A while ago, I acquired a 28mm M42 mount extension tube (unknown make) but never really thought about using it, so I decided to try it mainly just to see if the image quality suffered by using the tube.
The film used was 35mm FomaPan 200 (ISO 100)
The developer used was Kodak D23 (stock for 71/2 minutes 20°C
Now the tube is added, this has moved the lens 28mm further away so in theory more exposure will be required to compensate for the light loss through the extension tub, the question I was faced with was, just how much extra exposure is required.
Quick and dirty calculation
The lens I used is 58mm and the tube is 28mm so I figured the focal length is now 86mm
Without using all the EF/FL calculations, I tried to break it down into simple terms by thinking in f/stops.
58mm lens = f/5.8 (let’s call it f/5.6 as we are used to that figure)
58mm lens + 28mm Tube = 86mm (let’s call it f/8 as we are used to that figure)
Difference between f/5.6 and f/8 = 1 stop ( so let’s give it one extra stop over and above what the light meter says)
For all still life I use an LED 5600k dimmable panel. Does film react differently when used with LED lighting?, I don’t know, I haven’t done any testing and that’s another rabbit hole I have to go down at some stage.
All metering was done using incident metering, with 1/2 the dome catching the LED light and 1/2 the dome catching the shadow area.