I just came across this terrific chart (below) that clearly illustrates the impact choices of F stop (aperture), shutter speed and ISO makes on the photographed image.
For camera people this is all fundamental stuff that must be grasped in order to shoot, but for many, it can be confusing and opaque. So for non camera peeps, here is the chart that makes it all much simpler.
Aperture, or ‘F stop’
This is a setting on the lens that opens up and closes an internal iris made of metal blades. This increases or reduces the amount of light that passes through the lens and hits the sensor in the camera, which in turn creates the image we that see.
The basic rule is this, the more light, the greater the depth of field.
The depth of field is the area that is in focus in the image.
A large depth of field means objects near and distant will be in focus. A shallow depth of field means only images the lens is focussed on will be sharp.
The depth of field can often be controlled by the aperture (though in practical reality, the aperture is more often controlled by the amount of light on set).
Many people like the shallow depth of field look as it’s considered cinematic, but it’s also a major pain in the ass on set as to keep things in focus when they move is a challenge– and let’s be clear, what we are often focussed on are the creative and emotive humans we call actors. They will move, and so they should. And when they do move, it has an impact on shots with a very shallow depth of field as the focus must be ‘pulled’ so that the image remains sharp on the actors.
Of course the amount of light available plays a huge part too – shooting outside in summer sunshine offers massive amounts of light (and whole new set of issues) where shooting on the street at night offers very little light.
When there is lots of light, the aperture is usually very small (F32 on the chart with a very deep depth of field so everything is in focus), and when there is little light the aperture is often very wide (F1,4 on the chart with a shallow depth of field, so only the subject is in focus).
It’s rare to change the shutter speed when shooting, unless you want to get a specific ‘look’, like the opening of ‘Saving Private Ryan’. Most films are shot with shutter speeds of a 50th or 60th of a second. The chart here is designed for stills photography but the principle remains the same.
The basic idea is that the shorter the shutter speed, the less time there is for an object to move in frame, so it begins have less motion blur and appears sharper. Our eyes are set to expect a certain amount of motion blur in a film (due to having seem hundreds of thousands of hours of material over our lives) so when that changes, we notice it.
Most movies are shot at 24 frames per second now, but changing frame rate can also change how the image looks to us. The Hobbit films were shot at 48fps and to my eye, it looked horrible. Sharp, smooth and much like video. My advice is stick with traditional shutter speeds / frame rates except for specific and targeted effects (like slow motion or high shutter speeds to get a ‘look’ during fight scenes). The bottom line is, the higher the shutter speed, the less motion blur.
One note on shots that you choose to shoot in slow motion – the slower you can go, the better it will look. Rule of thumb stuff there.
As the ISO increases, so does the cameras sensitivity to light. Meaning, if you are shooting in low light scenarios, you can increase the ISO to get more detail and a proper exposure.
BUT… this can increase noise and create more grainy images.
Sometimes it’s unavoidable, and on the whole, audiences don’t mind or even notice occasional grainy shots and scenes (and you can do stuff in post to reduce the impact of noise too, though that has other implications).
Usually, there will be an ISO agreed for a shoot (negotiated between the DP and director), and it’s likely to be as low as is possible so as to reduce grain and noise.
Bottom line, the higher the ISO, the less light is needed to shoot (even candle light at the extreme end) but so the noise and grain will increase. The general rule is to shoot with the lowest ISO you can get away with.
All three of these variables – aperture, shutter speed and ISO will work in concert on your set, the ratios depending on scenario, time of day, amount of lighting available and creative choices.
In practice, the ISO is often set and locked (at least from scene to scene), as is the shutter speed, leaving aperture as the only variable that gets changed from shot to shot. Occasionally the ISO will be increased as the sun sets or if locations change to more dimly lit spaces. The ability to increase or decrease the amount of light on set is also a huge factor, and in most instances, aside from using ND filters, not much be done.
Again, in practice as you can see from the Vimeo clip, shooting with a very high ISO only becomes a huge issue when presenting your work on a very large screen and problems are often most visible in shadows and out of focus areas. Basically in places audiences are not usually looking (they should be watching your actors, more specifically their eyes).
Here are some common scenarios you may find yourself in
- Shooting interior
Lighting a scene should allow shooting on a low ISO (nice and grain free) and with a fairly wide open aperture, yielding a solidly cinematic image.
- Shooting interior with no lights
Or rather practical lights (lights that occur in the world of your story such as desk lamps, candles etc). Likely you will shoot on a higher ISO and the aperture will still be very wide making shooting a challenge. Get it right though and it can look very pretty, authentic and of course you can shoot very quickly as there is little lighting setup. Windows and light spilling in will always be a big consideration.
- Bright sunny day
Shooting with low ISO, the DP may add neutral density filters to the camera to reduce the amount of light entering the lens so the aperture can remain more open to yield a more cinematic image. Bright days cast deep shadows so bounce boards and even some lighting may been needed to fill those shadows.
- Overcast days
Can still shoot with a low ISO and wider aperture. In many ways, an overcast day is the fastest environment to shoot in, though you will still need bounce boards to fill in shadows.
- Exterior nights
Pretty much you will always be pushing up the ISO in order to get an aperture that has a decent depth of field. Night shoots are both cinematic dreams and practical nightmares. One tip, shooting into a light can hide a myriad of sins and it often looks more cinematic, so long as there is a logical reason for a light source to be present in the story.
- Green Screen
Where possible shoot with the lowest ISO possible as noise and grain are a pain in the ass in post. Also, get as much light as possible on the subject and screen as most often you want a deep depth of field. Get your subject as far from the green screen as possible to avoid reflected green ‘spill’.
I am running my final two day Guerilla Filmmakers Masterclass in London in the Summer and tickets are £69 for the two days. This is the last time I am running it so if you want to come, you can sign up at www.GuerillaMasterclass.com
Onwards and upwards!
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