View Full Version : How to calculate bellows extension from the bellows factor?

I sometimes like to play "arm chair" LF using my ancient Palm Pilot. I run pCAM (now a neat iPhone app it seems) and Bob Wheelers extensive Vademecum.

I am thinking about using my Wolly Raptar 480mm for some (sharp) portraits on 8*10.

So pCAM gives me the distance from the lens to the sitter (a shoulder/head portrait) through it's nice schematic scene preview function, and the DOF function of the Vademecum gives me the DOF limits and the bellows factor (the correction in stops or time to compensate the bellows extension)

Now my question is how I can get from a Bellows Factor of 3.2 to the extension or total length of the bellows for said shot?

yes I know I can mount my camera with that lens and so on, but I still would like to know how to calculate the bellows draw in a given situation, also when I am considering buying a new lens, for which I do not have enough bellows to begin with.

thanks in advance,

Best,

Cor

memorris

2-Mar-2011, 09:22

At infinity the focal distance is the focal length of the lens. Assuming the nodal point is on the same plane as the lensboard, the bellows extension is 1 * f. When the object is 2 * the focal length form the lens, the focal length is 2 * f (1:1 image size). Between these 2 distances, there is a linear curve so calculating bellows extension is basically an algebra equation. Without knowing the distance of infinity for the particular lens you can not make accurate calculations, but you should be able to get fairly close.

Heroique

2-Mar-2011, 16:18

Now my question is how I can get from a Bellows Factor of 3.2 to the extension or total length of the bellows for said shot?

I’m no good at doing LF w/ palm pilots, iPhone apps, pCAM, or Vademecum, but maybe I can help you with some very, very, very simple guidance:

Why not focus on your subject, note the length of your bellows, and add an appropriate amount “bellows correction” to your metered exposure?

A useful rule of thumb – don’t worry too much about “correction” unless the distance to your subject is 10x your focal length, or closer. With your 480mm lens, this threshold would be 10 x 480mm = 4800mm (that is, 4.8 meters, or closer).

If this is the case, the next rule of thumb is to add ˝ stop for every 25% increase in your bellows extension (beyond infinity).

-----

If you really want to start with a bellows factor, and use it to calculate the extension amount, just reverse these rules of thumb. So, if you somehow already know that your bellows factor is 1.5x (or about ˝ stop), then a workable bellows extension would be 25% beyond infinity, or 600mm in your case (480mm + 25%).

take your focal length in inches (18.8") then measure the distance (in inches) from the film plane to your lensboard with the camera focused on your close subject. Pretend your focal length is an f stop. If it's 32 inches (f32), that's a difference of roughly (f16.5 to f32) 1 1/2 stops. This rule applies to any focal length.

At infinity the focal distance is the focal length of the lens. Assuming the nodal point is on the same plane as the lensboard...

Not exactly.

The distance from the film plane to the front of the lensboard when focused at infinity is the flange focal length (FFL) of the lens, which is on the lens data sheet.

It's generally close to the focal length only for lenses of moderate FL.

On wide angle lens, the FFL is usually longer than the FL, to prevent slamming the lens into the ground glass, e.g. 73.4mm for the Fujinon SWD 65mm/5.6.

On long FL lenses the FFL is generally less than the focal length (e.g. 261mm on the Nikkor-T 360mm/8), so you don't need as much bellows draw.

In all cases, the bellows extension beyond the FFL required for a 1:1 shot equals the lens focal length.

- Leigh

memorris

2-Mar-2011, 18:42

It is measured from the nodal point and that is different from the plane of the lensboard, or flange but as a quick and simple way to do the calculation and get in the ballpark, it works.

Admittedly I am no lens expert but was taught that the number is when the distance required to focus is 2 X f that yields 1:1.

...the number is when the distance required to focus is 2 X f that yields 1:1.

That's correct from an optical design standpoint. However, the amount of bellows extension may be substantially different.

In the case I cited above for the Nikkor-T 360/5.6, its flange focal length is 100mm shorter than its optical focal length, a difference of almost 4 inches. That's a lot.

You're correct that lens focal length is measured from the rear (second) nodal point to the film plane along the optical axis.

However, most data sheets don't give the location of that nodal point or of the second principle plane, on which it resides. They do give the FFL.

- Leigh

If this is the case, the next rule of thumb is to add ˝ stop for every 25% increase in your bellows extension (beyond infinity).

-----

If you really want to start with a bellows factor, and use it to calculate the extension amount, just reverse these rules of thumb. So, if you somehow already know that your bellows factor is 1.5x (or about ˝ stop), then a workable bellows extension would be 25% beyond infinity, or 600mm in your case (480mm + 25%).

