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monkeymon
12-Oct-2009, 14:44
Hello!

I would like to know how to calculate, or if someone have already done this, a bellows compensation "ruler" for my 14" commersial ektar on 8x10 camera. Half stop intervals would be enough for me. For long now i have just added a stop or two for close ups, but would really like to be more accurate on my exposures from now on. So, help would be appreciated, since i got a bit confused while googling for an answer.

Dan Schmidt
12-Oct-2009, 15:09
try this:

http://www.salzgeber.at/disc/index.html

basically you put something in the field of view that you know the size of. Then by measuring its size on the ground glass you will be able to determine the magnification. From the magnification you can get the exposure compensation factor.

domaz
12-Oct-2009, 15:19
You could make one pretty easily using this spreadsheet (http://www.largeformatphotography.info/Bell_Ext.xls) on the main LFInfo site. I find measuring bellows extension and using a premade ruler or simply doing the calcuation (Bellows Length/Focal Length)^2 much easier than messing around with discs that you end up leaving in the picture.

Lynn Jones
12-Oct-2009, 15:36
Easiest way, but buy a cheap calculator:
Focal length squared divided by bellows extension squared equals a factor less than 1. Multiply the film speed by that factor and you now have a film speed including the bellows factor. EG: 6" lens, 12" bellows, 36 (F) divided by 144 (BE) =.25 If the film speed is 100 X .25, the new film speed including bellows extension factor (BEF) is 25 (or t f stops). The reason this is easier is that when you don't have nice even numbers to deal with is may be difficult to figure it in fstops when the numbers are inverted, even though the result is the same.

This is the reasonb, lets say that you have a 135mm lens and 13 inches of bellows. fist convert inches to bellows or mm to inches. When that is done with BE squared divided by F squared you have a factor of 6, now how much is that in f stops? If you invert the problem the answer is .165. Multiply a PXP film speed of 125 by .165 and the answer is a tiny bit more than ISO 16 (an amount not worth considering).

By the way, if you convert everything to inches, the numbers are smaller or convert the mm to cm the numbers are smaller.

Lynn

monkeymon
12-Oct-2009, 15:44
thanks for the quick reply, does these numbers sound about right for 355mm lens? :

435mm = 0.5 stop
503mm = 1 stop
562mm = 1.5 stop
615mm = 2 stop

Mark Sawyer
12-Oct-2009, 15:52
Easiest way, but buy a cheap calculator:
Focal length squared divided by bellows extension squared equals a factor less than 1. Multiply the film speed by that factor and you now have a film speed including the bellows factor. EG: 6" lens, 12" bellows, 36 (F) divided by 144 (BE) =.25 If the film speed is 100 X .25, the new film speed including bellows extension factor (BEF) is 25 (or t f stops).

This is not the easiest way, and for the life of me, I can't figure out why all the text books, manuals, etc. keep pushing that method.

As usual, the easy way is the most direct way. Measure the bellows extension, and while you've got the ruler or tape measure in your hand, measure the aperture through the front element. Divide the former by the latter and that's your f/stop. Period. No squaring this, then squaring that, then getting a factor for reducing film speed or figuring the new f/stop.

If you really want it to be easy, write the measurements of the f/stop diameters on your lensboard and put a scale on your camera bed or rail, (and for style points, do the division on a slide rule!)

(The weird thing is, in all my time in large format, I've never met another photographer who uses this method. Do you folks all just love squaring numbers?) ;)

monkeymon
12-Oct-2009, 16:10
damn, all these easy ways really seem confusing.. my intention was to put a simple guide ruler on the rail of my 8x10, i use primarily only one lens.. and my 8x10 stays mostly inside on my home studio.. so i just needed a quick and easy way to check my bellows correction. like it's on mamiya rz67.

Pete Roody
12-Oct-2009, 16:30
This is not the easiest way, and for the life of me, I can't figure out why all the text books, manuals, etc. keep pushing that method.

As usual, the easy way is the most direct way. Measure the bellows extension, and while you've got the ruler or tape measure in your hand, measure the aperture through the front element. Divide the former by the latter and that's your f/stop. Period. No squaring this, then squaring that, then getting a factor for reducing film speed or figuring the new f/stop.

