Pinhole puzzle

White Rose

White Rose

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Richard
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I'm planning to have a go at pinhole photography using an old body cap on my Nikon d50. My plan is to frame the shot using a lens and then swap to the body cap to take the shot. The puzzle I've got at the moment is which focal length to use to replicate the same angle of view as the pinhole. Any ideas?

Cheers
 
If I remember Optics from my A-level Physics (more than quarter of a century ago) - the focal length is the same as the distance between the aperture and the screen/film/sensor - so think in terms of 25-35mm... the optimal size of the hole is something like f/25 - f/100 (1mm-0.25mm diameter hole) so fairly small and I think the way to do it is to cut a large (inch?) hole from the centre of a body cap and glue something opaque* and thin over that large hole and pierce that for your pinhole (think foil/similar) otherwise a 1mm thick plastic lens cap is going to introduce other effects than pure pinhole to the picture.

*Make sure the glue joint is opaque around the edges too though.

Edit: looking a bit deeper that's cobblers :) - the diameter of the hole is proportional to the square root of the focal length. Try looking at http://www.pinhole.cz/en/pinholecameras/pinhole_01.html for a starter.

Rearranging the formula for f (focal length in this case) leads to it being approx 500 times the square of the diameter of the hole and the f-number would be 500 times the diameter of the hole. So for a 0.25mm hole...

focal length would be about 30mm and f-number 125

Code: 
d	f	f-stop
0.25	31	125
0.33	54	165
0.5	125	250
0.7	245	350
1	500	500
 
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I can't get my head around that - the bigger the hole, the higher the f-stop - surely a bigger hole would be a lower f-stop?

Edit: My bad - I didn't look at the focal length. 2nd edit: Surely though, the effective focal length will be the distance from the hole to the sensor (even if that means the image will be out of focus)?
 
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But a pinhole doesn't work like a normal lens... the variation in the aperture changes the field of view (as would the focal length with a normal lens)
 
Yeah but whether it changes the FOV or not, at a given fixed distance from the sensor (which is what you'll get if you're using a body cap and no tubes), a bigger hole will equate to a lower f-stop.
 
I'll defer to Rayleigh who developed the equations and I could have made a schoolboy error in developing the one for the aperture (although since the appent focal length is varying inversely with the square of the diameter of the hole and the aperture is varying inversely with the diameter of the hole - it seems to hold).
 
I think the problem is that because a pinhole does not focus the same way that a lens does, there isn't a specific focal length for a hole; but for a given hole diameter, there is an optimal 'focal length' (distance from hole to sensor) for image sharpness - and so those focal lengths given in the table above are not physical facts but suggested distances. With the body-cap model, the focal lenth will remain fixed and not increase with hole diameter and thus the f-stop number will decrease with increased hole diameter not get bigger. We're probably saying the same thing in a roundabout way :nuts:
 
I agree that the distance between the hole and the film/sensor plane is fixed - however the whole point here is about apparent focal length (as in approximating the focal length seen by the field of view) and the apparent aperture (which is a function of the apparent focal length)
 
Nah - you've lost me again - I can't see how or why there is a point in an apparent focal length by "approximating the focal length seen by the field of view" - I can only see that being affected by the thickness of the material that the hole is in (i.e. what angle of light will pass through the hole) and I take it that by apparent aperture you mean f-stop (as opposed to the actual diameter) - that is not a function of apparent focal length but is absolutely determined by the actual focal length, i.e. the distance from the hole to the sensor.
 
The pinhole has two properties:

1. apparent focal length (ie. what a standard lens would have to be to give similar effect) from the equation is inversely proportional to the square of the diameter of the pinhole

2. the apparent aperture (ie. what a conventional lens would have to be to give the same effect) which is inversely proportional to the diameter of the pinhole.

None of this has any bearing on the size of the sensor/film used - because as the pinhole gets larger, the image cone gets smaller - so you will find that as you make larger holes then the image cast onto a sensor at any given depth from the pinhole will get smaller too.


Any Physics teachers in the house? :D

EDIT:

I should have looked in a Physics textbook and not on t'interweb...

The equation I set out from has one purpose only - to calculate the optimum diameter for the hole in your pinhole camera.

d = (1.9).sqrt (f.l)

f= hole to image plane distance (say 30mm a reasonable estimate - not sticking a ruler into my camera to find out :))
l = wavelength of the light of interest (normal to use 550nm as a guide)
d = optimum size of the hole

That gives an optimum hole diameter of 0.244mm (call it 0.25mm for cash)

None of which bears any relation to the apparent (as if replaced by a conventional lens) focal length or aperture. But DOF can be regarded as infinite in the first instance...
 
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Going back to White rose's original question, "what focal length to use"

Well, as you're using a DSLR as your pinhole rather than a film camera why not do a test shot with the pinhole attached, preferably a subject that gives you defined edges, like a building. Then look at the results on the monitor then attach your lens, assuming it's a zoom lens and match it?
 
I read this thread and thought I'd have a quick go:)
Excuse the bad quality of picture, its late and im tired:razz:

Using a cheapo extension tube set , I simply put a piece of kitchen foil between two rings and punched a hole using the smallest needle I could find.

DSC_6272_01.JPG


this produced the image:-

DSC_6268_01.JPG


vs

DSC_6270_01.JPG


exif shows 50 mm.
 
White Rose said:
I'm planning to have a go at pinhole photography using an old body cap on my Nikon d50. My plan is to frame the shot using a lens and then swap to the body cap to take the shot. The puzzle I've got at the moment is which focal length to use to replicate the same angle of view as the pinhole. Any ideas?

Cheers
Click to expand...

Your D50 should have a little symbol on the top plate - a small circle with a horizontal line going through it - that is the line of the sensor inside the camera - used to be called the film plane indicator before digital arrived and it shows the exact line of the sensor inside the camera body.

If you put your body cap on and hold a ruler over the top of the camera and measure the distance from the film plane/ digital sensor symbol to the front of the lens cap externally you will get your focal length.

As a previous poster stated - don't put a ruler inside the camera body

My Canon is as close to 50mm as makes no difference and with a pinhole diameter of about 0.3mm I get an fstop of 166. Pinhole work is not an exact science :)

If you are making your own silver foil pinhole - just use the very tip of the smallest needle you can find - not the main body. You are looking to make a pin prick of a hole of less than a third of one millimetre in diameter.

Just experiment until you get the results and exposure you want - it is a lot of fun
 
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Thanks for all the advice. I've drilled the body cap and tried making several different pinholes using cooking foil. My initial thoughts are:

1. A smaller hole equates to a 'sharper' image.

2. None of my images so far are actually sharp enough to print.

3. The angle of view is less than I had imagined it would be. Although I haven't measured it, I would guess it's about the same as a 50mm lens.

4. Dynamic range on the images seems noticeably worse than an image captured with a lens. This maybe due to my camera and the longer exposures necessary with the pinhole.

5. The pinhole really shows up the dirt on your sensor!

It's fascinating trying to get a decent image. I can see this becoming a bit of an obsession.

Thanks again for the help.

Cheers
 
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