Home-made exposure guide

[richard_l]

According to his Chart A, he's using EV as an absolute, independent of film speed. I.e., his EV is really LV (EV for ASA 100). According to Chart B, an LV of 4 for ASA 100 would require the same exposure as an LV of 2 for ASA 400. (LV 2 would be 2 stops less bright than LV 4, but since ASA 400 is 2 stops faster than ASA 100, the exposure settings would be the same.)

EV, however, is a usually a measure of exposure (f-number and shutter speed), whereas LV is an absolute measure of reflected light (or brightness of an object, see this link ). EV only coincides with LV when ASA is 100. I just remember that EV 0 is f/1 @ 1 sec (or equivalent). EV 1 is 1 stop less exposure than EV 0. EV 2 is 2 stops less exposure than EV 0, etc., etc.

If the correct exposure for ASA 100 is f/2 @ 1 sec (EV 2), then the correct exposure for ASA 400 would be 2 stops less, or f/4 @ 1 sec (EV 4).

 

[dreilly]

Okay, I get it. I think. Let me see if this makes sense:

I'm metering out my window on my Weston meter. Set to 100ASA, I get an LV/EV of 12. When I shift the ASA up to 400, the EV reads 14. That's the right EV for 400ASA, but the LV is still 12. To go back to a simplified LV chart like I'm using, I'd have to subtract 2 from 14 to get 12, and read the exposure from there. Right?

My mistake was in confusing LV and EV and Fred Parker using LV consistently in the second chart.

I get it!

Wait, do I?

 

[richard_l]

Okay, I get it. I think. Let me see if this makes sense:

I'm metering out my window on my Weston meter. Set to 100ASA, I get an LV/EV of 12. When I shift the ASA up to 400, the EV reads 14. That's the right EV for 400ASA, but the LV is still 12. To go back to a simplified LV chart like I'm using, I'd have to subtract 2 from 14 to get 12, and read the exposure from there. Right?

My mistake was in confusing LV and EV and Fred Parker using LV consistently in the second chart.

I get it!

Wait, do I?

Yes, that's right. I knew you understood it perfectly from your first post explaining it. Parker's second chart threw me for awhile too. Parker is the best exposure reference I've seen, but Ken Rockwell's page (http://www.kenrockwell.com/tech/ev.htm) nicely clears up the confusion about EV and LV.