question about two lightmeters and foot candles

[weasel]

I have 2 lightmeters, both analogue, a sekonic l 28, and a spectral star 1degree spot. The sekonic scale reads in foot candles, and then the dial calculator does its calculator thing giving you f stops and EV readings. The spot meter scale reads in EV, then the dial calculator gives you exposure info.
Here is my question, hoping it makes sense. Is the relationship between foot candles and EV constant at a given iso? Meaning, say I set the sekonic at asa 100 and take reading. that gives me say125 foot candles. On the scale I get an EV reading that matches those foot candles. Is that calculation a constant, meaning can I extrapolate that scale to figure out foot candles with my spotmeter?

 

[BrianShaw]

The user manuals generally provide a conversion table.

 

[ic-racer]

I have 2 lightmeters, both analogue, a sekonic l 28, and a spectral star 1degree spot.

One of your meters will give a reading that can be converted to foot candles of illuminance and the other will give a reading that can be converted to foot candles of luminance.

 

[Deleted member 88956]

I would suggest getting this little book by Todd & Zakia. I would call it beyond advanced enough for any photographer who needs to know everything there is to know about how it all ties in together in photography.

And I will add, that as complicated & boring as some books on sensitometry are, this is a rather easy read.

 

[DREW WILEY]

No, they're not directly equivalent units of measure. Different systems. Kinda like fahrenheit versus centigrade - they converge at a certain point, but otherwise need to be mathematically converted.

 

[Stephen Benskin]

I have 2 lightmeters, both analogue, a sekonic l 28, and a spectral star 1degree spot. The sekonic scale reads in foot candles, and then the dial calculator does its calculator thing giving you f stops and EV readings. The spot meter scale reads in EV, then the dial calculator gives you exposure info.
Here is my question, hoping it makes sense. Is the relationship between foot candles and EV constant at a given iso? Meaning, say I set the sekonic at asa 100 and take reading. that gives me say125 foot candles. On the scale I get an EV reading that matches those foot candles. Is that calculation a constant, meaning can I extrapolate that scale to figure out foot candles with my spotmeter?

You might find the Wikipedia article on Exposure Value to be helpful. The main problem is you are dealing with Illuminance with one meter and Luminance with the other. You will need to know the Reflectance of the metered surface.

My Pentax digital spot meter's user's manual had a conversion table for an 18% gray card. Without a table, you could possibly get an idea using an 18% gray card and the three equations below to determine the luminance and to cross check the two meters. Note: 18% gray cards don't have Lambertian surfaces and therefore do not represent a perfect diffuser. In other words, depending on how it's set-up, the gray card might not actually be reflecting 18% of the light.

Also, as you can see from the second equation, the EV value will change depending on the ISO speed value even though the lighting conditions haven't. Meters use a fixed value of ISO of 100 to determine the EV number. The f/stop and shutter speed for film speeds other than 100 are determined by the meter's calculator. If you are checking your meters, you will want to use 100 for S.

 

[DREW WILEY]

IF you are lucky enough to own a gray card that is actually 18% gray. They can even vary batch to batch even in the same brand, or have faded. I once measured a whole stack of them form different brands with a high quality industrial spectrophotometer, and NONE them were correct. Some were way off. But the mid neutral gray patch on my carefully stored MacBeth Color Checker Chart was dead on. And some of the premium quality modern gray balance discs that I've got ahold of were accurate too (not all of them). One also has to be wary of gray cards that are glossy, with excess reflectivity.