[pure-silver] Re: bellows contraction factor
----- Original Message ----- From: "Shannon Stoney" <sstoney@xxxxxxx> 
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Friday, April 20, 2007 2:53 PM
Subject: [pure-silver] bellows contraction factor
Hi, just wondering if there might be something called bellows contraction factor. When I use my new wide angle lens, which of course necessarily is closer to the film plane in a view camera, my negatives look overly dense, even when I expose and develop the same way that I do with the 150mm lens (for a 4x5 camera). I had the lens checked to make sure the shutter was timed right, and anyway I am timing it myself most of the time in seconds rather than fractions thereof. Could it be that contracting the bellows causes more light to hit the film, just as extending the bellows causes less light to hit the film plane? 

--shannon
There are several places for errors. First your 150mm lens and shutter may not be correct. Its easy to check the aperture although harder for a short lens because the aperture is small. The window that admits light is called the entrance pupil and it is in general not the same size as the physical aperture because it is the _image_ of the aperture as seen from the front of the lens. The glass in front of it may magnify it or reduce it depending on the power of the front cell. The size of the entrance pupil is pretty easy to measure. You need a small plane mirror, a first surface mirror is ideal but a shaving or makeup mirror will work _as long as it is flat and not a magnifying mirror_. You also need a fairly large card with a small hole near the middle. Also, a small light source, a pencil flashlight works. The last thing you need is a translucent screen, a sheet of drafting paper or even writing paper will do. The maeasurment can be done on a view camera but I will describe how to do it free standing so to speak. An additional item will be some sort of holder for the lens and card. First, the lens is autocollimated which will focus it at exactly infinity. This is done by placing the flat mirror over the lens. Then set up the white card in back of the lens with the pencil flast in back of the hole. Now adjust the distance of the lens from the card until the image of the hole is sharply focused. You will have to tilt the lens slightly so that the image does not fall back on the hole. The lens is now focused on infinity. Now replace the mirror with the translucent screen. The distance is not critical because the light from the lens is now collimated, meaning it appears to come from infinity. The flashlight will project a circle of light onto the screen. This circle is the entrance pupil. Measure its diameter at various settings of the stop control. Devide this value into the focal length to get the actual f/stop. This method works for any lens. By using the same technique of autocollimating its also possible to get an accurate measurement of the focal length of the lens but some means of holding lens and screen in alignment is necessary. A view camera works fine for this. The focal length of a lens can be found by its distance from the image at infinity focus and when adjusted for an exact 1:1 image of some object, i.e., unity magnification. The difference of the two distances is exactly four times the focal length. When the lens is returned to infinity focus a measurment of exactly one focal length from the image toward the lens will indicate the location of the rear principal point. This is useful for making certain calculations about the lens. The front principal point can be found by simply repeating the measurement with the lens turned around. There are a number of makeshift methods of measuring shutter speed but a simple speed meter of the sort sold by Calumet is very convenient and useful. Bear in mind that this kind of meter measures the total open time of the shutter rather than its _effective_ time. Because the shutter blades do not open and shut instantly the effective shutter speed will always be a little slower than indicated and will vary with the f/stop. Nearly all blade type shutters are calibrated at the higher speeds for the effective speed for the full clear aperture of the shutter. As a result the effective shutter speed for smaller f/stops will be longer. For the common Compur shutters the top speed is about 20% slower for small aperturs than the marked speed. Because the opening and closing times are fixed the error becomes smaller for the slower shutter speeds. 

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx


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