Comments at bottom.
At 2:09 AM +0000 12/5/03, olympus-digest wrote:
>Date: Thu, 04 Dec 2003 19:06:04 -0600
>From: "Curtis P. Hedman" <Curtis.P.Hedman-1@xxxxxxxxxx>
>Subject: [OM] OM Pin-hole photography problem - help!
>
>I decided to try pin-hole photography with my OMs recently... I bought a
>body cap with a drilled piece of stainless steel in the center. When I
>tried it out on my -4T, I expected a pretty long "open shutter" time, given
>the f/stop is on the order of 180 or so. To my surprise, almost every
>exposure I tried clicked off at about 12-14 seconds, regardless of what I
>pointed the camera at. Is there something I'm missing? Is there some tab or
>coupling pin on the back of the OM lenses that enables OTF metering - the
>modified lens cap obviously does nothing except engage the three bayonet
>lugs... Any suggestions on what I can do to make this work? I'm thinking
>about trying a short extension tube between the body and the cap...
>
>Any thoughts or guidance will be appreciated!
>
>Curt
>
>------------------------------
This should work.
At 2:09 AM +0000 12/5/03, olympus-digest wrote:
>Date: Thu, 4 Dec 2003 20:41:30 -0500
>From: "Walt Wayman" <hiwayman@xxxxxxxxx>
>Subject: Re: [OM] OM Pin-hole photography problem - help!
>
>Using the "Sunny 16" rule and an ISO 100 film, I come up with
>this, more or less:
>
>1/125 sec. @ f/16
>1/60 sec. @ f/22
>1/30 sec. @ f/32
>1/15 sec. @ f/45
>1/8 sec. @ f/64
>1/4 sec. @ f/90
>1/2 sec. @ f/128
>1 sec. @ f/180
>
>Am I missing something?
What's the f/ of the pinhole? It's computed as the hole-to-film distance
divided by the hole diameter, same as for a lens.
The film-flange distance is 46mm. The body cap is good for maybe 6mm, for a
total of 52mm. The hole is probably 0.5mm, so the pinhole probably has an
effective speed of f/104, so by Walt's table the exposure ought to be 1/3
second or so.
Walt's table is correct. For a given scene illumination and film speed,
ignoring reciprocity effects, all valid combinations will follow the following
equation: A^2/T=constant, where A is the f/ and T is the shutter speed in
seconds, and the constant involves film speed and illumination. The equation
is from section 3.1 of ANSI PH2.7-1973 "Photographic Exposure Guide", which
tells what's in the constant.
16^2/(1/125)= 32,000. Tx=104^2/32000= 0.338, or 1/3 second.
In another posting, Peter mentioned that his pinhole was 0.1mm at 5cm, which is
50/0.1= f/500, so let's do the shutter time computation for that pinhole:
Tx=500^2/32000= 7.8 seconds, which is within OM-4 territory, at least the max
shutter open time.
The TTL metering integrates light and when sufficient light has been collected
for the film (based on its speed, ISO 100 in this case), closes the shutter.
The lens setting (and thus the coupling pins) makes no difference to film
exposure, but is used to drive the viewfinder display.
Given the low resolution of pinhole cameras, use of faster film should cause no
loss of sharpness. John H suggested a combination of lens and pinhole he saw
in a model railroad magazine. This would allow the pinhole to be large without
loss of sharpness.
OMs with electronic shutter control sense only light hitting the film plane,
and shouldn't care about the lens at all. What happens if you fire this camera
with no lens, but in a dark room or with a hand over the lens opening? In my
OM-2n, the shutter will open, and stay that way for awhile, then close. The
timeout period varies with the camera., but 12-14 seconds is plausible. If one
removes the hand, the shutter immediately closes. Is the pinhole perchance
blocked? Or far smaller than 0.5mm (500 micron)?
Joe Gwinn
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