If you really want it to be easy, write the measurements of the f/stop diameters on your lensboard and put a scale on your camera bed or rail, (and for style points, do the division on a slide rule!)

(The weird thing is, in all my time in large format, I've never met another photographer who uses this method. Do you folks all just love squaring numbers?) ;)

Mark,

This is a typical way to use convertible protar lenses. Many times, the iris scale is in mm only. Otherwise, you would need 3 scales for a triple convertible lens and even more if you have more than 3 elements. And as you said, you have the bellows correction built-in.

Pete

Mark Sawyer
12-Oct-2009, 18:39
Mark,

This is a typical way to use convertible protar lenses. Many times, the iris scale is in mm only. Otherwise, you would need 3 scales for a triple convertible lens and even more if you have more than 3 elements. And as you said, you have the bellows correction built-in.

Pete

Interesting... I've never seen a Protar (or any other lens) marked in mm. But then, I haven't seen many Protars...

You might need two scales for a double or triple convertible, one for having the front element on and one for it off, although I don't know whether the magnification factor would be significant...

wfwhitaker
12-Oct-2009, 19:18
As an alternative to Mark's method that doesn't require long division, think of your relative bellows extensions in terms of "f/stops".

For example:

If you have an 8" focal length lens and the bellows extension at your desired focus is 11", you need to open up one stop to compensate because the difference between f/8 and f/11 is one stop. Works for other focal lengths, of course, and is independent of format. For a 14" lens, you could probably round to 16 (f/16) in most cases.

Pete Roody
12-Oct-2009, 20:06
Interesting... I've never seen a Protar (or any other lens) marked in mm. But then, I haven't seen many Protars...

You might need two scales for a double or triple convertible, one for having the front element on and one for it off, although I don't know whether the magnification factor would be significant...

You can see a protar set with the mm scale on the 'bay right now. Do a search for 'protarlinse'. Typically these sets came from the factory with four elements. The aperture is dependent on the focal length so you would need scales for each single element and the combinations of elements you would use. Much easier to use the mm scale imho.

I never thought of using this technique for other lenses that have fstop scales.

Darryl Baird
12-Oct-2009, 20:29
As an alternative to Mark's method that doesn't require long division, think of your relative bellows extensions in terms of "f/stops".
For example:

If you have an 8" focal length lens and the bellows extension at your desired focus is 11", you need to open up one stop to compensate because the difference between f/8 and f/11 is one stop. Works for other focal lengths, of course, and is independent of format. For a 14" lens, you could probably round to 16 (f/16) in most cases.

I've used this method on a recent series of close-ups, ranging from 1:1 to 4:1 ratios and it has worked well for me. (9 inch lens and 34 inches of bellows = 4 stops)

Be sure to factor in any reciprocity failure for your film too.

Jerry Bodine
13-Oct-2009, 14:54
Here’s my chart showing the bellows factor as calculated by [(extension / focal length) squared ] for your 355mm lens, using closer extension intervals to make it easier to interpolate. It contains sufficient info to create your own scale on the monorail. It is independent of film format and film type. I find it easier to work with factors rather than stops, so that when I also have a filter in place then the bellows factor and filter factor can be readily multiplied to get the exposure time correction. Once I’ve selected the aperture that gives the desired depth of field, I don’t want to change it to account for extension and filter. However, if it’s helpful, I’ve included my calculations for converting from factor to f-stops. I keep all relevant data of this sort in my field records (pocket-size notebook), on pages that I’ve created with Microsoft Excel so as to fit the little notebook and make it a compact reference.

For 355mm lens:

Extension Bellows
mm inch Factor
355 14.0 1.00
380 15.0 1.15
405 15.9 1.30
430 16.9 1.47
455 17.9 1.64
480 18.9 1.83
505 19.9 2.00
530 20.9 2.23
555 21.9 2.44
580 22.8 2.67
605 23.8 2.90
630 24.8 3.15
655 25.8 3.40
680 26.8 3.67
705 27.8 3.94
710 28.0 4.00

Factor Stops
1 0
1.33 -
1.67 -
2 1
2.67 -
3.33 -
4 2
5.33 -
6.67 -
8 3
10.67 -
13.33 -
16 4
21.33 -
26.67 -
32 5
42.67 -
53.33 -
64 6

Lynn Jones
13-Oct-2009, 14:56
For me Mark it is the easiest way, that is after we got electronic calculators 38 or 39 years ago. I started using this system 55 years ago in Med Photo School rather than a really crazy system called "effective aperture". I remember doing all this this with a pencil a paper before calculators, horrible! However, what ever works for each of us is the best for us.

