Correct back focus..

[paul mc c]

Bit of help needed, my set up is 533mc, filter drawer and 0.8 reducer/flattener, I believe the correct distance is 55mm, is what I have ok.

Thanks Paul 

[symmetal]

You don't say what model of reducer/flattener you're using to confirm that it's 55mm, but assuming that's the case:

The 533 has 6.5mm 'backfocus' from the front of the camera body, or 17.5mm from the front of the attached 11mm spacer. Your ruler is showing the 55mm mark around 10mm from the camera front face so that's not quite right.

As the filter and camera protect glass will add around 1mm to the required back focus it's better to aim for 56mm back focus distance, so you want 49.5mm betweeen the rear face of the flattener and the front of the camera body.

I'm assuming the knurled ring at the rear of the flattener isn't part of the flattener itself, in which case your start point for measuring is correct though parallax is likely making it look a little off.

Alan

Edited August 29, 2023 by symmetal

[paul mc c]

1 hour ago, symmetal said:

You don't say what model of reducer/flattener you're using to confirm that it's 55mm, but assuming that's the case:

The 533 has 6.5mm 'backfocus' from the front of the camera body, or 17.5mm from the front of the attached 11mm spacer. Your ruler is showing the 55mm mark around 10mm from the camera front face so that's not quite right.

As the filter and camera protect glass will add around 1mm to the required back focus it's better to aim for 56mm back focus distance, so you want 49.5mm betweeen the rear face of the flattener and the front of the camera body.

I'm assuming the knurled ring at the rear of the flattener isn't part of the flattener itself, in which case your start point for measuring is correct though parallax is likely making it look a little off.

Alan

Hi Alan, thanks for replying,the reducer is Altair lightwave 0.8 ,the knurled ring is in fact part of the reducer, do you mean this distance should be 49.5.

[PeterC65]

The back focus distance should be measured between the reference face of the flattener and the sensor inside the camera. The reference face in this case is the scope end of the thread on the knurled ring which is part of the reducer. You seem to be measuring from the wrong side of this knurled ring. As mentioned, you should add one third of the thickness of any filters you have in your optical path to the stated back focus distance for your reducer. The thickness of filters varies from manufacturer to manufacturer.

 

[symmetal]

The flattener has an 'integral' T2 adapter, the knurled ring on the end so I would now assume that it is present in the 55mm spacing quoted, which is measured from the right edge of the ring, in which case the purple line is the 49.5mm required. If the adapter is not included in the flattener 55mm spacing specification then the green line is the 49.5mm required. I would think it is part of the flattener as Peter above has said so the purple line would be the one to use.

Note the front of the camera body is as I've indicated, and not where you've placed it. The sensor is 6.5mm to the right of the of the purple or green line end.

Note: pictures of the flattener on the web don't show the ring as knurled anymore, implying it's not usually removed, so it is now almost certain that the purple line is the one to use. 🙂

Alan

Edited August 29, 2023 by symmetal

[paul mc c]

2 hours ago, symmetal said:

The flattener has an 'integral' T2 adapter, the knurled ring on the end so I would now assume that it is present in the 55mm spacing quoted, which is measured from the right edge of the ring, in which case the purple line is the 49.5mm required. If the adapter is not included in the flattener 55mm spacing specification then the green line is the 49.5mm required. I would think it is part of the flattener as Peter above has said so the purple line would be the one to use.

Note the front of the camera body is as I've indicated, and not where you've placed it. The sensor is 6.5mm to the right of the of the purple or green line end.

Note: pictures of the flattener on the web don't show the ring as knurled anymore, implying it's not usually removed, so it is now almost certain that the purple line is the one to use. 🙂

Alan

Brilliant Alan,thank you much appreciated.

Paul