• Tom Lee

Pinhole Confusion

WARNING! – Reading articles like this one can get you labelled as a geek (even worse when you start writing them!)


I love the #nostalgic feel and sense of time that the look of pinhole photography gives you. Not so much digital pinhole, but #analogue with film. My preference is for wide format 6x12 format landscapes using Ilford Delta 100 film. Some #purists would have you believe that you should accept what the medium gives you; in other words, keep all the ‘warts-n-all’ #imperfections and other blemishes that working at slow speeds and no lens gives you. However, I’ve been looking at how I can exercise some control over the final result (albeit in post edit), to suit my #personal taste in the medium. To do this I need to explain where my difficulties lie and with the help of my friend Keith Ellul, explain the science behind the #pinhole genre and some of film #scanning quirks.

What is Pinhole Photography?

A pinhole (formerly known as #camera #obscura) was originally developed as a drawing aid using a mirror to project an image outside a darkened room or tent onto a table where the image could be traced onto paper. In time the dark room was replaced by a box and the mirror by a pinhole.


As the name suggests, it involves light passing through a minute hole (the pinhole) in a box or chamber containing light #sensitive material (film or paper). The film or paper will capture the image which when developed, shows the scene without the need for a focussing lens. The effect of a pinhole is a natural #phenomenon with some specific #idiosyncrasies including an inverted image of the scene on the sensitised material.





Diagrammatic representation of what happens with a pinhole image inside of ‘the box’

Other characteristics of pinhole photography include long #exposure times, slight blur to trees due to wind or missing people altogether, a soft image quality, almost infinite depth of field and no rectilinear distortion amongst others. Much of the foregoing will vary due to the size of the pinhole and depth to the sensitised material from it. This #essay however, is not concerned with how to take the pinhole image but how to deal with the results of it!


Conundrum #1 : Circles of Confusion

For most photographers this subject will be an #irrelevance, other than to understand why paying for a good lens on your digital camera is really well worth it!


But, how can a pinhole camera with almost #infinite depth of field deliver a soft image? My Noon 6x12 has an equivalent aperture of f/231. You could be mistaken in thinking that everything taken with this aperture would be pin sharp front to back, but then the #science interferes…..


When you look through a modern camera with a good lens, everything you shoot should be sharp, irrelevant of the aperture that you shoot at. This is because the lens on the camera is focussing the lightwaves entering the camera onto the exact position of your sensor chip or film plane. Most lenses are not capable of focussing in absolute terms, so instead of a precise dot we get a ‘#circle of #confusion’ (or small spot). The spot is caused by the penumbra (or shadow) created by the lens aperture. The smaller the aperture (f22) the greater the depth of field. If you have a substandard lens or one that doesn’t focus well, then the ‘confusion’ or spot on the film or sensor will be just in front or just behind where it needs to be; giving you a soft or even out of focus image if it’s really bad.


Focus point too close

Perfect Focus (smallest possible circle of confusion}

Focus point too far out

Diagrammatic representation of ‘circle of confusion’ on a sensor or film plane. The aim of any lens it to get the dot as small as possible for the greatest sharpness in an image. (source : Goliardico~commonswiki)

In its simplest analogy, when you look through a magnifying glass, you focus the lens so that whatever you’re looking at appears clear and sharp. You have minimised the ‘confusion’ to the best possible #focus making everything not only magnified, but clear.

In a cheap camera lens (think Holga or toy camera), often the result can be seen as ‘#chromatic #aberration’ in a colour image. This is because the red, green and blue light that make up the colours we see in the visible spectrum, travel at different #wavelengths and the circles of confusion do not focus on the exact same plane. In a black and white film the effect is still the same but the wavelengths are resolved into tones of grey on the film surface.









The different light wavelengths do not intersect at the same point on the focal plane, giving rise to multiple ‘confusion’ points (source : dpreview). This is rectified by having multiple groups of lenses to correct the ‘confusion’ of wavelengths.







When you pay for an expensive lens, this usually means that there are more optics or groups of elements capable of handling the differing light wavelengths, thus placing the circles of confusion more accurately onto the #film plane or sensor. More lenses and groups equal more expense and (usually) better quality.


Now the choice of film also has a part to play in how sharp the image looks. The ‘Callier effect’ is the variation in contrast that particular film types produce when exposed under varying light conditions. The more contrast that a film provides, the sharper the image appears, but should not be confused with focus!


Now back to the pinhole…….this type of camera has no lens at all! This means that the wavelengths are free to please themselves, however the pinhole (0.15 microns) means that the ‘confusion’ spot (or penumbra shadow caused by the small pinhole aperture) is extremely small and the dispersion of wavelengths is inconsequential, but does give rise to a soft looking image. The answer is to go with the smallest possible pinhole you can find, however this also increases your exposure times that could already be in the minutes (if not hours). The problem is therefore one of #compromise. I would say that now that I know what the issue is, I can accept the soft image for what it is – a function of the camera, technique and style.

Conundrum #2 : Grain Aliasing (in film)

What is Aliasing?

#Aliasing or digital noise is uniform and well known in digital circles; caused by the variation of brightness and colour information produced by the camera sensor. As the image is sampled and processed, the image is re-constructed for viewing on the screen or computer and an ‘alias’ will appear, which is different from the original. Typical examples would be #moiré patterns or jagged edges at boundaries of high and low contrast.


The silver halide grain in a film by contrast is completely random. They are either developed or washed out and vary in size, as does the distance between them. When we scan film (even at high resolutions), we are effectively re-sampling the image and interpreting the ‘grain’ in the film in much the same way that a digital camera sensor would do. Given that the grain is random, even if we are using fine grained film, any re-sampling will increase the effective aliasing in the scanned image. The aliasing effect will therefore be more apparent in coarser or faster film types.

