how do I calculate bellows extension?

[areliano_decotentin]

<p>Hi All,<br>

I'm getting a bit of a headache trying to do the math: How do I calcluate this? I have a 195mm lens focused at infinity. How far will this lens be from the focal plane when focused at say 1 meter? I'm making a bellows and I'm trying to figure out how much bellow I'm going to need..</p>

 

[steve_hamley]

<p>http://www.salzgeber.at/disc/index.html</p>

<p>Cheers, Steve</p>

<p>PS, Do remember to remove the disk before taking the photo...</p>

[chris_autio]

<p>bellows extension squared / lens focal length squared<br>

e.g. if 9 inches bellow (where your image comes into sharpness) with a 5 inch lens(128mm), then 81/25 =just under a factor of 3, or 1.3 stops<br>

It's best to do a trial for non flash, since most lenses have variances in actual speed of shutter.</p>

[john_a5]

<p>I originally answered the same bellows extension idea as Steve, but you are looking at something totally different. There must be a simple formula, but I do know that a lens, at 1:1 magnification is at the same distance from the ground glass as from the object being photographed. 1:1 magnification is at 2 times your focal length--or about 400mm in your case, which is less than .5 meters. I don't think you want too short a bellows and 400mm is only about 16 inches, which is pretty short for most view cameras.</p>

[john_a5]

<p>For those of you answering the bellows extension factor, that isn't the question here, but I have a very simple way to calculate extension factor, no math really, just a tape measure.</p>

<p>With a 5 inch lens, say a 120-128mm or so, the bellows extension works just like f-stops based on the extension. A 10 inch extension is the same as moving from f5 to f10=2 stops. If you extend to an 8 inch lens (210mm+-) to 11 inches, it is one stop extension factor and if to 16 inches, a two stop factor, just like if it were f-stops. I carry a tape measure, really small pocket one, and that is all I need to calculate the extension factor.</p>

[areliano_decotentin]

<p>John - You're right I wasn't looking for bellows extension factor, though that's a useful calculation for that too. My subject is misleading but I don't know how that question (calc) is phrased.<br>

I'm not going to use this for focusing at less than a meter, so probably 12" will be enough(?). I'm thinking Speed Graphic scale...</p>

[michael_briggs2]

<p>As already pointed out, the question isn't how to compensate exposure for a given bellows extension, but rather what the bellows extension needs to be to focus on a particular distance. This is discussed in the Lens Tutorial, http://www.photo.net/learn/optics/lensTutorial. The equation is 1/So + 1/Si = 1/f, where f is the focal length and So is the object distance and Si is the image distance -- the bellows extension that is wanted. Be sure to measure all three distances in the same units, such as mm. Also, the image and object distances are measured from the principal points, as discussed on the Lens Tutorial. For most LF lenses, and for most purposes, you can just measure from the center of the lens. For true telephoto lenses, which focus using a shorter than normal extension, you do need to know the location of the principal points. Datasheets from the lens manufacturers provide this information, but you probably don't need this refinement.</p>

<p>Using the equation for the values in the original question:<br>

f = 195 mm, So = 1000 mm ==> Si = 242 mm. So the lens has to be advanced 242 - 195 = 47 mm from infinity focus to focus at 1000 mm = 1 meter.</p>

[john_a5]

<p>Areliano, there is an easy way, actually, for you to figure this out almost perfectly! Just set up a light 1 meter from the lens, facing the lens. Then, with the lens wide open, take a piece of white cardboard and move it behind the lens until the light appears in focus on the cardboard--that is the length you need to make the bellows. This can easily be done with the lights on in most rooms.</p>

<p>or use the formula that was posted as I was writing this!</p>

[areliano_decotentin]

<p>Michael - Awesome - thanks much.</p>

[areliano_decotentin]

<p>John thanks - that's exactly what I would do as I tend to be a cut twice, measure once kind of DIYer, but the lens in question is on its way in the mail so I'm in the world of theory!!</p>

[pvp]

I was going to post a response but the robots are dropping all of my linefeeds and paragraphs so I can't make it intelligible.

 

Some other time, perhaps.