Extensions & width of field

[Ljusdahl]

Ugh, it seems every page I read says different things! Friggen...

I'm looking for a formula to calculate the Width of field with a certain lens, with and without a certain extension tube.
Also, how do you know what the new closest and farthest focus is(called front- and back focus, right?) with an extension tube attached.

I'm trying to figure out what lenses and tubes I should get for my Bronica Sq-ai kit. I like having to option to go close if I want to, for example tight head shots and such.

Well, I'm off to let my head cool down.
Thanks in advance.

-Ljusdahl

 

[Dan Fromm]

Width of field? Do you mean the distance from left to right (or the other way 'round) that will be in the shot? It depends on magnification. Learn what magnification means.

Near and far focal limits? Or depth of field? Please clarify.

Front- and back-focus are neologisms coined by idiots who have autofocus cameras to describe their cameras' failures to focus where they'd like. Why they just can't say "my really stupid, possibly broken, camera focuses in front (or in back) of where I want the plane of best focus to be" puzzles me. Especially since back focus, in French tirage optique, has been used for centuries to refer to the distance between the film plane and the rear of the lens when the lens is focused to infinity.

 

[Ljusdahl]

Yes Dan, if I've understood correctly "width of field" is what people call it. Maybe there's a better name for it.
Yes, focal limits. What it sounds like.

Thanks for clearing the back and front thingy up.

 

[Ole]

So what you want to know is this:

With a given combination of lens and extension tube, what is the maximum and minimum focus distance? And what is the field of coverage with said combination, in terms of the size of the object field covered?

You can either work it out from the basic equation (1/F=1/u+1/v), or - if the SQ-Ai is anything like the ETRS - look it up in the camera manual.

 

[Ray Heath]

g'day Lj

why make it complicated?

are there so many options in the available equipment that you could calculate and then obtain specific gear?

try a few different focal lengths and realise that focal length doesn't only determine subject size, it's also about perspective and angle of view

Ray

 

[David A. Goldfarb]

"f/calc" is a shareware program that does field of view calculations among other things. Google should turn it up for you.

That said, with most 6x6 cameras, a lens of about 135-150mm and an extension tube of 10-15mm is right for most portrait subjects.

 

[Nick Zentena]

Isn't the answer buried in the manual some place -) I'm fairly sure Bronica has a page showing mag and focus distances.

 

[JBrunner]

Especially since back focus, in French tirage optique, has been used for centuries to refer to the distance between the film plane and the rear of the lens when the lens is focused to infinity.

This is what we refer to this distance as, when dealing with video and cine equipment. If somebody is using it in another way, it is, as Dan says, simply another ham handed usurpation by cranial rectosis afflicted hackers.

 

[Ljusdahl]

Thanks for the answers!
I checked the manual. At first I thought the numbers given were the closest and farthest focusing with a tube, but when I looked closer I realized that the longer lens you have, the smaller number you get. This contradicts the experience I have with my ETR equipment, so what are these numbers?

For instance, a 40mm with S-18 gives 0.43 - 0.60, while a 80mm gives 0.22 - 0.35.

What are these numbers then?

 

[Nick Zentena]

magnification but I didn't check the manual so...

 

[panastasia]

Thanks for the answers!
I checked the manual. At first I thought the numbers given were the closest and farthest focusing with a tube, but when I looked closer I realized that the longer lens you have, the smaller number you get. This contradicts the experience I have with my ETR equipment, so what are these numbers?

For instance, a 40mm with S-18 gives 0.43 - 0.60, while a 80mm gives 0.22 - 0.35.

What are these numbers then?

They look like minimum - maximum focusing distances in meters

 

[Ole]

The numbers make sense if they're reproduction ratio.