Back focus question

[chris_g]

Someone correct me if I am wrong or tell me it I am right.

The Canon T3i has 44mm of distance from the sensor to the mount point on the body, according to some research on the web. The T-Ring specifically for the Canon has 11mm, as measured by my ruler, once it is completely threaded to the reducer corrector. Do I need any of the multitude of extension tubes that I bought to achieve the needed back focus of 55mm as 44mm plus 11mm = 55mm? My whole confusion at this point, do I measure from the sensor or do I measure from the mount point on the body of the camera?

[KathyNS]

No need to measure at all! TL;DR: Your setup is fine as it is.

There is a reason that many reducers/flatteners/coma correctors specify 55mm as their design back focus. It is because 55mm is the depth of *ALL* DSLRs when they have their correct T-ring installed. The whole point of designing the devices with a 55mm back focus is so that you don't need any other adapters. The measurement is from the mounting flange to the sensor.

The T-mount standard was developed by Tamron (hence the "T"), a maker of SLR lenses. They wanted to make their lenses interchangeable among different brands of cameras, so that a photographer who used both Canon and Nikon cameras, for instance, didn't need to buy two different lenses of each focal length. To do that, they needed adapters for each camera mount type. They also needed the optical spacing to be the same for all cameras, even though each manufacturer had their own body depth, so that the lenses would focus properly. The solution was to have the mount adapter compensate for the body depth. The result is that all standard T-rings make their camera 55mm deep.

The fact that your camera with its T-ring is the right depth is not a coincidence. The system was designed that way.

[chris_g]

Thank you for the info Kathy, that will get logged into my journal. One more question if you don't mind. What devices or when would there be a need for extension tubes?

[KathyNS]

Thank you for the info Kathy, that will get logged into my journal. One more question if you don't mind. What devices or when would there be a need for extension tubes?

There are focal reducers / flatteners that require 85mm or 105mm of back focus. To use one of these with a DSLR, you would need 30mm or 50mm of extension tubes, respectively.

Or, with your 55mm reducer, you might some day decide to use it with a dedicated astro camera that uses 12.5mm of back focus. So then, you would need 55 - 12.5 = 42.5mm of extension tubes.

I replaced my DSLR with a filter wheel / CCD combo. My coma corrector required 57mm of back focus: 2mm for the necessaty thread adapter and the standard 55mm for a DSLR. To make up for the missing camera body depth with the new equipment, I had to use 19mm of extensions.

[chris_g]

Thank you for the info Kathy, that will get logged into my journal. One more question if you don't mind. What devices or when would there be a need for extension tubes?

There are focal reducers / flatteners that require 85mm or 105mm of back focus. To use one of these with a DSLR, you would need 30mm or 50mm of extension tubes, respectively.

Or, with your 55mm reducer, you might some day decide to use it with a dedicated astro camera that uses 12.5mm of back focus. So then, you would need 55 - 12.5 = 42.5mm of extension tubes.

I replaced my DSLR with a filter wheel / CCD combo. My coma corrector required 57mm of back focus: 2mm for the necessaty thread adapter and the standard 55mm for a DSLR. To make up for the missing camera body depth with the new equipment, I had to use 19mm of extensions.

Thank you again, I do plan on switching to a dedicated astro camera, though that is a ways away.

[pixelsaurus]

Great thread. I just purchased a T adapter for a Pentax DSLR and after ordering it, I suddenly had a vision of receiving a bog standard M42 to PK adapter, the one that fits completely into the throat of a Pentax dslr in order to use M42 thread camera lenses. I was pleasantly surprised to find my new T adapter was about 10mm thick. That plus the Pentax register (45.46mm) comes to roughly 55mm. Another T adapter mystery sorted.

[KathyNS]

I was pleasantly surprised to find my new T adapter was about 10mm thick. That plus the Pentax register (45.46mm) comes to roughly 55mm. Another T adapter mystery sorted.

I would bet money that an accurate measurement of the T-ring would show that it is 9.54mm thick.

[pixelsaurus]

I would bet money that an accurate measurement of the T-ring would show that it is 9.54mm thick.

Close enough, my digital calipers say 9.57mm.