DX Coding - Why no speeds below ISO 25?

Scheimpflug · Mar 21, 2011

Just a curiosity question-

Does anyone know how the range was decided for DX coding on 35mm film cartridges? In particular, I'm wondering why the range is from ISO 25-5000 (in 1/3 stop increments), and not a more useful range such as 16-3200?

I've done some searching, but I can't find any examples of ISO 4000 or ISO 5000 film ever being produced. Yet the DX coding supports these speeds...

However, films of ISO 20 and below were common in the past (example: Kodachrome A at ISO 16, or Kodachrome F at ISO 12), and some are still available today (example: Addox CMS 20), yet these slow speeds cannot be represented by DX codes.

:confused:

Steve Smith · Mar 21, 2011

I suppose it's because it was intended to be used in consumer 'snapshot' cameras where the standard was 100/200/400 and even 25 would have been considered lower than necessary.

Also there was probably a pre-digital expectation that film speeds would rise significantly in the near future.

Steve.

Scheimpflug · Mar 21, 2011

Steve Smith said:
I suppose it's because it was intended to be used in consumer 'snapshot' cameras where the standard was 100/200/400 and even 25 would have been considered lower than necessary.

I've heard that mentioned before, but is there any truth to it?

I was always under the impression that the biggest benefit for DX coding was for the photo labs - to simplify processing by letting the machines auto-detect film speed, manufacturer, and type.

And while many consumer snapshot cameras eliminated manual control of ISO when DX coding became popular, remember that so too did some mid-range SLRs (for example, the Pentax P5/P50). Quite a few top professional SLRs used the DX coding by as well, with the option to override it if necessary. For example, the Canon EOS-1 line has used it since 1989, and the Nikon F4 had it in 1988.

Steve Smith · Mar 21, 2011

Scheimpflug said:
I've heard that mentioned before, but is there any truth to it?

I don't know. It was a supposition.

Scheimpflug said:
I was always under the impression that the biggest benefit for DX coding was for the photo labs - to simplify processing by letting the machines auto-detect film speed, manufacturer, and type..

That's not very plausible though as both C41 and E6 are standard development processes which don't need to know the film's speed (or manufacturer).

Steve.

Photo Engineer · Mar 21, 2011

At the time DX coding was introduced, there were no plans for commercial production of anything lower than ISO 25. Also, there were drawing board plans for films out to 3000 as we know today, and these could have been used as is or with a push to give speeds to 5000. Although not applicable to this topic, R&D was underway on instant products with an ISO value of 3000 and other available light materials were being considered.

PE

Vlad Soare · Mar 22, 2011

Too bad these plans didn't come off. An ASA 3000 film would have been very nice.
I take it these would have been real ASA 3000 films, not just 800-something pushable to 3000, right?

Photo Engineer · Mar 22, 2011

I'm not sure. The highest speed films that I saw were ISO 25,000. I have posted this before. Keeping was a real problem if you went much over 1000.

PE

cmacd123 · Mar 22, 2011

Steve Smith said:
I suppose it's because it was intended to be used in consumer 'snapshot' cameras where the standard was 100/200/400 and even 25 would have been considered lower than necessary..

At the time, you could still buy Kodachrome 25. and it was considered a "mainstream" product. I also remember that there used to be some very fast films, the name "Ektapress" comes to mind. (and I vaguely remember a Konica c-41 film that was also fairly speedy.

Scheimpflug · Mar 24, 2011

Thanks for the replies everyone, and thanks to AgX for the PM.

Interestingly, it sounds as if the DX coding wasn't really "forward thinking" enough - it covered what was available and planned at the time, but not really too many steps beyond that range to cover the future possibilities.

I see that there are gaps in the DX bit patterns as well, so they could have added more speeds without even requiring any more contacts. I haven't seen the actual specification itself, I wonder if there was a reason for this?

BetterSense · Mar 24, 2011

DX coding also has a bit for "high" or "low" latitude and for length of the roll. Did ANY cameras out there ever implement these features?

Vlad Soare · Mar 24, 2011

Yes, EOS 1V (and probably the rest of the 1 series, too) uses those. The number of remaining frames on the film is displayed in the viewfinder. This information is taken from the DX code.
Also, although I have no proof for this, I believe that the "evaluative" metering mode tries to err on the side of overexposure with negative films and underexposure with slides, using the exposure latitude encoded in the DX to figure out whether the film is negative or slide.

pnance · Mar 24, 2011

Wasn't the original Polaroid b&w film ASA 3000?

lxdude · Mar 24, 2011

BetterSense said:
DX coding also has a bit for "high" or "low" latitude and for length of the roll. Did ANY cameras out there ever implement these features?

As I understand it, that information was primarily there to be read by processing machines.

Jeff Kubach · Mar 24, 2011

Other than a pro, how many consumers go below 25 or above 3200!?

Jeff

Steve Smith · Mar 25, 2011

lxdude said:
As I understand it, that information was primarily there to be read by processing machines.

In a processing machine, the films are out of their containers and clipped together into a continuous strip all going through the same process, so this information isn't going to be of any use to the machine.