Thanks for the feedback of my odd question. Perhaps I phrased it clumsy, so I'll try again, this time starting with the hardware:

I have this 480mm lens I would like to try on my Toyo 8*10 for a head/shoulder portrait. My max bellows draw on the Toyo is about 650 mm.

If I plunge this figures in the Palm I arrive at a bellows factor of 3.2.

So how much bellows will I need ?

If I follow above rule of the thumb: say bellows factor of 3; 1 1/2 stop; 75% beyond infinity: 480 + (.75*480)=840mm

So not enough bellows, but I knew that, I am considering constructing a top hat lens board (is that the correct description?) and I know more or less know what extra length I neeed, which is considerable..not sure if that will work..

best,

Cor

Doremus Scudder

3-Mar-2011, 02:07

Bellows extension exposure factors are calculated using the following formula:

Ext˛ / Fl˛ = Exposure Factor

Where

Ext = Bellows extension measured from film plane to lens nodal point (middle of the lens for most designs) and

Fl = The focal length of the lens.

Simply rearranging the equation to solve for Fl results in:

Ext˛ = Fl˛ * Exposure Factor

So, you would multiply 480˛ by 3.2 and then take the square root of the product.

I get 858.65mm

That should be the answer to your question.

Best,

Doremus Scudder

That's were I was looking for, thanks again, Doremus!

Best,

Cor

Bellows extension exposure factors are calculated using the following formula:

Ext˛ / Fl˛ = Exposure Factor

Where

Ext = Bellows extension measured from film plane to lens nodal point (middle of the lens for most designs) and

Fl = The focal length of the lens.

Simply rearranging the equation to solve for Fl results in:

Ext˛ = Fl˛ * Exposure Factor

So, you would multiply 480˛ by 3.2 and then take the square root of the product.

I get 858.65mm

That should be the answer to your question.

Best,

Doremus Scudder

aduncanson

3-Mar-2011, 10:16

Cor, I am trying to understand your question, but I am having trouble figuring out how your Palm software comes up with a bellows factor of 3.2. Among many other useful identities discussed above, Bellows Factor = (1+ Magnification)^2. So a Bellows Factor of 3.2 means the Magnification is 0.79x. That implies that the field of your head & shoulders portrait is an awfully tight 9.8 inches by 12.4 inches. Is that the kind of cropping you are looking for? What did you enter into the Palm that it used to determine magnification or field size? Thanks

Cor, I am trying to understand your question, but I am having trouble figuring out how your Palm software comes up with a bellows factor of 3.2. Among many other useful identities discussed above, Bellows Factor = (1+ Magnification)^2. So a Bellows Factor of 3.2 means the Magnification is 0.79x. That implies that the field of your head & shoulders portrait is an awfully tight 9.8 inches by 12.4 inches. Is that the kind of cropping you are looking for? What did you enter into the Palm that it used to determine magnification or field size? Thanks

I I use pCAM for a preview of a head/shoulders portrait I get a distance of 1.2 meter, and a "frame" around the subject of horizontal 29 cm and vertical of 37 cm.

(still talking about that 480 mm lens on 8*10).

If I use that distance of 1.2 m in the Vademecum (DOFu) I get a Bellows factor of 2.8 (less than I said first, probably I choose a tighter crop, only the head the first time)

Oh the magnification is 0.66 than, so you were correct, that 3.2 is too tight.

It's all armchair LF but in practice these figures usually fit nicely, and at the moment I am preparing a top hat construction for the extension of my bellows,

best,

Cor

Drew Bedo

4-Mar-2011, 05:16

Hello Cor,

Every focal length of extension beyond infinity requires two stops of additional exposure. It follows then that 3.2 stops is equivilant to 1.6 focal lengths beyond infinity. If the lens is 480mm, the extension beyond infinity will be 760mm and total extension will be 1248mm.

Did I do that right? I use a verry small 4x5 and my longest lens is a 210: A meter and a quarter(50 inches) of bellows seems like a lot to me.

Drew Bedo

4-Mar-2011, 05:21

Ok, 2.8: That means 672mm beyond infinity and 1,152mm total.

Post a few shots when you are done? I'd like to see the Top-Hat mount and the portrate.

aduncanson

4-Mar-2011, 07:50

Drew, Cor is expressing Bellows Factor as a multiplier applied to exposure time rather than as a number of stops compensation. [I'm not sure how I picked up on that, but post no. 8 in this thread is pretty clear.] This seems to be one common convention, but clearly care must be taken to avoid confusion about how bellows factor is being expressed.

