Shutter calc

[Rmaydana]

I have a question, I am calibrating 4x5 shutters, and I have the following doubt.
I have the reading of the shutter speeds in ms, it is easy to know if the shutter is slow or fast. What I can't do for lack of knowledge is to find the compensation for each stop. is there a formula for this or a spreadsheet that can be shared. thanks.

 

[Focomatter]

Shutter speeds are normally expressed in fractions of a second, e.g. 1/4 sec which equals 250 ms. Like the aperture values the numerator is not written on the dial. The values are 1 stop apart on the dial.

 

[MattKing]

I have a question, I am calibrating 4x5 shutters, and I have the following doubt.
I have the reading of the shutter speeds in ms, it is easy to know if the shutter is slow or fast. What I can't do for lack of knowledge is to find the compensation for each stop. is there a formula for this or a spreadsheet that can be shared. thanks.

When you speak of "compensation", are you referring to a measurement of how the actual shutter speed differs from the marked shutter speed?

 

[Rmaydana]

When you speak of "compensation", are you referring to a measurement of how the actual shutter speed differs from the marked shutter speed?

yes

 

[Rmaydana]

Shutter speeds are normally expressed in fractions of a second, e.g. 1/4 sec which equals 250 ms. Like the aperture values the numerator is not written on the dial. The values are 1 stop apart on the dial.

thank you very much for the input. Clarifying the issue is the correction value, example for 1/4 (250) I am reading 267 there is a difference between the nominal 250 and the reading which is 267. for this example how do you calculate the correction setting.

 

[OAPOli]

The error in EV is log2(measured time / nominal time ). The error in your example is approx. 0.1 EV.

How are you measuring the times? For leaf shutters the exposure time is longer in the centre of the shutter (large f/stop) than it is on the periphery of the shutter (small f/stop). Thus is relevant for the fast speeds only.

 

[Focomatter]

thank you very much for the input. Clarifying the issue is the correction value, example for 1/4 (250) I am reading 267 there is a difference between the nominal 250 and the reading which is 267. for this example how do you calculate the correction setting.

That difference is insignificant with respect to actual ISO values of film that are plus or minus 1/3 of an f stop. It was only when Kodak started selling pro versions of some of its color films with more precise ASA values that this got smaller (a few decades ago). For example Ektachrome 64 (EPR) could have had an EI of 80 (one third stop faster than box speed) written in red on the piece of paper that came with the film. There was still some approximation here as the EI values incremented in 1/3 stops (same as increments of 1 DIN).

Most mechanical shutters are off a bit so I would not be too concerned. It is often that just the fast speeds that are the most off so they are best avoided. The only exception in my experience was when shooting with a Hasselblad from a small plane at low altitude so I was compelled to use 1/500. As well the aperture used affects the effective shutter speed so that it is effectively faster when used wide open. With view cameras this is much less of an issue since one tends to use long exposure and small stops.

 

[ic-racer]

From ISO 516 on shutter speed testing:

So, if 'n' = 1, 2, 3, 4... you get the whole stops.
If 'n' = 1.5, 2, 2.5, 3, 3.5 you get half stops
if 'n' = 1.3333, 1.666666, 2, etc you get 1/3 stops etc.

Pragmatically, one can make or download a table of 1/3 stop intervals as milliseconds and match up your numbers to the chart.

To mathematically take your milliisecond numbers and convert to fractions of a stop is also possible, but the numbers you get might be hard to use with an exposure meter. Like what would one do with 3.0864ms =1/324 = 1/256 + 1/3.45 stop? Might be easier to have looked at a chart and see 3.0864 is close to 3.10 = 1/256 + 1/3 stop.

 

[ic-racer]

What I do, is to see if the shutter falls under ISO tolerances. If so , I use it as is, if not, the shutter goes to the repair bench.

 

[Rmaydana]

What I do, is to see if the shutter falls under ISO tolerances. If so , I use it as is, if not, the shutter goes to the repair bench.

View attachment 364430

Thank you, and according to the table, I have to send the plug to the technician.
the speed for 1/4 (250), is shot at 13,6 (about 1/60). very bad.
Well at least a revelation and a speed not to use. Thanks
additional data is a copal 3S

 

[Doremus Scudder]

There's an active thread over on the LF forum with exactly this question (you aren't Andy F., are you?)

At any rate, I posted my conversion table of shutter speeds in milliseconds to actual shutter speeds in 1/3-stop increments over there. You can get it there along with the explanation of how to use the compensation from my posts.

Here's the link: https://www.largeformatphotography....ert-Milliseconds-To-Standard-Camera-Fractions

Best,

Doremus

 

[Rmaydana]

There's an active thread over on the LF forum with exactly this question (you aren't Andy F., are you?)

At any rate, I posted my conversion table of shutter speeds in milliseconds to actual shutter speeds in 1/3-stop increments over there. You can get it there along with the explanation of how to use the compensation from my posts.

Here's the link: https://www.largeformatphotography....ert-Milliseconds-To-Standard-Camera-Fractions

Best,

Doremus

Check the link, the information is useful. The author of the other post is someone else.