Focus problem with Opteka 500mm mirror lens

Question

Ok, I'll lead with the fact that I know this is a cheap lens and they're notoriously difficult/impossible to focus well.

But ... something is strange here.

I took this lens out to try and get some pics of the Jupiter-Saturn conjunction tonight. Camera is a Canon T7i. Used a tripod and a remote shutter release. Focused on the moon to start with, and used the live view zoom to really get a pretty good focus.

Then, still on live view, I click the shutter release. When the image displays, it's obviously unfocused relative to the live view. It's not terrible, but the live view is definitely better. I tried a range of ISO & shutter speeds. Found the conjunction, and got the same thing - could see moons when zoomed on the live view, but the pictures are nothing but blur.

So why is the focus changing between live view and the shot?

Just for kicks, here's a shot of the moon with the 2X teleconverter on as well. Saw the same sort of focus shift.

Answer 1

Your focus isn't changing. It's blur caused by vibration, possibly of the mirror and/or shutter. If you're not using a wired or wireless remote, it's also due to pressing the shutter button.

Beyond that, where I was located there was quite a bit of upper atmosphere turbulence shortly after sunset. With the planets low in the west, they're in the area of the sky most affected by the temperature differential along the terminator.

_{EOS 7D Mark II + Kenko 2.0X Teleplus PRO 300 DGX + EF 70-200mm f/2.8 L IS II. ISO 800, f/5.6, 1/1000. Wired remote with mirror lockup.}

This shot of the Moon¹ taken during the conjunction, shows a lot of atmospheric turbulence. Parts of the Moon are much sharper than other parts that are all the same distance from the shooting position.

Or your exposure time may be too long and you're seeing blur due to the motion of the planets during the exposure.

_{¹ Postprocessed using a CT of 2500K (even when finishing in monochrome, CT will affect relative contrast of the different areas of the Moon), Monochrome, red filter, and adjustments to contrast, highlights, and shadows. Moderate NR and sharpening.}

Answer 2

Your ISO should be no higher than 100 or 200. Shutter speed should be approx. 1/200th or faster on a sturdy tripod with the mirror locked up. Your high ISO is destroying detail.

BUT honestly, the Opteka is not a good performer. I don't think you will get the sharp pic you desire from it even after you minimize vibration and lower noise. Sigma and Quantaray mirrors are much better and sell for $50 regularly. Tamron's are excellent.