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DSLR Video Weekly: Changing your f-stop

This is article #36 in the DSLR Video Weekly series.  If you’d like the whole thing in one shot, check out the book Creating DSLR Video: From Snapshots to Great Shots.

Once you get the hang of video, be sure to monetize it by becoming a contributor to Adobe Stock.

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Low light shooting really pushes your lenses. So, I usually switch to prime lenses in these scenarios. I typically carry a 28mm, 35mm and 50mm prime lens for low light shooting. These are affordable lenses and offer a range of coverage. If I know I’ll to need to shoot at some distance, I’ll add a 200mm prime lens to my kit.

You might think that purchasing these four lenses sounds out of reach until you hear that I bought three of the four from a local camera store as used lenses. Older prime lenses work great for video and are often very affordable. Just be sure to take your camera body with you when you’re shopping. Most stores will let you attach the lens and test it out in the store. I’m always on the lookout for older primes with low f-stops, manual focus and aperture rings.

How low can you go?

Even though your lens may have a very low f-stop, you may not be able to use it. As you open up the lens to let in more light, your depth of field will decrease. If you’re dealing with a stationary subject and have a tripod, open up the lens wide.

But if you or your subject is moving, having a focal plane less than a foot deep makes focusing impossible. Remember to use all three sides of the exposure triangle. Set your aperture wide, but also readjust your ISO and shutter speed (more on both next).

I opened up my lens all the way to let in the most light. even though I’m only a few feet away from my subjects, I’m only able to keep one of the two in focus at this f-stop. I slowed the shutter speed to allow more light in (because my subjects weren’t moving and I was using a tripod), which let me drop the ISO and avoid more noise.

Manual controls

These two shots were taken only 20 minutes apart. When the sun goes down, it’s often a fast transition. The top shot was shot at f/9.0, whereas the bottom was at f/2.8. The same lens was used for both shots

Why would you want to change the camera’s aperture in the middle of a shot? Well, there are lots of reasons. Perhaps you’re panning from one area to another where the lighting varies. Or maybe cloud coverage is blowing through and your light levels are varying greatly. Or you might be shooting at sunrise or sunset when lighting levels change quickly.

Most new lenses rely on the camera to control aperture using its dials or menus. The only problem with this approach is that you can’t really make a change without stopping the current file that’s recording. If you look for prime lenses or older used lenses, you may find some that have a manual aperture ring, which allows you to make changes to the aperture while shooting.

The aperture ring may need to be unlocked to work. You’ll often find a notched switch, which can be unlocked so that you can turn the aperture ring freely to change your f-stop.

Most aperture rings need to be unlocked in order to rotate them manually. Look for a bright tab near the back of the lens.

Unwanted changes when zooming

Most zoom lenses have a variable aperture, which means that the more you zoom in, the less light that passes through. One of my favorite lenses covers a wide focal range, the 28–300mm lens. This is a great lens for shooting all day long when on vacations or walkabouts because I can travel light. However, it is a terrible lens to use for low-light shooting for two reasons.

  • First, all zoom lenses will have higher f-stops than primes.
  • Second, as I change composition by zooming, I have to remember to adjust my ISO and shutter speed to compensate for changes in aperture. This constant juggling is necessary; otherwise, there’s more color correction to do in post to adjust for the variance in exposure.

If you really want to use a zoom lens and shoot in low light, plan to pony up some money for an expensive lens. You can get lenses with a constant aperture, but these lenses often cost (much) more than your original camera body.

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