5 replies to this topic

[sharky1961]

Hello,

I own a Nikon F4 in Subal housing and use the nikon 17-35mm with the Subal Fisheye Port. Subal lists the extensionring 60mm with this domeport and a +2 diopter. as I was trying out this combination I found out that the lens also focusses from about 20cm from the domeport to motiv without the diopter as I phoned Subal they said that the diopter was nescessary for the the corner sharpness is this through? I have read some other people use the 33mm extensionring and a +4 diopter and claim to have great sharpness, but what about the distorsion or other effects of such a +4 diopter (alterations must be greater with a +4 then without a diopter or am I wrong?
what are your opinions about this?


Rob

[Alex_Mustard]

First a general comment. One reason you hear about several different dome setups for a lens is that people all have different tollerances of what they call acceptable performance. Some are happy if the image looks sharp in the corners under normal conditions (e.g. at mid range apertures). Others want to extract the perfect setup out of their system - and want perfect optics in the corners with the lens wide open - even though they will never use it as such. But argue that it is always worth achieving the best optical setup.

More specifically the 17-35mm will work without a dioptre (and a dome port for that matter). As will any lens. Here is a picture (attached) I took yesterday with the 10.5mm through a flat port (OK, the big flat glass window on Giles's semi-submarine) - and it has come out. But the image quality and coverage from the lens is much degraded compared with when I jumped in the water.

So to the point! The reason the dioptre helps corner sharpness is that the dome port creates a virtual image with a curved plane of focus and this is much closer to the lens in the corners than in the middle of the frame. Without the dioptre the lens can focus in the middle but not on the corners - they are two close. Two things can bring the corners into focus - 1) a close-up dioptre and 2) a small aperture - that increases depth of field.

So while you may get sharp corners without the dioptre at small apertures (with distant subjects) you will not get sharp corners when the lens is wide open.

Alex

Attached Images

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[sharky1961]

Hello Alex,

thank you for the quick repley. I have both the 33mm and the 60mm extensionring.With what set up I will get me the best Images using the 33mm with a +4 diopter or the 60mm with a +2 diopter?? I think you can better judge sharpness than I can so your advice will be very helpfull and because I own both extensionrings I am very flexible.

Rob

[onokai]

I am getting this lens for my subal f5 procase- subal chart says to use this for f4
AF 17-35/2.8 D
DP-SWPRO + EXR-P55
DP-SWB + EXR40
AF only
4BN003
4ZN023
+4/72 mm
Check it out at subal web site for lens/chart
Mark

[sharky1961]

hello Mark,

I own the DP-Fe2 fisheye domeport, so Subal says ext 60mm with +2 Diopter.
what about the quality of the diopter, would an achromatic diopter ( I think only Canon has one the d500) give a sharper image or will the B+W and heliopan diopters give just the same quality image. I am looking for the highest quality here so a difference of $100,- doesn't care but it must be worth the extra money


Rob

[onokai]

Quote(but what about the distorsion or other effects of such a +4 diopter (alterations must be greater with a +4 then without a diopter or am I wrong?)

I feel you are onto something here how can a relitively cheap diopter make it better. The problem is with the dome design and placement. Alex explains it well above 'here is my take-

I just ordered the big dome port DP-FE2 and the 60 ext ring like yours. Heres the deal. This is from 20+ underwater photo years. I do not know much about digital I only do film. We buy the best lens. Yep I have all 2.8 lens and only 2,8. and putting any diopter over them is a degragation in quality. That said subal says a +2 is needed. They want to cover themselves if problems happen- I say pool test it with and without diopters with film then you will know. I am very picky on image quality as I do not want to photoshop- chop -alter my images. If you can not use a diopter you are better off- if you have to use one the lower the # seems to be prefered.Some digital guys will jump all over this but how is a $100 piece of glass sharper than a $1500 lens that is engineered to it's fullest without the add ons.. That said the highest quility diopter is obviously better than a cheapy. I helped develop the exact extension rings needed for the aquatic f/3 housing back in the 80's and have some idea of optics. Subal has great engineers but they fall a little short when it comes to optics. a port designed well with the right rings should need no diopters .That said I'm pool testing mine next week. In a sense digital is alot easier many problems can be fixed later. With film that is not the case . Hope that helps. Mark