Some Photography Exposure Considerations
Controlling the exposure of our image is one of the most important aspects of photography and fundamental to the principle of our photographic knowledge. To really master photography exposure we must move away from the auto settings on our cameras and enter what the manufacturers call the creative controls.
Photography exposure is simply a relationship, it is controlled by 3 things - the ISO rating (commonly referred to as film speed), the aperture and the shutter speed. It is this relationship that controls the photography exposure. Although it is simply a relationship it is a very involved area of photography, time spent understanding exposure will be time well spent. It is a continual learning process, one photographers top image may end up in another photographers Recycle Bin.
Correct photography exposure is really a misnomer, what we are aiming to achieve is an exposure that accurately records to the digital file what we want to record. If it is darker or lighter than we intended then we have failed to master exposure.
Landscape work is more often than not done on a tripod, this holds the camera still, enables considered composition and allows a small aperture to maximise depth of field. As we are using a tripod we can use a slow ISO setting, say ISO 100, a slow speed will produce an almost noiseless image. I would select this as a maximum speed for 90% of my landscape work.
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With the ISO speed set we can now look at the relationship between the aperture and shutter speed. Remember changing one will necessitate changing the other to maintain this relationship.
Depending on the subject matter we have choices to make. If we want to cover a grand vista that is pin sharp from foreground to infinity we will need a small aperture, a large f stop number. If the converse applies and we want to isolate a flower or tree in the foreground and render the remainder of the image diffused and out of focus then we need a large aperture, a small f stop number.
These examples are extremes at either end of the f stop choices, reality states that we should use the most suitable f stop to portray the image as we wish it to be recorded.
By selecting Aperture Priority on the camera controls we are allowing the camera to select the shutter speed to match our preferred f stop to determine the exposure. We can of course over-ride this by adding or subtracting compensation to make the image lighter or darker. This is entirely under our control, the camera is our tool.
Take a look at the images to the left we can see the result of choosing a different combination of aperture and shutter speed. Both are exposed as I intended and are used to illustrate the effects of the choices available.
If we recall that our cameras digital sensor can record up to 6 stops of dynamic range we need to ensure that our scene can be contained within this range. If it cannot then we run the risk of either blowing out the highlight detail or blocking up the shadow detail. Our eyes can distinguish a contrast range of about 10-12 stops of light, almost twice that which our digital sensor can record. With this in mind we need to try and visualise a scene the way our camera will record it. If your camera has a built in spot meter or you have a hand held spot meter then it can be used to measure the extremes from light to dark (IMPORTANT : NEVER POINT YOUR CAMERA DIRECTLY AT THE SUN - IT WILL BLIND YOU IN AN INSTANT - YOU HAVE BEEN WARNED!)
At times the contrast range will exceed 6 stops, if so we can either use a neutral density graduated filter to lower the contrast of the sky or alternatively we can blend two differently exposed images in Photoshop.
© Phil McDermott
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Articles by Phil McDermott Isle of Mull Photographer for commercial and wedding photography.
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