Sekonic L-398M User Manual

 

As can be noted from the above formula, if the subject luminance is fixed and the 
magnification changes, the film plane brightness (illumination) varies in the inverse 
proportion (M + 1)

2

. With a large subject distance, M is small and this effect 

becomes  nearly absent, but as the subject distance decreases, M becomes larger. 
In an actual photographic example, with a magnification M of 1, film plane 
brightness becomes (M + 1)

2

 or 4. This is 1/4th that of a subject distance at infinity 

and exposure must be  increased 4 times. When the measured exposure value is 
1/60 sec shutter speed, this must be compensated to 1/15 sec (1/60 x 4 = 1/15). In 
closeup photography, (M + 1)

2

 is referred to as the compensation factor. 

Hold meter as close as possible to the subject and point Lumisphere toward the 
camera. If light source is very close to the subject, temporarily remove the subject 
and hold the Lumisphere in the subject position. The light then striking the 
Lumisphere becomes exactly the same as that  illuminating the subject. To obtain 
exposure compensation for closeup:  
1. Determine exposure by normal incident light method. 2. Measure 
subject and image sizes (groundglass). 

3.  If the image is larger than the subject, it is enlarged, while a smaller image is 

compressed. Divide the larger value by the smaller value.  

4.  Obtain compensation factor from Table 1.  
5.  Apply compensation factor to measured exposure value to produce 

compensated exposure value.  

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