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-   -   help bellows extension (http://www.film-and-darkroom-user.org.uk/forum/showthread.php?t=13883)

big paul 21st July 2021 02:15 PM

help bellows extension
 
can anybody explain the easiest way to measure bellows extension factor ,I have Dyscalculia so calculating anything is hard so it must be simple please

Lostlabours 21st July 2021 03:11 PM

There are Apps for smart phones or bellows extension tables so no caclualtion is needed.

I marked the focus track of my wista with 1/3 stop interval lines on the brass with a sharp felt tip pen (for my 150mm Sironar N) and they are still there 30+ years later.

Ian

MartyNL 21st July 2021 03:37 PM

Ian's right, there are some very simple and free apps for phones which are pretty much fool proof. Just try a few and see which you like. I use bellows factor for my android phone.

The quick and dirty calculation, I believe, is 50% of the focal length is equal to 1 stop. For example, a 300mm lens focused at 450mm will be +1 stop.
The problem is that the numbers are not usually so convenient and this is where the app makes life easier.

John King 21st July 2021 05:19 PM

Working it out.
 
Use a calculator for any figures. Then I would sit down and work out the bellows/extension tube and write them down.

It is only really necessary to work the exposure increase to 1/4 stop. Any film/digital medium will cope with that quite easily. Going any further will not really show any great difference

Terry S 22nd July 2021 11:24 AM

It's interesting that Paul asked this question, as it popped into my mind last night, that I wanted to use a Bronica extension tube for some pictures.

I presume that there is an easy equation / an app available, for exposure compensation for that too?

Terry S

alexmuir 22nd July 2021 06:36 PM

The tube should have come with instructions that give the factors. They may also be in the Bronica Manual by William Cheung. It’s a useful book if you use Bronica Sq/Etrs/GS1 cameras. I have both somewhere. Which size tube is it?
Alex


Sent from my iPhone using Tapatalk Pro

Tom Kershaw 23rd July 2021 09:05 AM

I have always found this document useful: https://people.umass.edu/~rbhome/bellows15.pdf

photomi7ch 26th July 2021 10:38 AM

I would have said the cameras light meter is the easiest way. Having read the comment above I suspect that the cameras involved do not have this ability. Just a thought has anyone tried using a hand held meter to the viewfinder to take a reading?

Molli 26th July 2021 01:38 PM

Hi Paul, which lens are you using?

Here is how I work it out using a calculator.
I have a quick and dirty method which I'll post beneath this post.

I'll use my 215mm lens as an example.

The equation I use is

(B/F) squared.

So,
B = Bellows Extension
F = Focal Length

If I measure from the film plane to the front of my lens and that's 320mm, the equation would look like this :

(320÷215)
= 1.4883

Squared (i.e. 1.4883x1.4883) = 2.215

That last number is the Bellows Factor - how much you need to multiply your metered time.

If I metered the scene as f/8 1second, I would multiply 1second by 2.215

My time compensated for Bellows Factor will be f/8 2.215 seconds.

I tend to round up ☺️ so f/8 2.5 seconds.

Molli 26th July 2021 01:52 PM

Quick And Dirty:

Turn your measurements into F Stops (millimetres or inches, it doesn't matter).

So, my 215mm lens would be f/22.
A bellows extension of 320mm would be f/32.

That's a one stop difference.

In inches?

8½inch lens ≈ f/8
12.59inches ≈ f/11 (Rounded to the nearest whole stop).

Again, one stop more exposure required.

As mentioned, I tend to err on the side of more exposure so I round out to a wider aperture or longer exposure time.


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