Pitfalls of Pinhole Aperture as Aperture Diffractions
Images from Pinhole Aperture Cameras capture image details of a scene from near to far without any focus point whatsoever. Though Pinhole camera images have charm, any quest of sharp image details is not possible by this method alone. We'll show the pitfall of small aperture photography and how Aperture Diffraction is a relevant concern for aperture choice selection.
In the link Understanding Depth of Field we suggest how you can make a working Pinhole camera with your dSLR camera. We also show you one of two reasons why depth of field can not deliver sharp details, because light is not refocused onto the image plain. A lens does this by redirecting light to a sharp point.
Where aspiring photographers may have the urge to seek high resolution images of details from near to far by using ever smaller aperture settings, there is also destruction of your images taking place through Aperture Diffraction, as you venture into smaller aperture sizes
On a large format image camera, (ie:4x5, 5x7, 8x10"), apertures like f64 are not as destructive because the actual size of the negatives are so large, and require only small amounts of magnification for the most common enlargement sizes that might be used by peoples in living spaces, so the effects are so small the viewer does not experience the negative destructive effect.
When you use a 35mm image format and small aperture sizes, the size of the image format can reveal this damage of aperture diffraction at even fairly minimal enlargement sizes.
Consider the 35mm image format. If you enlarge a 24x36mm image to an 8x10 inch enlargement size, you have enlarged it 59.73 times. Where if you shoot a 8x10 inch large format negative, there is no enlargement at all to an 8x10 print size, you got a contact sheet. Enlarge an 8x10 negative 59.7 times, and you got a whopping big print of 39 x 49 feet.
Image from dSLR used as pinhole aperture
Simulating a pinhole camera
Canon 5D Mark II, Tinfoil with pinhole
1.3 sec @ ISO 400
Pinhole Aperture - Randy Smith Photography © 2011.
A Pinhole Aperture camera is a good example of the effect the aperture size has on an image by reducing size of Circles of Confusion so that details are revealed, there was no lens used to make this image. If you are not familiar with the concept of Circles of Confusion, see Understanding Depth of Field. The image to the above is a Pinhole image taken from my dSLR camera.
I used tinfoil over an extension tube, and no lens to take this picture. I pricked the tinfoil with just the tip of a sewing needle for my aperture pinhole.
A lens will refocus light onto the image plain, the pinhole camera will not. The aperture opening only restricts light down to narrower Circles of Confusion.
Q: So why not shoot all my shots at a the smallest aperture?
Pinhole Aperture Aperture Diffraction
It turns out there is yet more physics in dealing with light and the aperture opening, particularly when we are working down around Pinhole Aperture sizes.
We think of light traveling in straight lines, but the course of light can be influenced in several ways:
Think of a wave of water as an example. When a wave on the surface of the water reaches an object that sticks out of the water, or waves pass a through a slit in a barrier or a wall, the waves will spread out after passing the small opening in the wall, the waves will even rap around the wall. Light will do this also, although to a much smaller fraction of what water will.
Imagine how this will effect the details in your scene some of the light starts altering it's course after it passes through the small aperture opening.
There is not a strong presence of this, but their is a visible influence on subtle details, especially when you enlarge the image 59 times.
Pinhole Aperture - Aperture Diffraction
Pinhole Aperture Airy Disk
This image is enlarged from the Pinhole Camera image shown above. The picture has many Airy Disk all over the place in that image. In a science lab setting, these are replicated by using intense light coming through a single small hole in a plate that is in front of a light target or light sensitive medium.
Admittedly my self made pinhole aperture diameter is likely much smaller hole than most Pinhole cameras use, which helped lead to the formation of Airy disk in the presence of strong specular light.
The width of these bands are related to the wave length of light, in this frequency range. Light theory suggest that light cycles between positive phases and equal negative phases. Frequency wave length is the measurement from the top of a peak in one phase, to the top of the next peak in that same phase.
The bright bands of light are caused by having numbers of photons that happen to be in the same phase, when this happens the energy is additive and so it gets brighter. The bands that are dark are not the absence of light, they are equal numbers of photons who are near perfectly out of phase with each other, that is equal numbers of positive and negative phased photons. They cancel out each other's energy, making it appear as dark to us, but potons are there.
The effect of an Airy Disk forming is related to the width of my aperture opening (having been created by putting a small hole in tin foil), the width of which being somewhat close to the actual light wave length frequencies passing through the aperture opening. I was surprised to see these, I did not plan this.
Sunlight was the source of this light that made these Airy Disks, light came back at the camera at an incident angle from a glass business building just out of frame to the right. Light off those windows were reflecting light back to my camera. The results of many windows had made quite a few airy disks on the image.
Pinhole Aperture - Airy Disk
Pinhole Aperture Pitfall
What is important here !
Smaller apertures influence a small portion of photons by changing their direction travel, and this starts mucking up your image details. So the smaller the aperture, the greater depth of field but also the greater potential for details to loose resolving definition. Images get less sharp.
What a strange combination, on one hand we think we are getting better images and then we find out they are getting a little worse.
Let me refine this statement more. Not only does the Depth of Field resolutions loose more detailed definition, but also there is also definition of details being lost on the plane of focus, so all of the details suffer.
Choose which star is less sharpest?
Can you see which star has less defined details?
Place mouse over image to read answer.
This poses a question as to at what point do images start getting less sharp. That is, at what aperture setting can I maximize the best of both image sharpness and depth of field.
As it turns out, there is such setting, and part of the answer has to do with the design of the lens.
As you decrease aperture openings from the lens widest opening you are reducing the Circles of Confusion sizes which has a huge effect on DOF presentation, but you also are moving closer to the settings where the presence of Diffraction effects become more apparent. So the resulting answer is a balance of these two factors, your chosen Circles of Confusion size, and Diffraction.
Sharpest Aperture Setting for ResolutionA setting of the aperture two stops down from the lenses widest opening (or vary close to it), equals the best resolution of the lens and depth of field for image details. If you are shooting flat art, this is where you want to be.
An important point - Pitfalls of Pinhole Aperture
This is vary important to remember, for getting the best results for taking pictures of any flat art work. Adding depth of field beyond this point will not help you get higher resolving images.
As a ball park guideline, there are some landscape photographers that resist when ever possible at shooting beyond f16, so as to maintain a good image sharpness of details for enlargement options. This is another notable benchmark to keep in mind with working out your depth of field options. However, testing your own lenses for what happens under these conditions is something that never hurts.
But again this is something to consider while working with the 35mm camera. As you use larger format sizes you can use smaller apertures and suffer less negative effects of while trying to gain more depth of field, because you are not likely to enlarge those image files less than you would the small 35mm image format to reach your desired enlargement target size.
This concludes the four page section dedicated to aperture. And at this point you have been introduced to concepts of working with aperture that many photographers don't learn for years trying to pick up photography on their own so you are way ahead of the learning curve.
If you are reading these pages in sequence, the next topic combines working with Shutter Speed and Aperture for balancing exposure.
& Slow Shutter Speed
And Story behind it!
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