Re: [Rollei] Incident v. reflective

[Bob Shell <bob >]

on 4/12/02 10:45 AM, Kotsinadelis, Peter (Peter) at peterk   wrote:

> In answer to your first paragraph it depends on what film you are using.
> Velvia is not ISO 50 but between ISO 32 - 40. When I set my ambient meter
> I set it as ISO32 and its works perfect.  For my camera ISO40.
> I too did this once using the ISO50 with ambient meter and pictures were
> dark. 
> 

Velvia is, in fact, ISO 50.  There are very strict standards and procedures
for determining ISO speeds.  The fact that many photographers like the
results better when they rate it at 40 or 32 does not change that.  When you
set your camera to a number other than the rated ISO number that is not an
ISO speed but an EI, Exposure Index.

> The .5 stops difference can be attributed to the gray card.  Although all
> claim 18% I have seen a slight variance between alleged gray cards.
> 

The Kodak Gray Card is manufactured to incredibly tight standards and is
precisely 17.8something in reflectance.  I don't recall the exact number,
but they have it specced very precisely.  The only time a Kodak Gray Card
will have a different reflectance is if it has faded from being left exposed
to light.  It is a good practice to replace them ever few years.

I'm not sure what .5 stop difference you are speaking of above, but the
difference between a gray card and standard reflectance to which meters are
calibrated is .5 stop, the difference between 12.5% reflectance and 18%.
This is why the Gray Card instructions tell you to make a half stop
correction from your reading off the card.

> Last, the 1/3 stop for reflective meters is consistent with what I have
> found as well.  It may seem odd but perhaps the camera makers are
> adjusting the meter to be slightly more overexposed. For this reason
> when I am not with meter I use a variation of the Sunny 16 rule.
> I set the shutter to the next higher speed for a given ISO and use F11.
> The reason for this is that is allows for about a 1/3 - 2/3 overexposure
> compared to the Sunny 16 rule. It also accounts for shadows.
> 

Generally point and shoot and low end SLRs are set to overexpose on the
theory that they are being used with color negative film.  Pro cameras are
usually set up for proper exposure on transparency film.

> Even meters side by side can give slightly different readings. I have a
> Sekonic L328F and L508.  Side by side in the same light the difference
> between the two can be 1 to 2 tenths of a stop. Now before someone says
> I need to have them calibrated, let me say I already did just that.
> Apparently there is a slight tolerance when adjusting them.

Yes there is an adjustment tolerance, but you should never see more than
about a tenth of a stop difference.

Bob

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