Technical Query

[Don Hunter]

I have a Sony A7RV camera in a Nauticam housing with a 140mm glass dome port, and am trying out a Canon EF 8-15 f//4 lens with a Sigma MC-11 adapter.  I am very happy with the quality of the images this produces, but have a major problem.  The Canon lens is supposed to have a minimum focal length of 140mm but my lens focuses on the inside of my dome port as well as any bubble on the outside.  I reduced the recommended port spacer from 30mm to 20mm bringing the lens to within 30mm of the dome, which is not recommended, but this does not fix the problem.  However on checking the minimum focal distance I find that the lens focuses at 17mm!!  How is this even possible? Do any other users have the same problem? Is this a problem on my particular lens?  I have tried using different focus modes, but this does not solve the problem. Obviously keeping the port in pristine condition and cleanliness is optimal but it is distressing to have a dive or two where it keeps focusing on that one particle of dandruff that inadvertently crept in.  Have any other users experienced this problem. 

[Chris Ross]

The minimum focal ldistance is specified from the focal plane at the sensor, you need to subtract the flange length and the length of the lens and adapter.  This works out to be focusing very close.

 

I think you possibly need to keep the dome clean and brush off bubbles if they form.  Also consider change to just using just the centre or a single focus point and place it on something with decent contrast if you are not doing this already.   If you use back button focus you an control what the camera focuses on better,  if needed focus and re-compose is easy this way.

 

I use that lens on my setup and it rarely gives me problems.

[Don Hunter]

Thanks Chris. It would be better if it was a little more tolerant and you could use wide tracking for fast moving subjects though. 

[Davide DB]

Hi Don,

Welcome aboard!

[Chris Ross]

8 hours ago, Don Hunter said:

Thanks Chris. It would be better if it was a little more tolerant and you could use wide tracking for fast moving subjects though. 

Are you talking about subject recognition?  that shouldn't recognise dust and bubbles.  I would think you would need to develop a routine of some type, don't know if you use back button focus, but this solves lots of issues.  Cameras generally will jump to the background rather than foreground in my experience at least so you should be able to take advantage of that.  If you're troubled by a dust bunny or something, move the focus point away so the camera doesn't see it.  If you have tracking focus that works use that and AF when you are over the subject then let tracking takeover. 

 

If you are talking fast moving big animals, pre-focus on  your fin tip which will get you closer to the right focus point.  Plus a blower bulb is your friend, blow out your dome just before assembling it.

[Don Hunter]

On 2/28/2025 at 7:19 PM, Davide DB said:

Hi Don,

Welcome aboard!

Thanks

 

[Don Hunter]

On 3/1/2025 at 1:33 AM, Chris Ross said:

Are you talking about subject recognition?  that shouldn't recognise dust and bubbles.  I would think you would need to develop a routine of some type, don't know if you use back button focus, but this solves lots of issues.  Cameras generally will jump to the background rather than foreground in my experience at least so you should be able to take advantage of that.  If you're troubled by a dust bunny or something, move the focus point away so the camera doesn't see it.  If you have tracking focus that works use that and AF when you are over the subject then let tracking takeover. 

 

If you are talking fast moving big animals, pre-focus on  your fin tip which will get you closer to the right focus point.  Plus a blower bulb is your friend, blow out your dome just before assembling it.

Thanks