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albert kok

Member Since 02 Sep 2006
Offline Last Active Apr 09 2014 12:53 AM

Posts I've Made

In Topic: Fish eyes compared: Tokina versus Panasonic (test 2)

07 February 2014 - 01:18 PM

The second picture below  is taken in the  same conditions, but now with Tok set at 10mm, F11, 1/50.  Shark at  1 meter and divers below at around 5 meters. Next month I hope to show you some pictures taken with Oly/Pana and the 4 inch dome taken in similar conditions. 

Attached File  carr sharks.jpg   151.04KB   8 downloads 

In Topic: "Souvenirs "from Lembeh OMD -EM5

07 February 2014 - 12:08 AM

Very nice shots. I liked the third picture most: the shrimp with the purple scissors.  'Mes felicitations'!

In Topic: Mounting an Oly housing on an Ikelite tray

02 February 2014 - 08:40 AM

Dear Bill

You certainly added some useful gadgets! thanks for the O ring advice, Ill  give the 73X3mm ring a try first. Yeah, I think a removable second lever would be nice, but probably costly. Maybe something for a future Olympus housing? The vacuum leak is also an useful addition. I have one on my Ikelite which was easy to install, because you can screw it an unused function button. I noticed that your sentry needs a  separate drill in the back of the housing.  I guess you have described the costs on your website?

I tried the 73X3mm O-ring today. It definitely makes a difference. Its easier to mount the 4 inch Precision dome, and remove it later from the Oly housing than with the 74X3 mm O-ring. Highly recommended! 

In Topic: Blue Shark, Azores Islands.

21 January 2014 - 11:49 PM

Very nice shots!

In Topic: Fish eyes compared: Tokina versus Panasonic (test 2)

18 January 2014 - 01:22 PM


Perhaps a somewhat late reaction to your comment on the smaller  sensor size enhancing the sharpness of the Pana shots. I think that overall, the topside results showed only  small differences in sharpness of foreground and backgound objects between the Pana and Tok pictures, at identical apertures. I assumed this was because  the crop factor (1.3) multiplied with the smaller focal length of the Pana lens created almost equivalent focal lengths, and consequenty identical field sizes (or: angles of view) of the Pana and Tok images.  Hope I was right ..  



A note of depth of field and sensor size.

its interesting to calculate the dept of field of the two camera/lens combos with the DOF calculator I found on Internet


Focal length 10 mm for the Nikon lense and 8 mm for the Oly lense, same aperture F8, and distance to subject 15 cm.  About the same values I used for my  topside pictures shown earlier. Total DOF was 7.07 cm for the Nikon/Tok and 8.6 cm for the Oly/Pana combo. 

When you insert in the calculator  arbitrary identical focal lengths (12 mm), and apertures (F8) at 20 cm distance, total DOF becomes larger for  Nikon (8.74 cm) than for Oly (6.42). This means that with identical apertures and focal lengths the bigger sensor of  Nikon yields a larger not smaller DOF than the smaller sensor of Oly.  It also suggests that  the greater DOF of the Oly/Pana combo (with real focal lenghts) must have resulted from the smaller focal length (= the extreme wide angle) of the Pana lense. But it must be kept in mind that these DOF differences can be easily changed by selecting slightly different  subject distances: for instance selecting 13 cm instead of 15 cm brings the DOF of Oly down from 8.6 cm to 6.28 cm. The same holds for aperture: a larger aperture for Oly/Pan of 5.6  will  also bring down the DOF considerably from 8,6 tot 5.86 cm.

Cheers A



Back to reality

A somewhat limited DOF of  of Nikon/Tok  for small  and  very close subjects should not present a big problem for UW photographers working in  UW conditions with blue water and larger objects a bit further away. Here I have an example of Nikon D7000 and  Tok set at 12mm, with  the 5 inch dome lense  and  a small aperture (f11). In this uncropped picture the shark on the foreground (at around 1 meter)  is crisp,  although it is moving a bit too fast for my 1/50 shutter speed. The limited visibility and  the slight blur of  the sharks on the background  (at around  2-3 meters) only increase the suggestion of depth. According to the DOF calculator (set at  12 mm focal distance, 1 m subject distance and f11 aperture)  total DOF would be infinite.