Test Strip Exposure with Flash?

[Phil Moreton]

Hi Everyone,

First post for me, looking for advice on how to calculate a test strip.

I'm doing a test strip with my 8 x 10 camera. I have an F4 lens and I'd like my middle strip on the test to calculate towards F4.0.

I'm using flash so I need to measure my first pop (flash) at a certain f. stop of light intensity. I only want one flash per strip so that the final 10th strip on my photo would have captured 10 flashes.
So for example:
1st flash = F? which is the base exposure.
2nd flash = F?
3rd flash = F?
4th flash = F?
5th flash = F4.0
6th flash = F?
7th flash = F?
And so on....till the final 10th flash.

What should I set my initial base exposure of flash to so that my 5th exposure reaches my target of F4.0.

I hope this makes sense and that someone can help me calculate what I need to set my metered base exposure flash to, so that by the time I hit the 5th exposure would technically be an intensity of F4.0.

Thanks,

Phil M.

 

[John Koehrer]

For base you might try the old GN charts. With a given film speed the GN is calculated @ 10 ft./3M
Because shutter speed isn't a consideration you're working only with aperture. Testing would start at f4
and then in steps open and closed. Flash meter would help for anything other than 10' the charts or calculations
get you close.

 

[Pieter12]

How fast is your lens?

 

[Phil Moreton]

Hi John,

For base you might try the old GN charts. With a given film speed the GN is calculated @ 10 ft./3M
Because shutter speed isn't a consideration you're working only with aperture. Testing would start at f4
and then in steps open and closed. Flash meter would help for anything other than 10' the charts or calculations
get you close.

Hi John,

Thanks for the reply. I do have my light meter so I know when my flash intensity is at F4.0. But in regards to this form of test strip i'm just trying to figure out what my first base flash should read at in F stops so that my middle exposure (the 5th flash exposure) would calculate to a total of F4.0.
This is just so I can get a good indication on the results once developed of my speed of film and where my exposure sits.

 

[Phil Moreton]

How fast is your lens?

Hi Peter,

Lens is an F4.0

 

[Chan Tran]

f/1.8

 

[Phil Moreton]

f/1.8

Hi Chan,

Firstly many thanks for your reply.
So, F1.8 for my base flash light exposure setting.
What are you basing this on and what are the increments in between?
Just so I can understand it for future tests.

Thanks

 

[Ozxplorer]

Phil, your progression will look like:-
Working down from aperture f/4 in half stops & every second aperture is the half stop!
f/4, f/3.4, f/2.8, f/2.4, f/2, f/1.7, f/1.4, f/1.2 & f/1
Use this graphic for more information....

 

[MattKing]

Welcome to Photrio.
I am slightly confused as to what you are doing.
Are you varying the intensity of the flash and using one flash ("pop")? per strip?
Or are you varying the number of flashes ("pops") per strip?
And are you attempting to achieve a particular tone at some part of the set of strips?

 

[Phil Moreton]

Phil, your progression will look like:-
Working down from aperture f/4 in half stops & every second aperture is the half stop!
f/4, f/3.4, f/2.8, f/2.4, f/2, f/1.7, f/1.4, f/1.2 & f/1
Use this graphic for more information....
View attachment 245131

Hi Fred,

Thanks for your reply too.
So are you saying my base exposure and therefore every flash is equalled to half a stop of light?
Should my base exposure then be F2?

Meter my flash to the Intensity of F2 = base exposure.

1st flash = F2
2nd flash with another flash of intensity of F2 = F2.4
3rd flash with another flash of intensity of F2 = F2.8
4th flash with another flash of intensity of F2 = F3.4
5th flash with another flash of intensity of F2 = F4
6th flash with another flash of intensity of F2 = F4.8
7th flash with another flash of intensity of F2 = F5.6
8th flash with another flash of intensity of F2 = F6.7
9th flash with another flash of intensity of F2 = F8
10th flash with another flash of intensity of F2 = F9.5

Is this correct?

Thanks,
Phil

 

[Phil Moreton]

Welcome to Photrio.
I am slightly confused as to what you are doing.
Are you varying the intensity of the flash and using one flash ("pop")? per strip?
Or are you varying the number of flashes ("pops") per strip?
And are you attempting to achieve a particular tone at some part of the set of strips?

Hi Matt,

Thanks for the welcome!!

I’m not changing the intensity at all on the flash.
All I am doing is after each flash I am opening the film plate by 1/10th
So that by the end of the 10th flash the 1st 1/10th of the plate would have been exposed to 10 flashes and the last part 10th of the full plate would have been exposed to only 1 flash.
Each time I open the plate by 1/10th it will only be 1 flash.

