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DuikKees

Flatport length issues

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Hi I am struggeling a bit with the length of my flatportcombination with the 105VR2. I have the impression that there is too much space in front of the lens for the optimum use of wet diopters.
I have a Nauticam D750 housing and I now use a compact base, 20mm extension and the 30 compact port. The effective internal length from base to glass is 90mm. Since the 105 sticks 80mm out in front of the housing, there is a full 1cm room between lens and port glass.
Couple of questions:
1. Is this too much (for the use of wet diopters)?
2. There is a dedicated port for the 105. The macro port 87. What is the exact internal length of this port? Is this 87mm? 

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With the 105mm VR and the Port 87 you still have space to add a "dry" dipoter. However, there is no problem with wet diopters.

Same with the 60mm AF-S and Port 60. I have no try dipoters with this lens.

You can also use the 105mm VR wit the Port 60 and a 20mm extension.

Regards,

Joss

 

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Another way to look at this flat port length issue Is to ask look at the basics, then do the real-ife test.

1. Is the distance between the front of the lens and the inside of the port window important? 
Let us consider what the light does: The light from the subject being photographed goes through the port glass, hits the lens and most of it goes through the lens to the camera's sensors.  A small (very small) amount of light bounces  back from the lens to the inside of the port window, and back to the lens, in ever diminishing, sub microscopic amounts, thanks to the coating on the lens and inside of the port window being clean.  This amount of ping-pong bouncing light is minute, and is not really affected by absorption by the air inside the port, nor by the distance between the port and the length, in the short, millimeter distances that we may realistically encounter.  (Two practical limitations:  1. A small amount of light bouncing around is unavoidable due to the unavoidable built-in entropy of the system. 2. If a port is too long or its inner diameter too small, the inside of the port will show in the camera's image.)

In other words, for quality optical glass, the light transfer from port to lens is so efficient that we can ignore the distance between the inside of the port glass and the front of the lens and, within limits, can use a flat port to enclose the lens without worrying about how long it is.

We can also concluded that the performance of an external, wet diopter is not affected by the distance between the port window and camera lens.    

2. The operational, hands-on, approach is to assemble the camera, lens, housing and port in question.  For the answer, look in the viewfinder and take a photo. The quality of the viewfinder image and of the photo will tells us if the assembly, including port and lens, are working as they should.

 

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