RE: Pinhole and SLR

[Wayde Allen <allen@xxxxxxxxxxxxxxxx>]

On Thu, 29 May 1997 hannu.jarventaus@xxxxxxxx wrote:

> >         could you briefly describe how this works please?
> >(converting an SLR to a pinhole). 
> >I assume you remove totally the lens from the front.  Is it 
> >something that can be achieved with any 35mm SLR camera?
> 
> The method is very simple.
> You guessed right, the lens has to be removed.
> 
> Take one extra cap for the lens opening in the camera, and
> make a 4-5 mm hole in the middle of it.
> Then tape one piece of aluminium folio on the hole
> and make a little pinhole in the folio.
> I have seen some advertising of readymade pinholes,
> maybe you could use one of these too.
> 
> Because the pinhole is very little the viewfinder image
> is very dim, but in daylight you should be able to see something...

I recently made a pinhole for my Pentax K1000 using an old body cap. 
Simply drill a fairly large hole through the body cap, and then glue or
tape your pinhole over this opening.  My pinhole was made from the bottom
of a throw-away pie tin.  The hole was punched with a straight needle. 
Basically, I did what Mr.  Jarventaus did.  You then have a pinhole
lens for your 35mm camera. 

The problem with this arrangement is that the natural focal length
(distance from the pinhole to the film plane) for a pinhole is roughly: 

   f = s^2/wavelength  where s is the radius of the pinhole

so for visible light (around 550 nm) the optimum pinhole diameter is
approximately:

   D=0.047 * sqrt(f)

where:  D is the pinhole diameter in millimeters

        f is the focal length in millimeters 

Assuming a 40mm spacing from the film plane to the pinhole, the optimum
pinhole size as about 0.03mm.  (Using IR wavelengths will increase this
size very little.) If you make the pinhole yourself you probably won't get
a hole much smaller than 0.2mm which will significantly degrade
resolution.  I would suggest purchasing a laser drilled pinhole if you
want to get the best resolution possible.  You can get these from various
optical companies. 

I'm VERY surprised that Mr. Jarventaus can see anything in the view finder
of his camera.  His pinhole must be fairly large.  However, I can't say
I've really tried looking through the one I built so will have to give it
a try.

> If you will use the cameras exposure metering, you have to
> find out how your camera works with stopped down metering.
> 
> The exposure time can be very long, so you may have to give some
> extra exposure (reciprocity error).

Yep, you are talking about very big f-numbers.  For the pinhole described
above ( f=40mm, D=0.03mm) the f-number is:

   f-number = 40/0.03 = 1,333

For a hand drilled pinhole of say 0.2mm you'd have:

   f-number = 40/0.2 = 200

Also, a normal camera lens has some sort of linkage that tells the
internal meter what the f-stop of the lens is set to.  Since a homemade
lens usually doesn't have this kind of mechanism, the internal camera
meter has no idea what the lens f-stop is so it isn't terribly useful. I'd
recommend an external meter.  You could also meter with a normal lens, and
convert the meter reading to an exposure for the pinhole. 

> For the IR photographs you need some kind of red filter.

Yep, and that will cut the light level even more.  Exposures are going to
be really long.  Better get out the tripod!

Hope this helps.

- - Wayde
  (allen@xxxxxxxxxxxxxxxx)

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