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Some thoughts on TTL


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#1 james

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Posted 10 September 2003 - 05:36 AM

Hi Gang,

On the last 30 or 40 dives I've done, I've shot manual strobe power on all except 2 of them. Those two dives, I used TTL with the 105 to shoot fish portraits to greater than 1:1

I have gotten passably skilled at using my strobe power settings to get good macro shots and fish portraits. What I have decided is that I get a higher percentage of well exposed "macro" shots using the TTL meters in my S2 than if I set my strobe power manually.

I think the reason for this is that the camera can adjust strobe power a lot quicker than I can - especially for fish portraits. The difference between a fish being 18" away and 6" away is one full stop of light - or one power setting on my strobes. With swimming fish, or other inverts, I simply cannot adjust that fast. Heck, it just may be a commentary on my poor response times...but I doubt it...

I would say that I get >90% good exposures when using TTL, versus perhaps 50% to 75% when setting power manually. I also have never had TTL "ruin" a shot by overexposing it.

So I think TTL is a tool that helps me get better photos.

Cheers
James
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#2 chrisg

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Posted 10 September 2003 - 06:20 AM

One issue with ttl is that it actually provides the "wrong" exposure for digital cameras being operated in raw mode. Basically, if the exposure looks
"right" in the preview image on the lcd, its actually "wrong" in terms of preserving the highest possible quality shadow detail. The best possible image quality comes from over-exposing the picture as much as you can get away with without causing any "clipping" in the highlights, and then adjusting the exposure upon raw image import.

This kind of means that for the highest quality, you have to evaluate your exposures more from the histogram and absense of (on some cameras anyway) blinking over-exposed pixels. I don't think this is practical for wide-angle because you are attempting to balance 2 light sources (flash+ambient) in a pleasing fashion, which makes reviewing the preview image's exposure important, but in the case of macro where the only light source is the strobe, it seems quite practical.

#3 james

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Posted 10 September 2003 - 07:31 AM

Great post Chris!

Most people think that the best exposure is one that is slightly underexposed. Not so! That is the SAFEST exposure.

The best exposure is one that is slightly overexposed, BUT with the histogram JUST TOUCHING the right side. That means maximum saturation of colors in your shot.

Here's my procedure for tuning exposure in using TTL and Flash Compensation on the S2:

1) Set camera in Preview w/ Histrogram for the start of the dive. (This mode will show you the histogram, but requires you to hit "OK" after each shot to write it to media. It's a flaw in the S2 firmware IMO)

2) Take a shot at 12" or so and check the histogram. Shot is underexposed.

3) Dial in +1 stop of Flash Compensation and try again. Shot is good

4) Put the camera in Postview mode which automatically writes to the card. Otherwise if you hit the shutter button after taking a good shot, it is lost.

Notice that the above procedure does NOT use the LCD postview of the shot to gauge exposure. You can use the LCD as a general guide, but it MUST be calibrated in order to do this. Most LCD's brightness is designed for use in bright sunlight so they are too bright underwater.

Cheers
James
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#4 jimbo1946

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Posted 10 September 2003 - 08:16 AM

I second James' thoughts about TTL for macro and close-up. On my only trip so far with the D100, I used TTL entirely for the 105mm shots, and I was generally very pleased with the exposures. I used the LCD review with the histogram overlaid on the image, and this was very helpful in recognizing a good exposure.

I shot in RAW mode, and typically adjusted exposure upward by 1/2-stop or so.
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#5 craig

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Posted 10 September 2003 - 06:23 PM

I always judge exposure using the histogram. I'd like others to chime in on what exposure produces best color. From what little color perception I understand I believe that saturation vs. brightness varies depending on the color you're talking about, so the best exposure for saturation may not be the brightest. Still, exposing all the way to the right without touching maximizes SNR and is what I try to do. Make sure your color balance entering the lens is good or you'll only be exposing one color channel properly.
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#6 AndreSmith

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Posted 10 September 2003 - 08:05 PM

. Basically, if the exposure looks
"right" in the preview image on the lcd, its actually "wrong" in terms of preserving the highest possible quality shadow detail..

keeping this in mind would you recommend adjusting the LCD brightness setting? The Canon 10D has 5 levels of brightness - which one would you recommend for underwater photography?

Thanks Andre