Tested on Canon 6D II - works great in TTL Mode, had to enable flash in camera and run it at 1/200 sec shutter.
Flash works, disperses good light. takes 4 AA batteries
Here's a good review and some info from here: https://www.pentaxforums.com/accessoryreviews/vivitar-550fd-m-p-o.html
Note that this listing does not include the Stand seen in the above URL
This review shares a lot of text with my review of the Vivitar 2800-D. I'm mostly writing this for those that have found an old flash in an attic, yard sale, or thrift shop and want to know what it is and how to use it.
The Vivitar Auto Thyristor 550FD M/P/O is a dedicated flash that supports TTL. It's closely related to the Vivitar 2800-D family of flashes - the body is almost identical, but it has an additional TTL mode. The Vivitar 28FD Auto-Dedicated is the same as the 550FD. Vivitar must have had someone whose job was to rename existing products. The Vivitar 530FD is similar, but with no bounce head and slightly less power. Considering that these seem to be the only Vivitar flashes with an F in the name, the F might be for the Four operating modes.
My manual says "Printed in 1984", which sounds about right. Vivitar sold them for quite awhile, but keep in mind that this is an early 1980s design. There is nothing digital about it. A thyristor is an electronic device to cut off the flash without wasting battery power. Previous automatic flashes would short out the capacitor, using a full charge for every flash regardless of how much light was emitted.
The "D" in the name and "M/P/O" on the back means that it is a dedicated flash for Minolta, Pentax, and Olympus cameras. By "dedicated", it means the flash tells the camera when it is charged, via a Ready pin on the hotshoe. That is all. The flash does not set the shutter speed or f-stop. The body may detect the "ready" signal and select the shutter speed, but this depends on the camera body, not the flash. There are also "C/R" and "N" versions of the 550FD, which are dedicated for Canon/Ricoh (as in 1980s Ricoh manual focus film SLRs) and Nikon, respectively. A Pentax body probably won't detect those flashes. Sellers don't always check, so be sure to look closely at the back for "M/P/O".
TTL mode: this is not P-TTL. This is the 1980s TTL, which required a sensor in the camera body. When the sensor got enough light, the camera told the flash to shut off. The only Pentax DLSRs with a TTL sensor are the *ist-D, *ist-DS, and *ist-DS2. With a Pentax A lens on a *ist-DS, I was able to get TTL to work fine - your results may vary. With any later DSLR, or earlier lenses, you should use Auto or Manual modes on the flash and manual mode on the body.
Auto mode (A1 or A2): the flash has a built in sensor that will shut off when it feels the scene has enough light. A1 is for "far" subjects, and A2 is for "near". There is a table on the back of the flash to determine the correct aperture for a given mode and ISO (assuming shutter is set to the camera's flash sync speed). The camera has to be manually set to match the table. Changing the ISO slider on the flash just adjusts the table display - it does NOT communicate anything to the camera. Again, this is 1980s technology!
Manual mode: Full power only. Use the table to figure out your shutter/aperture/ISO settings and set them manually on the camera using M-Manual mode. You can reduce the output by pointing the bounce head upwards, using a diffuser, etc.
I used one of these flashes for years on film bodies, and later on my K100D and later bodies (no TTL). Now I use them on radio triggers for off-camera flash. The flash head can take diffusers/soft boxes designed for the Canon 420EX, or you can glue on velcro, use speed straps, etc. Vivitar also made some snap-in color filters (clear/blue/yellow/red/orange) that fit the 550FD, 2600/2800, and 2600-D/2800-D families of flashes. I made a snap-in diffuser from the side of a plastic milk jug that cuts output by about 1 f-stop.
The guide number at ISO 100 is 24 meters or 80 feet (meaning at full power, ISO 100, and subject at 6m/20ft, set aperture to f/4). The trigger voltage on every unit I've checked was 4 volts, which is safe for DSLRs. You may want to check first, just to be safe.
There are many, many more powerful or sophisticated flashes out there, but if you want to shoot "old-school" with something cheap, reliable, and fairly compact, the 550FD is a pretty decent flash - look how many still work after all these years. Most of the ones that don't work just need to have the contacts on the battery door cleaned (these things ate alkaline batteries, which then usually leaked). You can easily find one for less than ten dollars, or even 99 cents if you shop carefully.
Update: I can confirm a previous reviewer's issues with the K-x. It appears that the flash puts voltage on or grounds the "Digital hotshoe pin" (the one closest to the lens, in the one o'clock position looking down at the camera), which confuses the K-x. Putting a slip of tape over that pin solves the problem.
