Variable INON s-ttl exposure

I am using a Canon A710IS, Ikelite housing and INON D-2000

I am finding that when set up for s-ttl (Av mode, Main selector in S-TTL (and S-TTL Low), EV switch=B) I get good exposures from f2.8 through to f5, but as soon as I hit f5.6 and lower the exposure seems to drop significantly with the histogram moving significantly to the left. Not a disaster, but not ideal and can be confusing and irritating.

Can anyone explain why this is happening and how I should fix it?

Thanks, Andrew

Not knowing anything about your subject distances...here is one thought.

Is it possible the low sTTL setting does not have enough light to get adequate exposure at the smaller Ap? If this were the case, moving to the higher sTTL setting might help.

I tend to use the low sTTL setting with subjects that are close to the camera/strobe and the 'regular' setting with subjects a bit further away.

Just a thought.

Does seem curious to have that much difference between f5 and f5.6 however. One would think the dropoff in light would be less than one stop which is not what your entry suggests.

Maybe posting an example of the issue at f5 and f5.6 would help the experts address your issue.

Not knowing anything about your subject distances...here is one thought.

Is it possible the low sTTL setting does not have enough light to get adequate exposure at the smaller Ap? If this were the case, moving to the higher sTTL setting might help.

I tend to use the low sTTL setting with subjects that are close to the camera/strobe and the 'regular' setting with subjects a bit further away.

Just a thought.

Does seem curious to have that much difference between f5 and f5.6 however. One would think the dropoff in light would be less than one stop which is not what your entry suggests.

Maybe posting an example of the issue at f5 and f5.6 would help the experts address your issue.

I've since deleted the set of test images unfortunately...

What I did was I set up an 'airside' experiment. So I was in a darkened room taking a photo of a phone handset in marginal light. I sat about 1m away so according to G=A/D with G=20 (air, 1m) anything from f20 down sould have been within the capabilities of the strobe, so f4, 5.6, 8, 11 etc should all have resulted in equivalent exposures. Once I noticed the difference I tried:

1. Changing from s-ttl low to s-ttl
   
2. (Back at s-ttl low) moving the EV switch from B to A
   
3. Moving the EV switch from A even more to the left
   

I was rather surprised that the latter two didn't lighten up the image.

Later on in the day I took the camera out of the housing and used it with its own on-board flash to do the same thing. I still noticed a (smaller) drop-off in exposure as we moved through f5.6 and beyond so I guess its the camera to a large degree.

There is supposed to be no significant difference between different cameras of the same model but its not always true.

Yrs ago, I had a Nikon 6006. Its supposed to NOT have mirror lock up. However, if you took a picture with the camera upside down, the mirror would not reset, thus allowing a method of obtaining mirror lock up. I tried other 6006's and they did not perform this way. I liked having it as I could take night photo's from a tripod with a locked up mirror... as long as the camera was upside down, which was not a problem with that tripod. It cost me one print on the film camera to make the lock up.

Maybe there is something different about your camera.

You might contact Canon and see what they have to say about it.

Im pretty sure the only dial settings that make a difference in S-TTL is 'A' 'B' or 'C'.

I did the same thing of thinking turning more would make a difference but the manual seems to say these are the only ones to try, ie they're slight adjustments rather than a range being available.

So try S-TTL and A, think that option hasnt been done yet?

Using the cameras internal flash compensation might be the go? Or just go manual on the strobes.

Actually might be worth trying manual on strobes and checking you can get over exposure at F8 or F5 on? Maybe the strobe is struggling for some reason. Strobe angle is the only other thing I can think of, but Im guessing you're pointing right at it.

Otara

Im pretty sure the only dial settings that make a difference in S-TTL is 'A' 'B' or 'C'.

 

I did the same thing of thinking turning more would make a difference but the manual seems to say these are the only ones to try, ie they're slight adjustments rather than a range being available.

Actually the manual I have (Basic Operation) in section "Adjusting strobe output in S-TTL auto" (pages 35-39) describes how adjustments can be made all along the yellow line for EV adjustment. So there's 3 brightening adjustments from 12 o'clock to 9 o'clock position, and 5 darkening adjustments from the 12 o'clock to the 5 o'clock position.

I'll have another go air-side on S-TTL (full) position but I'm thinking that the camera is behaving differently after about f5.6. It isn't a complete disaster as the images are just a bit on the dark side, but there's no or little clipping at the black end so it's all recoverable. I was just concerned that the strobe is being quenched too early and colour will be lost underwater.

Next calm day (haven't had too many of those lately) I'll get into the (12 degree, brrr) sea and see what the effect is with water in the way...

Actually the manual I have (Basic Operation) in section "Adjusting strobe output in S-TTL auto" (pages 35-39) describes how adjustments can be made all along the yellow line for EV adjustment. So there's 3 brightening adjustments from 12 o'clock to 9 o'clock position, and 5 darkening adjustments from the 12 o'clock to the 5 o'clock position.

 

I'll have another go air-side on S-TTL (full) position but I'm thinking that the camera is behaving differently after about f5.6. It isn't a complete disaster as the images are just a bit on the dark side, but there's no or little clipping at the black end so it's all recoverable. I was just concerned that the strobe is being quenched too early and colour will be lost underwater.

 

Next calm day (haven't had too many of those lately) I'll get into the (12 degree, brrr) sea and see what the effect is with water in the way...

Yes I was a wee bit over literal on reading instructions at another site it looks like.

It sounds like its just a question of going through all the combinations, including checking exposure with the camera by itself, and then deciding whether you can live with it if you cant fix it. Good luck with the dive!

Otara

Yes I was a wee bit over literal on reading instructions at another site it looks like.

 

It sounds like its just a question of going through all the combinations, including checking exposure with the camera by itself, and then deciding whether you can live with it if you cant fix it. Good luck with the dive!

 

Otara

BTW, the camera strobe +/- exposure compensation (Func Set button) when in Auto strobe when in Av/Tv/Auto/P modes can also control strobe output in sTTL above and beyond the power knob on the D/Z/S model strobes. This increases considerably your exposure adjustment range. You also still have the camera, overall, exposure compensation on the exposure compensation button plus or minus two full stops.

James