How Shutter Speed affects studio flash
What are your settings?
That's the question that disheartens so many photography instructors. The settings are particular to the situation. I know that many photo magazines list settings along with the photos. But until the digital age with that information in the EXIF data, I'm willing to guess that 50% or more of the "settings" listed with a photo were a guess. I was there. I never took detailed notes on my photos while making them. If someone asked me about the settings used on an image from a week/month/year ago I would have had to guess. I think most others would have too.
But enough of that rant. The question is still going to be asked. So, lets talk about one of those settings. Shutter Speed when using flash in a studio situation. When doing a workshop I often get to hand over the flash trigger to a student and give them some settings to get started. The important ones are ISO (how sensitive the film/sensor is) and Aperture (how much light the lens lets through).
Then there is shutter speed.
A difficult concept for some is that within a range, the shutter speed doesn't much matter in the studio when using flash. You have the ISO, the Aperture, and the power of the flash. There's your Exposure Triangle for the studio. You can usually set your shutter speed somewhere between 1/30 and 1/125 of a second and be good to go.
But what about blur? Won't my image be blurry from camera shake or subject motion if I shoot at 1/30 of a second? In most cases the answer is No. A studio setting is usually dark enough that even at 1/30 nothing will be recorded. The duration of the flash becomes your "shutter speed." While the shutter is physically open for 1/30 of a second, the flash duration may be 1/500 of a second or faster (depending on the particular flash you are using). Yes, in a bright studio or if you are working at a high ISO and at a wide open aperture you might experience some motion blur. You can do a quick test to see what's up. Set the ISO and Aperture for the flash exposure and take a shot WITHOUT firing the flash. In most cases the image will be black. That's good. That's what we expect. If there is something recorded, you can go to 1/60 or 1/125 of a second.
But my camera manual says it syncs at 1/250 of a second. Well, yes. It will do that with a dedicated speedlight/lite. The flash duration on those little things can be really fast and work at 1/250. But when you go to studio strobes you have to give a bit on the shutter speed. For example, the Canon 5D MkIII (used for these tests) specs say that its X-Sync shutter speed is 1/200 of a second. But on page 104 of the manual you will find some additional information:
That's good advice. Test for yourself. That's what I did this afternoon and here are the visual results. Next to each image is the Shutter Speed (SS) used (80=1/80 sec.) and the info reading for the spot on the shoulder identified in the first photo. You will see that at 1/80 to 1/125 sec. the image is pretty clean/similar. But at 1/160 and 1/200 sec. the bottom of the image starts getting darker. At 1/250 sec. there is a clear black band across the bottom of the image.
It might be easier to see the comparison side-by-side. Here are the images that were taken at 1/125, 1/160, 1/200, and 1/250 sec. to more easily see the shadow of the shutter curtain starting to impinge on the bottom of the photo. The image at 1/200 sec. is also starting to be darker overall.
In many situations you might not notice the darkness on the bottom of the image (or on the side if holding the camera in vertical orientation) at these faster shutter speeds. But it is there. And it may be troublesome if you have taken the time and energy to create an even background light on the scene and then find one side of the image is darker when you get the file into your computer for editing.
If you are using a wireless flash triggering system with your Canon or other compact flash gun you may also be limited to slower shutter speeds. I don't have such a system to test with. If you do, please let us know your test results in the comments section below.
What happens if you go slower than 1/30 sec? A few things might happen. If there is enough ambient light in the studio you might pick up some color variation and a bit more exposure as the ambient light will then be adding to the overall exposure. Here you can see images taken at 1/60 sec. and 1/8 sec. at the same ISO, Aperture, and flash power level. The image at 1/8 is slightly brighter and a little more yellow from the tungsten modeling lamp in the flash.
So, when in a controlled studio setting using studio strobe units you should heed the warning in the camera manual and work at around 1/60 sec. to sync the camera and strobes.
When outside doing syncro-sun flash the shutter speed becomes more important. But that's for another article.