Tips On Buying an External Flash
The first thing that you ought to know about external flash or speed lights is that they are much more powerful than your built-in pop-up flashes.
There are some upsides as well as some downsides to that. While undoubtedly more power means you can properly light up a scene way better than you could with your built-in flash, on the downside if you do not manage that power properly you are going to end up with harsh lights that will either wash out the scene or create strong shadows.
If you are in the business of portrait photography either of those will be unacceptable.
The following few tips will come handy when you are in the market for buying an external flash:
1. Guide Numbers
Simply put, guide numbers are an indication of the power output of your flash. They apply similarly to both built-in pop-flashes as well as to the external speedlights.
Guide numbers are calculated by multiplying the aperture value with the subject distance from the flash (not the subject distance from the camera). Please note the difference, because this is going to impact your exposure.
Based on the guide number you can come up with the right aperture value to use for a specific subject distance. Let’s say the guide number of your speedlight is 60. The subject distance is 20″. That means you need to use an aperture of f/3 to get a good exposure.
TTL signifies ‘through the lens’. It means the speedlight can speak to the camera and knows when to fire and how much power to use based on the brightness of the scene and the average reflectance of the subject.
For non-TTL speedlights, you cannot control the flash output using the camera’s metering system. However, you can manually control the output by tweaking the controls at the back of the flash.
If you are buying a speedlight ensure that it has TTL mentioned and is compatible with the specific camera model you have. Most OEM speedlights will have TTL functionality for their own DSLRs. But the same cannot be said about third party speedlights. Check the literature to confirm.
Sync speed is the maximum shutter speed that your DSLR can use while ensuring the flash is able to expose the picture properly. Let’s say the maximum sync speed of your DSLR is 1/160th of a second.
If you use a shutter speed of say 1/320th of a second, your pictures would be under exposed. This happens because the entire picture is not exposed at the same time.
At slower shutter speeds the camera has more time for the flash to fire and the sensor to pick up the light before the shutter mechanism closes allowing for a properly exposed picture.