Can I use my Canon Speedlight 199A with my Canon T2i?

Question

I have a 199A from my days with my AE-1.  Both work!  Can I use the 199A with my T2i?

Answer 1

Automatic flash exposure with an AE-1 (btw, I still have my AE-1) and modern Canon E-TTL of today are VERY different.  
While you can use your flash as a "manual" flash... you may want to invest in newer gear.
On your 199A, there was no "TTL" capability of the flash.  That flash has 3 "auto" power levels... a high, medium, and low.  It merely told the camera to set the f-stop to either f/2.8, f/5.6, or f/11.
The flash unit has a thyristor sensor on the front that measures the light return to the flash and then cuts off the flash when it thinks it's received enough light.  Highly reflective surfaces could fool it into thinking it had recevied enough light and shut down even if the subject of interest was underexposed.
Modern E-TTL II flash is MUCH MUCH smarter.  As the name implies, it is actual "TTL" metering... meaning the camera and flash are have bi-direction communication.
E-TTL does the following:
It meters the scene with no flash whatsoever -- using "evaluative" metering and noting the amount of light present in each zone of the scene.
It fires a pre-flash at low power level.  Typically the default is 1/32nd power, but you can configure the flash to change that.
While the pre-flash is illuminating the scene, the camera meters the scene again.
The camera then compares each zone of the scene WITH the pre-flash to each zone of the scene WITHOUT the pre-flash.  Respective zones with very big difference indicate reflective objects (e.g. mirrors).  Zones with almost no difference indicate probable light sources (e.g. a light source in the scene... such as a table lamp, street light, etc.).  Zones with a more reasonable difference indicate actual elements of the scene benefiting from flash.  Having isolated zones which aren't reliable indicators of flash... the camera concentrates on calculating needed flash power by looking at the remaining zones.
If the flash is mounted on-camera and also pointed directly ahead (and ONLY if both of these conditions are true) THEN the E-TTL system will also request lens focusing distance information and take this into account when calcuating flash power.  This is because the "inverse square" law can be applied.  If the flash is off-camera then the computer cannot assume that the lens distance and flash distance are the same.  Also if the flash is being bounced then the distance the light will need to travel will not match the lens focus distance.  Hence, it only uses this info if both conditions are true.
With all of the above considered, the computer decides how much power should be used for the "real" shot and the camera opens the shutter and the flash fires using the calcuated power level.
All of this happens so fast that most people presume the flash just fired once.
Thyristor flash would be fooled by a strong light source (e.g. shooting into the sun and trying to use the flash as "fill" would fool the unit into thinking enough light had been delivered -- there are many things that could trick a thyristor-based auto flash.
Modern E-TTL does much more... that was just the basics, but you can see how the system is much more clever about figuring out how much power is really needed and knows how to isolate areas of the scene that used to fool auto-flash technologies of the past.
Modern flash can provide high-speed sync (rapid successive pulses of light as the curtain shutters sweep across the sensor at speeds faster than the flash "sync" speed.
Modern flash can also provide "2nd curtain" sync... where the flash fires just before closing the shutter rather than just after the shutter opens.  When shooting a subject to imply motion while using flash, this provides more directional blur that the eye expects.
Some (but not all) modern flash can provide "multi-strobic" flash... where the flash pulses several times at a slower speed (you control) during a very long exposure.  E.g. if I photograph an athelte "throwing a ball" you get images of them at many different points throughout the "throw" all in one exposure due to multi-strobic mode.
Most modern flash can participate in multi-light shooting scenarios where one flash acts as a "master" to tell the other units when to fire (and at what power level) and the others act as "slaves" (taking orders from the master) to provide better lighting (read specs carefully... some units can only be "slaves", some can only be "masters", and some can be either.)
Modern flash technology really has evolved and refined what you can do with flash.
Tim Campbell
5D II, 5D III, 60Da