Asa notches and Cameras

Post by **marc** » Sat Jun 21, 2003 8:37 pm

Well, I am only speaking on behalf of my own personal experiences with the two stocks and those of a friend of mine. I am fortunate that my camera overexposes as opposed to underexposing as is the case with some people's cameras. Deductively, I would guess that the processing of Kodachrome allows a certain amount of masking of the grain structure since it is added after the film is exposed. When you underexpose Kodachrome a certain amount the dies that represent the given colors are more saturated and thus can cover the grain structure. When overexposing Kodachrome the colors are more washed out and thus reveals more grain structure. I would guess that it is not so much that Kodachrome is grainier than other films when overexposed but that it appears less grainy when slightly underexposed because of the very stable saturated colors. With respect to the other films, More grain is washed away when overexposed and thus reveals a grain structure that appears to be less visible.
With the filter compensation, The film is not being overexposed, it is being exposed "properly" based on the amount of light that is lost through the use of a filter. If you were to drop the asa reading and expose it without the filter it would definitely be overexposed. Now, when the compensation is made with the amount of light lost through the filter, the camera is compensating based on the 160 asa which is underexposure instead of the 125 asa which is what the film is rated as. So there is compensation for the filter but you are still back where you started because that compensation is based on and extra 35 asa deviation

Post by **Actor** » Sun Jun 22, 2003 2:47 am

So how come the ASA of the film drops, or does it at all?

This is my understanding of this subject and if I am wrong I welcome correction by those with more expertise than I.

OK. Regarding the "drop" in ASA of color reversal films. The ASA does not drop when the filter is switched in and the film exposed in daylight. What happens is that the filter soaks up 2/3 stops of the light, effectively reducing the film speed accordingly. With built in light meters that sample the light through the lens this is not a concern. The correction will be made automatically since the amount of light reaching the meter will be reduced just as the amount of light reaching the film is reduced. The so-called "drop" in film speed is primarily of interest to those using external meters and setting the exposure manually.

It is interesting to me that still photographers do not seem to think in this way. Films have one speed and that's it. If the photographer is using an external meter he is expected to know the "filter factor" of any filters he is using and correct accordingly. But then still photographers seem to be more filter crazy than cinematographers.

Now with the B&W reversal films Plus-X and Tri-X the situation is different. For instance Plus-X is rated as ASA 40 for tungsten. But for daylight the film speed is ASA 50 if you do not use the filter. That is because the spectrum of daylight is wider than tungsten, effectively making the film more sensitive. With daylight you gain 1/3 stop.

However if you switch in the filter you lose 2/3 stop. So the rating for Plus-X w/filter is ASA 32.

Tri-X is ASA 160T, ASA 200D w/o filter, ASA 125 w/filter.

This all assumes that the light meters involved are Panchromatic.

Post by **ericMartinJarvies** » Sun Jun 22, 2003 3:35 am

S8 Booster wrote:Check:

http://lavender.fortunecity.com/lavende ... tspec.html

http://lavender.fortunecity.com/lavende ... ching.html

Most camras are notched for 40 & 160 ASA by befault.

200 ASA will normally be exposed as 160 ASA
50 ASA will normally be exposed as 40 ASA (Without the 85 filter)

Some higher end cameras may set 50 and 200 ASA corretly but is most likely no big deal anyway exposurewise.

R

hello,

i am not understanding this ... on all of my beaulieu cameras, i see where on the camera there is a plastic set notch, then on the gate there is an adjustable spring loaded piece that appears to hold the film upwards, tight against the perf claw, thus to avoid the jitters. but no where else does any of these cameras have any sort of pin or otherwise to adjust according to the cart. perhaps otehrs coul take a picture of their camera's special pins so i can see? so in the case of beaulieu cameras, it does not matter what cart is inserted, you will always have to manually adjust your iso setting, correct?

also, perhaps someone could offer assistance. i am still struggling with the relationship between the filmstock asa, the on-camera iso variable settings, and the lens' iris stop setting. lewt me try to give some examples wherein i am filming, and i get stuck and am not certain if i am making a logicol adjustment/setting, based on lighting conditions:

ok, lets say i am using a pro8mm 50d cart, which is a 50 asa daylight balanced film. this would mean i set the camera's iso to 50, correct? now then, the company claims that the film has latitude of 7 stop exposure with this particular filmstock, 50d. what does that mean? does it mean the iris could be 7 stops either direction? or 3.5 either direction? or what?? alsom understanding that f5.6 to f8 is the best place fo the iris to be in terms of picture quality, what happens if you cannot acheive this with the existing lighting, and you change the on-camera ISO variable adjustment control to a higher setting, like 160 for example?(i am usng this example because right now i am sitting in front of my computer, and there is natural light coming in, i have the 50d cart loaded, and the on-camera iso knob is set to 50, and the lens' iris all the way to f22 and the on-camera meter is still not satisfied. now then, if i move the on-camera iso setting to 160, the light meter is satisfied. so what exactly happens at this point by moving the on-camera iso setting to 160 instead of leaving it at the suggested setting of 50, and on top of that having the lens' iris cranked all the way to f22? i need to know what happens to to the image doing this, as i still do not understand the whole film asa, camera iso, and lens f-stop workings and relationships. thanks

Post by **S8 Booster** » Sun Jun 22, 2003 5:26 am

The Beaulieus does not use the notch system as it sets the film speed manually by the film speed dial so you do not need to bother with this.

