Below the waves with the Samyang / Rokinon / Bower 7.5 mm f 3.5 lens

[Klaus]

I want to share information and images on using a MF fisheye lens for the M43 system through this post. This concerns the Samyang / Rokinon / Bower 7.5 mm f 3.5 lens – it is sold under these three brand names (at least) but as far as I know all of them are identical; mine happens to be a Rokinon. It is very slightly shorter than the Lumix 8mm fisheye and can focus close (9 cm from the sensor plane). While that should make it suitable behind a small dome, I could not find any details on its use under water on the internet. When I had a chance to source a fish-eye dome (Athena ~ 10 cm) for a good price, I decided to simply give it a try. I am using it in combination with the Olympus E-PL5 – certainly not a new camera, but the raw images might live up to the ones from the more recent E-PL10; both have a 16 Mpix sensor, though probably not the identical one.

I did the „half-filled box“ already in a previous post on this forum when I found the Geomar-publication on this – you can look it up if you are interested; @DreiFish has meanwhile developed the approach to perfection, mine was very simple. The conclusion was that the dome should be a bit further out, but that is not desirable because of vignetting. I am now posting real-life pictures so that anyone interested can judge for him/herself. I have never used the Lumix or Olympus M43 fish-eye lenses, so I cannot say anything about relative strengths.

There will be two posts follwoing this one: One for the technical setup and one for the first UW images I took with the lens. Maybe this can be helpful for someone.

 

[Klaus]

1)      Setting up the lens

I could acquire the Athena fisheye dome port, desigend for the Lumix 8mm fisheye, in the classifieds locally. This port has a enough distance to the port hole on my housing (PT-EP05L) that it does not require any modification to the silly target light button. The other available fisheye ports for the Pen-housings (Zen and AOI) come a bit closer.

There’s nothing specific about attaching the Samyang/Rokinon lens to the body, I am describing what I did to use MF in the housing (PT-EP05L) via the zoom dial. I acquired a Olympus zoom gear (seoncd hand) for the 14-42 mm kit-lens. A friend helped to machine this out and widen it such that it can be slipped onto the lens from the mount side. A small step machined in the rubber prevents the aperture-ring from turning at the same time as focus, but when I pinch the rubber at the gear level, I can change the aperture. This piece is not easy to modify without a machine shop (and even with), 3D-printing this part is likely another option. The lens is also usable with fix-focus setting and no focus control at all (see below).

null

(left side: widened piece, right side normal Olympus zoom gear)

 

For setting the aperture, I painted a small white dot on side of the aperture ring, this can be seen from the mount together with the original red marking and the F-stop can thus be verified. My copy of the lens has its „sweet spot“ at f8 – f11, which is quite useful under water I think. The F-stop needs to be set before the lens is attached to the body and cannot be changed during the dive.

null

(towards the top – here, the white dot ist just a bit off to the right side from the red dot, this corresponds to the intermediate click between f8 and f11.)

 

For the focus, I start with a „fix-focus“ setting that will result in a depth of field from about 20 cm (from the dome) to infinity with the aperture setting shown. That corresponds to this distance setting on my copy of the lens:

null 

I placed some marks on the lens barrel to get this dialed in quickly.

With the camera set up like this, I place it in the housing. I painted a white mark on the zoom ring, which I turn straight up before placing the camera inside. That way I can quickly return to my „home“ setting during the dive.

nullnull

In addition, the marks placed on the lens barrel can also be seen through the port just in case. I covered the white labels on the front of the lens with electrical tape to prevent reflections.

null

 

So how does this all work now? First a dry image:

null

(uncropped frame)

There is a small amount of vignetting in my „home“ position. This lens is not perfectly centered; I am using an E-PL5 in the housing designed for the E-PL3 (very minor modifications necessary), but the centering is not better when I use an E-PL3 body (PM me if you want to see that). Once cropped, the image ends up at about 14 Mpix rather than 16 Mpix, the field of view is corresondingly reduced. By far the worst consequence, however, is that I cannot discuss corner sharpness in this forum ☹ !!!

One way of dealing with the vignetting is to pretend shooting in full frame i.e. setting the camera to 3:2 ratio. The vignetting is almost completely gone then (will only affect the jpg, raw stays full sensor). When the lens is focused even closer, the front element moves out and this also reduces vignetting in close focus wide angle images. All things considered, vignetting is not a big problem and the other Pen fisheye domes (Zen, AOI) should have less vignetting.

