An easy way to figure bellows factor

[Leigh]

Yeah. When you go back and edit your previous post, you can correct any mistake with impunity.

"Edited the post above for clarity."???
More like correcting an error and covering your posterior.

So I edited my reply and included your entire previous post as it was before you edited it.
Caches are awfully handy.

- Leigh

[Corran]

Whatever Leigh. I never think of it in stops. In my example it was 2 stops, as noted, which is why I put it in there.

The formula is correct. Get over it.

[Corran]

And you'll have to get on Ken Lee's case about it too. And I quote from his site:

How Many More f/stops are Required ?
Compensation = (B / F) stops
where B = Bellows extension and F = Focal length

Given a 300mm lens, 450mm of bellows draw and a 2-second exposure, by how many f/stops should we adjust the exposure ?

Compensation = (450 / 300) stops
Compensation = 1.5 stops

We adjust exposure by an additional 1.5 f/stops

http://www.kenleegallery.com/html/tech/bellows.php

[ImSoNegative]

its cool how everyone pretty much has there own way of figuring it and how it all comes out about the same

[Joe Smigiel]

OMG! Math! Fractions! Exponents!

Leave the calculator home. There's also a way to visually determine extension corrections using a light meter dial. (I've posted a pic of one below to follow along with.) If using a 150mm lens focused at 300mm think of the distances as f-stop readings. (See the red dots on the pic.) Think of 150mm as f/15 and note the shutter speed. Then think of 300mm as f/30. Whatever shutter speed is aligned with f/30 will be your corrected exposure for the bellows extension. In that example, if your meter read 25 seconds at f/15 (see the pic below), you should find the shutter speed at f/30 to be 100 seconds (= 1 2/3 minutes) and that would be the corrected exposure time for the extension factor at the original f/stop. Your exposure becomes 100 seconds @ f/15 (or equivalent). I've picked these numbers because I know an extension of twice the lens focal length gives an extension factor of 4x or 2-stops and if you look at the numbers in the example you'll see that the corrected exposure (100 seconds @ f/15) is 2-stops different than the original meter reading (25 seconds @ f/15). (I apologize for not having a scan with simpler alignments to post.) A Pentax Spotmeter V is great for this. I'd never buy a digital meter because of this trick.

If you can use such a meter dial you don't really have to calculate anything. You just need to know your lens focal length and the total extension and you can eyeball it on the meter face. For example a 90mm lens @ 120mm extension with an original meter reading of 9 seconds @ f/9 produces the corrected result of about 15 seconds at f/9 just eyeballing the meter.

[EOTS]

Hi guys,

why not use the QuickDisc?
Simply print this out and measure on the ground glass: http://www.salzgeber.at/disc/disc.pdf

Best regards,
Martin

[Wayne]

Rofl. Then you can afford to cart around a full-time assistant to do your calculations for you.

That works fine for a couple of lenses.

My 4x5 kit has about 18 lenses in roughly 15 different focal lengths.

- Leigh

[StoneNYC]

Rofl. Then you can afford to cart around a full-time assistant to do your calculations for you.

Yea, holy shit man haha

[vinny]

Hi guys,

why not use the QuickDisc?
Simply print this out and measure on the ground glass: http://www.salzgeber.at/disc/disc.pdf

Best regards,
Martin

doesn't work well over water, sand, snow, or anything else you don't want disturbed. I have one and use a cloth measuring tape instead.

[EOTS]

doesn't work well over water, sand, snow, or anything else you don't want disturbed. I have one and use a cloth measuring tape instead.

Yeah, that's right ...
I also have a spreadsheet printed (x axis extensions, y axis focal lengths) and measure with a tape and look it up ...