A spot meter is the most accurate way of determining the correct exposure of the scene in front of you, with the use of digital SLR’s this is not really required as the histogram is a great way of understanding exposure however for film users there is still no better choice. There are various ways of using a spot meter and calculating the required exposure, although I am only going to explain my method as I have always found it to work very accurately even with slide films such as Velvia which have a limited exposure range.
I use a Sekonic L-558 which has served me well over the last five years and like any spot meter there is more than one way to set it up, you can choose to use half stops or third stops depending on your camera and lens, my Hasselblad lenses for instance have half stop increments so I set my spot meter the same to keep things simple. You then have the choice of selecting the exposure and metering for the aperture or selecting the aperture and measuring the required exposure, the latter is the method I use and is shown on the Sekonic L-558′s screen as [F].
Ok, let me explain how this works. If I was looking at a typical landscape scene (rather than the inside of my office at present) I would set the aperture on the spot meter to f11 and then take a reading from that darkest point of the scene, this may be the underside of a rock or a dark shadow essentially whatever is the darkest point in your shot lets say the reading was 2 which means 1/2 a second @ f11, press the memory button and store this exposure. Next I will find the brightest part of the scene most commonly a clear bit of sky near the horizon or the running water in a waterfall etc. I then repeat the process as before of taking and storing a reading and lets say this time it is 60 or 1/60th of a second.
So now that I know the exposure readings at the darkest part and the brightest part of the scene I can calculate (1) the exposure range and (2) the ideal exposure. The exposure range is simply the number of stops between 1/2 and 1/60 which is 5 stops (1/2 -> 1/4 -> 1/8 -> 1/15 -> 1/30 -> 1/60), if I am shooting black and white or colour negative film I will have no problems capturing the 5 step range, if I am shooting Fuji Velvia or another transparency film I may have either some dark shadows or light highlights as the scene is about 1/2 a stop greater than the film can record.
To determine the correct exposure based on the readings above and a black and white film such as Tri-X I would aim to put my ideal exposure in the middle of the exposure range which would be 1/10th @ F11. If I was shooting with a SLR I can simple input these values and take the shot, however my Hasselblad only has whole stops for exposure time so I need to make the half stop difference up by changing the aperture – for example 1/8th @ F13 (also known as F11 1/2) or equally 1/15th @ F9.5 (also known as F8 1/2) – knowing this I can now make my shot.
There are a few other considerations that you need to think about when calculating your exposure, generally for negative films you are better with slightly more exposure than indicated assuming the highlights don’t go too far of the scale, however when shooting transparency films you are better being slightly under exposed. If you are making long exposures then reciprocity failure comes into the equation, this is a failing off of a films sensitivity over time and happens to most films when the exposure exceeds a second, most manufacturers include a table with adjusted exposures in the film packaging for example a 1 second exposure on Kodak Tri-X would actually require 1.5 seconds and a 8 second exposure would need 36 seconds to get the correct exposure – the longer the exposure the greater the reciprocity failure.
And just one final thought, if you have metered the scene and it exceeds the exposure range of the film you either have to sacrifice some of the shadow detail or if you have a bright sky in your image you can control this using a ND (Neutral Density) filter, but I will come back to this in a later post.
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My name is Mark Voce and I’m a fine art photographer specialising in landscape, architecture and night photography.
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Hi Mark,
I just have a question regarding the reflected metering. If you measure the darkest and brightest parts of the scene, do you compensate for that reading the light meter gives you?
For example, if you meter a white cloud, the reflected reading can easily be +2 stops more than the average gray.
Hi Martin,
The spot meter will always give you a reading based on zone 5, so if you read the darkest shadow to be 1 second and you wanted this to be on zone 3 you reduce the exposure by two stops to give you 1/4 of a second. If you read a bright cloud to be a 60th of a second and you want to then place this on zone 7 you would increase the exposure two stop to give you a 15th. If that makes sense.
The idea with a spot meter is to make multiple measurements to get a accurate picture of the scene. My usual method involves metering the darkest shadow and then placing this on zone 3, the metering the brightest parts to give me an idea of dynamic range.