This is a technical question that’s been coming up for me and I haven’t been able to wrap my head around why there are different understandings of EV and it’s role in exposure. The two schools are essentially that EV is either constant or relative and treated as such depending on how your meter works. I was taught and have always treated it as a constant which has always worked very well for me in how I work. I work primarily in analog processes (film and alternative processes) and I meter using analog and digital spot meters (Pentax V and Pentax Digital). I also use concepts of the Zone System when considering how meter readings relate to tone in the negative and print.
The following is gonna be a little long as I attempt to describe the issue as I’m seeing it. I’m trying to be very specific so that I have the best chance of getting clarity.
With an analog light meter (and the same holds true for the digital variants that I’m familiar with using), settings for aperture, shutter speed, and ISO are all set independently as dials on the meter. They are not mechanically or electronically connected and they do not effect the EV read by the meter. You read the EV measure of light, through the meter, in the area of scene you want to determine exposure for (these meters measure a 1 degree area or light so that you can compare exact illumination of different parts of a scene), and then you place that EV value provided by the meter on the meter’s scale and you get a combination of exposure settings that are correct. Using the Zone System method with this is very simple and involves only shifting where the EV value falls on the scale relative to the zones of the Zone System (Zone V being the measured EV value centered on the scale with lighter and darker zones adjacent to that value).
What I just described makes complete sense to me and has served me well. Using that system and meters that work with it, EV is constant. Changing exposure settings does not effect the EV value read by the meter. This is where I get confused as to why some meters behave differently. Say for example I measure light on an 18% grey card in bright sunlight. I use one of my spot meters and let’s say I get an EV reading of EV 15. I can change the ISO setting on the meter to my heart’s content (maybe I decided to change from ISO 100 film to ISO 800) and none of that changes the fact that the light hasn’t changed and so the EV reading remains the same. Changing from ISO 100 to ISO 800 has no bearing on the light falling on the grey card and I can make that change on my light meter dial and see how the correct exposure settings will change by the 3 stop difference but when I measure the light falling on the card again, the EV value will be the same because the light has not changed. It’s all still correct and the EV value has never changed. To describe the scenario another way, let’s say I set my meter to ISO 100. I measure the grey card and get EV 15 and I see that I can use f-16 @1/125. Now I decide to change to ISO 800 (three-stops faster than ISO 100). I set ISO 800 on the dial of the meter and I measure the light on the grey card again in lighting that has not changed and is exactly the same as when I measured it previously. The meter of course still reads EV 15 because the light is the same and it doesn’t care about the exposure settings I may or may not change on the dials (aperture, shutter speed, and ISO). This makes sense to me.
Now the confusion. Many common digital meters, the kinds that measure incident readings and reflected, will show a change in the measured EV reading when the ISO is changed. Using equivalent lighting and testing criteria as described in the previous example, with the meter set to ISO 100 it will read EV 15. Changing the ISO on the meter to ISO 800 will then cause the meter to denote a change to EV 18 even though the lighting remains unchanged. For some reason, this style of meter is programmed to change the EV output of identically lit scenes when the ISO setting is changed. This makes no sense to me and I’m hoping to better understand why.
If anyone with a solid understanding of why the different treatments are occurring could explain what’s happening and why there is different behavior under the exact same conditions, that would be very helpful and very appreciated.
The following is gonna be a little long as I attempt to describe the issue as I’m seeing it. I’m trying to be very specific so that I have the best chance of getting clarity.
With an analog light meter (and the same holds true for the digital variants that I’m familiar with using), settings for aperture, shutter speed, and ISO are all set independently as dials on the meter. They are not mechanically or electronically connected and they do not effect the EV read by the meter. You read the EV measure of light, through the meter, in the area of scene you want to determine exposure for (these meters measure a 1 degree area or light so that you can compare exact illumination of different parts of a scene), and then you place that EV value provided by the meter on the meter’s scale and you get a combination of exposure settings that are correct. Using the Zone System method with this is very simple and involves only shifting where the EV value falls on the scale relative to the zones of the Zone System (Zone V being the measured EV value centered on the scale with lighter and darker zones adjacent to that value).
What I just described makes complete sense to me and has served me well. Using that system and meters that work with it, EV is constant. Changing exposure settings does not effect the EV value read by the meter. This is where I get confused as to why some meters behave differently. Say for example I measure light on an 18% grey card in bright sunlight. I use one of my spot meters and let’s say I get an EV reading of EV 15. I can change the ISO setting on the meter to my heart’s content (maybe I decided to change from ISO 100 film to ISO 800) and none of that changes the fact that the light hasn’t changed and so the EV reading remains the same. Changing from ISO 100 to ISO 800 has no bearing on the light falling on the grey card and I can make that change on my light meter dial and see how the correct exposure settings will change by the 3 stop difference but when I measure the light falling on the card again, the EV value will be the same because the light has not changed. It’s all still correct and the EV value has never changed. To describe the scenario another way, let’s say I set my meter to ISO 100. I measure the grey card and get EV 15 and I see that I can use f-16 @1/125. Now I decide to change to ISO 800 (three-stops faster than ISO 100). I set ISO 800 on the dial of the meter and I measure the light on the grey card again in lighting that has not changed and is exactly the same as when I measured it previously. The meter of course still reads EV 15 because the light is the same and it doesn’t care about the exposure settings I may or may not change on the dials (aperture, shutter speed, and ISO). This makes sense to me.
Now the confusion. Many common digital meters, the kinds that measure incident readings and reflected, will show a change in the measured EV reading when the ISO is changed. Using equivalent lighting and testing criteria as described in the previous example, with the meter set to ISO 100 it will read EV 15. Changing the ISO on the meter to ISO 800 will then cause the meter to denote a change to EV 18 even though the lighting remains unchanged. For some reason, this style of meter is programmed to change the EV output of identically lit scenes when the ISO setting is changed. This makes no sense to me and I’m hoping to better understand why.
If anyone with a solid understanding of why the different treatments are occurring could explain what’s happening and why there is different behavior under the exact same conditions, that would be very helpful and very appreciated.