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# Thread: Jasons' Bellows Factor Compensation Thingy

1. To find the distance from the infinity position:

distance = (focal length in mm)* square root(f/stop change wanted) in mm
distance = (focal length in mm)* square root(f/stop change wanted)/25.4 mm/inch in inches

Example:

distance = 135mm * square root (4 f stops) = 135mm * 2 = 270mm

I used this to mark off changes in f/stops from 0.0 [infinity] to 4.0 in 0.5 increments.

Steve

2. the version I made from JBrunner's idea has a 1.5" square target. I don't recall if I did any scaling though.

3. Originally Posted by silveror0
... f-stops cannot be multiplied. ...
f/stops are added rather than multiplied.

4. Originally Posted by Sirius Glass
To find the distance from the infinity position:

distance = (focal length in mm)* square root(f/stop change wanted) in mm
distance = (focal length in mm)* square root(f/stop change wanted)/25.4 mm/inch in inches

Example:

distance = 135mm * square root (4 f stops) = 135mm * 2 = 270mm

I used this to mark off changes in f/stops from 0.0 [infinity] to 4.0 in 0.5 increments.

Steve
But if you want a 1 stop change, you get 135mm, and that should be a 0 stop change. Or am I missing something?
For a 135mm lens I calculate a 1 stop difference at 191mm, 2 stops at 234mm, and 3 stops at 270mm.

5. Redundant

7. Originally Posted by johnnywalker
But if you want a 1 stop change, you get 135mm, and that should be a 0 stop change. Or am I missing something?
For a 135mm lens I calculate a 1 stop difference at 191mm, 2 stops at 234mm, and 3 stops at 270mm.

I cannot get it to line up here, but here it is ...

Enter Lens Focal Length 135

Bellows Scale f/stop
Extension Distance Change

60 0 0
65 0 0
70 0 0
75 0 0
80 0 0
85 0 0
90 0 0
95 0 0
100 0 0
105 0 0
110 0 0
115 0 0
120 0 0
125 0 0
130 0 0
135 0 0
140 5 0
145 10 0.25
150 15 0.25
155 20 0.5
160 25 0.5
165 30 0.5
170 35 0.75
175 40 0.75
180 45 0.75
185 50 1
190 55 1
195 60 1
200 65 1.25
205 70 1.25
210 75 1.25
215 80 1.25
220 85 1.5
225 90 1.5
230 95 1.5
235 100 1.5
240 105 1.75
245 110 1.75
250 115 1.75
255 120 1.75
260 125 2
265 130 2
270 135 2
275 140 2

Steve

8. Math makes my head hurt. I learned the algebra in the '60's at a photo school. I always had a headache. Then I found out about the f/stop/inches method that Steve Sherman describes. I've been pain free ever since.

I keep half a dressmakers tape in my bags and studio roller cabinet, that lets me hit my exposures within a third of an f/stop. But even using the guesstimate in your head method it puts you in the half an f/stop range. Then you just go over for negs and under for trans.

9. The only time Salzberger's "Quick Disk" has failed me is the time I left it in the middle of a frame... nicely focused and perfectly exposed.

10. I crunched the numbers backward to find length and marked a small retractable metal measuring tape accordingly for each lens (3) in a different color permanent ink. It is attached with a key ring to the strap of a spotmeter. Its a no-brainer -- just measure from the lensboard to the film plane and see which exposure factor is the closest .

http://www.jeffreyglasser.com/

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