jking@xxxxxxxxxxxxxx wrote:
> ok, I may be about to open myself up to calls of idiot but here goes
> anyway....
>
> I would like to calculate how much light loss there is depending upon the
> lens focusing distance and lens focal length. This is bcause I mainly use
> an incident light meter and want to be able to compensate for the light
> loss at close focusing distances e.g. macro but also taking photos of
> statues at close range etc.
-----------------------------------------------
I just reviewed what John Shaw (Closeups in Nature) has to say on this
subject. He recommends calibrating your lenses using the camera's TTL
meter. Then use the results to build a correction table for use with
the hand held meter. I'll quote the relevant portion here from the
section titled "Using a hand meter for extension closeups". Not quoted
is the bit about why you will go nuts trying to do this mathematically.
Chuck Norcutt
---------------------------------------------------------------------
The easiest way to note the light lost due to extension on any lens is
to meter TTL with and without the extension. The difference in stops is
the amount of light the extension costs on that lens. Here's how to do it.
Set whatever lens you want to use at infinity focus. Without adding any
extension, take a meter reading of an evenly lit subject in unchanging
light. An easy subject to meter is an evenly lit wall of your house.
Pick a shutter speed that gives you an f-stop in the middle of your
aperture range. Then add the extension and take another meter reading.
Change the aperture on the f-stop ring to get back to correct
exposure. Do not change the focus setting at all during this procedure.
Now note the difference in stops between the two readings. This is
the amount of light in stops used by that extension on that lens. Do
this for all the extensions you you plan on using with this particular
lens. When you are in the field, add the total number of stops for
whatever extensions you're using and open up this number of stops from a
hand meter reading to get the correct exposure.
You might also want to try this using only the built-in extension in a
lens mount. Focus and meter with the lens set at infinity and then rack
the lens out to its closest focusing distance and take another meter
reading. You now know the amount of light that the lens loses in its
own focusing mount.
I've made a little chart for my most used lens and extension
combinations which I carry in my wallet. This way I can compare a TTL
meter reading with a hand reading if I want. Or I can take a hand meter
reading, and without even taking a TTL reading I can compute the correct
closeup exposure. For example, on my chart I've noted that my 180mm
lens loses one full stop of light between infinity focus and its closest
distance (it loses a half stop getting to a focused distance of 10 feet,
and another between there and the closest distance of 5 feet). A Nikon
PK-13 extension tube (27.5mm long) on that lens costs one stop. So if
I'm shooting a flower blossom and the lens is focused at a marked 10
feet but it is mounted on the PK-13 extension tube, I know I have a
total of one-and-a-half stops of light loss. If my hand meter tells me
that correct exposure is 1/8 sec. at f/8, I open up one-and-a-half
stops, which is 1/8 sec. at f/4-5.6 or any equivalent exposure.
-----------------------------------------------------------------
==============================================
List usage info: http://www.zuikoholic.com
List nannies: olympusadmin@xxxxxxxxxx
==============================================
|