Infinity Focus Issue

[Startinez]

I've been noticing an issue with my Canon EOS 1100D that seems to pass over between both lenses (18-55 & 70-300) so I'm thinking the issue is with the camera itself, although the 70-300 is a Tamron lens so not actually Canon.

 

The issue is that stars just won't finely focus. This camera has a small screen for previews and post shot review and whilst on this screen the captures look good, but as soon as they're on a full sized PC screen you can immediately see that they're not perfectly focused. I've tried cleaning the cameras internal imaging sensor with a proper kit and cleaning methods to no avail. It seems to me that the issue is that the lenses just don't quite focus far enough to infinity to perfectly focus... I'm thinking about a factory reset before I'm out on the next attempt but I wondered if anyone has come across the issue or anything similar before?

[happy-kat]

Hi. With the kit lens on that camera it will do infinity focus. Did you try stopping down and how stable is your mount and what are you using to trigger the shutter?

Also dew on the lens will effect the sharpness of the stars.

Edited August 17, 2019 by happy-kat

[Startinez]

I haven't tried stopping down (that's going from say f/4 to f/8, right?) but that's a good call. Mount isn't anything special but it's certainly stable enough, and using a cabled intervalometer to trigger the lens.

[happy-kat]

It is worth stopping down to say f5 or so but also keep exposure very short to remove tracking issues if a tracking mount. If your tripod is on decking don't move and hold breath as decking wobbles.

[Startinez]

The thing is it's happened on multiple occasions in different locations, so that's what's pointing me towards an issue with the camera body itself. I'll try stopping down although I think my lenses only go to a max of f/5 ish anyway. No tracking and only 10-20s exposures so I shouldn't be losing focus there with such a wide frame.

[michael8554]

It's not clear to me whether you are manually or auto focusing? 

I've not imaged with a lens but would think that accurate autofocusing on stars is challenging for the camera? 

I've read here of successful manual focusing using a bright star to start with, then dimmer and dimmer ones. 

Michael 

[don4l]

What method are you using to get focus?

I've recently acquired an EOS250, and although it makes an attempt to autofocus on Jupiter, or the moon,  the results are unusable.

The only way that works for me is to manually focus.

 

[Startinez]

Apologies, I am manually focusing. 

[geordie85]

10 minutes ago, Startinez said:

Apologies, I am manually focusing. 

It could very well be user error. 

Try taking multiple shots, whilst moving focus in between each shot ever so slightly, then reviewing them on your pc. 

It can be very difficult to achieve perfect focus by just looking at a star on the preview screen. 

Are you using any aids to help with focusing? Like a micro focuser or bahtinov mask? 

[happy-kat]

On the kit lens infinity focus is just in from the full focus travel of the lens, same for many lenses.

[Uranium235]

If the camera is modded (as in filters removed, but not replaced), the sensor may need to be re-shimmed to work properly with some lenses. I had to do it to avoid the hard infininity stop with the Samyang 135mm.

Or, you can try using EOS utils to get a proper look at stars while focusing.

Typically, a modded camera in my experience has a difference between what you see through the viewfinder, and what you see on the preview screen  - the preview screen is the one that tells the truth (for daytime photography anyway).

[ollypenrice]

It helps to focus using a star located at one of the four intersections of the one third lines. (Imaginary lines on the chip, parallel to its sides and a third of the way to the opposite side.)

Another often-quoted technique is to look at the faintest visible stars. These will only be visible at all when you are in good focus. Out of focus they disappear.

Olly

[Startinez]

Thank you all for your help. I now have a few things to test out next time I get some clear skies!

[Alien 13]

Use the camera features to your advantage, grab a bright star and get rough focus with the live view screen, zoom in X5 and focus again but take notice of the dimmer stars that pop into view, finally use the x10 zoom and get the realy faint stars to appear....

Alan

P.S. you can temporally boost the ISO to help with star detection and also initially set the timer for 20 seconds rather than bulb to get the best live view.

Edited August 18, 2019 by Alien 13

[DorsetBlue]

Its annoying sometimes when lenses have a "Buzz Lightyear" function i.e. to Infinity and Beyond. 😀

[MattJenko]

I found that Canon kit lenses (and others) would easily slip slightly when in manual focus mode. Meaning you get great focus and then when coming to start imaging, it gets a knock and only has to slip a tiny amount. Using some duct tape when doing the focus actually helped me fix that focus position and made sure it wasn't slippage when moving the camera which caused the slight defocussing.

[carastro]

Quote

If the camera is modded (as in filters removed, but not replaced), the sensor may need to be re-shimmed to work properly with some lenses

This is a good point.  Should not be a problem using the telescope though. 

Have you got a mini Bahtinov mask for camera lenses?  They are not easy to get hold of so small but if you have a 3D printer you should be able to make one.  I normally stick a blob of Blue tac onto the focusser to keep it in place and re-check it after applying the blue tac.  

Live view on the computer is easier than looking at the back of a tiny LCD screen and also avoids having to bend in awkward positions.

Carole 

Edited August 27, 2019 by carastro

[happy-kat]

I use back button focus coupled with switch to auto focus after setting focus as it doesn't slip then

[michael8554]

9 hours ago, DorsetBlue said:

Its annoying sometimes when lenses have a "Buzz Lightyear" function i.e. to Infinity and Beyond.

Perhaps I'm missing something here? 

The only way to be sure that you're focused on infinity is if you're able to pass through best focus, and then reverse back to best focus?

Michael 

[DorsetBlue]

On ‎22‎/‎08‎/‎2019 at 19:00, michael8554 said:

Perhaps I'm missing something here? 

The only way to be sure that you're focused on infinity is if you're able to pass through best focus, and then reverse back to best focus?

Michael 

I was referring to there being further adjustment available after the infinity marking on the lens.

[michael8554]

Yes, why is that a Bad Thing?

Michael 

[happy-kat]

It is a good thing as temperature can effect infinity position I've read on here so you want focus capability past infinity to be sure you can get infinity by it being back from the full extent of lens travel plus there's lens construction accuracy differences.

[Alien 13]

The ability of an AF lens to go far beyond infinity is vital for video use, it would not be good for the motors if they were always hitting the end stop.

Alan