Canon RF 50mm f/1.8 STM or Canon EF 50mm f/2.5 Macro + EF/RF adapter in a Flat Port?

[JayceeB]

Folks,

 

Has anyone tested the Canon RF 50mm f/1.8 or Canon EF 50mm f/2.5 Macro + EF/RF adapter in a Flat Port?  Looking for feedback on AF and IQ.

 

Thank you!

Edited February 5 by JayceeB
Added EF 50mm

[humu9679]

4 hours ago, JayceeB said:

Folks,

 

Has anyone tested the Canon RF 50mm f/1.8 or Canon EF 50mm f/2.5 Macro + EF/RF adapter in a Flat Port?  Looking for feedback on AF and IQ.

 

Thank you!

 

I used the EF 50mm macro years ago above water. Slow focusing and very loud (and I wasn't shooting videos). Maybe on a newer body it will show improved focusing speed, but this is a 30-something year old lens.

[JayceeB]

18 hours ago, humu9679 said:

 

I used the EF 50mm macro years ago above water. Slow focusing and very loud (and I wasn't shooting videos). Maybe on a newer body it will show improved focusing speed, but this is a 30-something year old lens.

Thank you for sharing your experience!

[JayceeB]

I picked up a refurbished RF 50mm f/1.8 from Canon for $160 USD.  It is tiny.  Image sharpness above water is more than adequate by my standards.  Autofocus speed above water is average, but I wasn't expecting class leading performance.  This lens isn't listed on the Marelux port chart, but they confirmed that the 140mm dome port should work with no extension.  I'll post my experience and samples once I get this combination underwater.

[JayceeB]

I tested this setup today with the RF 50mm and 140mm dome port with no extension.  It did not go well.  The lens would not autofocus on subjects less than 20' away.  In most cases, when I pushed the back button autofocus, the focus box (Canon R5) would just turn red, and it wouldn't even attempt to focus hunt.  The specs on this lens give a minimum focus distance of 30cm.  Could it be that the virtual image of the dome is too close to the lens?  I'm wondering if adding some extension would solve this.

 

Test shot of a manta I had to back off by 20' to get focus. (Some color correction, but no sharpening)

 

 

[Adventurer]

4 hours ago, JayceeB said:

The specs on this lens give a minimum focus distance of 30cm.  Could it be that the virtual image of the dome is too close to the lens?  I'm wondering if adding some extension would solve this.

Yes, the MFD of 30cm is too high. You will need a dome diameter of approx 52cm for it.

 

What you can do as a solution is to put a +4 diopter on the lens to improve the focus on the virtual image.

 

The lens entrance pupils sits approx 3.3cm too far inside the housing for optimal position of the entrance pupil behind the dome.

 

Hydronalin in Germany has a solution for making affordable INON Glas domes work for that lens and the RF 35mm, if the +4 diopter does not do the job for you.

Edited February 16 by Adventurer

[JayceeB]

18 hours ago, Adventurer said:

Yes, the MFD of 30cm is too high. You will need a dome diameter of approx 52cm for it.

 

What you can do as a solution is to put a +4 diopter on the lens to improve the focus on the virtual image.

 

The lens entrance pupils sits approx 3.3cm too far inside the housing for optimal position of the entrance pupil behind the dome.

 

Hydronalin in Germany has a solution for making affordable INON Glas domes work for that lens and the RF 35mm, if the +4 diopter does not do the job for you.

Thanks, @Adventurer.  Let me ponder on the +4 diopter suggestion.  Not sure how far I want to pursue this option as I was looking for a lightweight travel setup for Malapascua Thresher Sharks.  I have tested the 140mm dome + RF 14-35mm successfully at the 35mm end.  I think that will likely be my choice, as AF is fast and reliable.  I'll utilize the 50mm for above water use only.

Edited February 17 by JayceeB

[Adventurer]

19 hours ago, JayceeB said:

I was looking for a lightweight travel setup

Well, a +4 diopter should not break the bank and also add less than 30 gramms to your travel weight.

 

As long as you do plan on doing splits (over under shots) the diopter will make your lens AF much faster, as the virtual image never appears at infinity when the dome is dipped in water.

It will pull the AF to a working area that is more likely to happen underwater. Having said that, you may also improve your RF 14-35 with a minor diopter when working behind more compact domes.