Since my last post I have been deliberating over my OM-1 body cap lens and how I want to move forward with it. I had a couple issues with the images produced by the 1mm pinhole and sought to rectify those by the second attempt.
I wanted my images to be slightly sharper than that of my first test run. During my research I came across a formula for calculating the optimal pinhole size and decided to have a look into that. This method was first attempted by Josef Petzval and is as follows:
d=2√2fλ
Where d= pinhole diameter, f= focal length (distance from pinhole to focal plane) and λ= the wavelength of light. For standard black and white film, a λ corresponding to yellow-green (550nm) should yield optimum results. Once all values are converted to mm, for my particular OM-1 body cap lens the formula looks like this:
d=2√(2)(42)(0.00055)
d=0.43mm
Seeing as my reasoning for using a pinhole in the first place was the blurriness of the images, I didn't want a completely sharp image as that would defeat the point (in my eyes) of using a pinhole for this project at all, but this figure of a 0.43mm pin hole was a good reference point for future lens construction. For this reason I decided to construct a new body cap lens with a pin hole diameter of 0.5mm and see where that left me. I tried to shoot some similar images to those that I shot with the 1mm lens, focusing more on the castle ruins as I felt the atmosphere of the pinhole images leant itself very nicely to the stone ruins as seen in the last test and spoken about in my previous blog post.
The first image above is the one taken with the 1mm hole, and the second is with the 0.5mm hole. There is a slight difference in the sharpness, the second has a slightly more defined edge to it, but where the 0.5mm really shines through is the enhanced detail in the stone work. It is much easier to pick out the individual pieces of stone in the second image, without it being so clear that a regular manufactured lens could have done the job. I feel that the 0.5mm pinhole strikes the balance between sharpness and the classic fuzz of a pinhole lens that I was looking for when crafting it.
Here is another one of the 0.5mm images that I feel showcases the improvement on the 1mm very well. The range of tones and the contrast shown in the 0.5mm images is much closer to what I want aesthetically than that of the 1mm pinhole lens. Being able to see just that little bit more detail in the stone walls, whilst still leaving enough of the hazy merging of stones creates such an air of intrigue and mystery that pulls the viewer in to the image and leaves them searching through it to a greater extent than any of the 1mm images I captured.
As with the 1mm test roll, I was metering the scene with my light meter, then converting the reading so that it corresponded with the f-stop of the lens I was using (for this 0.5mm pinhole lens, the f-stop was calculated to be 84). This time however I was using an online converter (https://www.scantips.com/lights/exposurecalc.html) instead of the paper wheel I had crafted, mostly due to the weather conditions on the day. It was incredibly wet, windy, and cold and I was worried about damaging the wheel or it getting blown away (as well as wanting to keep my hands inside my pockets as much as possible). Due to the lens having a much smaller hole, not only was looking through the view finder and seeing an image that much more difficult, but the exposure times needed were that much longer. The weather on the day obviously also played into it, but on the test roll with the 1mm I didn't take a single shot that lasted longer than 1s, this image above was an 8 minute exposure and wasn't even the longest of the day!
A feature of film that I have had to take into account with these longer exposures is something called the reciprocity failure. This is the name given to the phenomenon that the longer light hits the surface of the film, the less reactive the silver particles on the film are to said light. This reciprocity failure is something that comes into effect when exposures are longer than roughly 1s. Each different film stock has its own different reciprocity failure compensation value, for the film I am using (Ilford's HP5+) this value is 1.31. This means that to get the correct exposure time and compensate for this failure I have to do the metered reading to the power of 1.31 like so:
Tc = Tm^1.31
Where Tc is the calculated exposure time, and Tm is the metered exposure time. The metered reading and subsequent conversion to account for my given f-stop was 256s (about 4.3 mins), which when put into this formula gave a calculate exposure time of 1428s (23.8 mins). The first image in the slideshow above was taken at the 4.3 minutes the meter reading gave, and the second at the calculated 23.8 minutes. In my opinion, a number of the images on this roll seem slightly overexposed, even the shots that I didn't have to compensate for the reciprocity failure, so I believe perhaps the online meter reading converter wasn't as accurate as the handheld constructed wheel I had used for the previous roll of pinhole images. However, out of the two images above I feel that the longer second shot is the most successful of the two. The film grain is much more visible in the second image, something that I am very fond of aesthetically as it gives off a very eerie vibe.
Overall, I feel that the 0.5mm hole for this body cap lens is the better of the two for what I am looking for. I will have to do another test roll using the handheld exposure convertor to see if the overexposure issues I was having was down to the online converter I was using or not. The good thing about this body cap lens's design is that to change the size/configuration of the actual hole, all that needs to be done is the small square of aluminum is taken out and a new one put in and secured down with black tape. This means it will be very easy for me to experiment further with pinhole shapes/designs for this camera without having to invest in multiple body caps and whilst still being able to go back to a previous configuration.
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