Lynn

Dan Fromm
13-Oct-2009, 15:23
thanks for the quick reply, does these numbers sound about right for 355mm lens? :

435mm = 0.5 stop
503mm = 1 stop
562mm = 1.5 stop
615mm = 2 stopI don't know how you calculated these numbers, but they seem wrong unless extension at infinity is 265 mm.

At infinity, extension (distance from the lens' rear nodal point to film plane) is the lens' focal length. Should be roughly 355 mm from the diaphragm to the film plane at infinity. Magnification = 1:1 requires 2 stops more exposure than nominal. And 1:1 requires 2f extension (distance from rear nodal point to film plane). For a 355, that's 710 mm, not 615. 615 mm extension is 260 mm more than at infinity.

Where are you planning to measure from?

Jerry Bodine
14-Oct-2009, 15:39
Monkeymon,

The tabulated info in my last post is really messed up and difficult to read, and I've not been able to get it to post in a readable format. However, if you click the "Quote" button in my post the columns are adjusted to read the way I intended. You can then copy/paste that display if you wish. For some reason the formatting is OK when using the "Preview Post" button, but then gets screwed up when submitting the post.

Jerry

c.d.ewen
14-Oct-2009, 16:49
Here's a factor vs f/stop chart lifted from B+W's literature.

Charley

15-Oct-2009, 05:17
To elaborate on wfwitakers post - I used the method he touches on for year doing commercial work and it's as accurate as you need to be. You have a compensation scale on the front of every lens you own, if you can't remember the normal f stops in your head. Simply convert the focal length of your lens into inches, remembering that every 25.4mm is 1 inch. So for a 355mm/14" lens there is no compensation needed with up to 14" of bellows. Here's where the f stop scale in your lens comes in. Round your lens lenght to the next nearest f stop(in this case f16). The next stop is f22 - so with 22" of bellow out you need 1 extrat stop, with 32" of bellows, 2 stops, 45" - 3 stops and so on.
Tim

15-Oct-2009, 06:32
http://lh6.ggpht.com/_OR3U2BmIDuk/Stci_-PfG3I/AAAAAAAACHk/t9zfsqY_Yss/s800/2inch.jpg
The Old Kodak manuals have a scale on the edge of a page (photocopy it). Put a 2 inch whatever in the set and hold the scale against it on the ground glass. My scale lives rolled-up inside the 2 inch tube.

15-Oct-2009, 09:30
Monkeymon,

I thought I would avoid this thread because the only thing worse than a bellows factor discussion is one on depth of field, but curiosity got the better of me.

For use with a single lens, I think that your approach is the simplest, but Dan Fromm provided an important correction to your math. The correct numbers for your table are as follows:

422mm = 0.5 stop
502mm = 1 stop
597mm = 1.5 stop
710mm = 2 stop

These distances can be generated using "image distance = 355 x 2^(number of stops correction/2)". This formula will generate image distances for 1/3 or even 1/10 stop corrections if desired.

Dan also alluded to the difficulty in directly measuring the image distance. You might want to find some features on the front and rear standards which are nearly 355mm apart when the camera is focused at infinity. If you will be using significant tilts & swings, it would be good if those features are near the axis connecting the lens and the center of the focusing screen. Then make all of your subsequent measurements between those features.

Mark Sawyer
15-Oct-2009, 09:40
Maybe it's just the crusty old curmudgeon in me, but I still like the basic method of what an f/stop really is...

f/stop = focal length / aperture

15-Oct-2009, 10:02
There is nothing wrong with your formula Mark except that:

1) You mean "image distance" not "focal length",
2) You cannot usually measure image distance directly,
3) You mean "entrance pupil" not "aperture",
4) It is not really possible to precisely measure the entrance pupil either, and
5) It requires doing division.

But none of that is to say that your process does not work, but...

It is simple enough to create a scale for a single lens that reads the correction directly in stops.

