Exposure is almost a form of art and once you know how to do it well, then your rate of 'keeper' photographs will increase several-fold. If using manual I usual look for a midtone subject which is a tone from the scene your photographing which YOU deem to be somewhere in the middle of the tones of your exposure. You eye and brain is capable of 'seeing' about 12 stops of tonal range (from detail less black to detail less white). Negative (print) film can record about 8 stops of tonal range and transparency film or most digital cameras can record about 4 stops of tonal range. Already you can see that the camera records light differently from what your brain remembered the scene or subject.
If you are photographing in 'perfect' light at noon then the sunny F16 rule is a good benchmark. It means that if using an aperture of F16 then a good shutter will be the same as your ISO speed rating. i.e. F16 at ISO 125 would give a shutter speed of 1/125th of a second. If you stop down the lens to F22 which is one stop smaller than F16 then you 'open' up one stop on the shutter, which would be 1/60th of a second. If you open up to F11 which is a stop faster than F16 your shutter speed would be 1/250th sec. Of course this is all at ISO125. This is the Bunsen Roscoe law of reciprocity. E=IxT Exposure (E) equals light intensity (I) multiplied by time (T).
Most digital cameras have quite sophisticated light meters and if using manual exposure mode with a colour matrix meter as found in some of the higher end Nikons works quite well.
But!!!!
This would work well for a landscape with equal amounts of sky and land. Say there is some green grass of a hillside and some nice blue sky with fluffy white clouds then 9 times out of 10 it would be a good exposure. Note I say 'good' and not correct as there is no such thing as a correct exposure, just good exposures and bad exposures, but then again you may want to do something different like silhouetting a building against the sun and underexposing would create a desirable result.
This type of meter matrix patterns work when there is a fairly balanced range of tones. If we made an exposure of only white snow then this is where the problems start. White snow is pretty much two stops lighter than midtone. So the meter would have a tendency to under expose the snow making it look very grey.
Colour matrix meters do recognise that snow should be white and often expose it as say one stop above midtone. A non colour matrix meter would not recognise the white snow as being white and would probably expose it as midtone (two stops less than it should be rendered). I would normally spot meter there snow and open up two stops from midtone using spot metering. The same problem comes into play with the sky, as this is, during the day rarely midtone, being between say half a stop and two stops above midtone. To render a white nimbus cloud white I would use spot metering (measures the light from only one small portion of the viewfinder area) and open up two stops on the brightest part of the cloud.
I often photograph birds in flight using aperture priority and 3d colour matrix metering (Nikon as I believe Canon's system is called evaluative metering) and I dial in exposure compensation as the background sky changes. If it is all white clouds I would set this to between 0.7 and 1.3 stops over dependant on the amount of light about. Blue sky at about a 45 degree angle to the earth is about 1 stop above midtone but the colour matrix does a pretty good job and I may only need to add 0.3 to 0.7 stops over compensation. As you get towards the horizon the sky is more cyan and much lighter than the darker blue sky. It is maybe one and a half stops above midtone in reality but compensation of about one stop over would do.
Using spot metering is really not a good idea if you are using a programmed exposure mode (aperture/shutter priority or P which is programmed mode) on a constantly changing background. For birds in flight or moving subjects I always use Aperture Priority on matrix metering and ‘dial’ in compensation as required. A subject like a
Red Deer against lush green grass I would not have any compensation ‘dialled’ in as the subject and grass are both ‘fairly’ midtone.
If I’m shooting landscapes or when the camera is pretty static I use manual exposure mode and spot metering... taking my time..
I know how to recognise tonality of subject and I’m pretty good at it through experience. I shot transparency film for many years before going digital, a year and a half ago. I still use transparency film with medium and large format (5”x4”) view cameras at college. The benefit of digital is great, but just looking at the result on screen and judging the ‘good’ or ‘bad’ exposure by that alone is a recipe for disaster (as a friend found out recently). If you look at your screen in bright sunlight the screen looks darker than it should be and you end up over exposing. The opposite happens when it is dark and the screen looks brighter than it is and you end up under exposing.
This is why god invented the Histogram!
The histogram is perhaps the best thing to come out of digital for many people. It shows the distribution of pixels as a tonal value. In short the tones and amount of tones in an image. Highlight warning is also great but they do have a built in 'safety' margin so often flash indicating the highlights are ‘blown’ or ‘burnt out’ (detail less white) when in actual fact checking the histogram shows the highlights are not clipped.
There is of course 'centre weighted' metering which takes an average reading from the centre of the frame and I use this rarely. In fact the only photographer I know who uses it a great deal is Niall Benvie.
I think I've covered the basics here and....
I could go on and explain ‘reading’ and understanding the histogram but my hands are sore from typing now. If anyone wants me to do this I will. Just ask.
Linear Mode
