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# Thread: Bellows factor conversion method????

1. Tom, another method to try is to figure the factor and then divide your film speed by that factor. For example, if you use a 150mm lens drawn out to 300mm then (300/150)²=4, then (ISO/4)=new ISO. If your film is TMY-2 and you rate it at 400, divide that by your factor: 400/4=100. Measure your scene at 100 to get the proper exposure.

2. Originally Posted by jimgalli
Oh, let's see. 13.5 is about 1/3 stop more than 11, and 18.0 is about 1/3 stop more than 16. I'll bet one stop would work just fine. Spoiling the fun you all want to have with your gizmos. I know.
I have this chart in my bag with me. If I do the math of (180/135)²= 1.7. This is 2/3 of a stop. This chart shows that the difference between f/13.5 and f/18 is 2/3 of a stop.

3. That's a good match to the actual number of 0.83 stops! A 1/6-stop error is more than acceptable.

4. Originally Posted by Greg Davis
Tom, another method to try is to figure the factor and then divide your film speed by that factor. For example, if you use a 150mm lens drawn out to 300mm then (300/150)²=4, then (ISO/4)=new ISO. If your film is TMY-2 and you rate it at 400, divide that by your factor: 400/4=100. Measure your scene at 100 to get the proper exposure.
Cool, that makes sense.

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