Mark Sawyer
15-Oct-2009, 18:59
There is nothing wrong with your formula Mark except that:

1) You mean "image distance" not "focal length",
2) You cannot usually measure image distance directly,
3) You mean "entrance pupil" not "aperture",
4) It is not really possible to precisely measure the entrance pupil either, and
5) It requires doing division.

But none of that is to say that your process does not work, but...

It is simple enough to create a scale for a single lens that reads the correction directly in stops.

Picky, picky, picky... ;)

Turner Reich
15-Oct-2009, 22:35
This is not the easiest way, and for the life of me, I can't figure out why all the text books, manuals, etc. keep pushing that method.

I fully agree with Mark, I'd rather guess and bracket than to stand there with a calculator while the sun goes down.

16-Oct-2009, 06:46
.... It is simple enough to create a scale for a single lens that reads the correction directly in stops.
The Kodak Bellows Factor Scale (shown above), works for any lens and is a no-brainer.

16-Oct-2009, 08:09
The Kodak Bellows Factor Scale (shown above), works for any lens and is a no-brainer.

I agree, that is an excellent solution, particularly for the studio. Because it measures image size directly rather than bellows extension, It works without adjustment for lenses with especially long or short back focus (and I have both.)

As a practical question, do you think that it would work well if it were scaled down for use with a common 35mm film canister rather than your piece of 2" iron pipe? Or would the reduced scale make accurate reading difficult? (That answer might depend on whether one is shooting B&W negatives or color transparencies.)

Greg Blank
17-Oct-2009, 09:54
From what I have read related to the posted responses, it would seem that not one person has posted the Ansel Adams formula, which by all my experiences is the simplest and quickest way to calculate using a ruler and a calculator. I would add the following thoughts: Lenses up to and including 150mm lenses don't usually have bellows factor becuase you seldom extend the bellows beyond the lens focal length.
150mm sometimes :)

Indicated (Metered) f stop x Bellows extension / (divided by) Focal Length = Effective Fstop.

So: F 11 (Metered) x 300mm extension / 240mm (Lens focal length) = 13.75

Meaning you should open up 3/4 of a stop or close down slightly and add a full stop to time if possible.

The next stop up from 11 being 16, look at the relationship of 11 and 16 or whatever your Metered and or desired F stop is and go from there.

Hello!

I would like to know how to calculate, or if someone have already done this, a bellows compensation "ruler" for my 14" commersial ektar on 8x10 camera. Half stop intervals would be enough for me. For long now i have just added a stop or two for close ups, but would really like to be more accurate on my exposures from now on. So, help would be appreciated, since i got a bit confused while googling for an answer.

wfwhitaker
17-Oct-2009, 10:30
...Lenses up to and including 150mm lenses don't usually have bellows factor becuase you seldom extend the bellows beyond the lens focal length.
150mm sometimes...

How do you figure that? I suppose you never focus closer than infinity?

Merg Ross
17-Oct-2009, 10:47
How do you figure that? I suppose you never focus closer than infinity?
When I use a 150mm it is usually at double or triple extension; I do a lot of close-ups.

Will, I use your method, I prefer hands-free photography!

17-Oct-2009, 16:19
... do you think that it would work well if it were scaled down for use with a common 35mm film canister rather than your piece of 2" iron pipe? ...
Aluminium tube, not iron pipe! I would guess the 35mm canister is about the same size. and more practical (if you posess one). But paint it orange so as not to forget it in the set.

Greg Blank
17-Oct-2009, 20:35
I did say some times, I use a 150mm as a "Normal" lens. When I do close up work I primarily use a 240 or the 300.

How do you figure that? I suppose you never focus closer than infinity?

Mark Sawyer
18-Oct-2009, 00:06
I did say some times, I use a 150mm as a "Normal" lens. When I do close up work I primarily use a 240 or the 300.

Presuming 150mm as a "normal" means a 4x5 format, you must get one heck of a bellows draw using a 300mm for close-ups...

Greg Blank
18-Oct-2009, 05:32
Since I have two cameras I usually use the 8x10 for the three hundred and 240, but both lenses will work on the 4x5.

Presuming 150mm as a "normal" means a 4x5 format, you must get one heck of a bellows draw using a 300mm for close-ups...

Jacques-Mtl
18-Oct-2009, 06:12
You can have a look at this web site. I find the section of John A. Cook the easiest one.
http://www.largeformatphotography.info/bellows-factor.html