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horvendile

Fisheye focusing necessary?

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Hi!

I'm busy kitting my new (used) Sea&Sea Z6 rig. I have a Zen 230 mm glass dome port which I will primarily use for the Nikon 14-30/4 with Sea&Sea correction lens. However, I also happen to own the Sigma 15/.8 EX fisheye which I understand is generally appreciated for underwater use. Could be fun to use that one also.

Problem: on the FTZ adapter I won't get autofocus with the fisheye. I can buy a focus gear for something like $170 (in SEK) to do manual focusing. Question is, do I need it?

I'm thinking that if I'm at f/8 or so, with a fisheye focused at maybe 0.3 m pretty much everything will be in focus. But I'm not sure how this will play out underwater behind a dome port.

I'm also thinking that you guys probably have something experienced to say about this. Would my $170 be better spent on ice cream?

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Posted (edited)

Once you figure out where the virtual image pretty much not needed however the moment you change aperture the focus will change so you will be stuck with focus position and aperture for the dive

Edited by Interceptor121

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Posted (edited)
1 hour ago, Interceptor121 said:

the moment you change aperture the focus will change so you will be stuck with focus position and aperture for the dive

Do you mean it will change because of focus shift or changing depth of field? Either way being stuck at one aperture doesn't seem so bad.

Figuring out where the important part of the virtual image will be may be trickier if it is to be done before the dive, but I think I saw a calculator for that somewhere. I'll let Google be my friend.

Edit: google isn't as friendly as I hoped. I found calculator, but not really what I was looking for.

Edited by horvendile

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Posted (edited)

If you still have your SLR housing you could focus with it, turn the camera off, then check the focus distance on the surface. Use that distance with your Z6. You will need to use the same dome with both housings. I tried using this pre-focus approach early on in my digital underwater photography using the 14mm MF lens - got too many OOF pix of salmon so gave up on that lens. Got the 14 AF!!

The technique might work better for larger subjects like blue whales. Since you already have the lens it will not cost anything to experiment other than your time. You could write Nikon and complain about not having an FTZ that does screw-drive AF.

Edited by Tom_Kline

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In theory yes you probably could, in practice maybe not.  I was shooting my fisheye on m43 and found shots where I focused on the centre of the image but the edges were closer than that and they were marginal, so another layer of complication, what I needed to do was focus on the nearest part on the image. 

Regarding aperture change - some lenses will shift focus as you stop them down.

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Hmm, it does seem increasingly optimistic to manage without focusing.

After posting the above I was thinking that it could be enough to cover the virtual image for infinity distance with my depth of field. Or, explained in greater detail:

* Locate where the virtual image of inifinity will be. For the sake of argument, let's assume that's a focus distance of 40 cm. (I can find more exact numbers but as of writing this I haven't figured out if this is measured from the dome glass, from entrance pupil or from sensor plane.)
* Head to the trusty old dofmaster calculator: https://www.dofmaster.com/dofjs.html
* Find that according to the definitions of that site, at 15 mm f/8 focused at 30 cm your depth of field just about covers 40 cm distance.
* Near limit in this case is 23 cm.
* A near limit of 23 cm for the virtual image is likely too far for what is desirable, if I want to take a picture of something at short distance.

I would feel pretty stupid sitting on the bottom during a Maldives night dive with mantas swimming at 30 cm distance to have set the focusing distance too far away and not be able to correct it for the whole dive.

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Dome port work on virtual image
Your lens however may have a manual focus with meters and therefore the focus position doesn’t change


Sent from my iPhone using Tapatalk

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54 minutes ago, Interceptor121 said:

Dome port work on virtual image
Your lens however may have a manual focus with meters and therefore the focus position doesn’t change
 

Um... could you rephrase that? I didn't catch your meaning.

The Sigma fisheye does have distance markings yes, but that doesn't mean it doesn't have focus shift upon stopping down. If that's what you mean with the second sentence.

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I use semi-manual focus with my 14mm in caves, where it's usually too dark for fast autofocus. By this I mean I move to back button focus, focus once, then shoot repeatedly. On a normal dive I'd take 150 photos and refocus four or five times (mostly to check I didn't hit the focus button by accident and wreck things). I'm rarely doing CFWA so a focal distance of 1m to 5m works great. Usually shooting at f8 to f16 behind a dome.

From that experience I would say that if you can find the focus point first, you're good. But it may be hard to find...as you know the focus above water bears no resemblance to the focus underwater.

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12 hours ago, errbrr said:

From that experience I would say that if you can find the focus point first, you're good. But it may be hard to find...as you know the focus above water bears no resemblance to the focus underwater.

Under perfect conditions I think I could prefocus correctly as long as I'm not doing cfwa. After all, calculating approximate distance to the virtual image of infinity isn't that difficult. BUT... I decided it wasn't worth trying to save the $170. The focus gear is in the mail now.

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Probably saving yourself hours of frustration :)

I hope you take at least $170 worth of beautiful pictures with it!

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