what is back button focussing and how can i focus on stars?

[mojitomax]

2 questions, 1 thread - for those in the know -

firstly what is back button focussing? and how do i set this up on a 7D?

secondly how can i focus on stars for astrophotography?

I took this image as part of a set for some star trails. It was a bright night and i used a 30 second exposure on a 28mm 1.8 lens and a 7D. I used live view and manual focussing. i focussed on a bright star and it appeared to be in focus. However on the image i think the problem is that the exposure is too long and there are light trails - not necessarily a lack of focus?

I need to retry again with a shorter exposure. Any advice on exposure length for astro/milky way photography? This is fine for light trails where the motion blur is to be expected anyway.

Many thanks


IMG_3754 by mojitomax, on Flickr

[flat-planedCrank]

I'll leave the astro question for the seasoned folks


Back button focusing is separating the activation on the af from the normal shutter release- normally to a top thumb button on the back.

It has a number of advantages including pre-focusing, allowing you to meter and hold that ae lock and still use the af, stopping the af hunting in low light, using servo af as single-shot (by tapping the button), and so on...

It's a popular technique that can take a short while to get used too but i've never heard of anyone going back I've used it for years


Your 7D has an af-on button on the back - there's probably a custom function to set it to only use the af when that button is pressed.


Hope this helps

[RobDickinson]

For back button focusing switch off af on the shutter in customise buttons menu. Then af-on, it's much better.


Stars will trail when the exposure time is too long. This depends on pixel density, focal length / field of view, where you are pointing the camera and even your latitude.

Google the 500 rule

[DIW35]

The 500 rule essentially states that your exposure should be no longer than 500/focal length to avoid star trailing. Using the rule a 500mm lens would indicate a maximum exposure time of 1 sec, whereas a 50mm lens would allow 10 secs etc.

[mojitomax]

So practically how would one use back button focussing?

Would one half press the shutter button and lock exposure and then back button focus the subject and then take a shot?

Or does one focus first and then press the shutter button?

It sounds a bit faffy. Is this a tripod technique?

I'm still confused. If I'm just out taking street shots or family pics how does one use this technique?

Many tha ks

[Mr Will]

Put the selected autofocus point over the subject, focus, (if required) release the button and recompose, shoot.

All it's doing is giving you control over whether you want the camera to autofocus or not, there is no great mystery or complication to it but it ends up giving you all the advantages of manual focus AND autofocus combined.

Some examples:

Quick snapshots - Point camera at subject, press both buttons. Camera focuses and shoots as normal.

Photographs of my daughter riding her bike - Camera in AI-Servo (AF-C), keep the central spot over her, keep the back button pressed, press the shutter at the correct moment. The advantage here is that the exposure won't lock the moment that I start tracking, it will only lock at the instant I press the shutter, reducing the chance of incorrect exposures if she moves from sunlight to shade (or vice versa).

Street shooting - Focus on a spot on the ground where I want my subject to be. Recompose to get the rest of the frame how I want it. Wait for the subject to get to the correct spot. Shoot. The camera will not try to refocus, resulting in instant response and allowing me to focus on things that are not currently directly under an autofocus point. Also works well for sports and wildlife - focus on the apex of a corner or a bird table then just shoot whenever the subject arrives.

Portraits/macro/still life with an off-centre subject - focus on the subject, recompose, take several shots without having to refocus in between each one.

Does that make it any clearer?

[mojitomax]

Righto. I see. That explains it. Thanks

[Mr Snap]

For star fields, I wouldn't use autofocus. Try setting your focus manually on infinity instead.

In practical terms - unless your using an absolutely humungous telephoto lens - anything beyond a kilometre or two is at 'infinity'. Stars are a teensy bit further away than that...

[Mr Will]

Mr Snap said:

For star fields, I wouldn't use autofocus. Try setting your focus manually on infinity instead.

In practical terms - unless your using an absolutely humungous telephoto lens - anything beyond a kilometre or two is at 'infinity'. Stars are a teensy bit further away than that...

One small point on this one - many modern lenses can actually focus beyond infinity. This is done to make life easier for autofocus systems but makes things slightly trickier for astrophotography. Common solutions are either to focus using live-view or to focus on something on the horizon then lock the lens to manual focus for the actual shot.

[Mr Snap]

Mr Will said:

One small point on this one - many modern lenses can actually focus beyond infinity. This is done to make life easier for autofocus systems but makes things slightly trickier for astrophotography. Common solutions are either to focus using live-view or to focus on something on the horizon then lock the lens to manual focus for the actual shot.

You mean Buzz Lightyear was right all along?

[RobDickinson]

Back button focusing is very natural, it just screws up people who borrow your camera.

I use it all the time but if I am on tripod I use live view to focus.

[Mr Will]

RobDickinson said:

Back button focusing is very natural, it just screws up people who borrow your camera.

My top tip for that is that switching the camera to Auto moves focussing back to the usual place. Very useful if you are handing your camera off to family or friends for a shot or two.

[mojitomax]

I used manual live view for the stars and Focussed on the brightest one. I think that I've just had too long an exposure resulting in star trails.

[ExPat2B]

Best use for back button focus.

Set AF to central point only. Point central AF point at subjects eye, focus on that.

Now recompose the shot using rule of thirds ( typically move the subject off center so that it intersects the 1/3 lines ) , then use shutter to take the shot.

Result is a perfectly focused shot with perfect composition.

[RobDickinson]

Not with a fast lens you won't.

Focus and recompose is destined to fail because you change your subject distance.

Always better to use the closest focus point to the subject as framed

[mojitomax]

RobDickinson said:

Not with a fast lens you won't.

Focus and recompose is destined to fail because you change your subject distance.

Always better to use the closest focus point to the subject as framed

This is the issue I have when using fast lenses. Quite often the focus is slightly out. I've reverted to using f2.2-2.8 for general people/dog photos

[RobDickinson]

If things are static I've taken too shooting fast lenses in live view.

Compose the shot, put the recticle over the bit you want in focus, zoom in 10* and manual focus. Snap.

[mojitomax]

RobDickinson said:

If things are static I've taken too shooting fast lenses in live view.

Compose the shot, put the recticle over the bit you want in focus, zoom in 10* and manual focus. Snap.

Static on a tripod is no worries, I use live view as well, but it's the general family snaps etc that the camera mis Focusses on. It's damn annoying. I just need more practice I suppose and a smaller aperture.

[RobDickinson]

I do that hand held in low light with slow moving subjects..