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Overexposure iTTL issues


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#1 ATJ

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Posted 25 July 2007 - 10:33 PM

I have been having intermittent issues with overexposure using iTTL with my D70 in an Ikelite 6801 with one or two DS125 substrobes. The housing and strobes are new enough that they have the latest iTTL circuitry. Sometimes the photographs are well exposed and other times they are grossly overexposed. The overexposed images happen more frequently when I'm using a the Micro 60mm lens compared to zoom lenses. In fact, I'm not sure I ever had the problem with the 18-70mm kit lens, but it does occasionally happen with the 18-55mm II.

With the 18-55mm II, the problem comes and goes. With the Micro 60mm, I usually have to switch the master strobe off and on to "clear" the problem.

I don't have an iTTL flash for land use, so I can't determine if the problem happens without the housing or not.

Has anyone else had this problem?

#2 Islandbound

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Posted 25 July 2007 - 11:43 PM

Yes, I have had the same issue although I have never tried to "clear" the flash like you did. I end up usually just switching to Manual, turn the strobes up and then dial in a high fstop while keeping the shutter at 1/125. If its too dark, I dial down the fstop, too light then I turn the fstop to a higher number. I know its not scientific but I dont have the experience to do anything else.

Does your Ikelite housing have the knob on the back to turn the TTL up or down? (and the manual too). Do you know how to use it?

#3 ATJ

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Posted 26 July 2007 - 12:21 AM

There's no knob as such, but buttons that let me increase of decrease EV compensation for the strobe. I sometimes use that, although what I often find is that with 0 EV I get over exposure but with -0.3 EV I get underexposure. Last Sunday, while photographing a sea star I got massive overexposure at 0 EV, and still had overexposure at -0.7 EV. I then switched to -1.0 EV and was underexposed, -0.7 EV under, -0.3 EV under and 0.0 EV correct. For the next 30 or so shots, it worked perfectly.

#4 PRC

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Posted 26 July 2007 - 02:29 AM

Sounds to me like an intermittent in the chord.

Honestly, if you take the time to try fully manual then then you may never go back.

Though there is an issue in that the camera and strobes in my Ike setup - D200 - camera gets get very confused if I try to use manual on the strobes with all of the pins connected. I had similar behaviour with my D70 also.

My fix was to interpose a small piece of Mylar in the hotshoe to disrupt connection to all but the 'fire' pin.

Alternatively you can use the Manual mode on the housing but this does not allow you any other option than balanced flash when using two strobes.

The upside of fully manual is that you now only reliant on a single wire in the chord not for or five to be fully functional - my next move may well be to mod my chords so that all of the wires parallel up on the fire pin - fault tolerant / redundant connection !

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#5 ATJ

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Posted 26 July 2007 - 04:57 AM

I don't believe it is the cord. I have both single and double cords and the problem happens with both.

I was also able to reproduce the problem at home putting the camera in a copy stand, connecting through the housing with an SC-17 and mounting one strobe. This allowed me to change the lens without touching or moving anything else. I could switch between the kit 18-70mm which resulted in a well exposed image and the Micro 60mm which overexposed.

18-70mm
Posted Image

60mm
Posted Image

Now the interesting thing was that when I raised the camera away from the towel, I got the same exposure with both, which suggests camera or strobe distance to subject is playing into it.


With respect to fully manual, I used a Nikonos III for 20 plus years and then a CoolPix 4500 for 3 years, both fully manual. While I was getting pretty good at dialing the right EV in, I did get sick of the trial and error.

#6 rtrski

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Posted 26 July 2007 - 07:23 AM

You said it was happening in macro mostly, and your above example indicated that as you increased distance it got better.

I read something somewhere about TTL having issues with short subject-to-strobe distances because the flash can't be quenched fast enough to prevent overexposure. Is that basically what you're seeing sometimes?

Can you try moving your flashes 'back' with respect to the lens when shooting close?

