The CAT System.

We have seen the limitations off full manual flash in earlier posts. One engineering approach to improve electronic flash was Canons CAT system. The idea was simple take the distance information from the lens focusing and assume that the object that is in focus should as well be lighted correct. Then use a big (literally) potentiometer to adjust the flash output.

It worked with cameras that allowed matched needle metering. If CAT was activated the level of charge and the feedback from the focus information moved the indicator needle in the viewfinder, and it was matched by selecting the correct aperture. A good description is on the Mir site.

CAT was introduced with the Canondate camera in 1970 and followed in the FTb, and in the old F1 in 1971, EXAuto in 1972 and EF in 1974. While the first compact cameras with inbuilt flash where introduced in 1977 the pocket camera 110ED from the same year still used CAT with the ED flash. In the SLRs it was phased out by the introduction of the A Series in 1976 and finally died with the introduction of the new F1 in 1981.

The whole concept had a number of important draw-backs:

- The system worked only with a number of dedicated cameras. (We are used to that by now, but at the time it was seen as a restriction.)

- The system works with even fewer flashes. In fact apart from the ED for the 110 pocket cameras there was a model D for the Canonet and the AUTO EX Cameras The only model sold in larger quantity was the 133D which was sold with Cameras like the FTb the EF and the old F1. In 1978 Metz introduced the Ueberblitz CT45. After that date there was really no good reason for anyone to buy a 133D which was embarrassingly the "professional" flash for the old F1. To counter the situation Canon came up with the 500A hammer head style flash for the Old F1 which was a total flop. Very few units were produced and sold and it is rare to find today. In typical Japanese fashion the embarrassment got the silent treatment and there is no hint or mention of the 500A in any official Canon Literature anymore. Some people even doubt that they existed, but there are collectors who have some pieces. (The link seems to be dysfunctional now. Earlier there were pictures. A Japanese ad here.)

-The system worked only with four lenses the 50 f1.4 the 50 f1.8 the 35 f2 and the 35 f3.5. With all other lenses the flash was fully manual like ED above. That was really the nail in the coffin of the whole idea to apply CAT to SLRs. While the system worked reasonably well with compact cameras without interchangeable lenses, I don't know how Canon could ever think it was a good idea for the FD lens system when only two focal length worked. The FD lenses had to be fitted with a flash coupling ring that fitted into the front bayonet. There is a small pin on the focusing ring that coupled into the flash ring. When you moved the focus ring the potentiometer in the flash ring was moved and set to the "right" level. Obviously that works only with lenses with a similar diameter but it was also much more restricted by optical lens design. The same ring only works with lenses where the movement of the focus ring produced similar distance settings and the focus movement was in the same relation like the square fall off of light. That explains why it worked only with few FD lenses. Shortly after its introduction the CAT system started to compete with the first "Computer" flashes that worked with all cameras and all lenses. Specifically after the introduction of the Metz 45er it must have been more and more difficult to sell a 133d and a flash ring to Canon users.

- Another disadvantage of CAT is the method of bringing the flash to the "right" level. Actually it did not! The potentiometer simply set the needle in the viewfinder according to distance and the aperture matched the full blast from the flash. Each shot the energy stored in the capacitor is fully released. The flash recycling times were the same regardless of what distance you took the picture. And they were not too fast.

- The mir site perpetuates one marketing myth, the Canon guys came cleverly up with when the automated flash units started to appear and eclipse sales of the CAT flash units. Which is that the CAT system allows to use the flash before it is fully charged and is therefore quicker. As with all good marketing B*** S*** there is a seed of truth in that you can actually fire the flash before it is fully charged. which frankly is the case with all flashes. (see also measuring the voltage here) . However matching a moving needle as the flash is charging along resembles more a video game and the results as things change while you might recompose or refocus make results even more unpredictable and even more ugly as it might result in over exposure.

At some point of time I intended to buy a FTb and a 133D. Somehow I ended up with the 133D alone. I do not intend to buy an FTb just for the sake of CAT so there will be no experiments. The 133D can be used with any camera as a manual flash with guide number 18. As I have quite frankly enough from manual flashes we move quickly on to the "Computer-Electronic Flash" in the next posting .