 

nullnull

(vignetting cropped in 4:3 ratio)

 

OK, so how much did it cost me? All prices second hand, including shipping.

Athena dome: 130 € (really good deal I think, looks like new)

Rokinon lens: 70 €

Zoom gear: 23 € + 2 beers for widening it up

In sum 225 € (not counting the housing and camera, these can be sourced for about the same price together)

As you can tell, I don't know how to insert the images at their right place, but I hope it's useful as is.

[Klaus]

1)      So how does it work under water with an MF lens?

I think reasonably well. The biggest limitation is probably that the aperture cannot be changed during the dive. I have not taken it out for a lot of dives yet and will start with a picture from a „natural pool“. No tiles for meauring distortion, but a straight out-of-camera jpg, uncropped, with the front oft hat step quite close to the dome. Here I also used a single YS-27DX strobe (with diffusor) for extra light.

 

null

 

The following shot was taken freedivig with the rig (no strobe). The rock wall was reasonably parallel to the sensor plane:

null

 

I have no comparison to the Lumix or Olympus 8mm fisheyes, but to me this seems to limited more by visibility than the lens not „getting along“ with the dome.

null

Getting closer, the rock wall is imaged with at least decent quality. Newer bodies may give better rendering, crystal-clear water probably would have helped as well. But the viz not certainly not bad here.

null

This is an example of a (moderately) close focus wide angle shot. It is possible to get sharp images from about 15-20 cm to infinity.

 

null

And here is a split shot – certainly the waterline is not flat, but in this particular shot one could consider it a feature rather than a bug. It is possible to get the waterline crisp in focus, but then the topside background (Flores Island, Azores) becomes slightly blurred (aperture set at the click between f8 and f11).

 

So, what about AF vs. MF?

It’s still early, but I think I can get by with only two focus setting – my „home“ one (20 cm – infinity) and a „close focus“ one with ¾ of a turn on the zoom dial (I placed another mark there). This will then have everything in focus from about 5 cm away from the dome to about 1,5 m. Switching is possible but not fast – cleary, AF is better. I might in the future get a lumix 8mm lens, but maybe more for setting the aperture than the focus. That said, here’s a picture where MF is clearly too slow:

null

(1x YS-27DX strobe)

Focus is perfect on the eyes … of the diver below the triggerfish (Mrs. K.). I am not sure the AF on my E-PL5 would have mastered this one either, though. The triggerfish was attacking the dome glass.

IQ is good enough to identify the dive computer model on my wife’s wrist. Remember those?

 

Here is the „making of“ that particular shot, just before the „Ouch!“:

null

 

The rig is quite compact, in particular in combination with the YS-27DX strobe. This, btw, is comparable in intensity and recycle time to the YS110a, but a bit more compact. The coverage of both (with difusers) is also comparable but that is based gut-feeling rather than testing.   

 

Finally, a shot from our local quarry (Echinger Weiher). The Samyang/Rokinon lens has received very favorable comments about its resistance to flares in top-side tests. I am not sure if this shot would be prone to flares with the Lumix fisheye, but it definitely was not a problem with my copy of the Rokinon lens.

null

(1x YS-27DX [left] and 1x YS-110a [right] for the foreground)

 

Conclusion:

Taking pictures with a fisheye is a lot of fun and not comparable in any was to what I have been using before (14-42 kit zoom with Inon UWL-H100 w/o dome, i.e. ~100° field of view). This was definitely worth the rather moderate investment. So far, the MF has not been a big issue – fish-eye lenses are very forgiving on that. That said, fast moving fish swimming up to the dome are not ideal. I may invest another ~300 € on a used Lumix 8mm fish-eye in the future, but dialing in a different aperture and getting the live view with the OPEN aperture are probably more important reasons for the upgrade than the AF.

Edited August 18 by Klaus
typos + duplicated image

[humu9679]

@Klaus Nice work! I had both the Olympus and Lumix fisheyes and actually preferred the Lumix because of its smaller size. Both worked well in Nauticam housings and 4.33” port. Olympus needed an extension if I recall correctly.