Shutter Priority vs Aperture Priority Modes
There are 3 elements in play when considering exposure: f/number, shutter speed and ISO sensor sensitivity. This essay compares the shutter priority (Tv) and aperture priority (Av) modes. Canon terminology is used here and I assume that ISO is set to auto.
In Tv and Av modes, one of these 3 elements is set by the photographer (let’s call it the “primary” control), one is set by the camera (the “secondary” value) and the third is always at its minimum value (also set by the camera). Recognising that the camera always sets one of the elements to its minimum value is key to understanding the modes.
The minimum values are:
- The minimum f/number is, obviously enough, that obtained with the lens wide open.
- The minimum shutter speed is a preset within the camera’s menu. For Canon cameras it is within the “ISO speed settings” menu. It can be set to a fixed value or set to auto, in which case the camera will select a value depending on the focal length of the lens attached.
- The minimum ISO is generally 100 but can be adjusted.
In bright, well-lit conditions, the camera will set the ISO at its minimum (100). There is then very little technical difference between using Tv and Av. In Tv, the photographer sets shutter speed (the primary control) with the rotary dial and the camera sets the f/number (the secondary value) according to the lighting. Av is just the same, except the photographer sets the f/number and the camera sets the shutter speed. Note that for any given situation, the pairs of values of shutter speed and f/number to give correct exposure will be exactly the same in Tv and Av; the only difference is which is treated as the “primary” control. In either mode, immediate control is available of depth of field (via f/number) and the freezing of action (via shutter speed), although one is always traded off against the other.
Not Well Lit
By “not well lit” I mean photographing in poor light and/or using a lens with an inherently large f/number – typically long telephoto lenses with f/numbers of 5.6 or so. In these conditions, the difference between Tv and Av becomes more marked. Consider what happens as the light falls through and then below the “well lit” range…
In Tv, as the light level drops within the “well lit” range, the camera will open up the lens to get a correct exposure (while holding the ISO at 100). If the light continues to fall, at some point the lens will be set wide open and we reach a limit. As the light falls further, the camera changes its logic to one of holding the lens wide open and increasing the ISO value. Shutter speed remains the “primary” control but ISO is now the “secondary” value, with f/number pegged at its minimum. If we look through the viewfinder and rotate the top dial, we see pairs of values of shutter speed and ISO to give correct exposure. We still have immediate control of freezing the action via shutter speed but we have no immediate control of depth of field via f/number. In particular, we cannot increase depth of field except by manually increasing ISO, which takes time and may be distracting.
A similar logic apples to Av. As the light level falls within the “well lit” range, the camera sets a lower shutter speed (while holding the ISO at 100) until the preset minimum shutter speed is reached. As the light falls further, the camera changes its logic to one of holding the shutter speed at its minimum value and increasing the ISO value. Rotating the top dial sets pairs of values of f/number and ISO to give correct exposure. Now we still have immediate control of depth of field via f/number but no immediate control of shutter speed. In particular, if our subject suddenly starts moving rapidly, we cannot increase shutter speed except by manually increasing ISO.
The key to choosing which of these 2 modes to use perhaps lies in understanding what control is given up when operating in “not well lit” conditions. In Tv there is no way to stop the lens down and increase depth of field. In Av there is no way to increase shutter speed and freeze the action. In either of these cases, we have to manually increase ISO, which takes time.
Light Falls Further
When using auto ISO, we need to keep in mind that increasing ISO will result in increasing noise and decreasing colour fidelity in our image. Digital cameras tend to offer a maximum ISO value that is well beyond what can be used in practice with acceptable image quality. We therefore need to preset the maximum value that the camera can use in auto ISO – typically something around ISO 1600, depending on camera model.
We have seen above that as the light level decreases in “not well lit” conditions, the camera increases the ISO to achieve correct exposure. As the light falls lower still, eventually the ISO will reach the maximum we have just preset and we reach another limit. Beyond this limit, if the light falls further, the image will be under-exposed. We can protect against this situation by setting the preset menu “Safety shift” to “Tv/Av”. With this option set, the camera will reduce shutter speed to achieve correct exposure (violating the preset minimum shutter speed if necessary).
The logic in this condition is the same in both Tv and Av – when the automatic ISO would otherwise exceed the preset maximum value, the shutter speed is reduced as a “last resort” to achieve correct exposure. The difference between Tv and Av lies in the f/number at this stage. In Tv the lens will already be wide open at its minimum f/number, while in Av the camera will retain the selected f/number to the bitter end.
Which to Choose
We have seen that in well lit conditions there is little to choose between these 2 modes. In either, the ISO will be at the minimum and we have immediate control of both shutter speed and f/number, regardless of which is nominally the “primary” control.
In “not well lit” conditions, Tv gives us immediate control of shutter speed to freeze the action in a dynamic and rapidly changing situation. However, depth of field will be at a minimum and we have no way rapidly to increase f/number and depth of field. Conversely, Av gives us immediate control of depth of field for creative effect. However, shutter speed will be at its preset minimum value and we have no way rapidly to increase it if our subject suddenly takes off at high speed.