# Formulas for camera obscura (correct sub forum?)

• Stargazing
• Ravensong
In summary, the conversation discusses setting up a military surplus command tent as a camera obscura/pinhole projector to observe a full solar eclipse. The problem is the lack of personal knowledge on the formula needed to determine the correct focal length to hole diameter. The solution is to request assistance from actual scientists rather than relying on sources like Britannica and Wikipedia. There are also warnings and advice given for getting good photos and film of the event, such as the potential for lower light levels to affect photo quality and the recommendation to minimize photography during the first eclipse to fully experience the atmosphere.
Ravensong
Experiment / demonstration: Set up military surplus command tent as camera obscura / pinhole projector for observation of full soar eclipse.

Problem: lacking in personal knowledge of the formula needed to determine correct focal length to hole diameter

Solution: Request assistance from actual scientists instead of Brittanica and Wikipedia

Thank you in advance for the help, I'll no doubt have further related questions on this project as I start attempting to do the maths.

The main driver of the size of the hole is the amount of light you will need, not so much the focus. Larger holes reduce the focus unless the hole is so small that optical effects blur it. The rule of thumb given in http://www.cameraobscuras.com/camera-obscura/faq/ is that the hole diameter should be 1/100th the distance from the hole to the image screen.

Ravensong

Any warnings / advice on getting good photos & film of the event? (mostly wondering if the lower light levels will mess with photo quality)

Film can be sensitive to low light reciprocity error, where the exposure time has to be longer than the normal calculation. It happens fast and changes from full sunlight to night, so it would be tricky. Automated digital photography is probably easier.

PS. I attended a meeting of the Fort Worth Astronomical Society where they were planning their trips. Someone here can probably give you a reference in your area. One of the recommendations from some avid eclipse chasers was to minimize the photography (or don't do any) during your first eclipse and just absorb the atmosphere. Apparently, it is very spooky and memorable. Save the photography for later eclipses.

## 1. How do I calculate the focal length for a camera obscura?

The focal length for a camera obscura can be calculated using the formula: f = (d x D) / (D + d), where f is the focal length, d is the distance from the pinhole to the viewing screen, and D is the distance from the pinhole to the object being viewed.

## 2. What is the ideal size for the pinhole in a camera obscura?

The ideal size for the pinhole depends on the desired sharpness of the image. A smaller pinhole will produce a sharper image, but it will also require a longer exposure time. A pinhole with a diameter of 0.3 mm is a good starting point.

## 3. How do I determine the correct exposure time for a camera obscura?

The exposure time for a camera obscura can be determined using the formula: t = (D x D) / (d x f), where t is the exposure time, D is the distance from the pinhole to the object being viewed, d is the distance from the pinhole to the viewing screen, and f is the focal length.

## 4. Can I use different materials for the viewing screen in a camera obscura?

Yes, you can use a variety of materials for the viewing screen, including paper, fabric, or even a wall. The material should be thin and semi-transparent to allow light to pass through and create an image.

## 5. How can I make adjustments to the camera obscura to improve the image quality?

To improve the image quality, you can make adjustments to the pinhole size, focal length, and distance between the pinhole and the viewing screen. Experimenting with these variables will help you find the optimal settings for your camera obscura.