What makes a lens's focal length?

[sevo]

How come that light doesn't bounce off the inside walls of the camera and fog the paper somehow?

It does. That is why you paint the cameras insides black and create the surfaces to have light trapping properties where misdirected light is reflected towards a long chain of absorbing surfaces rather than towards the film.

Sevo

 

[Roger Hicks]

It does. That is why you paint the cameras insides black and create the surfaces to have light trapping properties where misdirected light is reflected towards a long chain of absorbing surfaces rather than towards the film.

Sevo

(For Jeremy, obviously, not you)

It's also why flare factors vary from close to unity to 4 or more.

The flare factor is the difference between the brightness range of the original scene and the brightness range of the focused image, where the darkest areas have been 'filled' by all the stray light bouncing around.

Scene brightness range 1000:1, image brightness range 800:1, flare factor 1.25. Image brightness range 500:1, flare factor 2. Image brightness range 250:1, flare factor 4.

This is why some lenses (and cameras) are contrastier than others.

Tashi Delek,

R.

 

[JeremyLangford]

It does. That is why you paint the cameras insides black and create the surfaces to have light trapping properties where misdirected light is reflected towards a long chain of absorbing surfaces rather than towards the film.

Sevo

Oh yea, that makes sense.

 

[HankOsaurus]

Greetings, Forum.

Jeremy, I'm almost 62 and I am learning from the talented and knowledgeable folks contributing to this thread as you are ... even though I have been using manual cameras since the mid 60's. Thanks for asking the questions that spawned this excellent discussion.

Chris, you set me to thinking about pinhole cameras, diffraction, and effective aperture.

I read a year or so ago that someone set out to make the world's largest pinhole camera. If I recall correctly, it was an airplane hangar! I wonder what the diameter of the "pinhole" was, and what its effective aperture was at the distance of the width of an airplane hangar. I would guess that it would work out to a pretty big F number, counting the pinhole diameters it would take to get across that hangar to the opposing wall.

Happy day.

 

[HankOsaurus]

Hi Folks.

Just found a link with some technical details about that big pinhole camera I mentioned upthread:
http://news.softpedia.com/news/The-World-039-s-Largest-Camera-and-Photograph-62423.shtml

According to the link, the aperture of the pinhole was 6mm. The distance from it to the back wall of the hangar was 79 feet, six inches. That translates to 24,231.6 mm. Divided by 6 mm, that comes out to an F number of 4,038.6 !

 

[Chris101]

Dang Hank.

I used to live in El Toro in the '60's. And I just spent the last week in that area. Had I the foresight to check RFF before going on vacation, I could have made this hangar a destination! Talk about past/present/future!

Thanks for that link. The problem with cameras that big is that you can basically only take one picture! Oh, and I'd crop it.

 

[Merkin]

Jeremy- a really fun way for you to begin to get a handle on this would be to turn your room in to a camera obscura. If you aren't familiar with a camera obscura, check out the wikipedia article (there is also a really good indie folk/pop band sort of akin to belle and sebastian of the same name). All you will need is some opaque black plastic sheeting from lowes or home depot, a knife, and a roll of actual honest to goodness gaffer tape (blue painters tape *might* work in a pinch, you just need a tape that isnt going to remove paint or wallpaper or leave a residue behind). tape up the cracks around your door, and cover your window or windows with the black plastic sheeting. Get the room to the point where it is so dark, even at midday, that you can't perceive your hand right in front of your face when you wave it about. Then, cut a small round hole in the plastic sheeting over a window and watch as the image of whatever is outside your window appears on the opposite wall, upside down and backwards.