P.S. Normally all pictures in the Blog are taken by me. If I ever use a picture from the net in exceptional circumstances I will clearly link to the Source. The Picture of the old F1 with 133D is from here

Correction: CAT was actually introduced a year earlier, than I thought. It came with the Canonet 17 QL (new) in July 1969. This was NOT a laps of memory (come on I am not THAT old) rather than a lapse of research.
The error might be forgiven when I show you how I looked like around 1970 in the picture below:

The Little Canon Flash

The small REVUEtron flash has been retired forever. In the last blog I sang his praise which was appropriate as he did indeed good service and it was his retirement speech after all. He is now gone and we can confess openly that he could be a pain in the ass at times.

The manual flash is not very precise as distances have to be estimated. (In advance.) It is completely unpractical in fluid situations when distances change rapidly. In Situations when there is sufficient time to calculate exposure and aperture, like in macro or still photography, the distances from Objects to flash and/or camera are so short that small errors of estimation make a noticeable difference, so one actually needs to measure them.

The (photographic) world was longing for some flash automation.

I planned to make some experiments with the little manual flash and then introduce two automated flash systems. Unfortunately the little REVUEtron left us so that I will jump right to the first flash system, CAT from Canon.

The idea of the CAT system was simple: The lens of the camera is focused anyway to the most important part of the picture, so one can use the focusing information from the lens and translate it somehow into an electronic signal to vary the flash output. (When the DSLRs appeared and the film based TTL did not work anymore. The technique of reading the focusing information from the lens was re-introduced.)

One catch of the Canon CAT system was that it worked only with cameras which where built specifically for it (a trend started by Canon that has become widely the norm today) and it worked only with a few select lenses.

For SLRs the CAT flash was the Speedlite 133D.
For compact cameras without interchangeable lens it was the Canolite D
For the Datematic and the Canon 110 Pocket camera it was the Canolite ED.

The Canolite ED, or short ED, is .... you might have guessed "The Little Canon Flash". I think I never owned a camera which would work with ED. So I really can not remember why and how I bought it. I am however pretty sure that it must have seemed like a good idea at the time. (In the picture ED is connected to a Mecalux 11 optical trigger from Metz.)


It works perfectly in manual mode and has a guide number of 14m. I will use ED for all the experiments I intended for the REVUEtron. ED is still in perfect working condition and I guess that it has to do with the fact that ED is powered by two AA batteries. ED has a flash ready lamp on the back and a guide number table for ISO 100. Any other ISO must be calculated. ED has no manual release switch! The recycle times with fresh batteries are about 3 to 4 seconds. Much faster than the REVUEtron Not bad.


I will not use the flash on a film camera because I am too lazy and to cheap to make the many intended test shots on film. Therefore I needed to trigger ED with a digital camera.

First a word of WARNING. Old flashes can have VERY high voltages on the contacts. Do not ever attach an unknown and/or old flash to your modern digital camera. Always consult your manual about the voltage your camera can take and then measure or look up the flash voltage.

A second word of warning: Your measurement might be very imprecise, but looking it up might not be safer as you put your trust in results that might be manufacturers specs of a new unit. Internet search for trigger voltage might produce a result, but it was measured by someone you don't know. And you do not know how good his equipment and method was.

How does ED measure. My Multitronic measurement device is very slow and therefore not very good in this situation. The problem is, that the flash fires the moment you connect the measuring probes of the Multitronic. So my readings when first charging the flash and then measuring were about 85 V. However I tried a second method by leaving the measurement device attached and then switching the flash on. It will charge and the voltage raises. At some point it will fire.
The reading in this case was about 165V. Hmmm that is the voltage when the flash will EARLIEST fire. I still do not know the voltage when it is FULLY charged, which might be considerable higher.

The Handbook of the Nikon D300s specifies 250V maximum. Sorry ED you will never be attached to it!

At this point I got sidetracked by the question how best to use photocells to fire ED when using Nikon i-TTL.

I have two models of all round photocells available: The Metz lux 11 and the China Import DS-1. The China Import claims to be specifically suited for digital flash systems with pre-flash. The half page manual is in a bit of a garbled English but makes a lot of sense once you ignore the grammar.

I tried three i-ttl flashes on my D300s: SB-800, SB-900 and Metz MZ54 4i

I got some expected and some unexpeced results:

The Metz photocell performed as expected. It did not work at all with any flash. The ED was fired by the pre flash and contributed nothing to the picture.

A flash picture of a flash! So far so good.

But listen to this: I tried the China gadget and it does NOT work with the SB-800 however it does work reliably and repeatedly with either the SB-900 or the MZ54 4i.

Why? I have no clue !


SB-800











SB-900: Hurray the ED fired in time!


More fun with the ED in the next post.