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  #1  
Old 27th December 2008, 09:38 AM
Dave miller Dave miller is offline
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Default Pinhole Exposure

I have just noticed that the pinhole exposure notes that came with the Bulldog were written for the 5x4 version. That raises a question, that, as your collective brains no doubt need exercising after the Christmas excesses, I shall put to you.


The optimum aperture / focal length is given as 125mm at f252 for the 5x4 format. So, my question is, does that hold for the 10x8 format, and if not, what should change? The aperture, or the focal length, or both.
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Old 27th December 2008, 11:17 AM
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You'll find your answers at the following:

http://www.photostuff.co.uk/pinholec.htm
http://www.mrpinhole.com/

Just give it lots & lots & lots
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Old 27th December 2008, 12:36 PM
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Not a clue Dave but many thanks to Carl for the links I'm planning to have a play with pinhole cameras in the new year after seeing Paul Mitchells pinhole pictures and the links are a great help



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Old 27th December 2008, 03:03 PM
Dave miller Dave miller is offline
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Yes, thanks for those links Carl, very useful; unfortunately they did not answer my question. Never mind I worked it out myself, it's 4 minutes at f252.

Actually that wasn't the answer to my question, rather the exposure I worked out, in fact it was a slight under exposure, but I wouldn't be sure until the negative dries. It's all down to the devil reciprocity failure since the metered reading was 15 seconds.

My original question stemmed from the idea that if a 5x4 negative required a certain exposure would a 10x8 need 4 times as much? I came to the conclusion that negative size was immaterial since the same amount of light would be hitting all parts of the negative, so the answer was no.
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Old 31st December 2008, 12:23 AM
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Quote:
Originally Posted by Dave View Post

My original question stemmed from the idea that if a 5x4 negative required a certain exposure would a 10x8 need 4 times as much? I came to the conclusion that negative size was immaterial since the same amount of light would be hitting all parts of the negative, so the answer was no.
Isn't it only true if you use the same "focal lenght" (bellows extension) ?

Otherwise wouldn't you be changing the effective apperture ?

Martin
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Old 31st December 2008, 01:15 AM
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Martin is correct.
The basics of how to work it out are:

The effective Fstop is the Focal Length divided by the diameter of the aperture. So if you know the bellows extension, then you have the focal length and you can either measure the aperture diameter or you know it from the spec of the pinhole which you bought or made. But I think Dave already knows this.

So to the original question: Just making the film format bigger but keeping the same extension and Fstop means the exposure will be the same for 10x8 as it is for 5x4. Except I think you may suffer more from the inverse square law meaning you may well get more light falloff towards the edges of the image. i.e. apparent vignetting at 125mm extension, not real vignetting. But I could be wrong about that. Doubling the extension should reduce that but of course you would get much smaller effective Fstop and much longer exposure times.

Don't know enough about pinhole photography to comment on whether using a bigger pinhole would make things better at larger extension or not, but my very non scientific judgement tells me that an aperture of around 6mm is the smallest you want to go with a lens before diffraction really kicks in. But that sounds much bigger than pinhole cameras use. But then they always show a lot of diffraction.
Anyone know what happens if you make the pinhole that big?
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Old 31st December 2008, 03:58 AM
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http://www.photostuff.co.uk/pinholec.htm

Having looked at the above website, it does seem to answer your question. It gives the image diameter as output. For the 125mm extension with your F252 which is a pinhole of .5mm, then the output image diameter is 162mm. Well on a 4X5 neg the corner to corner distance is around 162mm. But that won't cover 8x10.

So you need to play with the numbers in the calculator to give an output image diameter covering 10x8 which is around 325mm. And infact, doubling the extension to 250mm does it exactly except that it recommends a 0.7mm pinhole and not 0.5mm.

So I think I may have been correct about the vignetting due to inverse square law but I don't understand why cos its a pinhole and not a lens. Perhaps thickness of pinhole material is a major factor. Probably since I heard the best are made from gold leaf which is incredibly thin but totally lightproof, (except for the hole).

(a pinhole material of 0.5mm thickness with a 0.5mm hole will provide a cone of usebale light which is 90deg before any vignetting due to light hitting the sides of the hole. So thinner the material, the bigger its useable cone of light).

New years resolution: Must stop rambling...
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Last edited by Argentum; 31st December 2008 at 04:14 AM.
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Old 31st December 2008, 07:32 AM
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I understand that the best pinhole apertures are made from as thin a material as possible usually by thinning the material at the edge of the hole in someway. The use of gold or silver foil may assist with this. I believe the pinhole which was provided in the Bulldog kit is laser cut, since there is no sign of edge relieving, although it is in very thin material supported on thick card. The pinhole being centred over a 5mm hole through the card.
It looks like it is probably 0.5mm diameter, so the “tube effect” may give the vignetting that you propose. I shall make exposures at both 125 and 250mm and compare the negatives today; light permitting.
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Old 31st December 2008, 07:45 AM
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The thing is that those pinhole calculators must be making assumptions about the thickness of the pinhole material as its not a parameter. That could be one reason why different calculators give significantly different numbers. But the thinner the better as there will be less light being scattered from the inside edge of the pinhole. This is one reason why I pay no attention to theoretical values for diffraction from a lens. They never take the aperture profile into consideration when calculating it. A lens iris with several blades is much thicker than your pinhole material so I suppose it figures that a pinhole can be much smaller before diffraction becomes significant.
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Old 1st January 2009, 04:10 PM
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The reason Gold leaf is such a great material to make Pin Hoes from is it can be beaten into very thin sheet of only several molecules thickness.

Therefore the Coning effect of the tube is virtually none existent – an almost infinite width/depth ratio

Laser Cutting is ideal for Pin Holes as is produces no flange edges - it just punches straight through with no edge deformation

Isn't the fall off in illumination towards the edges of a Neg the Cos^4 effect?

Martin
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