If you know how to use your camera don't bother reading on, you probably know more than me. If however you (like myself) purchased a camera to take underwater photos but were never an accomplished land photographer, then this article forms the basis to starting out.
In this blog we will go through the some photography basics. Taking photos underwater it is all about controlling the limited light you have available to create a well balanced image.
There are three elements that allow you to expose your image correctly. Shutter speed, aperture and ISO. You could put the camera in auto mode and let it's algorithms decide what it thinks is best but your camera doesn't always automatically produce the effect you want. Let's tackle these three elements one at a time so you can learn how to balance them to take get the perfect shot.
Note: Some of this may only make more sense once you understand all three - so stick with it until the end.
Shutter speed:
Shutter speed is the simplest to grasp. It is basically the length of time that your camera's sensor is exposed to the light. This is measured in seconds or almost always a fraction of a second. Examples of shutter speed: 1/30 or 1/160 or 1/400
It determines whether you freeze the action or create a blurry effect.
Think of it as a curtain that opens up and reveals your sensor to the light only when you hit the take-photo-trigger. It remains open for the amount of time you determine before closing again. The faster the shutter speed the crisper your image will be. If you have a longer exposure and you or your subject are in moving, blurring is likely to occur. This can be used for great effect if you have a tripod and want to a photograph of hazing flowing water, but is not great underwater when your subjects are often swimming away from you and you are bobbing around in the water. You can control the shutter speed in ways. One: by controlling everything manually (M) or two: put your camera's shutter speed priority mode (S). This mode allows to to pick the shutter speed you want and let the camera figure out the rest (aperture and ISO) to create the best effect. This is useful when you know what you are wanting to photograph. For example, you know a dolphin is going to be moving fast and so to get a crisp picture you will need a faster shutter speed. If you are not comfortable shooting manual, put the camera in S mode and let it help you. If you are shooting a nudibranch or some coral you know it is going to be still or not moving much so a slower shutter speed can be selected to allow more light into the camera. You will see why that is a good thing by the end of this blog.
Aperture
This one is is counter-intuitive so stick with it. It is also super simple once the penny drops. Aperture is a hole in the lense that allows light through - like your pupil in you eye. When its dark your pupils expand to allow more light in. When its sunny your pupils contract so you can focus without being blinded by the light.
Aperture is represented by F numbers ie: F/1.8 or F/8 or F5.6. Here is the tricky bit. The larger the F-stop number - the smaller the opening. So an F/4 is wider than an F/8. Smaller numbers represent bigger holes. I think of it as a fraction. Half of the cake (1/2) is more cake than a quarter of the cake (1/4). That seems weird because you think of 4 being bigger than 2... but if you make an imaginary fraction of the number it will make sense. That said, why is the size of the hole important?
A smaller aperture (higher F-Stop number) will give you a greater depth of field. A larger aperture (smaller F-stop number) will give you a shallower depth of field.
So think of those portrait images where the the person is in focus and the background is a nice blurry haze. That is a photo taken with a shallow depth of field. In photography that blurry background is called bokeh. These types of photos are typically achieved by having a larger opening (aperture) or smaller F-number - F/1.8 or F/2 or F/2.8
But in some cases you want depth of field. If you are shooting a fish you might want the face and the body in focus so you want stop down your aperture to F/8 or F13 or higher.
The problem with increasing your depth of field AKA: increasing the F-number is that your "pupil" gets smaller and less light gets in, and remember that in order to get a crisp image you have selected a faster shutter speed. So great! Small hole, fast shutter - awesome dark photo...not! Segway to ISO, the third consideration in controlling light.
ISO
ISO is like booze to problems. It helps you cope ... sort of ... but comes with nasty side effects and is best applied in moderation.
When you need a fast shutter speed to freeze a moving subject, the result is a limiting of the time your sensor is exposed to the light. Hence less light.
When you need depth of field because you want to take a crisp picture of the whole fish, not just a clear nose and blurry head/body, increase the F-number (decrease aperture.) The result is less light entering your lens.
Less light + less light = even less light.
ISO is the only other variable that you can control to increase the light and create something other than a black photo.
Senors come with a base ISO which increases the sensors sensitivity to light. Most cameras are optimally equipped to deal with an ISO level of 100. This means that at ISO 100 your image quality will be best. All cameras allow you to increase the ISO to give the camera more light, but this comes at a price ie: lowering the image quality and if used too much will make your photos appear grainy or in camera speak: introduce noise.
When you are in low light conditions you will have try to increase the ISO to take a reasonable photo. But remember that you are sacrificing image quality to introduce this light. I typically shoot between 100 - 600 even though my camera boasts ISO capabilities of up to 51200.
If you want more info on ISO here is a below is a link to a great article that explains this concept in more detail.
https://photographylife.com/what-is-iso-in-photography
The Balancing Act
You want the lowest shutter speed that will still give you a crisp image
You want the highest F-number (lowest aperture) to get everything in focus unless you are going for a bokeh effect.
You want to lowest ISO so you get greater image quality
Balancing this is the art of photography and being underwater in low light conditions makes this a real challenge because you have less ambient light to play with. Strobes or torches can add light but regardless, these 3 factors (Shutter speed, Aperture, ISO) are critical to taking great photos.
Tips
Practice on land as possible to get to know your camera.
If you have no idea what you are going to encounter on a dive and your plan is to shoot up, down, left and right select auto mode. Changing settings takes time and you will miss shots trying to re-configure constantly.
If you know what subjects you are planning to shoot go for shutter priority (S) or aperture priority (A) and let your camera figure out the rest.
If you going for that natgeo pic manually control all variables.
SET a limit on your ISO if you are shooting in auto mode - *use (P) instead of (AUTO). Your camera may automatically boost your ISO through the roof if you don't control it. Experiment with your camera and set your max ISO to 400-800 so that even in P mode you can avoid grainy noisy photos.
* (P) is like auto mode but it allows you to set parameters like ISO. AUTO will override anything you set.
Check out my blog on Underwater Lighting Techniques to learn all about ambient light and using strobes underwater.