Forgive the perhaps off-target reply - I'm just learning myself so really have no place posting. In fact I'll shut up now. :)

Current rig: Sony SLT-alpha55 in Ikelite housing, Sigma 105mm f2.8 DC Macro w/ Ike 5505.58 flat port or Sigma 8-16mm f/4.5-5.6 DC HSM behind UWCamStuff custom 5" mini-dome. Dual INON z240 Type IVs triggered with DS51 for TTL mimicry, or DS51 alone with home-made ringflash assy for macro.

 

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#7 acroporas

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Posted 26 July 2007 - 08:36 AM

My old Iklite housing for Canon Digital Rebel had similar problems. I eventually determined that the problem stemmed from moisture causing a short in the rusty parts of the housing's TTL circuitry. In my case the initial damage was caused by a minor flood (so minor that neither the camera or lens were destroyed). After the flood, TTL would work great the first few dives of each trip, but the longer the trip, the more often it would get stuck of full power. If I cleaned the circuitry with a tooth brush it would last longer, but still eventually quit working.


To pinpont the problem, try to duplicate the exposure with the housing in manual mode and determine if the strobes are giving a full-dump on the over exposed images, or if they are in fact quenching but just not soon enough. A full dump indicates something is probably broken, while quenching too late indicates that the circuitry is probably fine and this behavior is just to be expected and you should be able to fix the problem by dialing in some flash exposure compensation.

A full power flash often overexposes macro, but rarely overexposes WA, so the problem can seem like it is lens related when it is actually just a problem with the circuitry.
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#8 dhaas

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Posted 26 July 2007 - 10:10 AM

Amigos,

Try this explanation on for size:

When shooting macro most (if not all) shooters stop down to F11, 16, 22 or whatever to maximize the limited depth of field at high magnification ratios.

At a camera's lowest ISO (ISO 200 for the Nikon D70, right?) even at f16 or f22 your DS125 strobes have a MINIMUM distance they can possibly quench to less output than a full power dump on TTL. I have customers who keep pushing their strobe(s) in closer when shooting macro, and this almost always overexposes the shot.

TTL is usually only discussed about having a MAXIMUM working distance based on ISO, chosen f-stop and of course working distance. Any strobe cannot put out MORE than a full power dump. But TTL also has a MINIMUM flash to subject distance, based again on ISO, F-stop and strobe to subject distance.

For macro shooting, I tell people that as you get closer, pull the strobe head(s) BACK a bit to at least 1.5 feet so TTL can quench in time to avoid overexposure.....

YMMV :)

dhaas


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#9 Canuck

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Posted 26 July 2007 - 11:27 AM

For macro shooting, I tell people that as you get closer, pull the strobe head(s) BACK a bit to at least 1.5 feet so TTL can quench in time to avoid overexposure.....


Or, aim the strobes sideways somewhat and use the periphery of the strobe output.

Or, use diffusers on the strobe.

BTW Dave, awesome octo shot :)

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#10 ATJ

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Posted 26 July 2007 - 03:06 PM

Thanks for the replies.

I had thought it might be related to the strobe to subject distance and inability to quench, too, however, it is far from consistent. I can take seemly identical shots with the strobes at the same distance with some normally exposed and some overexposed.

Here is a sequence of shots from the weekend (taken with the 18-55mm II). They have been batch processed and only resized. The only thing that changed between them was the EV compensation in the housing (for the strobe).

0 EV
Posted Image

0 EV
Posted Image

-1.0 EV
Posted Image

-0.7 EV
Posted Image

-1.0 EV
Posted Image

-1.0 EV
Posted Image

-0.7 EV
Posted Image

-0.3 EV
Posted Image

0 EV
Posted Image

0 EV
Posted Image

Plus one more at 0 EV that was well exposed.


As I said, it happens most often when using my Micro 60mm, but I can get through a whole dive with that lens and have few or no overexposed images. Another dive may give me a much higher number.

If I look at my last three dives with the Micro 60mm:

05 May: 129 shots, only 3 overexposed
30 May: 35 shots, zero overexposed
31 May: 93 shots, 7 shots overexposed

The dive on the weekend with 18-55mm II, I took 139 shots and 12 shots were overexposed.

