The Math of Photography

[jtimmons]

Sorry if I am asking too many questions, figuring all of this stuff out is pretty hard. My next question is.....

Is there a cheat sheet for the math of photography??

Ex.

I bought 3 kenko extension tubes that fitted together will equal 68mm. So to get an image with a 1x magnification (1:1) the lense will need to be zoomed to 68 mm, what is the minimum focal distance that I can achieve.

Also, (since I only have a 18-55mm lense right now, with the addition of 68mm of extension tubes, will I have a magnification of 1.25X or am I doing something wrong.

Thanks for all of the help guys.

 

[gruZ]

I think this site should help out with the minimum focal distance question. If not, it is still a very useful site to have, with printable tables. Click.

 

[jann1033]

just subcribing to easedrop

 

[mabas9395]

Another forum I visit had a question just like this a while back. He got some answers that are over my head but might help you out.

http://photography-on-the.net/forum/showthread.php?t=19350

 

[MarkBarbieri]

I bought 3 kenko extension tubes that fitted together will equal 68mm. So to get an image with a 1x magnification (1:1) the lense will need to be zoomed to 68 mm, what is the minimum focal distance that I can achieve.

You're headed the wrong direction with this. The tube length doesn't necessarily match the focal length to get a 1:1 magnification.

All lenses have a minimum focusing distance. If you move towards your subject, the focal point moves behind your sensor. The focus mechanism corrects this and brings the focus point back to your sensor. If you keep moving towards your subject, you eventually get to the limit of your lenses ability to move the focus point. The extension tube helps by moving the focus point forward, which in turn allows you to focus closer.

It looks like the formulas in the page mabas9395 sites are roughly what you need. The one thing to be careful about is that these formulas apply to a simple lens and not a real multi-element camera lens. To make them work right perfectly, you would have to apply them to the nodal point of the lens. My bet is that when manufacturers list near focus distance, they do so from the front element rather than the nodal point. Honestly, it probably doesn't matter for most purposes, but if you go out and measure things and they come out a bit off, that's probably why.

 

[MarkBarbieri]

Here's another good site on basic lens theory:

http://www.cambridgeincolour.com/tutorials/camera-lenses.htm

 

[jann1033]

You're headed the wrong direction with this. The tube length doesn't necessarily match the focal length to get a 1:1 magnification.

All lenses have a minimum focusing distance. If you move towards your subject, the focal point moves behind your sensor. The focus mechanism corrects this and brings the focus point back to your sensor. If you keep moving towards your subject, you eventually get to the limit of your lenses ability to move the focus point. The extension tube helps by moving the focus point forward, which in turn allows you to focus closer.

It looks like the formulas in the page mabas9395 sites are roughly what you need. The one thing to be careful about is that these formulas apply to a simple lens and not a real multi-element camera lens. To make them work right perfectly, you would have to apply them to the nodal point of the lens. My bet is that when manufacturers list near focus distance, they do so from the front element rather than the nodal point. Honestly, it probably doesn't matter for most purposes, but if you go out and measure things and they come out a bit off, that's probably why.

thank heavens, i thought one more thing i don't get