Tri-X - half a stop overexposure

[Patrick]

Just asking for peoples opinions on how acceptable overexposure looks on Tri-X. Obviously, this is very subjective. Ive never shot on Tri-X on super before vthough I have used Tri-X on 16mm and with that format, I exposed it correctly. Although Tri-X is rated at 200asa for daylight, the cartridge will 'instruct' the super 8 camera to expose it as 160asa which is about half a stop of overexposure. For those that do shoot Tri-X on a semi-regular basis, does this amount of overexposure look okay? I'm guessing that the exposure latitude of this stock must be a little generous. Out of curiosity, has anyone had Tri-X 'pulled' by half a stop during developing? If so, what are the results like? With pull=processing however, there is a reduction in contrast which is no good thing for black and white film.

[bolex]

Hello,

If you exposed the tri-x film as 160ASA it is 1/3 overexpose exactly on daylight! At tungsten light is correct expose is 160ASA!

[woods01]

I've shot a mainly Tri-X since Kodachrome was cut. Initially I was very careful with exposures but in the last few years have just let the autoexposure do the job in my Canon 1014xls. The results are terrific. I've occasionally blown the odd shot with tricky lighting like snow or sun reflecting on water but overall its been very good. In the summer be sure to have a ND filter in your bag or consider experimenting with colour filters to increase contrast and cut down the light.

[Patrick]

If you exposed the tri-x film as 160ASA it is 1/3 overexpose exactly on daylight!

Yea that just shows how bad my maths is. I was recently at an event shooting a mix of Ektachrome 100D and Tri-X. Switching between film stocks, I thought I recall there was about half a stop difference in the light readings - when taking a reading from the same subject (unless my memory is playing tricks on me. ) Camera used was a Canon 1014E.

In the summer be sure to have a ND filter in your bag

I used an ND filter with both film stocks at this event. As it turned out, pretty much all of my filming was in the shade and silly me - I forgot to remove the ND filter. If not for my absent mindedness, I could have had a bit more depth of field to play with.

[VulGerrity]

So you're saying you can't adjust the exposure on the camera?

If that's the case, I would do what other's have said, break out the ND filters. Since Tri-X is a reversal film, it's better to under-expose than over expose. If you over expose your film, you're going to loose detail in your highlights that you can never get back. But if it's processed as negative, then a little bit of over exposure is good so you don't loose detail in your blacks.

[carllooper]

A calculator can be used to work out stop differences in relation to ASA numbers

To work out the difference, in stops, between rating a film at 200 ASA and rating it at 160 ASA, you would calculate the log base 2 of each ASA number, and then the difference between those two numbers.

On a Calculator

Firstly a calculator, as a rule, calculates logs in base 10. So if you were to just calculate Log 200 (or Log 160) on a calculator you would get the wrong number. To get the right number you need to divide the result by Log 2

So for example, using a calculator (which calculates in base 10) you will get

Log 200 = 2.301029996
Log 2 = 0.301029995

So you divide Log 200 by Log 2 to get the right number:

Log 200 / Log 2 = 7.64385619

and do the same with 160:

Log 160 / Log 2 = 7.321928095

You then calculate difference between these two values, which will be the difference in stops:

7.64385619 - 7.321928095 = 0.321928084 stops

ie. a tad shy of 1/3 of a stop

You can also do the reverse of this. Suppose you know that you need to rate Tri-X, for daylight, at the equivalent of third stop less sensitivity. What would be the required ASA rating (ie. assuming we didn't already know)

We would calculate the log of 200 with

Log 200 / Log 2 = 7.64385619

than subtract 0.333 (ie. subtract one third of a stop) with:

7.64385619 - 0.333 stops = 7.31085619

And then convert this back into an ASA number, which is done using the inverse of log, which is to raise the base we're using (2) to the power of the log number (7.31085619):

2 ^ 7.31085619 = 158.7767862 ASA

ie. a tad shy of 160 ASA

[carllooper]

Another example of using logs

What ASA should we rate Tri-X at when using it with a Bolex RX.

Richard Tuohey told me it was 120 ASA.

Richard is correct. But lets assume we don't have a Richard we can ask.

We want to accomodate for the loss of light going to the Bolex viewfinder, and the loss of light that occurs as a result of a 135 degree shutter vs the 180 degree shutter assumed by light meters.Consulting information on the Bolex we find that 20% of light is lost to the viewfinder, and that the shutter is 135 degrees (rather than 180 degrees).

This means 80% of the light entering the lens is heading towards the film (the rest going to the viewfinder). Of this 80% a certain percent is lost by the difference between a 135 degree shutter and what our light meter assumes.

The percentage passed by the shutter will be 135/180 x 100% = 75%

So we need to calculate 75% of 80% (ie. what the shutter passes, of the light remaining after loss to prism) the answer being:

0.75 x 0.80 x 100% = 60%

In terms of stops, the loss will be:

Log 100% - Log 60% = 6.64385619 - 5.906890596 = 0.736965594 stops

Subtracting this from 200 ASA is:

Log 200 - 0.736965594 stops = 7.64385619 - 0.736965594 = 6.906890596

Converting back to ASA:

2 ^ 6.906890596 = 120 ASA (exactly)


Or more simply 60% of 200 = 120

[Patrick]

So you're saying you can't adjust the exposure on the camera?

As noted in my previous post, the camera used was a Canon 1014E which was the most advanced super 8 camera that Canon produced at the time (early 70s.) Manual exposure, auto exposure, single frame capability and 54fps are just some of the features that you'll find on this camera. And by the way, I nearly always shoot on manual exposure.

I would do what other's have said, break out the ND filters.

Read my last post.