EV changes with ISO on Minolta IVf???

[DonF]

On numerous light meter apps I have used on my Iphone, the EV remains constant for a given metered scene regardless of ISO setting. I use this fact with wet plate collodion exposures where normal metering techniques are generally unreliable due to variations in collodion speed and the UV response of the plate.

What I normally do is take a test plate, note the EV of the test scene, determine the best exposure settings, and adjust the exposure based on changes in EV for various other scenes.

However, when set to EV mode, my Minolta IVf gives different EV values depending upon the ISO setting. What's up with that?

EV is defined as the log (base 2) of the f-number squared divided by the time in seconds. ISO is not part of the equation. This makes sense to me, as a quantity of illumination is defined by EV, regardless of the film speed.

The behavior of the Minolta meter makes it rather useless for the relative metering I do with collodion, unless it is "correct" at a specific ISO.

Am I missing something important?

Best,

Don

 

[wiltw]

EV values change when I change ISO on my Minolta Vf also.
But the same thing applies for the lightmeter app that I use on my Android smartphone, too.

 

[chriscrawfordphoto]

EV changes with ISO on all light meters. It has to to give proper exposure. The EV numbers your meter gives are designed to be used with cameras like the old Hasselblads that have the ability to set exposure by EV. The correct exposure for a scene might be, for example, 1/125 at f16 on ISO-100. That's EV 15. If you change to ISO 400, that same light would require an exposure of 1/1500 at f16, which is EV 17.

 

[wiltw]

Part of the issue is due to the that that EV is a term with two definitions

1. The original use of the term was associated with Light Value, or how much light was hitting the scene
2. But later, 0EV got distorted to mean 'a specific combination of f/stop and shutter speed'. "Exposure value" indicates combinations of camera settings rather than the luminous exposure (aka photometric exposure)

https://en.wikipedia.org/wiki/Exposure_value

 

[DonF]

My old selenium meters had no EV scale, so it’s a new way of looking at camera settings for me. I checked the three exposure meter apps on my IPhone. Two show the same EV for a given scene regardless of ISO setting (LightMeterW and Lux). The third (Pocket Light Meter) actually shows two EV values, one value labeled “EV100”, the other is the EV for whatever ISO the app is is set to. Clearly the first two apps assume a reference ISO of 100 and do not correct that for ISO changes from 100.

Since the EV value seemed the same for any given lighting condition, I was treating it as a measure of the light. For example Sunny 16 light always read as EV 15, regardless of ISO setting. Thinking about it more carefully, that is only true if a reference ISO (100) is assumed. Otherwise, EV is just a representation of mechanical setting combinations of aperture and shutter speed on a camera, its original intent (as I have read).

So, I can continue using the Minolta as I have been for measuring relative light levels for wet plate collodion work. I just need to set the ISO in the meter to 100 and set the EV mode.

Thanks for correcting my misconceptions.

Best,

Don

 

[DonF]

After reading the Wikipedia article, I see that EV can be used independently of ISO number to define a single number for multiple equivalent combinations of f-number and time.

However, EV only becomes related to luminance when plugged into the ISO-defined equation for correct exposure. That formula adds the ISO number and makes it possible to calculate the luminance if the f-stop, exposure time, and ISO number are known. Or, with a little formula juggling, if the EV and ISO number are known.

Don