Pinhole exposure time mumbo jumbo

<p>Hi,<br>

My name's Damien. The introduction is done in the New Users Introduction thread, so here's my first question.<br>

I'm planning to build my first pinhole camera. The aim is to do it right on the first shot (i.e. learning as much as possible along the way and avoid finishing with a totally unusable brick). I intend to make a wooden pinhole camera with a focal length of about 4-5cm to be used with 120 film. For now, the exact focal length is not critical. I'll calculate the optimal pinhole diameter in regard to the exact focal length once the body is done.<br>

I am using Mr.Pinhole for my calculations. This is a nice website that I'm sure all of you know about. The thing is, for a pinhole camera with a focal length of about 4-5 cm an a pinhole sized accordingly (say between 0.2-0.3mm), Mr. Pinhole says exposure time on a sunny day shoud be around 1 second.<br>

Here comes the weird part. For a ready-made pinhole camera like the Holga Wide 120 that shares much of the specifications of my camera to be (focal length and pinhole size), the recommended exposure time on a sunny day is about 7-9 seconds.<br>

I understand that pinhole building is about experimentation, but damn, going from 1 sec to 9 sec is quite a leap.<br /> What is it that I don't get?<br>

Thanks for your help!</p>

<p>Possibly a different ISO of the film/paper?</p>

<p>It's true that Holga doesn't specify a film speed for the Holga Wide 120, but anyway, I guess it's a common one. Even between 100 and 400 ISO, I don't see anything justifying such a difference in exposure time. Still wondering...</p>

<blockquote>

<p>Even between 100 and 400 ISO, I don't see anything justifying such a difference in exposure time.</p>

</blockquote>

<p>Maybe reciprocity failure?<br>

<a href="http://en.wikipedia.org/wiki/Reciprocity_(photography">http://en.wikipedia.org/wiki/Reciprocity_(photography</a>)<br>

When I did night photography at long exposure I experienced first hand that changing the exposure from 5 or 50 seconds does not always make a big difference.</p>

<p>Ok, here we go... A 5cm (50mm) focal length with a .2mm pinhole in front of it gives a f number of 50/.2 or 250. The sunny F16 rule for ISO100 film is a shutter speed of ~1/125. F250 is close to 8 stops smaller than f16. 2 seconds is about 8 stops slower than 1/125.</p>

<p>So the time you've been given is ball park correct. Reciprocity failure varies widely depending on the film you're using. T-MAX 100 corrected would be about 3 seconds. Other films may need no correction or more correction, depending on the manufacturer's recommendation.</p>

<p>Okay, so if I follow you right, considering the above-mentioned specifications of my future camera, exposure time on a sunny day sould be around 1 sec. Now, due to reciprocity failure (thanks for the Wiki link, Niels), there is a high chance that exposure time will be longer than what is specified by Mr. Pinhole and confirmed by Bob.<br /> Actually, I am afraid of an exposure time of 1 sec. This is way to short (in terms on pinhole photography indeed) for what I intend to build. I plan to have a shutter that opens manually, but 1 sec is quite quick. Below 2 seconds is too quick actually, with a high risk of shaking the camera around and blurring the image. You don't suffer as critically of that issue when you can calmy open the shutter manually for about 10 seconds... I obviously intend to use a tripod.<br /> Do you think that I should rethink my camera design or do you think that I will be safe with my design and that thanks to reciprocity failure, the exposure time will be longer anyway?<br>

Oh, by the way, I have a full box of Ilford XP2 Super 400ISO in 120. I understand that reciprocity failure is relative to each brand and type, but I am such a newb that I don't know if I'll have some latitude about exposure time with that specific film. ISO400 is maybe quite high, but that's what I have around, so...</p>

<p>If I was building a pinhole camera for 12o film, I'd probably try for a 40mm focal length with a .2mm diameter pinhole. Exposure with ISO 100 film would be around one second. This is easy to manually time. Incorporating a cable release in the camera can reduce camera shake, but shouldn't be necessary with a rigid tripod. One way around moving the camera is to rest the camera on anything reasonablly stable, hold a sheet of black cardboard close in front of the pinhole, open the shutter, and time the exposure with the cardboard.<br>

For determining pinhole diameters I prefer <a href="http://www.pinhole.cz/en/pinholedesigner/">http://www.pinhole.cz/en/pinholedesigner/</a> with a user constant of 1.4. A site dedicated to pinhole photography is <a href="http://www.f295.org/site/">http://www.f295.org/site/</a>. Another site with much information is <a href="http://home.online.no/~gjon/pinhole.htm">http://home.online.no/~gjon/pinhole.htm</a>. It also has links to many other good sites.</p>

<p>Please note that in October 2015 my webpages on pinhole photography and building large format cameras move to: jongrepstad.com</p>

<p>Jon Grepstad</p>