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scorpio_fish

How Nikon TTL works

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I recently participated in a photo workshop in Death Valley conducted by Nikon guru Thom Hogan. We covered several areas in an outdoor classroom setting. My focus for the workshop was not on the technical side, but on the artistic side. I am no longer interested in techie information unless it applies to my ability to capture a scene the way I want to. Whenever I try to retain technical knowledge nowadays, my brain replies with a "hard drive full, unable to save". I probably need to run some type defragmentation software.

 

I did save some technie questions for the breakfast table and car rides. One thing that has been bugging me is metering and TTL flash. I now have the answer. The explanation is Nikon specific, but I have a suspicion it applies to Canon as well.

 

The camera allows the user to set metering to either matrix or spot. Center weighted metering is not recommended with the D100 (or S2) as the perimeter metering may be outside the digital cropped area. I generally use matrix metering because it works and works quite well. I will use spot metering in high range scenes to check bright and dark areas or if I want to peg a point of middle or bright light range.

 

I kept wondering what effect metering mode had on TTL strobe exposure. The question was how does the strobe meter the scene to determine when to shut down? The answer was that the TTL sensor/strobe uses a 5 point metering system to determine the strobe exposure regardless of what metering is set on the camera. Whatever way you meter with the camera is an ambient light reading. The flash is using its own algorithm to determine the flash exposure.

 

What's that all mean? First, there are two types of flash situations: 1) Flash is the primary source of light 2) Flash is used for fill.

 

For macro photography, flash is the primary source of light. Regardless of the way the camera is set for metering, the resulting flash is going to use its 5 point metering system to determine flash exposure. The results will generally be quite accurate unless you've got relatively large variances in strobe to metering point distances.

 

Wide angle would be considered a fill flash situation since part of the scene will be beyond the strobes reach and be subject only to ambient exposure. Your camera will meter the ambient exposure (in M mode or A mode w/ camera set to slow sync). The flash will be used to light up the foreground. In TTL, the flash is still looking at all 5 points (even if your camera is in spot meter and staring at a foreground object. Hence, the likely poor exposure using TTL for a wide angle shot. Sure, we all knew this trueism about TTL and wide angle, but I was still confused about the camera spot meter vs. how the camera tells the strobe it has had enough.

 

Some other tidbits from the flash class. Nikon flash, even film flashes, send out an inperceptable pre-flash which is used in combination with the sensor that looks at the film plane.

 

Nikons 3D Matrix Balanced super duper fill flash mode does all the thinking for you. The problem is you don't how it thinks or what it thought. You don't know what it did. If you don't like what it did, adjusting the flash or exposure may not help, because it will think you are stupid and compensate for you and you will again never know what it did. So, when used as fill flash, always set your flash to plain old TTL or DTTL.

 

We discussed the law of inverse squares, something I was well versed in, but for which I didn't fully comprehend the impact on lighting. We spent some time with strobe aiming and measurement to realize the importance. If you set up your camera aiming directly at a wall with your flash on top of the camera, the distance from the flash to the edge of your scene vs. the center of your scene may not seem great, but can make a big difference in how much flash reaches each area. Food for thought when aiming your strobes even at what may be appear to be a flat surface or at even a slight angle.

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Cool!

 

Thanks Scorpio. I have definitely noticed the "light falloff" when shooting large scenes. It's because the strobe to edge distance is greater than the strobe to center distance then....not because the angle of coverage on my strobe is too small.

 

Cheers

James

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