This is probably why when making a traditional print direct from the negative, the aliasing or random nature of the grain is not apparent. When #scanning at a high resolution we are in fact re-interpreting the structure of the grain and magnifying the randomness of the effect, thereby giving rise to a coarse grain structure in the resampled image.

According to photoscientia.co.uk - 'What appears to happen, is that once the grain clumps reach a critical size (approaching pixel size) aliasing is unavoidable, and this effectively amplifies the grain, making it much more visually obvious and objectionable.'

Left : is the simulated film grain, before scanning. Centre : is a representation of the tri-linear sensor grid of the scanner CCD. Right : is the aliased film grain. (This is the result of combining the film grain image and the CCD sensor grid image in a way that simulates the pixelisation process.) Source : photoscientia.co.uk

What can we do to reduce the problem?

The one sure way to #circumvent grain aliasing problems, is not to use film that provokes those problems in the first place. (fine grained slow film). This won’t completely eradicate the issue, but will be better starting point. Another is to use a large negative! Scanning from a 35mm negative to achieve a 20” print is far more destructive than a similar scan from a 120 film for the same size print. This has always been the case even in conventional darkroom printing due to the amount of information contained in the original capture.

Increasing the #resolution of the scan can actually exacerbate the problem of aliasing, and may need some experimentation due to the variables in scanners and software. Some people have tried throwing the scan slightly out of focus which has the effect of reducing the noise but losing some detail overall in what is an already soft image. Not very desirable and tricky to do if your scanner does not have a means of focussing manually. One method is to use something like anti-newton glass used in slide mounting under the negative to diffract the scan slightly (although not tried). Vuescan, is third party software designed for older scanners that may have become obsolete and also for more updated scanners that do not have any #focus control and may be more suited to this application.

However, with the recent updates to scanner software, there are two new areas to explore – one appropriately enough is called ‘Grain Reduction’ and the other is called “Digital GEM’ but both essentially do the same thing. It’s important to note at this stage that there are degrees of low, medium and high (or light, medium, heavy) and both scanner updates need to be explored. I use an Epson Perfection V750 Pro scanner and the following is related to the controls provided with this model, although other models and manufacturers have similar controls.


I decided to give the new #software a thorough workout and chose a particularly grainy ISO400 HP5 negative from my old TLR camera for my test. The image below was my ‘control’, scanned before and after the software update with minimal control.

A comparison of the scans do not look much different at full size other than a slight change in tonal adjustment.
The images above compare the grain structure at 100x magnification of a small section of cloud showing the improved scan progression (left to right).

OK, other than a slight loss in #contrast, I’ve established that the scanner software improvements can help reducing the aliasing of scanned negatives (I’ll post my settings at the end of this article), but is there more we can do to smooth out the pesky grain?

Photoshop has some useful tools in the Filters section but to get the most out of them we need to use them in conjunction and in the right way. Firstly, copy the layer by using stamp visible (Ctrl+Alt+Shift+E for windows) or (Ctrl+Opt+Shift+E for mac). Go to Filter>Blur>Surface Blur and keep the Radius slider low and adjust Threshold. This is a little trial and error but you can do a pretty good job using this method. Remember your setting will depend on your original scan resolution so a Radius of 5 and Threshold of 15 is about right for a 1200 dpi scan off a 6x6 neg. Use a mask to apply it to the areas of the image that need it. In other words where the grain artefacts’ detract from your final image. You should only apply this at the end of your processing because any adjustments you make (including resizing) will amplify any distortion in the grain structure.

So, let’s compare my original image pre and post anti-aliasing methods…..

The original scan with defaults – note the heavy grain structure in the sky even after processing in Photoshop.
An adjusted scan with the grain reduction anti-aliasing technique described above. Although the grain structure has not been completely removed, it’s much smoother and less objectionable. This would be greatly reduced if my usual Delta 100 was used instead of the HP5 400.

As promised, my settings for the optimum scan are shown below:

In the main settings, its important not to have the resolution set too high as this will result in exacerbating the artefacts’ found in the film. The scanning quality however, should be the highest possible.

When in the #Advanced Settings, you should be able to adjust the histogram by clicking the Detailed Adjustments tab and fine tune highlights and shadows of the scan. You may want to adjust the Unsharp Mask and Dust Removal settings, both of which will affect the clarity of the negative. A lot will depend on how much grain you intend to remove. It’s not always necessary to further adjust the grain structure in Photoshop after scanning, it really depends on the quality of the negative. Remember I chose a particularly nasty example to see how far I could rectify the issue and we discussed briefly the #Callier #Effect that some films will have.


Conclusion

To finish off I will use the exact settings above to scan a 6x17 negative to produce a similarly large print from and check the grain structure in a true pinhole image. Film stock was HP5 Plus (400 iso) from a 6x17 Reality do Subtle Pinhole Camera with a 300µm (0.3mm) pinhole, giving an f/233 aperture.

The image above is scanned using my above settings and re-sampled to at 20” wide print at 300ppi. The red square was sampled and enlarged 10x to examine the grain structure.



The image left represents a 1000x enlargement of the original negative!

As you can see the grain is no longer a significant issue and has been much reduced. I haven’t even used a post-production ‘Surface Blur’ technique. If I were to use a slower speed film with finer grain it would show another significant improvement.


All things considered, I think it shows that scanning negatives, particularly high-speed film has serious problems due to electronic aliasing. The scanner manufacturers appear to have gone some way to rectifying the problem with software updates and I dare say, the more modern scanners will be better again. I will however suggest that scans of even large format negatives are kept between 600 and 1200dpi resolutions, with 35mm films at around 600dpi maximum.


Happy Scanning.

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