as well as curvature of field, which was already mentioned. I can't tell if there is any distortion as I don't see any lines to look at for reference. Blackent gave a great description of coma. For some simple explanations see the following links on coma, distortion and field curvature, and spherical abberation.
I have a suspicion that this lens might be a semi-fisheye adaptor... when you have it on above water do you get a slight fisheye effect? That would explain barrell distortion as accepted in the lens design.
There is NO appreciable vignetting in this image. As Blacknet said, neither he nor I nor any of the other photographers we showed the image to saw vignetting. This lens may be prone to exhibiting appreciable vignetting, but it certainly isn't visible in this image. Vignetting, sometimes known as "uneven field illumination," is light falloff at the edges of a photo. Vignetting is not loss of sharpness, its light falloff at the edges. Lenses are always sharper when stopped down (up to a point usually f/11 or f/16 diffraction overcomes) than when wide open. Vignetting often occurs with wide angles lenses and is also solved by stopping down the lens.
Here are some examples:
Image
Image With vignetting
As to the strobe, the only ways I can possibly imagine as strobe causing vignetting is if under the following conditions: misaligned strobe (not pointing where it should, bigger possibility of this problem at closer range if you arent paying attention) which would cause uneven vignetting on a particular side or corner, or if you happen to hav a flash that illuminates say only 90 degrees and then use a lens with a diagonal angle of view of say 110 degree, causing insufficient illumination on the very edges. This can be less of a problem at closer range I think. These problems are easily avoided if you check what you are doing.
The solution to the softness? Lose the junk lens as suggested is the only total solution. But that may not be an option for you at the moment. In the meanwhile, take the advice already given and stop down the aperture as this will minimize the abberations. I'd recommend 3-5 stops from wide open when you are willing to make it go away as much as possible. The minor losses from diffraction wll probably be outdone by the minimization of hte distracting lens defects. You might also consider not taking pictures at close range as close focus propably makes the abberations manifest themselves more.
Thanks, blacknet and Avatar for the descriptions and examples. I've always seen the coma effect referred to as vignetting.
BN...I totally understand about added elements...you lose a bit of quality with each one. But sometimes for underwater, because of budget and other things, we can't all posess Nikor quality lenses and the camera and housings they require. We do the best with what we have.
BN...I totally understand about added elements...you lose a bit of quality with each one. But sometimes for underwater, because of budget and other things, we can't all posess Nikor quality lenses and the camera and housings they require. We do the best with what we have.
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