Thursday, 18 September 2014

Fundamentals of Interior Flash Photography Explained

When it comes to lighting interiors using flashes and strobes most beginner photographers struggle. Let us discuss some of the fundamental principles that one need to understand in order to produce absolutely stunning images.

Interior Flash Photography Tips
Interior Flash Photography Tips: Photo Jijo John

When we are exposing a picture using flash, the important thing to remember is that there are two exposures happening at the same time, one for the ambient light and one for the flash. A proper understanding of how flash, shutter speed, aperture and ISO work together to create an image is an absolute must to be successful in interior photography.

Understanding Sync Speed

Sync speed is the maximum shutter speed at which your camera will be able to synchronize with your flash. Sync speed is different for each cameras, for most cameras it is 1/200th of a sec or 1/250th.
DSLR cameras have a shutter comprising of two curtains (each with 4 metal blades). In normal position the curtains will be closed and when the shutter button is pressed first curtain opens up to start the exposure and then at the end of the exposure second curtain closes the exposure. This holds true for shutter speeds below the cameras sync speed.

How flash works at shutter speeds below sync speed is very simple, fist curtain opens fully, flash fires – light reaches the sensor, then second curtain moves in closing the exposure. Things gets complicated at speeds beyond the camera's sync speed.

Remember the speed at which shutter curtains travel is fixed, higher speeds are achieved by shortening the delay of deployment of the second curtain; so faster the shutter speed, the sooner the second curtain begins its travel (it does not wait for the first curtain to open fully). In such cases the sensor is never fully exposed all at once, instead light reaches the sensor through a slit (between the first and second curtains) formed by the travelling curtains, it’s more or less like painting the sensor with light; the faster the shutter speed, the smaller the slit.

As there is never a moment where the sensor is fully exposed, firing a flash during such an exposure will only light up a small portion of the image. So basically you are limited in your ability to use shutter speed to control your exposure. Need to stay within the cameras sync speed at all times when using flash. There are ways and means like High speed sync to overcome such issues, but as interior photographers you need not even bother as you will never have to resort to such techniques. Interior photography is mostly low light photography and we are talking about exposure times in seconds, not fractions of seconds most of the time. The discussion above about sync speed is only for educational purposes.

First Rule of Interior Flash Photography – Shutter Speed does not affect flash

The first and foremost rule to remember when trying to light interiors is that the shutter speed that you set on your camera (sync speed or lower) has literally no effect on your flash exposures; wondering why?

Well assuming you are staying within your cameras sync speed you will be shooting at speeds below 1/250th of a second. Now take a look at flash durations at different power settings.

Flash in Manual Mode

Power Duration

1/1 – Full Power       1/1050 sec
1/2 – Half Power 1/1100 sec
1/4 - Quarter Power  1/2700 sec
1/8th Power 1/5900 sec
1/16th Power 1/10900 sec
1/32th Power 1/17800 sec
1/64th Power 1/32300 sec
1/128th Power          1/41600 sec

It is obvious from the table given above that even at full power the flash duration is 4 times shorter than your sync speed. By the time the shutter opens and closes the flash would have already discharged all its light (within a fraction at the beginning of the exposure time). So no matter what your shutter speed is your flash exposure remains the same.

Second Rule of interior flash photography – Aperture affects both ambient and flash

Aperture determines the size of the hole through which light needs to pass in order to reach the sensor. No matter what speed the light is travelling a larger hole would let in more light and a smaller one will only let in less light. So if you open up your aperture you will effectively capture more light both flash and ambient and if you close down your aperture you will only capture less light both flash and ambient.

Third Rule of Interior Flash Photography – ISO affects both ambient and flash

The role of ISO is very simple increasing ISO values increases the sensitivity of your cameras sensor and enables it to properly expose the scene with less light. Decreasing ISO does the opposite. The tradeoff here is digital noise, most modern cameras do fairly well at high ISO settings this opens up a whole lot of opportunities for the interior photographer which we will discuss next.

Practical Tips

When it comes to lighting interiors it could be done in many ways, using hot lights, small flashes, big powerful strobes etc. using flashes is the most convenient method. However perfectly lighting an interior using only small flashes requires perfectly tweaking exposure variables for the finest result. 

If you are aiming to light your interiors using flashes, you should first understand the concepts covered in this article How to Calculate the Optimum Plane of Focus and Necessary Aperture for Interior Photography.  Consult a depth of field chart and you will be surprised at how much depth of field you will get even at wide apertures like f/4 when shooting with wide angle lenses. Using the tips you should first find the Largest Aperture with Enough Depth of Field to Cover the Entire Room that you are planning to shoot. Using the largest aperture allows you to get the most out of your flashes.

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