5 May, 2007
In Part III
, I covered how to handle graduated filters. In today’s article I am covering some of the final adjustments you need to make – for filters and bellows length.
Firstly the easy bit – filters. Each filter has a standard exposure compensation factor. If you are not sure what yours is, you can check by spot metering on the same subject in similar light with and without the filter on. The filter I most commonly use is a polarizing filter. Mine has a two stop factor (regardless of how it is positioned) so I simply increase the exposure by two stops. i.e. adjust the aperture (e.g. from F16 to f8) or reduce the shutter speed (e.g. 1/250 to 1/60).
So far so good. But for those who work on large format there is an added complication that may require an adjustment – bellows factor.
Whenever a lens is focused closer than infinity, the image is darkened slightly. As a result, exposure compensation is needed. (Many 35mm lenses automatically adjust the aperture to compensate). The closer the lens is focused, the greater the darkening effect.
This sounds scary but in practice, the compensation required is miniscule until you focus closer then about 10x the focal length of the lens. At 10x the focal length, an exposure adjustment of about +0.25 is required. (For a 150mm lens, 10x is 1.5 meters - so we are talking about macro territory here.)
By how much do you compensate? As you might expect there is a mathematical formula – in this case based on the relationship between the focal length of the lens and the length of the bellows (from the film to the centre of the lens and called the ‘image distance’).
When the lens is focused at infinity, the image distance is the same as the focal length. (i.e. 6 inches on a standard 150mm lens). There is no compensation required. When the image distance is 2x the focal length (e.g. 12 inches on a 150mm lens), an exposure adjustment of +2 stops is required.
The shutter speed adjustment (exposure factor) formula is:
- Exposure Factor = (Image distance/Focal length)²
For example, (12/6)² = 4. So the shutter speed needs to be multiplied by 4. (equivalent to 2 stops).
The reality is that nobody in their right mind wants to do sums like this every time they take a picture. Try it in your head if the image distance is 8.5 inches! So there must be a better way, right? Well there are two easy ways to get the adjustment right.
The simplest way, firstly, is using something called a
‘Quick Disk’. The quick disk is a round disk which you print and cut out. You place this in the picture and then measure the diameter of the disk projected onto the ground glass using the measuring scale. This tells you the appropriate image adjustment. I carry a cut out version of the disk and measuring scale in my bag all the time.
The second method I picked up from John Cook at the Large Format Photography website on their page covering bellows factor. It requires a tape measure in inches. (I carry a small sowing tape measure in the bag). Measure the image distance. Compare this to the focal length of the lens (in inches). Assume both figures are apertures and work out how many stops of light there are between them. That is how much exposure compensation you need.
For example, take a 6” lens (150mm) when focused such that the image distance is 10”. The difference between f6 and f10 is about 1.5 stops. Increase the exposure by +1.5.
I use a printed copy of the following table to help me get it right.
3.5 4* 4.5 5 5.6* 6.3 7.1 8* 9 10 11* 13 14 16* 18 20 22* 25 28 32*
90mm = 3.5" 150mm = 6" 210mm = 8.26" 240mm = 9.4" 300mm = 11.8" 450mm = 17.7"
In Part V, I am going to look at how to get a more accurate midpoint – especially if you have a handheld incident light meter. Then in the final part, I shall bring it all together by discussing some practical, real life examples.
© Jon Brock 2007