Digital Voodoo

Even though my S2 has yet to see the water after reading several threads on this forum and elsewhere I decided to do a little testing of my own. It seems that a fairly substantial number of people have been complaining about TTL inconsistency with the S2 and I wanted to see if my brand new S2 exibited any of these symptoms.

What I did was set up a target area about six ft from the camera that was fairly average in reflectance, set my camera on manual, attached an SB-25 strobe set to TTL, and used a Gossen Multi Pro meter to measure the flash output as I varied the f-stops from f2.8 to f22. What I discover was that the results varied greatly depending on the type of lens used. I tried 2 different Nikon CPU lens: the AF Micro Nikkor 60mm 2.8 and the AF Micro Nikkor 105mm 2.8D and got very similar result. Here are the test results with the 105.

Camera f-stop ---- Flash meter reading

F2.8 1/60 --- 4.0.4

F4 ------- 2.8.9 - almost dead-on

F5.6 ------- 4.0.1

F8 ------- 4.0.3

F11 ------- 4.0.3

F16 ------- 4.0.3

F22 ------- 4.0.7

As you can see the flash output only increased by 6 tenths of a stop from F5.6 to F22. The curious thing though was that the digital image at f22 did not seem to be 5 stops underexposed. Each image from f5.6 to f22 was progressively darker but definity not what you would get with film that was 5 stops under.

Here's what happened when I tried an older Nikon 24mm manual focus AI lens:

F2.8 ------- U

F4 ------- U

F5.6 ------- 4.0.4

F8 ------- 5.6.1

F11 ------- 8.0

F16 ------- 11.0

F22 ------- 11.8

Although I was consistently under about 1stop the TTL acted in much the way I would expect it to. All of the exposures were pretty much the same as I verified with the histogram.

I'm not sure what to make of all this but one thing is for sure, with a CPU lens, TTL does not function in any way that I am used to. Does the digital chip on the S2 compensate in some way? I would think that a shot that was 5 stops underexposed would be almost totally dark.

This is a little disturbing to me. What do you all think?

BTW,my Multi Pro is accurate and according to the S2 owners manual the SB-25 should be fully compatable.

Have a look at your histograms for the shots. Post a few if you can, I'd like to see the f8 and f16 histograms.

When I shoot w/ the 60 or the 105, I REGULARLY dial in +.5 to +1 stops of flash compensation w/ my S2 and I recommend it to others.

Cheers

James

I did a "minimal" test last night, compared to what Lionfish43 did. I too have a brand new S2 that has not yet made it underwater. I used a Nikon 60mm macro lens and picked a fairly "average" subject. I did four shots at fairly different F-stops, but all within the range of the camera's built-in strobe. Going from F27 to F16 to F11 to F5.6 the image got progressively and noticeably brighter. Not extremely, but noticeably.

I then did the "standard" test that I used for my prior underwater cameras to test the TTL - did a shot wide open in a bright environment and got short flash burst, then did a shot closed down in a dark environment and got the signal that the strobe did a full dump and it wasn't sufficient. So based on the "extremes" the camera is performing some sort of TTL.

In both cases the shutter speed was 1/125th.

I didn't look at the histograms ... will do so tonight. But it certainly appears that the TTL is not working in the manner I would expect.

What metering mode was used for the test shot? Was the entire frame (exposed scene) with the flash? What settings were used on the SB-25?

Lionfish, I assume you used a constant shutter speed?

This does sound very curious to me. At the start of the year I did TTL tests with my D100 and SB80DX and found it very reliable over a range of f-stops. Something that has proved to be the case in the field/ocean throughout the year. I have never felt the need to dial in any exposure compensation with the D100 + SB80DX combination.

Alex

below are the pictorial results of my original dTTL tests (using a sigma 28-70mm):

Alex's test vis-a-vie the S2 is apples to oranges. Different cameras, different metering systems. No offense, Alex.

Mystery solved! I repeated my test with the 105 today only this time I set the metering to spot instead of center-weighted. The results were much different this time although the S2's TTL metering is not as accurate as my 8008s.

Camera set to manual, spot metering, 1/60, SB-25 set to TTL-normal.

Camera F# Flash meter reading

f4.....................f2.8.8

f5.6..................f4.0.4

f8.....................f4.0.9

f11...................f5.6.6

f16...................f8.0.4

f22...................f11.0.5

The S2 consistently underexposes to a greater degree as the F# increases peaking at f16 which is about 1.6 stops under. This would jive with James' observation that he uses +.5 or +1 compensation at higher F#s.

I think what was happening was that my flash meter was reading a pre-flash when the camera was set to center-weighted metering although I did not realize it would do that.

Here is my test shot at f5.6

Spongebob is happy. To my eye this is the best exposure at .6 under

F8 is just slightly darker although its hard to pick up on a small image.

I havn't figured out how to post the histograms yet

Many digital camera TTL systems are "calibrated" to slightly underexpose the shot. It is much easier to bring out the shadow detail in post processing than it is to try to recover data from blown out pixels.

With that said, the proof is in the pudding - for me, the TTL system works flawlessly underwater.

Cheers

James