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]

  On 2/5/2025 at 3:29 PM, 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!

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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]

  On 2/5/2025 at 8:00 PM, 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.

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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]

  On 2/16/2025 at 3:15 AM, 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.

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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]

  On 2/16/2025 at 7:35 AM, 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.

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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]

  On 2/17/2025 at 1:55 AM, JayceeB said:

I was looking for a lightweight travel setup

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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.

[JayceeB]

  On 2/17/2025 at 9:27 PM, Adventurer said:

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

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@Adventurer , I took your advice and purchased an inexpensive diopter lens kit (Vivitar 43mm Close Up Macro Lens Kit - $13).  It came with +1, +2, +4 and +10 diopter lenses.

 

I tried the +1, +2 and +4 on the lens within the housing above water.  

The +1 was the only one that would focus to infinity above water.  

 

I naively thought this would be the one to try.  I tested it on a dive, and the experience was completely different than above water.  Minimum focus distance was 30” with the +1.  Much improved compared to no diopter, but still not acceptable.  

 

Next, I tried the +2.  Minimum focus distance was 18” underwater.  

 

This weekend I tried the +4.  The lens focuses right up to the dome port glass as well as to infinity.  

 

I’m in the early learning stages regarding lens optics, so forgive me for not considering the impact of water in the equation. 

 

I can’t believe how spot-on accurate you were in your recommendation, and thank you so much for helping me out on this.

 

My setup is a Canon R5 + RF 50mm f/1.8 + Vivitar 43mm +4 diopter in a Marelux housing with 140mm fisheye dome (no extension). 

I would highly recommend this setup for anyone interested in this focal length.

Edited March 10 by JayceeB

[Chris Ross]

  On 3/10/2025 at 3:25 AM, JayceeB said:

 

@Adventurer , I took your advice and purchased an inexpensive diopter lens kit (Vivitar 43mm Close Up Macro Lens Kit - $13).  It came with +1, +2, +4 and +10 diopter lenses.

 

 

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Did you have a look to see what the image quality was like?  Cheap diopters were renowned for making images a little mushy.  Better quality diopters like those from B&W have a better reputation.

[JayceeB]

  On 3/11/2025 at 5:25 AM, Chris Ross said:

Did you have a look to see what the image quality was like?  Cheap diopters were renowned for making images a little mushy.  Better quality diopters like those from B&W have a better reputation.

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Attached are two images.  Not class-leading sharpness, but not horrible.  Looks like B+W makes a 43mm +3.

null

 

[Adventurer]

  On 3/10/2025 at 3:25 AM, JayceeB said:

I can’t believe how spot-on accurate you were in your recommendation, and thank you so much for helping me out on this.

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Thank you so much for this feedback.

I truly appreciate it.

 

It is especially rewarding after having had quite a few exhausting discussions in an online forum, without anybody saying „thank you“.

I am very glad I could help.

[SFEgr]

  On 3/10/2025 at 3:25 AM, JayceeB said:

 

@Adventurer , I took your advice ... I can’t believe how spot-on accurate you were in your recommendation, and thank you so much for helping me out on this.

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Yes, amazing. I wonder if there is any link some kind soul might offer to 'a dummies guide tutorial in optics'?