Focal Length problem

[Flotsam]

I wanted to make a simple method for approximating bellows extension exposure compensation. Maybe just a string with 1/2 stop marks on it, attached to the rear standard that could be drawn to the front standard to read out the compensation.

I focused my 150mm Rodenstock Geronar at infinity and measured the the distance from the glass to the shutter. It was close to 5.2". Nowhere near the 5.9" that I had expected.

Can someone give me a reality check? Should I figure my compensation based on my measurement or on the marked F.L.?

 

[Jorge]

Most likely you measured the string wrong. You missed the measurement by 0.7 of an inch which is about 17 mm. Not a big distance, I think maybe the focal plane of your camera back is not marked or you measured to the back of the shutter as opposed to the center or middle of it. I would say dont worry and go with the 150 mm string marked in half stops.
Theoretically you should measure from the focal plane to the nodal point, but heck if you are going to get that specific it defeats the purpose of what you are trying to do which is to simplify things.

 

[Flotsam]

I can understand how you came to that conclusion. I didn't really give much detail.

I focused critically on an object well into the infinity range of the lens. I removed the bellows, put one end of a stainless steel ruler ( that reads in tenths of an inch) directly against against the ground glass and measured to the plane of the shutter blades, figuring that was pretty much the center of the lens. Not absolutely accurate but it definately fell between 5.1" and 5.3". 5.9" fell out in front of the lens. I'm pretty confident of the measurement but man, that's quite a bit off the stated focal length.

 

[Jorge]

I agree with you not laser measurement but you should not be off that much. I am stumped also. Maybe the nodal point of your lens is at a different place than the middle, but I am not knowledgeable enough to say one way or another. Hope another member can pitch in.

 

[Ken Burns]

Why did you assume that the distance would be 5.9 inches? Just because that is the lens focal length in no way means that it would also be the flange focal distance. You need to go to the Rodenstock web site and see if they have the specifications for this lens. If not, then you might need to contact Rodenstock by phone or mail and get the specs. Depending on the optical configuration of a lens, it not unusual for this type of variation.

 

[steve]

I gave up measuring and calculating years ago because it's too much work. For $7.99 you can buy the Calumet Exposure Calculator from Calumet Photographic (www.calumetphoto.com) - it's catalog item number CC9201.

You place the measuring square at the subject position and measure on the ground glass with the exposure compensation ruler that is part of the kit. You get a direct reading as to how much more exposure is required.

 

[David A. Goldfarb]

The Calumet Exposure Calculator sounds like a $7.99 version of the Quick Disk, which available for free at:

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

Another approach is to put a ruler in the frame at the subject position, compare the field of view to the width of the frame to determine the magnification, then use an exposure table to determine the amount of compensation for the magnification factor. I keep such a table taped to the back of each of my LF cameras for this purpose. If all goes well, this table will be attached to this post as an MS Word doc file. The print is small so that it can be taped to the groundglass frame.

 

[Flotsam]

</span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (Ken Burns @ Oct 12 2002, 07:21 AM)</td></tr><tr><td id='QUOTE'>Why did you assume that the distance would be 5.9 inches?</td></tr></table><span id='postcolor'>
You're right. I just assumed that the glass to shutter distance when focused at infinity would be petty close to the focal length of the lens.

It's probably a good idea for me to contact Rodenstock.

 

[Flotsam]

I think that Steve and David are right. Magnification is probably the more accurate way to find the compensation. It will also give me a chance to double check that F.L. measurement.

Thanks all.

Neal