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Old 21st July 2021, 02:15 PM
big paul big paul is offline
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Default 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
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Old 21st July 2021, 03:11 PM
Lostlabours Lostlabours is offline
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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
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Old 21st July 2021, 03:37 PM
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MartyNL MartyNL is offline
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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.
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Old 21st July 2021, 05:19 PM
John King John King is offline
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Default 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
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Old 22nd July 2021, 11:24 AM
Terry S Terry S is offline
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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
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Old 22nd July 2021, 06:36 PM
alexmuir alexmuir is offline
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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


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Old 23rd July 2021, 09:05 AM
Tom Kershaw Tom Kershaw is offline
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I have always found this document useful: https://people.umass.edu/~rbhome/bellows15.pdf
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Old 26th July 2021, 10:38 AM
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photomi7ch photomi7ch is offline
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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?
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Old 26th July 2021, 01:38 PM
Molli Molli is offline
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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.
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Old 26th July 2021, 01:52 PM
Molli Molli is offline
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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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