Which isn't too bad, now that I count them... perhaps my expectations are too high.

#11 ATJ

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Posted 26 July 2007 - 03:52 PM

OK... after seeing such a low overexposure count on recent dives, I went back though my archives. The situation used to be a lot worse. Some dives nearly 50% were overexposed. One dive, I actually left the housing on -1.3 EV for the whole dive and even some of those images were a little overexposed.

What's changed recently? In April I got a new DS125. Previously, I had been using a combination of a DS50 and a DS125 with the DS50 as the master. (I had to use the DS50 as the master, but that is a whole other story in itself.) It would appear that using 2 x DS125 has partially solved the problem, although not completely, it would seem.

#12 Peter Schulz

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Posted 27 July 2007 - 05:15 AM

What metering mode are you using? The reason I ask is that in tests I ran, I found that I got more repeatable results with center-weighted metering than with matrix metering.
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#13 Viz'art

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Posted 27 July 2007 - 05:54 AM

ATJ, I would also take a look at the lens diaphragm actuating lever, its the small flat lever at the rear of your lenses, wiggle it up and down, the aperture should snap back into place when you let it go,

Sometime oil on the diaphragm blades or a bent actuating lever might cause overexposure so maybe check and compare your 60mm with some other lens, and BTW as the lens gets cooler (as underwater environment), the oil residue gets stickier and the problem becomes more acute, when warm the oil very often, will not be sticky enough to affect results that much.

Just a hunch :glare: been there done that type of thing.
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#14 dmoss

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Posted 27 July 2007 - 07:32 AM

Speaking of been there done that fixes...

You've got the sync cords with the blue band, right ?
Cords attached directly to the strobe (no manual EV controller in between), right ?

Sorry if this sounds too obvious.

Edited by dmoss, 27 July 2007 - 08:17 AM.

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#15 scorpio_fish

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Posted 27 July 2007 - 12:06 PM

What metering mode are you using? The reason I ask is that in tests I ran, I found that I got more repeatable results with center-weighted metering than with matrix metering.


The camera's ambient light metering system is completely separate from flash metering. Changing your metering mode will only effect the exposure if the scene has mixed lighting, i.e. part of the scene is ambient and part is exposed by flash). A macro scene fully lit by strobes will not be altered by the ambient metering.

It is my observation that the Ikelite TTL system tends to overexpose when little strobe power is required (e.g. short strobe to subject distances or lots of white sand).

As to the concept of minimum strobe to subject distance, we don't really know. I'm sure Ikelite has an exact number as to the minimum flash duration. The way to determine if minimum flash duration is the problem is simply take a shot that is overexposed and then switch the strobes to the lowest possible manual setting and take the same exact shot. TTL flash duration should be able to be as short as the lowest manual setting. I would suspect it could even be shorter.

One real problem that you have is that you were using the DS50 as the master and the DS125 as the slave. This is likely the primary cause of the overexposure. The more powerful strobe should be the master. This may be why the two DS125 setup is working better.

Even so, I've found that Ikelite's iTTL works pretty well, but isn't perfect or on par with the old TTL film days, but I also think that it is unreasonable to think that it could be.
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#16 ATJ

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Posted 27 July 2007 - 03:04 PM

Thanks again for the replies.

Peter,

Matrix metering.

Jean,

Diaphragm works very well and does not stick at all. I have never had any problems with shooting manual above water.

David,

They are new (bought last year) digital TTL synch cords with the blue stripe (4103.51 and 4103.52). The cords are connected directly from the housing to the strobe(s), with no EV controller.

scorpio_fish,

I very much agree that the DS125 should have been the master but unfortunately in that configuration things did not work properly. For example, with the DS125 as the master and the DS50 as the secondary, if I turned the DS50 off, the DS125 would not fire at all. With the DS50 as the master, it would work with the DS125 on or off. I tried all sorts of combinations and even emailed Ikelite but they didn't seem interested - my second email to them went unanswered.


I guess it is the inconsistency that bugs me. If it always overexposed, I could use EV compensation. It will just have to be something I live with.