Help with focus - Tamron 35-70mm F3.5 - Forums

Topic starter

Jan 25, 2022

Hi guys!

I'm writing to ask if someone can help me out with an issue I had last night, when I did my very first widefield astrophoto with an old 1984 Tamron 35-70mm F3.5 mounted on a Canon 1300D Super UV/IR Cut + CLS Astronomik filter and tracked with the Staradventurer 2i.

I attach to the post the result so you can see what I ended up with.

Basically, when I was trying to focus a bright star or constellation, I wasn't able to see anything (both from live view or camera connect through an iPad) even if boosting the ISO to my maximum (6400) and with the widest aperture possible (F.3.5).

I ended up taking a picture of the area > blind focus after seeing the picture > taking another picture to see if the focus was correct > blind focusing again > taking another picture to check the focus...and so on!

At the end I had an acceptable focus but not a perfect one (I was wondering to use my Bathinov mask but obviously it wasn't possible).

Do you know why I had this issue or anyone had a similar experience using a camera lens?

With my APO 60/360mm I don't have any issue focusing a star, so I tought that maybe the problem is the use of my light pollution filter at 35mm, too wide to see anything even with maxed out ISO.

Thanks a lot for the help!
Lorenzo

Engaging

andrea tasselli

Frequent contributor

Jan 25, 2022

70mm divided by 3.5 gives back 20mm of aperture so no surprise you couldn't see stars in LiveView unless they are of first magnitude . The CLS filter obviously didn't help either but it is not going to be a major factor.

Helpful Concise

Lorenzo Scagnolari

Topic starter

Jan 25, 2022

Thanks Andrea, this math makes a lot of sense and I didn’t think about it!

So the only way is to focus as I did? Taking test shots and then blind focus between a shot and another until I reach something acceptable?

Thanks again for the help!

andrea tasselli

Frequent contributor

Jan 25, 2022

Lorenzo Scagnolari:
Thanks Andrea, this math makes a lot of sense and I didn’t think about it!

So the only way is to focus as I did? Taking test shots and then blind focus between a shot and another until I reach something acceptable?

Thanks again for the help!

Yes, that's the way to do it and also the more precise way at any rate. I don't relay on the LiveView with either Canons or Nikons. The only ones I find acceptable in getting good focus are those of the Fujifilm cameras.

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