# Why Does Film Blur with Basic Box Camera? | Explaining Focus

• aviator
In summary, the conversation discusses the concept of focus in optical devices such as cameras and magnifying glasses. The thin lens equation is mentioned, which explains how the focal length, object distance, and image distance are related. The conversation also touches on the idea of conic projection and how it relates to focusing. It is concluded that a pin hole camera, which works based on the principles of conic projection, does not have a traditional form of focus due to the lack of a lens.
aviator
this is what i would expect from a conical perspective on a flat plane that is how a basic camera works, the farther the film from the hole the bigger the projection of the image but insted the image gets blurry, why?

in a close image the center of the picture is closer to the viewer than the corners how comes that if the center is foucused the corners are also focus when they are farther than the center

could someone explain me or point me to a place where is explained focus?

For a thin lens, as in you simple box camera image formation is modeled by the thin lens equation.

$$\frac 1 f = \frac 1 o + \frac 1 i$$

where f is the focal length of the lens
i is the image distance and o is the object distance.

In a simple box camera the film is placed at the focal plane of the lens so any image which is near infinity will be focused on the film. The usual approximation for lens is that anything greater then 10xf is at infinity. So a camera with a 2cm focal length lens and the film at the focal length would be reasonably focused for any object greater then about .2m in front of the camera.

thanks a lot integral

but still seems to me that optics is not a conic projection since in conic projection there's no focusing so what kind of projection follows the view of objects?

i would say that in the interior of the eye or camera all in front of you is represented on 3d in your eye and depending how you move the film or retine you caught well what's closer or farther, like a 3d inverted hologram with the peculiarity that infinitum size is represented in the hologram finite by shrinking in half with double distance

anybody knows a web about optics where i can learn on the subject?

I am sorry but your last paragraph does not make any sense.

Do a web search on "geometric optics" you should be able to find some good information.

from what i know the "3d ness" that we see is really only there because of shadows, and the fact that we have two eyes. as far as I know its not connected to how the individual eye works, as in when something is close to you and it gets blurry, it would be blurry if there were no shadows, or if you had one eye, etc.

and i missed what you said about conic projection, but all 'focusing' really means is if you think of a thin lens (converging lens), and light going through it, then light coming in parrallel 'focuses' on one point. if you put a piece of paper there or something you see a good image. if you move the paper too close or too far you are cutting the cone shaped light not at the tip, and you see it as blurry.

The next time you get your hands on a magnify glass, instead of looking through it, Hold it above the table top beneath a ceiling light and watch the spot of light formed by the lens. Move the lens up and down watching the spot of light. You should be able find an image of the light bulb (or source) This will work for any relatively bright source of light.

To Aviator
as Integral said this is geometric optics ( that means straight line ray tracing )
To understand this you need to understand what a simple lens does .
Take a point object emmitting light towards a simple lens , on the far side those rays will come to an almost point also but at a given distance . But only at ONE distance .
That is the object to lens distance changes the point at which the rays focus .

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• Lens.jpg
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it think i know the answers to my questions now:

in a box camera with no lenses there's no focusing and by moving back the film you zoom and by moving it forward you increase the angular

with a magnifying glass you can focus light in a spot but if you take the glass away of it there's no way you can concentrate light therefore there's no focusing when a hole

besides in the formula 1/f = 1/o + 1/i if the i and the f has to be the same so the image estays focused would imply 1/o = 0 which is imposible

aviator said:
it think i know the answers to my questions now:

in a box camera with no lenses there's no focusing and by moving back the film you zoom and by moving it forward you increase the angular

with a magnifying glass you can focus light in a spot but if you take the glass away of it there's no way you can concentrate light therefore there's no focusing when a hole

besides in the formula 1/f = 1/o + 1/i if the i and the f has to be the same so the image estays focused would imply 1/o = 0 which is imposible

Could you rephrase this in english?

my point is that a box camera works as a perfect conic perspective with no focus.

theres no focus because a hole can't refract light as a lens does.

the image distance to the lens and the focal length has to be the same in order for it to be focused but that implys that if you use the formula for thin lenses then 1/(objects distance) must be equal to 0 which is imposible.

is it because I am wrong and the I and the F don't need to have the same value for the object ot stay focused?

i hope to be clearer now but english is my second language

aviator said:
my point is that a box camera works as a perfect conic perspective with no focus.

theres no focus because a hole can't refract light as a lens does.
By "box camera" you mean a pin hole camera, you are correct. A pin hole creates an image without a lens, therefore is in focus everywhere. The further from the pin hole the larger the image, but it also gets dimmer with distance.
the image distance to the lens and the focal length has to be the same in order for it to be focused but that implies that if you use the formula for thin lenses then 1/(objects distance) must be equal to 0 which is impossible.
Not quite, for every object distance greater then the focal length there is an image distance where an image if formed. So if you have a magnifying glass (convex lens) if you move the lens closer to a source you will find a focused image at the distance predicted by the thin lens formula, the closer the object is to the focal length the further back the image will be. If the object is at the focal length the image will be formed at infinity. On the other hand if you move the object away from the lens the image will move toward the focal length, so when the object is at infinity $\frac 1 o = 0$ as I said in my original post any distance greater then 10 focal lengths is approximately infinity as far as the thin lens equation is concerned.
is it because I am wrong and the I and the F don't need to have the same value for the object to stay focused?

i hope to be clearer now but English is my second language

OK, now I understand, sorry for the last post.

I am sure your English is orders of magnitude better then my abilities in your native language.

thanks a lot for your time integral i apreciate a lot the freedom of thought and expresion that there is in this forum which makes it the best i know of

## 1. Why do films blur with a basic box camera?

The main reason for film blur with a basic box camera is due to the fixed focus lens. Basic box cameras do not have adjustable focus, so they are limited in their ability to capture sharp images. This can result in a lack of clarity and focus in the final photograph.

## 2. Can I prevent film blur with a basic box camera?

While it is difficult to completely prevent film blur with a basic box camera, there are a few things you can do to minimize it. These include using a tripod to keep the camera steady, shooting in well-lit conditions, and choosing a slower shutter speed to allow for more light and a sharper image.

## 3. How can I improve the focus of my photos with a basic box camera?

If you want to improve the focus of your photos with a basic box camera, you can try using a different film speed or ISO. Higher ISO films are more sensitive to light and can produce sharper images in low light conditions. You can also experiment with different lighting techniques, such as using a flash or reflectors, to enhance the focus of your photos.

## 4. Are there any limitations to using a basic box camera?

Yes, there are some limitations to using a basic box camera. As mentioned, the fixed focus lens can result in film blur and limit the sharpness of your images. The lack of manual controls also means you have less control over exposure and focus. Additionally, basic box cameras do not have the ability to zoom in or out, so you are limited to a fixed focal length.

## 5. Can I use a basic box camera for professional photography?

While basic box cameras may not have all the advanced features of professional cameras, they can still be used for professional photography. Many professional photographers use basic box cameras as a creative tool, as they can produce unique and nostalgic images. However, for more technical and precise photography, a basic box camera may not be the best choice.

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