I'm fascinated by all these amazing detailed posts about the inner workings of digital sensors but I always come away from reading these threads with no clear idea of how to use this information to make my images better.
I'd love for the people that seem to have a deep technical understanding of how digital sensors work to talk about practical strategies to apply the knowledge to actual shooting because I, apparently, am too dumb to figure it out on my own.
You may be employing some strategies, already, without knowing it. A photographer who has been shooting with a camera for some time tends to develop a sense of which settings deliver a more pleasing image when doing certain kinds of photography. So, give yourself some credit. That said, it is to our benefit to be more intentional about these things.
I generally recommend starting by choosing a focal length and exposure settings that will deliver a shot with the framing (focal length), depth of field (f-stop) and degree of motion-freezing (shutter speed) you desire. These are the creative decisions that - along with choosing your perspective on the subject and when to press the shutter - go far in determining the quality of the resulting image.
With that out of the way, what ISO does the camera suggest you use? Of you're in full manual, the camera's recommended ISO will be that which produces a metering display parked at 0. The photo will be, on-meter. Now, in this situation, I'm treating ISO as the smoke indicating a possible fire. It's more an indicator than an end unto itself.
The question to keep in mind is, is ISO within a range where you typically get good results? If so, you're all set. If the ISO is outside this range, it's time to start considering your options. The acceptable range will vary, depending on the camera and your personal aesthetic. But generally speaking, if your settings will deliver enough light to the sensor to merit an ISO at base or 2-3 stops above base, you're in great shape. At 4-5 stops above base, things are still good. Higher than this and you probably want to invest some time rethinking your exposure settings.
As mentioned, the ISO range indicating exposures that deliver acceptable results will vary according to your tastes. For the sake of discussion, let's assume the ISO that corresponds to an on-meter display is higher than you like. Keeping in mind that ISO is an indicator (like smoke is to fire), the next step is to consider the f-stop and shutter speed settings. Will opening the aperture or slowing the shutter speed (either of which will deliver more light to the sensor) compromise your creative vision for the shot. If the answer to that question is, yes, there are other options to consider.
For one it's always an option to wait to press the shutter release until the shot improves. In this context, that may mean clouds passing to put more light on the subject, the subject moving from shade to sunlight, the sun moving to a position in the sky that more directly illuminates the subject, you moving to a new position from which the subject will be better illuminated.
Can you improve lighting by using the on-camera flash. an external speedlight, a reflector or other light source? If the answer to all these questions is, no, the best decision may be to wait...wait for another opportunity that comes with improved lighting.
Or, consider the option of making a different photo. For example, when I'm photographing elk grazing in a meadow at sunset, I'm keenly aware of slowing my shutter speed only so much. Once I've hit a threshold beyond which there's just no reasonable chance of freezing motion and capturing enough light for a good exposure, I'll sometimes go the other direction and start using shutter speeds so slow that I have to manually pan to follow the animal's motion during the length of the shutter actuation. If I'm successful, the resulting photo will feature a focused subject that's sharp in the face and an obviously blurry background. It's an aesthetically pleasing alternative to my standard approach of using a faster shutter speed to freeze motion.
By using a slow shutter speed, I'm emphasizing motion...calling attention to it. I'm also putting more light on the sensor, increasing exposure and improving the image IQ.
We could explore a host of possible scenarios and the variety of available solutions. But I'll stop, here, in the hope I've illustrated a thought process one can employ to get the shot and enough light to make the shot worth getting. In summary...
- Start by choosing exposure settings that will deliver the photo you envision.
- Check the ISO recommended by the camera as delivering an on-meter exposure.
- In this context, ISO is an indicator we use to identify potential problems with the exposure settings. It's like smoke (indicator) to fire (problem).
- If ISO indicates a possible problem, consider your options for changing exposure settings and go with what you consider the best adjustment or option.
- Remember, it's OK to decline to press the shutter. It's also OK to improvise an alternative creative vision for the shot; one that works better with the available light.
