EffectiveLV why?

[blackest]

I am kind of puzzled.

I am looking at EffectiveLV  tag on a number of Pentax DNG files

-1 :FC00:-1024
-2 :F800:- 2048
-3 :F400:- 3072
-4 :F000:- 4096
-5 :EC00:-5120

-1.5:1D18:-1536
-4.5:EE00:-4608

I can't see how you get the EffectiveLV value from whats stored in the tag.
I have managed to figure the displayed value relates to shutter speed at a particular ISO

So if a given value gave a shutter speed of 1/45th at iso 100 then it would be 1/90th at iso 200 and 1/180 at iso 400.

The EffectiveLV numbers seem odd from +ve to -5 then it becomes 26.5 26 ... as shutter speeds get slower and slower.

Also while i'm here, there are 77 metering segments on the k5 how many are used  for spot metering i know its an 11 by 7 grid
and 6th from the edge on the 4th row should be the centre of frame. are all the adjoining segments used to calculate the lightvalue for setting the exposure?
   

[Phil Harvey]

You can check the ExifTool source code to see what is happening.  The value is divided by 1024.

I can't answer your question about which metering segments are used by the camera.

- Phil

[blackest]

Thank you I will take a look. It may be a bit of a red herring for what i'm looking for.

That value seems to link to a shutter speed and iso value. If it is value 'a' then then the shutter speed doubles for each doubling of iso.

e.g if the shutter speed is 1 sec at base iso

1 sec at 100
2 sec at 200
4 sec ar 400
8 sec at 800
15sec at 1600
30 sec at 3200 iso
   
Is that helpful to know? probably not. I was kind of hoping it would give me an offset for the aperture used from the light measured at maximum aperture.

Then I thought about the metered segments for an accurate measure of light you need to know the aperture that you are metering at but it doesn't actually matter in practice.

For my da 18-55 it has an aperture of 3.5 at the wide end the camera knows this so it uses this as the aperture to calculate the actual light level and picks a shutter speed to expose mid gray properly. When I shot an approximate mid gray target it measured 9 pretty much on all the meter segments and gave 9.5 as the light level. I think kodak uused to say to add a 1/2 to the metered value because people are a little brighter than mid gray.  Anyway if you look up 9.5 on a ev table would you believe it comes up with the same speed the camera used

If you stop down 2 stops then the value for shutter speed is shifted 2 stops.

With a Km lens there is no way for the camera to know the max aperture so it assumes one and i think it uses F1.2 The coding system pentax uses on its lenses gives a max aperture of 1.2 stops, it doesn't really matter.
But so with an aperture of f1.2 the metered value might be 7.5 EV and where that meets with f1.2 is a shutter speed.

if it had the real aperture it would calculate the light level to be a higher value say 9.5 and when you go across the table to where the real aperture is it's the  same shutter speed as the first case.  So in practical terms it doesn't matter.

When you close the shutter a couple of stops then you slow the shutter down a couple of stops. Its relative not absolute values that matter.  It was absolute genius   that 1 f-stop represents a doubling or halving of the light value.

Anyway for an unknown max aperture and an unknown stopped down value.
If the metered value was say 7.5 (using the 1.2 as a base) then closing the aperture by 2 stops would give a measured value of 5.5EV  Therefore the shutter speed needs to be slower by 2 stops.

This is quite good now we can look up the shutter speed for the LV recorded + 0.5 at the f1.2 aperture and the actual shutter speed may be several stops slower. If its 3 stops slower then the lens was stopped down 3 stops if the max aperture was 2.8 then f4 f5.6 f8 was the aperture used.

But there is a bit of a problem there is no shutterspeed for max aperture recorded so we need an exposure table to figure out the shutter speed at max aperture  for the recorded light level to then see how many stops difference there was when the shot was taken.

I was kind of hopeful that the effectiveLV value was going to give the EV correction  for the exposure . e.g 3 Ev change would mean stopped down by 3 and knowing the Max aperture would mean translating that to an actual aperture.


EV  = log2(aperture2/shutter speed)
       
if EV= 7.5
then
7.5 = log2(aperture2/shutterspeed)

or
7.5 = log2((1.2)squared/ shutter speed)

i think exp(7.5) = aperture2/ shutter speed

or exp(7.5) x shutterspeed = aperture2

root aperture2 = aperture.