You may take a look at these linx for reference: (Other cams)
Cartridge notching on K40, E125, 50D, V200T, V250D, V500T
VISION 200T CART - CONFUSION ABOUT INCORRECT FILTER NOTCHING

With the "B" you simply set the fim speed for the film you use is. The built in exposure meter will compensate for the filter position.

The spring lever in the filmgate is for film transport stabilisation only and the other pin centering the cart is a guide for the cart positioning.

R

Post by **S8 Booster** » Sun Jun 22, 2003 5:36 am

Regarding the film latitude range this may give you an idea how it is determined or used:

http://www.kodak.com/US/en/motion/produ ... 5274.shtml

R

Post by **ericMartinJarvies** » Sun Jun 22, 2003 6:50 am

ah ... now its starting to make some sense. a little more every day

thanks s8 booster.
eric

ps - could you explain to me my other question posted above?

Post by **S8 Booster** » Sun Jun 22, 2003 5:38 pm

Your questions are basically far more complex than the answers but the best way to show the context is graphical which I seem to have no time to make - later maybe.

In the meantime I suggest you to look into this:

http://www.kodak.com/US/en/motion/students/handbook/

R

Post by **S8 Booster** » Sun Jun 22, 2003 5:47 pm

This table - although not directly linked to "our" stuff shows the relationship between film sensitivity and aperture for a given light intensity.

http://pages.prodigy.net/pam.orman/joem ... light.html

R

Post by **Technicolor** » Mon Jun 23, 2003 9:56 pm

I'm sorry about being totally thick but I still can't get my head round this.

Am I right now in thinking that if my camera was to expose Ektachrome at 160 ASA in daylight (using the built in meter) and I then engaged the filter this would not reduce the ASA because the cameras meter will simply open up the aperture a bit more to compensate for the light loss?

So was I originally wrong in thinking that if you underexposed the film at 160 ASA and then engaged the filter, the 2/3 of a stop light loss will reduce the ASA value?

So infact, the lower ASA ratings which are given to films when the filter is engaged is actually based on how their light/colour sensitivy is affected?

Post by **marc** » Mon Jun 23, 2003 11:36 pm

Yes,
When you read literature regarding drop in exposure, that may be with respect to cameras that do not make automatic compensation for the filter ( older cameras like regular 8). Some advice will be given as to how to compensate for the use of a filter. For amateurs it is easier to explain it in terms of ASA ratings. If you are using an external light meter then you just expose it at a lower ASA reading ( I think that it is 80 or 100 but I do not remember). It is easier for people to understand in these terms. With many Super 8 caneras the compensation is made automatically based on the existing ASA that the camera registers the cartridge as. Remember that when you have a lower ASA, the aperature has to be opened up more to let more light in. When you use a filter, you need to let more light in to compensate for the amount of light that the filter obsorbs.

Post by **Technicolor** » Mon Jun 23, 2003 11:44 pm

Thanks Marc, I understand now

How come Kodak bought out a filmstock like Ektachrome 7240 with a ASA rating of 125 which is unavailable to many cameras. Didnt they realise that most cameras would expose this film at 160 ASA instead like the original Ekta?

Post by **S8 Booster** » Mon Jun 23, 2003 11:55 pm

I can only give you an unverfied personal opinion on this from my own experiences particulary with the Canon 1014 Xl-S:

I am pretty sure that the film sensitivity drops by filter spectrum "choking" alone. Ref Kodaks specs for the films. Messing with the colour spectrum with a filter will cause alterations in the film sensitivity.

The filters will also cause a general light reduction factor wich is of less magnitude.

When I check the light metering system / f-stop reading on my "C" cam while I slowly put the 85 filter in position I can see a 2-stage alteration in the f-stop reading. It appears that the exposure control sets the film speed (Ref K40) to ASA 25 (Stage 1 - aperture opens - f-number reduces) while later in the process when the filter is in place the the exposure control compensates once more for the direct light loss (Stage 2).

From this it is my impression that there is a 2 stage adjustment in the process.

1) The cams exposure system will drop the film sensitivity from 160 to 100 ASA (40 to 25 ASA) for daylight exposure setting with the 85A filter and
2) The built in light metering system will compensate for the direct light loss from the insertion of the 85 filter.

Anyone correct me if I am wrong.
This is as said based on my observations and may not be correct.

R

Post by **marc** » Tue Jun 24, 2003 12:02 am

That 's correct because most super 8 cameras do this automatically. But with cameras where you have to set the exposure manually, it is easier to explain to the average user in terms of ASA droppage. John Schwind who suplies the regular 8mm film describes this difference in ASA when using the 125 outdoors because most regular 8 cameras have to be set manually and it is easier to descibe in this fashion. Many of the regular 8 cameras were made before the "auto exposure craze"