How does the EV number work?greenspun.com : LUSENET : Large format photography : One Thread
Now, I've had a Minolta Spotmeter F for the last 10 years or so, and I have never really worked in EV, I always used the f-number and aperture output information from the meter. However, I am aware that the EV is a relationship with variables associated with the ASA and light level of the scene being metered.
For example, the meter might say EV 10 when set to ASA 100, and with the same reading, it will shift to say EV 12 when set to ASA 400.
That's all fine and good, until... I just purchased a Pentax digital spotmeter. The two meters agree at ASA 100, but, wheras the Minolta will 'shift' the EV as the ASA gets adjusted, the Pentax EV reading remains the same regardless of the ASA.
Did I have an incorrect assumption about how the EV scale works? I always assumed the EV was based on some kind of ANSI or other standard, and that all meters would agree. It appears to me that EV is somewhat arbitrary in use, and that the the information (EV number) is only relevant for the particular meter that is being used.
One note, even though they display different EV numbers, they agree on the shutter speed and aperture combinations to make a proper exposure.
-- Michael Mutmansky (firstname.lastname@example.org), August 28, 2000
Mike... you wrote..
Did I have an incorrect assumption about how the EV scale works? I always assumed the EV was based on some kind of ANSI or other standard, and that all meters would agree.
You are absolutely right here... your understanding is proper...if you double the film speed, the EV reading should increase by one also...each represents a stop. I have no idea what is going on with your meter... I have never seen that meter before... But EV's are never independent of film ISO.
-- Bill Glickman (email@example.com), August 29, 2000.
I too have a Pentax digital spot meter and it functions exactly as you have stated. As Bill pointed out, your understanding of EV is correct, but where you may be missing something is in how the Pentax works. I believe it is calibrated for a single fixed EV. That is an EV based upon ~50 ISO. Once you meter a subject and get that EV, you then rotate the scales to determine the correct exposure for whatever ISO you desire. An electronic coupling does not exist between the rotating ISO ring and the meter. I hope that helps.
-- Pete Caluori (firstname.lastname@example.org), August 29, 2000.
Michael, Your Pentax reads and arbitrary EV value which is the same for each ISO (I think it's fixed on ISO 100). It changes only with the intensity of light. The difference in exposure is meant to be calculated by setting the film speed dial and choosing an aperture/shutter speed combination. If you are using two (or more) different speed films, you can simply calculate the differnce in exposure by knowing the difference between the different EI values for you films (e.g. EI 100 is one two stops slower than EI 400), or you can reset the film speed dial if that is easier. Since cameras and shutters don't come with EV numbers on them (anymore), it is impossible to transfer those values directly. That's what the dials on the meter are for, and why manufacturers aren't too concerned about using a standard EV system. Theoretically, the numbers could be any value whatever (lumens, foot candles, ergs) as long as the dials converted them to the correct shutter speed/aperture combination.
Also, consider yourself lucky that your Pentax and Minolta agree at ISO 100. Most meters aren't even close to each other. That's why we all have to calibrate (and recalibrate) when we change meters.
-- Doremus Scudder (ScudderLandreth@compuserve.com), August 29, 2000.
Your Pentax doesn't show EV (Exposure Value) numbers. Instead, it is showing LV (Light Value) numbers.
-- Alan Gibson (Alan@snibgo.com), August 31, 2000.
O.K., what's a Light Value? Is that a way of saying an arbitrary metric for measuring light? Arbitrary in the sense that Pentax picked ASA 100 as it's reporting basis, and the numbers are 'translated' into the correct shutter speed and aperture combination by the dials.
Actually, the Pentax manual is very clear in that it uses the letters "EV" for all of it's instructions, so I would expect that what is being measured is Exposure Value. However, as I have discovered, this is not the case, since the numbers only agree at the ASA 100 setting.
Ultimately, it's not a big deal, it just has meant that I have to be cognizent of this difference when I'm out in the field with others, or when I am using both meters at the same time.
-- Michael Mutmansky (email@example.com), August 31, 2000.
Yes, you've got it. LV isn't quite as well defined as EV, but the usual coincident value is at ISO 100.
With EV, 1 second at f/1 will be EV 0 (zero), whatever the film speed. But an LV value of zero is the amount of light that requiers an EV of zero at a particular film speed, usually ISO 100.
Unfortunately, some people do use "EV" to mean either EV or LV, after all they have only one letter different.
-- Alan Gibson (Alan@snibgo.com), September 01, 2000.