Steve.

cmacd123 · Mar 25, 2011

lxdude said:
As I understand it, that information was primarily there to be read by processing machines.

Actually, the type of film, (maker and model number code) is in the bar code. The bar code last digit also gives the length of the film, (3 is 24 exposures, 4 is 36 exposures)

The processors could sort on the bar code if they wanted, for example if they were planning to keep all the Kodak 400 in one batch. This was when optical printing required a separate "channel" for each type of colour film. The maker code is on the latent image bar code, as well as the frame number.

Scheimpflug · Mar 26, 2011

Jeff Kubach said:
Other than a pro, how many consumers go below 25 or above 3200!?

Probably not very many, as the films were not readily available... but as I mentioned before, I have never seen any evidence that DX-coding wasn't intended for both pros and average consumers alike.

I've been trying to piece together what films might have been available at the time when DX coding was conceived. From Wikipedia, I know that Kodachrome was available in ISO 25 from 1961-2002, and ISO 16 and 12 before that. I also am aware of Polaroid PolaBlue at ISO 8, which was mentioned in Popular Mechanics in 1988, but I do not know when it was introduced or discontinued. Kodak Technical Pan became available sometime around 1980/81 (don't have an exact date), and ran through 2004.

So it seems like somewhere along the line, a decision was made that even though ISO 25 film was available and in use, and speeds below ISO 25 were common in the past, that no future films would be slower than ISO 25. Space was reserved in the coding for higher speed films, but not for slower films.

It's a mystery.

Nicholas Lindan · Mar 26, 2011

OK - what films are available below ISO 25? BTW, many cameras won't recognize speeds below 50.

Specialty films like Tech Pan don't have DX coding.

Scheimpflug · Mar 27, 2011

Nicholas Lindan said:
OK - what films are available below ISO 25?

Addox CMS 20 is the only current film I am aware of at the moment, aside from bulk loads of specialty films (microfilm, motion picture film, etc) or older discontinued films.

Nicholas Lindan said:
Specialty films like Tech Pan don't have DX coding.

Well, they *can't* have DX coding if rated below ISO 25. Kodak recommended exposing Tech Pan at ISO 16-25 for 'Pictoral Photography'.

2F/2F · Mar 27, 2011

Because they would be 99 percent useless...like DX coding itself is for anything but completely automated point and shoot cameras and/or film shot at box speed. The only possible use would be for those "rigging" cassettes with DX stickers in order to cause their point and shoots to overexpose.

If you want to rate your film at something other than box speed, which must be what you are up to, DX coding is entirely useless anyhow, as you manually override it to do this.

What is it that you are after? How does DX coding benefit you as it is? How would extended low range help you?

Diapositivo · Mar 27, 2011

The DX code uses 5 binary values to hold film speed. That means that 32 different sensitivities can be recorded (2^5). The current implementation uses only 23 positions, so there are 9 positions left that can be used, in future, to indicate 9 more speed values without losing backward compatibility with current cameras when using "current" speeds.

The reason why those 9 positions were left "undefined" is probably to live room for greater distance between film sensitivities.

Next positions can be: 6400, 8000, 10000, 12800, 16000, 20000, etc;
But they can also be: 6400, 12800, 25600, 51200, 102400, 204800 etc.

Leaving the last positions undefined helps maintaining future use of the standard.

Fabrizio

Steve Smith · Mar 27, 2011

Some compact cameras do not have contacts in all of the positions and cannot differentiate between some values. I had a Nikon compact camera which would only be correct if the ISO was between 100 and 400.

Diapositivo said:
The current implementation uses only 23 positions, so there are 9 positions left

Call it seven usable positions as the all ones or all zeros ends of the range should be ignored as they would also be the 'code' for an un-marked cassette either all conductive or all insulated.

EDIT: I have just seen that ISO 5000 uses one of these end codes. And that there are 24 speeds in use rather than 23.

Steve.

Diapositivo · Mar 27, 2011

Steve Smith said:
EDIT: I have just seen that ISO 5000 uses one of these end codes. And that there are 24 speeds in use rather than 23.

You are probably right. I had read this page:

Dead Link Removed

which counts 23 positions but omits ISO 32 position.

Wikipedia is probably right in counting 24:

http://en.wikipedia.org/wiki/DX_encoding

vpwphoto · Mar 27, 2011

Pipe dreams... most of the cameras that relied on DX codes didn't have shutters capable of handling these high or low speeds you speak of. Shoot a Nikon F4 would be able to do much with IS) 12800 in sunlight.

cmacd123 · Mar 27, 2011

vpwphoto said:
Pipe dreams... most of the cameras that relied on DX codes didn't have shutters capable of handling these high or low speeds you speak of. Shoot a Nikon F4 would be able to do much with IS) 12800 in sunlight.

Some of the Canon SLR cameras have shutter speeds up to 1/4000 of of a second. Easier to do these days with microprocessor controlled shutters. Even my old EXA - 1 with it's top speed of 1/150 of a second would be able to use a 5000 ISO film as long as it was indoors without sunlight.

DX Coding - Why no speeds below ISO 25?

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