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tobyone

24mm Anamorphic lens in dome

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I am setting up a Sirui 24mm anamorphic lens in an underwater housing with an eight inch dome. My normal setup with this dome is for use with a 10-24 Fuji lens. I works great with a +4 diopter.The Sirui lens is about 44 mm longer than the Fuji, putting the front element about 30mm from the Dome apex. With the +_4 diopter on the 24mm I can achieve a close focus of 12 inches, in air with dome. I would like to get the focus down to about 10 inches. My question is that by putting a 12 mm extension on the dome, should this improve the focus on the aerial image?

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All it will do is move the virtual image 12mm further out from the end of the lens doesn't seem like this is enough to make a difference.  The main purpose of the extension is to place the entrance pupil at the dome centre of curvature.  I would be looking for about a 40mm extension to get it placed closer to the right spot. 

The 10-24 lens has a min focus of 24cm which mean working distance from front element is about 135mm, you already have 30 +44mm clearance to the dome so it can focus on a virtual image 60mm out from the dome surface which should allow you to focus about 110 mm from an 8"dome surface without a diopter.

On your 24mm lens - the virtual image will be about 29 cm from an 8"dome.  When you say you can achieve 12"is this the working distance from the end of the lens or the distance from the sensor?  Focus distances in specs are usually from the sensor plane. 

 

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Assessing from front of lens. The 24 anamorphic  minimum focus distance(assuming from focus plane)  is .6 meters or 1foot 9 inches without a diopter. I have not tested underwater yet, but I thought extending the distance of the dome even 12 mm would move the aerial (virtual)image back that much. This is a long lens for a 24mm,126.5 mms.

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8 hours ago, tobyone said:

Assessing from front of lens. The 24 anamorphic  minimum focus distance(assuming from focus plane)  is .6 meters or 1foot 9 inches without a diopter. I have not tested underwater yet, but I thought extending the distance of the dome even 12 mm would move the aerial (virtual)image back that much. This is a long lens for a 24mm,126.5 mms.

with the lens focusing at 12" that makes the the focus point 274mm from the dome.  That means underwater you will reach focus 20+m away.  Adding the 12mm extension brings the focus point to 254mm from the dome and this will let the lens focus on something 6m away UW.  So it improves things but not enough to make the lens useful I don't think. 

If you could find a 40mm extension which is about what's needed to get the entrance pupil close to the right position assuming it's in a similar position to the 10-24 lens in relation to the front element, you focus 234mm from the dome allowing to focus on something 2.0m away UW.  Switching to a 9"dome with the same extension would bring the close focus point in to 1.3m.

It seems like losing battle to me with the lens focusing so far away, you might get something close to acceptable using a big dome with the right amount of extension.  If you don't have the entrance pupil positioned properly it will introduce distortion into the image.

Have you checked what the diopter does with the lens - being anamorphic it might introduce some abberations??

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On 3/18/2021 at 1:54 PM, tobyone said:

I am setting up a Sirui 24mm anamorphic lens in an underwater housing with an eight inch dome. My normal setup with this dome is for use with a 10-24 Fuji lens. I works great with a +4 diopter.The Sirui lens is about 44 mm longer than the Fuji, putting the front element about 30mm from the Dome apex. With the +_4 diopter on the 24mm I can achieve a close focus of 12 inches, in air with dome. I would like to get the focus down to about 10 inches. My question is that by putting a 12 mm extension on the dome, should this improve the focus on the aerial image?

Why would you want to have an anamorphic lens underwater?

Anamorphic lenses give specific look and flares that underwater are generally not desired (we are all fighting flare) as it reduces contrast

Water has already a dampening and contrast reducing effect on the image it does not really need any help?

I am a bit confused by the purpose of this experiment.

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On 3/19/2021 at 12:54 AM, tobyone said:

I am setting up a Sirui 24mm anamorphic lens in an underwater housing with an eight inch dome. My normal setup with this dome is for use with a 10-24 Fuji lens. I works great with a +4 diopter.

BTW going by the specs, you should be able to focus on something 30cm from the dome without a diopter and adding a 12mm extension with the 12-24 should get that down to about 20cm. 

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Interceptor 121,

The main reason for shooting anamorphic is the wide aspect. Reef scapes and large pelagics would look more natural than with using an  ultra wide (weitwinkel) angle lens.  The flare is not the reason. Check out anamorphic video :

 

https://youtu.be/yNkCzbgGqOY

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