Slow Sync and Fill Flash - A Solution to a Vexing Problem

Ambient as Primary source, flash as fill

Hi folks. After we recorded episode 233 of the Make Better Photos and Videos podcast, Gordon asked if I could help with an issue that was vexing him. It is such a fascinating problem that we both felt it warranted an article.

Photographic Challenge

Gordon needed to make a photo that showed multiple possible selections for 50th Anniversary greeting cards. In Canada, the 50th Wedding Anniversary is referred to as the Golden Anniversary and so all the cards had some gold and often metallic surface treatment. They also all were on a sophisticated white card stock, in a variety of surface treatments.

Ambient as the Primary Light Source

Gordon knew that he wanted to use ambient light as his primary source of light. The light was coming in through a large window. The cards were set up on a table in the foreground with a white cloth on it, but to create a sense of home and space, Gordon wanted the room background to be visible, but darker than the cards.

In reviewing the image, we agreed that the foreground with the cards occupied about 65% of the overall scene with the remainder occupied by the room background which was darker, going to near black as distance increased.

Gordon was unhappy with the initial results. He found that the cards were not showing up as white, but more a dingy grey. Increasing the exposure using exposure compensation would get the cards to white, but then the background got too bright and became distracting. He was well aware that he could use masks and an adjustment layer in Photoshop to make the background darker, but wanted to get it right in camera. He also felt that the graphics on the cards themselves was lacking some pop.

Bringing in Fill Flash

So Gordon decided that to bring the pop back, and possibly make the cards their actual white without increasing the overall ambient exposure, he would use a fill flash. He has an excellent understanding of the inverse square law, and knew that the flash would fall off quickly.

To his chagrin however, the cards still appeared an unpleasant grey.

What to Do?

Gordon had already figured out the the amount of white in the scene was causing the built in light meter to propose an erroneous exposure because it was trying to make an exposure recommendation that would average to middle grey. He proved this assumption by using exposure compensation that rendered the cards white when he chose to overexpose. However, as the ambient was the primary source, it made the background brighter and that did not deliver the result he wanted. He also knew that TTL Flash is very accurate and very efficient. So why wasn’t it working.

effective but isolated. The only light source is flash

The Solution

The answer involved separating the exposure settings for the two light sources. Because he wanted the ambient to be dominant, and to retain the depth of field and lower ISO he wanted, this required a shutter open time far longer than what the flash would require on its own. The TTL flash would deliver the same exposure regardless of the shutter speed in use, so long as that shutter speed was equal to or longer than the flash synchronization shutter speed.

He wondered whether first or second curtain flash would make a difference, thinking that they might deliver different exposures. They do not. The only difference is when the flash fires, either at the beginning of the exposure or the end of the exposure. His curiosity on this subject is very common so I was glad that we got to clear that it made no difference to the flash exposure at all.

So why were the cards still greyish when the flash was added. It came down, as it always does, to the metering system in the camera. Many users fail to understand that their ambient light meter does the same through the lens metering as their through the lens flash metering. While they act independently, they use the same metering rules, so the flash metering was trying to do the same thing as the ambient meter. That is, turn the flash off such that the metered area would render to middle grey.

And it did. The system considered that 65% of the scene was bright and 35% of the scene was darker and produced a flash duration to achieve what would be middle grey. While our ambient light meter system will tell us this information, TTL flash metering does not tell us when it cuts the flash off. What Gordon needed, was exposure compensation for the overall flash response, specifically flash exposure compensation, independent of anything he was doing for ambient light.

Until you do this for a while, this is not immediately obvious, and even less so when you are mixing ambient and flash where the ambient is to be the overall exposure value defining case. Certainly you could make this kind of shot in all Manual mode for the ambient and with manual flash using an incident light meter for the ambient and a separate flash meter for the flash exposure, again in incident mode, but while this is optimal, it’s also a lot more work and requires additional gear. Fortunately you can achieve the same outcome using your automatic functions with only a few test exposures.

How To Do This

Do as Gordon did. He made the decision that the ambient light would be his primary source, and found exposure settings that were as good across the board as he could, and realizing that because of how reflected light meters work, his all white cards, would not be rendered white at all without compensation to add exposure. He did not want to do this because of the impact on the background which would make it more prominent than he desired. Now lock that exposure down.

Next introduce your TTL flash and take it off camera so you can direct where its light is going to fall. You may use a light shaper of some kind to change its quality, change its position to control its direction and perhaps add a gel to have the flash have a similar colour temperature to the main light. In Gordon’s scenario, the main source was daylight and so is the flash, so no colour correction was needed on the flash.

Now make an exposure using both ambient and flash and then adjust only the Flash Exposure Compensation to increase or decrease the flash duration as necessary to achieve the goal. In looking at the test images, I would have said adding two stops of flash compensation or +2 on the dial, would make the cards white instead of grey while not impacting the background. In his setup, it would also cause the red roses a foot behind the cards to pop more. The +2 is a solution for this scenario and should not be assumed to the one fix for all images to use.

Wrap Up

Mixing flash and ambient is a step of distinction between hope and prayer and taking control of the image making process. You may not need to do so all the time, but when you do, the difference in image success is apparent. I use fill flash as often as possible because of what it does to create definition and make colours pop, and in situations as I want it, does not look like I hit the subject with a flash. You can do the same thing, just remember to treat your two light sources AS two light sources that are coming together.

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