EXIF: 50mm | f5.6 | 1/15 s | ISO 100
When considering the relationship between aperture, shutter and ISO, the goal is always proper exposure based on your camera’s metering system. The camera’s built-in light meter determined how much light is necessary to achieve proper exposure and makes the necessary adjustments, based on whatever priority mode you are shooting in.
If you look really close you can see that this photo appears just slightly out of focus. It’s not, actually. What you’re seeing is blur from trying to hand hold at 1/15s shutter speed. Now, if I had a truly legit reason to shoot that slow, maybe I could justify it, but the fact is, I made a mistake. If you look at the EXIF data above you can quickly see where I went wrong.I shot in aperture priority, meaning that I told the camera to open the lens to f5.6 and fix it there, while adjusting the shutter speed to whatever was necessary to achieve proper exposure. The problem is, I didn’t check to see what shutter speed my camera decided was needed. Had I, I could have made changes.
My lens will shoot at two stops wider (or brighter) than f5.6. If I’d shot at f2.8 I would have doubled my shutter speed twice to 1/60 s (1/15 x 2 = 1/30 x 2 = 1/60s). That’s easily hand-holdable. Alternatively, I could have upped my ISO to 200, which would have also doubled my shutter speed. Had I shot this at f2.8 and ISO 200, my shutter speed would have been around 1/120 s, which would have totally eliminated any risk of blur. But I was in a hurry and while I do recall selecting f5.6, I don’t recall looking at my shutter speed in the viewfinder. My bad. The lesson here is to take care and, whenever possible, take your time. I’m really happy with this street portrait, which make me doubly disappointed with not getting a totally sharp shot.
F stop values: f2.8, f4, f5.6, f8, f11, f16, f22…
Shutter stops in fractions of a second: 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000 and so on.
ISO stops: 100, 200, 400, 800, 1600 and so on (you almost never want to shoot higher than 1600).
With a non image stabilized lens you can shoot at 1/60 shutter speed safely, as long as you’re not using a telephoto lens zoomed in.
With an image stabilized lens (Nikon calls theirs VR while Canon calls theirs IS) you can usually get away with 1/30 s and sometimes ever 1/15 s if you’re really careful.
For each stop you widen the aperture of your lens (i.e. shoot at a lower f-stop number) your shutter speed increases by a stop.
For each stop you up your ISO, your shutter speed increases a stop.
So the point is, you should always be checking your shutter speed to make sure it’s at least 1/30s or faster. If you are shooting people walking and you want to freeze them, then up your shutter speed to 1/120 s. If you want a bit of a blur on their legs to show movement, then lower your shutter to 1/30 s-1/60 s. If you’re shooting a portrait shot like the above, then up that shutter speed to ensure you don’t end up with a blurry photo.
Anyway, hope that helps. If anyone has anything to add, feel free to jump in.