TTL underexposure

[fred_fedak]

<p>When using the Gary Fong Lightsphere Collapsible with dome attachment, all is well as long as I am not firing it straight on towards my subject. If I do fire it straight on, the resulting image is underexposed by more than a stop, even though I am shooting TTL. Does anyone know why this is happening? It seems the camera, or flash, is being fooled somehow into thinking there is too much light, and is cutting back on the exposure.</p>

[lornesunley]

<p>You don't have the flash set to TTL BL do you?</p>

[b_christopher]

<p>I don't use a Lightsphere, but seeing photos of it, I wonder if direct flash is entering the lens?<br>

<img src="http://www.bchristopherphotography.com/temp/temp/pnet/pnetflash.jpg" alt="" /></p>

[douglas lee]

<p>+1 for B Christopher</p>

<p> </p>

[kevin_delson]

<p>Which flash are you using?</p>

<p>Several of the newer flashes (recognize) when the head is placed in a (bounce) position.<br>

The bounce configuration info is transmitted to the camera (with pre-flash data) with a corresponding increase in flash output to compensate...but only to a point. If you attempt to bounce off a ceiling that is 100 feet high, max flash output will result, (TTL) but with little effect.</p>

<p>The diffusing dome is another story.<br>

The camera recognizes the bounce configuration, but the light is not bouncing off anything; (no pre-flash registration) rather it is being sprayed in many directions.</p>

<p>You have (3) choices when using a diffusing dome.</p>

<p>1) Opening f/stop (Not the best choice) but will work..or,<br>

2) Ride the flash compensation in TTL. (Better Choice)<br>

3) Manual camera settings + Manual flash settings. (Best Choice)</p>

[dan_south]

<p>Are you using a lens hood?</p>

[fred_fedak]

<p>Thanks for the replies.<br>

Not using TTL-BL, just TTL<br /><br />Yes, the light could very well have entered the lens since I just realized that I neglected to attach the lens hood during that series of tests. However, that may not matter as it was with a Nikon 85mm 1.8, and the lens hood is not very deep.<br>

I am using an SB-900.<br>

Will test again and advise.<br>

Thanks again.</p>

[hans_janssen]

<p>What does your flash indicate under exposure after the flash on the flash display?</p>

[fred_fedak]

<p>I am not aware of any such indicator other than the ready light blinking, but that does not happen.</p>

[dan_ded]

<p>Long shot:<br>

Are you by any chance using a older, non-D-type lens? And is there a white wall or similar directly behind your subject? I find that my flash shuts off a little early in TTL and underexposes in such conditions. A lot of light is coming back and the camera aims for neutral gray, so I have to dial in a little + flash compensation. A D-type lens really shines in this particular situation and I usually don't need the compensation in that case. <br>

You may not get as much light coming directly back at you in the bounce situation, so maybe that's why that case looks better.</p>

[dan_ded]

<p>Actually, since you're using a diffuser, a D lens might not help. In that case, with a D lens, perhaps the camera fires what it thinks it needs for the distance, but the diffuser sends much of the light elsewhere. And perhaps in bounce mode it reverts to pure TTL, measures the light, and does all right.</p>

 

[hans_janssen]

<p>On the display of a SB600/800, and I assume on 7/900 too, is in the right top corner just after the exposure a triangle with a - or + and a number under it what gives the exposure, visiable.<br>

When you are shooting full power and there is still - there: wider aperture or higher iso.</p>

[fred_fedak]

<p>Lots of good guesses but it seems this may remain a mystery.<br>

I re-tested the scenario, both with and without the lens hood. This time there was no underexposure at all. (Very strange since the underexposure occurred multiple times originally) The only difference in the resulting images was that of quality of the light. The exposure levels were consistent, as expected. Hopefully, there will be no further re-occurrence.<br /> <br /> I was shooting with a Nikon 85mm f1.8D AF.</p>

[douglas lee]

<p>Next time try it with and without the diffuser attached and the flash head pointing forward. (Assuming the problem recurs.) Generally, diffusers are not intended to be used when the flash head is pointed forward. That is not a guess, that is a potential problem with shorter/wider angle lenses and on-camera flash. If I can find a reference, I will post it.</p>