Left-to-right: Vivitar 2800-D, 28FD, and 550FD rear panels, showing three modes
Left-to-right: Vivitar 2800-D, 28FD, and 550FD
Size Comparison with modern Speedlites:
Left-to-right: Pentax AF360FGZ II, Vivitar 550FD, and YongNuo YN-560 III
Read more at: https://www.pentaxforums.com/accessoryreviews/vivitar-550fd-m-p-o.html#ixzz8SPFzGQZ0
The Vivitar Auto Thyristor 550FD M/P/O is a dedicated flash that supports TTL. It's closely related to the Vivitar 2800-D family of flashes - the body is almost identical, but it has an additional TTL mode. The Vivitar 28FD Auto-Dedicated is the same as the 550FD. Vivitar must have had someone whose job was to rename existing products. The Vivitar 530FD is similar, but with no bounce head and slightly less power. Considering that these seem to be the only Vivitar flashes with an F in the name, the F might be for the Four operating modes.
My manual says "Printed in 1984", which sounds about right. Vivitar sold them for quite awhile, but keep in mind that this is an early 1980s design. There is nothing digital about it. A thyristor is an electronic device to cut off the flash without wasting battery power. Previous automatic flashes would short out the capacitor, using a full charge for every flash regardless of how much light was emitted.
The "D" in the name and "M/P/O" on the back means that it is a dedicated flash for Minolta, Pentax, and Olympus cameras. By "dedicated", it means the flash tells the camera when it is charged, via a Ready pin on the hotshoe. That is all. The flash does not set the shutter speed or f-stop. The body may detect the "ready" signal and select the shutter speed, but this depends on the camera body, not the flash. There are also "C/R" and "N" versions of the 550FD, which are dedicated for Canon/Ricoh (as in 1980s Ricoh manual focus film SLRs) and Nikon, respectively. A Pentax body probably won't detect those flashes. Sellers don't always check, so be sure to look closely at the back for "M/P/O".
TTL mode: this is not P-TTL. This is the 1980s TTL, which required a sensor in the camera body. When the sensor got enough light, the camera told the flash to shut off. The only Pentax DLSRs with a TTL sensor are the *ist-D, *ist-DS, and *ist-DS2. With a Pentax A lens on a *ist-DS, I was able to get TTL to work fine - your results may vary. With any later DSLR, or earlier lenses, you should use Auto or Manual modes on the flash and manual mode on the body.
Auto mode (A1 or A2): the flash has a built in sensor that will shut off when it feels the scene has enough light. A1 is for "far" subjects, and A2 is for "near". There is a table on the back of the flash to determine the correct aperture for a given mode and ISO (assuming shutter is set to the camera's flash sync speed). The camera has to be manually set to match the table. Changing the ISO slider on the flash just adjusts the table display - it does NOT communicate anything to the camera. Again, this is 1980s technology!
Manual mode: Full power only. Use the table to figure out your shutter/aperture/ISO settings and set them manually on the camera using M-Manual mode. You can reduce the output by pointing the bounce head upwards, using a diffuser, etc.
I used one of these flashes for years on film bodies, and later on my K100D and later bodies (no TTL). Now I use them on radio triggers for off-camera flash. The flash head can take diffusers/soft boxes designed for the Canon 420EX, or you can glue on velcro, use speed straps, etc. Vivitar also made some snap-in color filters (clear/blue/yellow/red/orange) that fit the 550FD, 2600/2800, and 2600-D/2800-D families of flashes. I made a snap-in diffuser from the side of a plastic milk jug that cuts output by about 1 f-stop.
The guide number at ISO 100 is 24 meters or 80 feet (meaning at full power, ISO 100, and subject at 6m/20ft, set aperture to f/4). The trigger voltage on every unit I've checked was 4 volts, which is safe for DSLRs. You may want to check first, just to be safe.
There are many, many more powerful or sophisticated flashes out there, but if you want to shoot "old-school" with something cheap, reliable, and fairly compact, the 550FD is a pretty decent flash - look how many still work after all these years. Most of the ones that don't work just need to have the contacts on the battery door cleaned (these things ate alkaline batteries, which then usually leaked). You can easily find one for less than ten dollars, or even 99 cents if you shop carefully.
Update: I can confirm a previous reviewer's issues with the K-x. It appears that the flash puts voltage on or grounds the "Digital hotshoe pin" (the one closest to the lens, in the one o'clock position looking down at the camera), which confuses the K-x. Putting a slip of tape over that pin solves the problem.
Left-to-right: Vivitar 2800-D, 28FD, and 550FD rear panels, showing three modes
Left-to-right: Vivitar 2800-D, 28FD, and 550FD
Size Comparison with modern Speedlites:
Left-to-right: Pentax AF360FGZ II, Vivitar 550FD, and YongNuo YN-560 III
Read more at: https://www.pentaxforums.com/accessoryreviews/vivitar-550fd-m-p-o.html#ixzz8SPFzGQZ0