I’d like to achieve the middle 5th strip to ideally relate to f4 of intensity of light that it would have been exposed to.

Hope this makes sense.

 

[Chan Tran]

Hi Chan,

Firstly many thanks for your reply.
So, F1.8 for my base flash light exposure setting.
What are you basing this on and what are the increments in between?
Just so I can understand it for future tests.

Thanks

Well you want 5 flashes to accumulate to f/4.0. I square f/4 because the intensity of the flash is proportional to the aperture squared so I get 16. Now you want 1/5 of that intensity per flash so 16/5=3.2. I take the square root of this and I get the aperture for 1 flash.
√(4² ÷ 5)=1.788854382

 

[wiltw]

Consider this progression first...

1. 1 flash = 0EV
2. 2 flashes = 1EV
3. 4 flashes = 2EV
4. 8 flashes = 3EV
5. 16 flashes = 4EV
6. 32 flashes = 5EV

We see that each full of exposure difference is half/double what its neighbor is. IOW, your desire for 'one flash per strip' is inherently not possible. For each step of your trial exposure you are doubling (or halfing) the number of flashes of the neighboring step!

 

[MattKing]

If you want half stop steps, the divisions need to be 1.414 times apart (square root of 2).

 

[Chan Tran]

I guess I didn't answer you question with the 10 flashes so here it is
1st flash f/1.79
2nd flash f/2.53
3rd flash f:3.10
4th flash f/3.58
5th flash f/4.
6th flash f/4.38
7th flash f/4.73
8th flash f/5.06
9th flash f/5.37
10th flash f/5.66
You will see that between the 5th flash and the 10th only 1 stop increased because adding 5 more flashes only double the exposure of the first 5 flashes.

 

[Chan Tran]

Hi Fred,

Thanks for your reply too.
So are you saying my base exposure and therefore every flash is equalled to half a stop of light?
Should my base exposure then be F2?

Meter my flash to the Intensity of F2 = base exposure.

1st flash = F2
2nd flash with another flash of intensity of F2 = F2.4
3rd flash with another flash of intensity of F2 = F2.8
4th flash with another flash of intensity of F2 = F3.4
5th flash with another flash of intensity of F2 = F4
6th flash with another flash of intensity of F2 = F4.8
7th flash with another flash of intensity of F2 = F5.6
8th flash with another flash of intensity of F2 = F6.7
9th flash with another flash of intensity of F2 = F8
10th flash with another flash of intensity of F2 = F9.5

Is this correct?

Thanks,
Phil

Every flash is not equal to adding half stop of light.
The second flash add 1 stop because it simply double the exposure.
The third flash add 0.6 stop and the 4th flash add another 0.4 stop so the third and fouth flash add another stop because adding 2 flashes to the existing 2 flashes double it.
So with every consecutive flash the amount of stops added is getting smaller and smaller.

 

[Chan Tran]

If you want half stop steps, the divisions nee

Hi Matt,

Thanks for the welcome!!

I’m not changing the intensity at all on the flash.
All I am doing is after each flash I am opening the film plate by 1/10th
So that by the end of the 10th flash the 1st 1/10th of the plate would have been exposed to 10 flashes and the last part 10th of the full plate would have been exposed to only 1 flash.
Each time I open the plate by 1/10th it will only be 1 flash.

I’d like to achieve the middle 5th strip to ideally relate to f4 of intensity of light that it would have been exposed to.

Hope this makes sense.

It would result in a not very useful strip as the density spacing between steps are not the same. There is the most density difference between the first and second step as the exposure is 1 stop different. Between the 9th and 10th there is only 0.16 stop difference in exposure.

 

[Ozxplorer]

Hi Fred,

Thanks for your reply too.
So are you saying my base exposure and therefore every flash is equalled to half a stop of light?

Phil, I’m sorry if any confusion was caused by my reply... I clearly misunderstood your “shout out” for help with your planned project. I think @Chan Tran is on the right track...

 

[Phil Moreton]

It would result in a not very useful strip as the density spacing between steps are not the same. There is the most density difference between the first and second step as the exposure is 1 stop different. Between the 9th and 10th there is only 0.16 stop difference in exposure.

Hi Chan,

You have been most helpful.
Yes I understand that the way I am planning this strip is not that helpful as I’m really only showing 4 exposures with an increase of only 3 above my base exposure. I guess I’m hopeful that I’m going to hit my correct exposure at my 5/10 strip. Hah!

I’m thinking now that I should revert back to the Inverse-Square Law of Light strip test. 1x, 2x, 4x, 8x etc. Just for the first test strip then I’d know how far off I really am.

Anyhow, thank you everyone for your replies to this thread. Chan, thanks again for your help and explaining the formula perfectly.