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Beginner question: Mininum focusing distance


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#1 Davide DB

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Posted 13 June 2019 - 07:21 AM

Sorry guys,

 

Never too late for... a stupid question  :monkey:

 

Underwater, a lens minimum focusing distance will increase or decrease?

 

Specific case: lumix 12-35mm behind Nauticam 6" dome port (its port according to Nauticam).

 

Sunday I made several nearly macro shots at 35mm with the subject very near. The camera indicated me focus acquired (single area pointed exactly on subject, AE lock, green square). Even on the boat, on GH4 display, images seemed gorgeous but at home on my PC i realized they were completely off focus. (I ate the dome cover).

 


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#2 TimG

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Posted 13 June 2019 - 07:31 AM

Hey Davide

 

I'm not familiar with the Lumix lens so others might have better advice. But the problem may well be taking macro pics with a domeport.

 

Domeports are essentially for wide-angle lenses. Inside a domeport the lens does not focus on the actual subject but on a virtual image that the dome creates and which bends. This is much closer to the lens and is why you need a very close-focussing lens. And the closeness of the focus then creates the problem of soft corners. 

 

A macro lens, on the other hand, needs a flat port so that you are focussing on the actual subject and not a virtual image.

 

The smallness of the camera LCD screen probably makes the images look fine. But, as you say, stick them on a monitor and..... 


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#3 Davide DB

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Posted 13 June 2019 - 09:20 AM

Thanks Tim

My bad I used the word "macro". Actually the 12-35mm is a light zoom (24-70 in 35mm).
I was filming very close some small corals at 35mm end. Probably too close. I was deceived by the GH4 AF which immediately locked focus so I thought everything was ok.
So now I wondering if minimum focus distance per lens specs is of any use underwater....
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#4 TimG

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Posted 13 June 2019 - 10:18 AM

No worries, Davide

I’d have thought whatever the minimum focus distance specified, if the cameras AF system locks on your subject, the focus should be good all things like, camera shake, shutter speed etc being equal. But then could the AF have possibly “found” something else in the frame rather than precisely where you wanted focus?

Was your shutter speed high enough?

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#5 Interceptor121

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Posted 13 June 2019 - 10:26 AM

Using a dome the focus distance should not change this is of course in theory

 

As you zoom through the dome port becomes more flat in general with flat ports the focus distance increases

 

It is possible that the camera is focusing on a reflection on the dome it is unlikely that it will lock focus and then it is not in focus unless you used some fancy multi area focus and is focusing on the reef instead of your subject


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#6 Barmaglot

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Posted 13 June 2019 - 10:39 AM

This may be relevant: http://wetpixel.com/...showtopic=61824



#7 Davide DB

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Posted 13 June 2019 - 02:02 PM

@Barmaglot Thanks informative post.
While definitively the 12-35 is not a macro lens form what I re ad I should get batter corner and a slightly wider image.

I used multipoint focus just one time several years ago ruinining all my clips. No thanks. I'm sure focus was locked. F8 iso 800. Maybe I was narked...

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#8 ChrisRoss

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Posted 13 June 2019 - 11:51 PM

What I find can happen with the contrast detect AF systems is they "look through" the subject to focus on what is behind if it has more contrast than the subject or if the subject is a little small.  Generally though I look for two things focus acquisition and the subject becoming more defined or snapping to focus.  The minimum focus distance is measured from the sensor plane and as stated above you are focusing on a virtual image, which is closer.  You can use this to Visualise  how far away you can focus:  https://oceanity.com.au/tools/virtual-image-calculator/ 

 

For a 6 inch dome (with a 3" radius of curvature ) at 20cm object distance from dome the virtual image is 8.1cm from the dome, which is as low as the calculator goes.  I would estimate the virtual image is 5cm from the dome with an object 10cm from the dome and it really should be able to focus on that.  If you were focusing on coral maybe the back of the coral head with coral branches against clear water had the better contrast and it focused there.  Just because it locked focus does not mean it focused on what you wanted to focus on.

 

Based on my Oly 12-40 which has the same more or less min focus distance you should be able to focus maybe 5cm from the port surface.  You could try assigning focus peaking to a function button and confirm it's peaking around the thing you want to focus upon?



#9 Interceptor121

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Posted 14 June 2019 - 01:27 AM

If a dome is well designed (not saying the 6" is) with a MFT the working distance won't change. Just measure from the camera marker finding the position on the housing to the front and you will be able to work out if it focuses or not alternatively do this in air it won't change massively.The point of a dome is to keep the field of view so the lens has to behave the same. The virtual image etc is an explanation on how this is accomplished but it does not relate to working distance once the dome is correctly designed


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#10 Davide DB

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Posted 14 June 2019 - 07:41 AM

 

Based on my Oly 12-40 which has the same more or less min focus distance you should be able to focus maybe 5cm from the port surface.  You could try assigning focus peaking to a function button and confirm it's peaking around the thing you want to focus upon?

 

I tried several times to switch for a moment to manual focus and engaging focus peaking (I have FP on Fn2). I've never been able to see the dots. Tried with several settings and colors. From my experience, even on land, peaking works only in few specific condition. I've never used FP with other camera but IMHO GH4 peaking sucks!

 

This is the shot I missed (rescaled to 1080p)

 

 

It's plenty of contrast for focus peaking. I think I was too close.

Other shots not so close are tack sharp. This lens is amazing. Minimum focus distance for this lens is 25 cm (10"). I guess is measured at 12 mm not 35 mm. This evening I will try at home.

 

I will try again next week. I hope to find it again with open polyps while eating...


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#11 Interceptor121

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Posted 14 June 2019 - 07:54 AM

 

I tried several times to switch for a moment to manual focus and engaging focus peaking (I have FP on Fn2). I've never been able to see the dots. Tried with several settings and colors. From my experience, even on land, peaking works only in few specific condition. I've never used FP with other camera but IMHO GH4 peaking sucks!

 

This is the shot I missed (rescaled to 1080p)

 

 

It's plenty of contrast for focus peaking. I think I was too close.

Other shots not so close are tack sharp. This lens is amazing. Minimum focus distance for this lens is 25 cm (10"). I guess is measured at 12 mm not 35 mm. This evening I will try at home.

 

I will try again next week. I hope to find it again with open polyps while eating...

 

I had the 12-35mm for some time if I remember correctly the lens extends and the nodal point will actually move backwards the dome will look for 'flat' and the working distance won't anymore be the same of wide end 

 

I have the same issue with the 12-60 however the dome port 7" is much bigger so the effect is less pronounced

 

I think a test in the bathtub is what you need...


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#12 Davide DB

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Posted 14 June 2019 - 08:05 AM

 

I think a test in the bathtub is what you need...

 

Yes, a bathtub is what I need (40°C in Rome today)  :evilgrin2:


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