Using that convention, a Bellows Factor of 2.8 implies 1.5 stops compensation and the total extension required is 803mm.

- Alan

Brian Ellis

4-Mar-2011, 07:56

No need to be that precise unless you're using slide film. All I've ever done is look at the subject and how much the bellows looks like it's extended. If he/she/it looks like they're closer than 8 times the focal length of the lens I don't worry about bellows extension. If the bellows looks like it's extended a lot more than usual for a particular lens I open up one stop. If it's really sticking out there I open up two stops. Works just fine for negative film.

I see Bellows Factor as either a time or a F stop correction ( I believe that is the consensus), so a Bellows Factor of 4 means 4 times longer exposure or open the lens up 2 stops. Bellows factor of 2.8 is indeed almost 3 times more time or 1.5 stops compensation.

I just happen to just finished the tophat construction (from mat board..;-)..to avoid to much strain on the front standard), and I could put theory in to practice on my 11 year old son, and it worked out fine (accept that his head is smaller that that of an adult) so total bellows draw is now 820 mm.

It will take some time to start shooting with this lens, I am curious how it compares to the head/shoulder portraits I made with my 300 mm Symmar-S. Especially if there is a lot of difference in way the face is rendered, a 300 mm close up to a face is supposed to change the perceptive a bit I understood, although I did not notice that too much.

At least the 480 lens is now at a nicer distance away from the subject.

Best,

Cor

Steve M Hostetter

4-Mar-2011, 09:55

use a ground glass gauge like from JBrunner you can down load it free

Drew Bedo

4-Mar-2011, 12:13

aduncanson: Thanks for that clarification.

I do not do portrates myself. Most of my work is still life or table-top compositions. Exposure compensation is always an issue. I usually shoot with a 210mm or 150mm lens. For the 210; every inch beyond infinity (8 1/4 inches) equals 1/4 stop of increased exposure. For the 150mm, each inch of extension equals 1/3 stop exta exposure. Usually this correction is made by opening up the aperature. I rarely actually do naything with correction factors or work out any math.

Ken Sinclair

12-Jan-2018, 11:19

Cor...

I just came across this a 'question' couple of days ago after having recently 'resurrecting' my Palm Pilot that had gone into 'hiding' after a house-move a number of year ago'.

When I was a 'working "Biological Photographer' my Palm with Photographer's Vade mecum was used on a daily basis... I don't know how I could have survived without it since close-up images of specimens were requested with an "I need this done NOW". I managed to find the Vade mecum programme on line and have just reloaded it to my Palm 505... all that I now need is to 're-learn' proper use of the stylus for entering the information required to give me the required 'corrections'

Ken

Hi Ken,

A blast from the past ! I still use a Palm Pilot, mostly as a timer for processing film..my original thread made me remember a project I never really finished: using a 480 lens on my 8*10 for portraiture ! I'll give it a try, thanks for the reminder !

Best,

Cor

Cor...

I just came across this a 'question' couple of days ago after having recently 'resurrecting' my Palm Pilot that had gone into 'hiding' after a house-move a number of year ago'.

When I was a 'working "Biological Photographer' my Palm with Photographer's Vade mecum was used on a daily basis... I don't know how I could have survived without it since close-up images of specimens were requested with an "I need this done NOW". I managed to find the Vade mecum programme on line and have just reloaded it to my Palm 505... all that I now need is to 're-learn' proper use of the stylus for entering the information required to give me the required 'corrections'

Ken

Daniel Casper Lohenstein

16-Jan-2018, 12:23

173793

In this chart you see the bellows factor on the right side of the upper chart. On the left side you find the bellows extensions sorted by focal length. Cut it out, stick it on your camera.

Regards

Drew Bedo

17-Jan-2018, 18:18

So this is not a camera-in-hand exercise: Yeah . . .I can't play chess in my head. I measure from the front of the lens board to the film plane and do the math. For the lens you mention, each 3 inches of extension beyond infinity requires about a third of a stop extra exposure. I did that pretty quick and that's an approximation, but should work.

On this board the pool of knowledge and experiences both deep and wide. Someone will tell you how to do this calculation to several significant figures . . . .I respect them.

Cheers

Jerry Bodine

17-Jan-2018, 21:58

It's just a simple calculation using algebra. The formula for calculating the bellows FACTOR is:

Extension squared divided by lens focal length squared. So if you already know the bellows factor, then solving the formula for bellows extension you get:

bellows extension = square root of [bellows factor times focal length squared].

I've never owned a Palm Pilot, but an inexpensive pocket calculator can do this calculation easily.

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