<p>Glad you are no longer having the problem.</p>

[fred_fedak]

<p>Douglas, I was testing both the flash, and the Lightsphere together, in every conceivable combination. It wasn't until I got to this configuration that I had a problem.<br>

Thanks, but I couldn't disagree with you more about diffusers not being intended to be used when the flash head is pointed forward. What is the lens on the flash head, but a diffuser in itself, not to mention soft boxes.<br>

True, a typical diffuser that comes with the flash and therefore is about the same size as the flash head, really doesn't do much other than to cut down on the amount of light reaching your subject. But that is a whole 'nother thread.</p>

[douglas lee]

<p>Disagree all you want. I was just trying to help. Obviously, you don't need it. My apologies.</p>

[fred_fedak]

<p>First, I acknowledged and thanked you for your input, then I added my input/opinion. Sorry if I offended you, I didn't realize you were the final resource of information here. You must be a real prince to live with.</p>

[douglas lee]

<p>Fred, your reply is a bit much, but, perhaps mine was as well. No need for the personal attacks.</p>

<p>Yes, my statement was overly broad. However, I did say "generally" and specifically referred to on-camera flash diffusers.</p>

<p>Let me say it this way -- if you are using on-camera flash with a translucent diffuser that extends close to or past the front element of the camera lens, then there is a very high probability that the result will be an underexposed flash picture, if using TTL, iTTL, or what have you.</p>

<p>You apparently dismissed that "guess", but did not say why. Seems to me, after looking at the Fong thingy, that particular diffuser could extend up to/beyond some lenses, especially with the dome attachment.</p>

<p>I hope your problem doesn't recur, but if it does, keep this "guess" in mind.</p>

<p> </p>

[fred_fedak]

<p>Douglas<br>

Sorry for delay, was not ignoring you, I have been swamped lately.</p>

<p>I agree, there is no reason for personal attacks. Your response was, at least a little sarcastic and it arrived at a particularity bad time, and I being one who responds to situations in the same spirit as they are presented to me, did the same to you. I think we are even, and that's behind us now.</p>

<p>With that said, I am also one that demands accuracy, especially in regards to words or thoughts others accuse of me. With that objective in mind, and sticking with the facts only........: I feel that your point about the high probability of underexposure if using TTL is certainly a valid one, however, my subsequent tests had already proven that theory not to have been a factor in my case since I was not able to recreate the problem. It has gone away as mysteriously as it arrived, and I will be most pleased if that remains the case.</p>

<p>You stated, <strong>"Yes, my statement was overly broad. However, I did say "generally" and specifically referred to on-camera flash diffusers."</strong> I assume you are referring to my mention of soft boxes, and I also was referring to the on-camera type. They are definitely meant to be used with the flash head pointed forward.</p>

<p>In your later response to me your statement of <strong>"You apparently dismissed that "guess", but did not say why" </strong>is way off base, for two reasons. 1. In your response of July 19th, you specifically stated ".....<strong>That is not a guess.....</strong>" So, there was no "guess" on your part for me to dismiss, and 2. B. Christopher did, in fact make a guess, based on the same principle, even going so far as to include illustrations to better enable me to get a firm understanding of what he was trying to say. I did not dismiss his guess at all, rather I acknowledged that it could very well have been the reason for my problem since I did recall that I neglected to attach the lens hood when the problem first appeared. That guess was the catalyst that caused me to perform the tests a second time. I obviously took action on it, and did not dismiss it.</p>

<p>Then, at the very end, you did put forth a "guess" when you said: <strong>"Seems to me, after looking at the Fong thingy, that particular diffuser could extend up to/beyond some lenses, especially with the dome attachment." </strong>I completely concur that that does indeed seem to be a strong possibility given the proper variances, (length of lens barrel, lens hood attached, etc.) but so far, with my equipment, seems to be unlikely, since when perched high upon my SB-900, and using the proper lens hoods, it appears to me that the lens elements are well shielded against that possibility. But I will most definitely keep that thought in mind for the future.</p>

<p>Good shooting to you, and thanks to you and everyone else for the input.</p>