1. ## Strobes and Pops

Hello! Calling on to the knowledge of the group.

How do I determine the correct amount of additional pops per stop?

Is my math correct?

1 pop; initial exposure e.g. f/8
2 pops; +1 stop e.g. f/16
4 pops; +2 stops. e.g. f/32
8 pops; +3 stops. e.g. f/64
16 pops; +4 stops. Etc.
32 pops; +5 stops
Etc.

So this being the case, if I needed to add 2 stops for my bellows compensation that means, I would have done 5 pops or 8 pops?

Thanks!

Jurgen

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2. ## Re: Strobes and Pops

What happened to f11 and f22?  Reply With Quote

3. ## Re: Strobes and Pops

I count stops and pops on my fingers

sometimes more than 10 fingers  Reply With Quote

4. ## Re: Strobes and Pops Originally Posted by jurgenestanislao Hello! Calling on to the knowledge of the group.

How do I determine the correct amount of additional pops per stop?

Is my math correct?

1 pop; initial exposure e.g. f/8
2 pops; +1 stop e.g. f/16
4 pops; +2 stops. e.g. f/32
8 pops; +3 stops. e.g. f/64
16 pops; +4 stops. Etc.
32 pops; +5 stops
Etc.

So this being the case, if I needed to add 2 stops for my bellows compensation that means, I would have done 5 pops or 8 pops?

Thanks!

Jurgen
Your math is partly right and partly wrong. The part that's right is 2 pops is + 1 stop, 4 pops is + 2 stops, 8 pops is +3 stops ..... The part that's wrong relates to the f stop increments. f 16 is two stops smaller than f 8, not 1 stop. Similarly, f 64 is 6 stops smaller than f 8, not 3 stops.

Let me make this waaaaay more complicated than it needs to be, just for fun.

The f stop is really just the simple ratio of the lens focal length over the lens diameter (or diaphragm diameter). f=L/D where f= f number, L= Lens focal Length, D= Lens or diaphragm Diameter

The amount of light coming through the lens is related to the diameter of the lens (or diaphragm). The smaller the diameter, the less the amount of light getting through. One stop is a doubling or halving of the amount of light. Adding one stop would be doubling the amount of light, and reducing by one stop would provide half the amount of light. The amount of light coming through the diaphragm is relative to the area of the diaphragm opening. The area of a diaphragm opening for a circular diaphragm is Pi * D^2 / 4 (where D= Lens or diaphragm diameter). Note that the diameter is squared, so it's not a simple doubling of diameter.

Now, if we want to cut the amount of light in half (reduce it by one stop), we need to reduce the area of the diaphragm opening in half. If we call the starting diameter Da, then the starting area Aa = Pi * Da^2/4. Half that area would be Ab = Pi * Da^2/8 which would be one stop less light. The diameter of this smaller diaphragm opening would be Db, and the area would be Ab, so Ab = Pi * Db^2/4. Now that we have two different equations for Ab we can combine them to solve for Db.

Ab = Pi * Da^2/8 = Pi * Db^2/4

The Pi on each side cancels out leaving,

Da^2=2*Db^2
Sqrt(Da^2) = Sqrt(2*Db^2) Note that Sqrt is the terminology for square root
Da = Sqrt (2) * Db

or Db = Da / Sqrt(2)

This means that for one stop less light (or half the light), the diameter of the diaphragm opening has to divided by the square root of 2.

Now, let's see how this affects the f number. Remember that f = L / D from above.

If our starting f number is fa, then fa = L / Da. This can be rearranged as Da = L / fa

Similarly fb = L / Db

and we already determined Db = Da / Sqrt(2) for one stop less light

Therefor fb = L / (Da / Sqrt(2) ) = (L * Sqrt(2)) / Da

and since Da = L / fa

fb = (L * Sqrt(2)) / (L / fa)

The L cancels out leaving fb = fa * Sqrt(2)

fb =fa * Sqrt(2) is our one stop smaller relationship

Let's say our starting f stop is f8, then one stop smaller is 8 * Sqrt(2) = 11.3 (or rounded to f 11)

One stop smaller than f 11.3 is 11.3 * Sqrt(2) = 16 (f 16)

One stop smaller than f 16 is 16 * Sqrt(2) = 22.6 (or rounded to f 22)

One stop smaller than f 22.6 is 22.6 * Sqrt(2) = 32 (f 32)

..... and so on

If you want to look at one stop larger increments, you divide the original f number by Sqrt(2).

fb =fa / Sqrt(2) is our one stop larger relationship

Let's say our starting f stop is f8, then one stop larger is 8 / Sqrt(2) = 5.65 (or rounded to f 5.6)

One stop larger than f 5.6 is 5.6 / Sqrt(2) = 4 (f 4)

.... and so on

Here are the traditional one stop increments

f 0.5, f 0.7, f 1.0, f 1.4, f 2.0, f 2.8, f 4.0, f 5.6, f 8.0, f 11, f 16, f 22, f 32, f 45, f 64, f 90

I'll admit, it's a little more painful than counting fingers, but a lot less painful than a root canal.  Reply With Quote

5. ## Re: Strobes and Pops

To keep it simple, double your previous number of pops for each additional stop.

In my experience, reciprocity failure starts to set in after 8 pops or so.  Reply With Quote

6. ## Re: Strobes and Pops

Are you looking at the f scale on a very old lens that doesn't show f11? If so, remember that is the old "U.S." scale, not the modern f scale. F16 is f16, but what says f8 is really f11 and so on.

Kent in SD  Reply With Quote

7. ## Re: Strobes and Pops

My bad, thanks for all the inputs!

The table I should have shared:

Initial exposure; 1 pop e.g. f/8
+ 1 stop; +2 pops; e.g. f/11
+ 2 stops; +4 pops e.g. f/16
+ 3 stops; +8 pops e.g. f/22
+ 4 stops; +16 pops e.g. f/32
+ 5 stops; +32 pops e.g. f/45
+6 stops; +64 pops e.g. f/64
... and so on.

My next question is if I needed +2 stops for example for bellows comp., my total pops should be 8 right? And not 7?

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8. ## Re: Strobes and Pops

From your own corrected table, +2 stops extra exposure would require 4 pops total. 2 stops is 4 times as much light (4 pops).

Bellows compensation is often expressed as a multiplier factor. A multiplier of 1 would be normal exposure with no compensation. A multiplier of 2 means you need twice as much light. That could be achieved by opening up the lens by one stop (for 2 times as much light), or multiplying the exposure time by 2. Similarly, a multiplier of 4 means you need 4 times as much light. That could be achieved by opening up the lens by two stops (for 4 times as much light), or multiplying the exposure time by 4.

To make this easier, maybe avoid drinking any wobble pops prior to analysis.   Reply With Quote

9. ## Re: Strobes and Pops

You have + in front of your number of pops. You’re not adding to the prior value in your table, just showing the number required.  Reply With Quote

10. ## Re: Strobes and Pops Originally Posted by Jim Michael You have + in front of your number of pops. You’re not adding to the prior value in your table, just showing the number required.
Yeah figured that out now, thanks!  Reply With Quote

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