Really confused over my camera's autofocussing

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trencheel303

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Quite a few cameras have problems with low-contrast subjects, particularly in low-light. Mostly it is degrees - the best cameras can be fooled too. There are things you can do to help yourself:

- Using a 'diamond' pattern (i.e. centre + points around, but biased to the centre) - available on most manufacturers AF systems can help, so there is a 'backup' AF point if the first can't get a lock. Obviously a problem if the distances of the surrounding points varies greatly from centre.
- Make sure (not sure what it is called on Canon) release priority for shooting is OFF - i.e. only allow a shot when you have a good AF lock
- half press and lock on a contrasty bit of the subject, recompose and fully press if all else fails.
- the brighter the light, the better - AF assist lamps on flashguns can help
- brighter lenses also help - an f5.6 kit lens will struggle more than an f2.8 premium one
- blow out your mirror box - crud on the AF mirror (behind the main one) or on the AF sensors will not help

Andy
 
Could you post some example shots for us to look at? And do you sharpen your shots at all?

That's the trouble of looking at shots at 100%, it's almost always going to disappoint. Even shots with the best kit in the world often don't look amazing at 100%. The slightest focussing error, or the tiniest bit of movement when taking the shot can mess it up.

The question is, do your shots look acceptable at whatever size they're viewed at, whether that's printing, hanging them on your wall, or on a computer screen?
 
There are invisible (!!) helper points on the canon 9 point AF system - which on some models you can turn on or off. Check that setting too (if the 450D has it...)

Andy
 
As people have already said, cameras will struggle in low light and/or on flat, low contrast surfaces.

Also bear in mind that the nifty fifty isn't at its sharpest at f1.8, even when the focus is spot on! It gets good from f2.5 upwards, and is a corker at f8.
 
.....but if I try manually focussing on a wall or otherwise rather flat object, and take the shot, I can find it's slightly OOF even although the camera said it was in focus.
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No autofocus system is going to make a very good job of focusing on a blank wall it needs some detail to work correctly. Pick something with some decent contrast and see how it works. AF systems are not miracle workers, you have to use a bit of user common sense as well. You need to master the camera not let it master you.
 
In my experience live view is more precise and I use it for close ups of things and also landscapes. However, it is excruciatingly slow and of no value for shooting anything with the slightest hint of movement.

I've not had any obvious complaints with my 450D along these lines. All cameras will struggle to focus on something if there is insufficient contrasting edges at the focal point. The 450D also has only the one central cross type sensor [picking up vertical and horizontal detail to aid focusing] whereas more advanced bodies have multiple cross type sensors to help in this area.

Focusing in live view works differently I seem to remember, which is why it takes so long. As you say, I suspect there's nothing wrong, just a limitation of the 450D's AF ability.
 
Deckard said:
In my experience live view is more precise and I use it for close ups of things and also landscapes. However, it is excruciatingly slow and of no value for shooting anything with the slightest hint of movement.

I've not had any obvious complaints with my 450D along these lines. All cameras will struggle to focus on something if there is insufficient contrasting edges at the focal point. The 450D also has only the one central cross type sensor [picking up vertical and horizontal detail to aid focusing] whereas more advanced bodies have multiple cross type sensors to help in this area.

Focusing in live view works differently I seem to remember, which is why it takes so long. As you say, I suspect there's nothing wrong, just a limitation of the 450D's AF ability.
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Liveview focussing, if your camera can do it at all, can happen in one of two ways. The first is the easiest - drop the mirror down, let the normal AF sensors do the job. The second is to use contrast-detect AF using the imaging sensor. This is the way compact cameras focus. The idea is the lens moves through the focus point. There will be a peak in the contrast of the image, detectable by the imaging sensor. The lens moves back and forth to find the exact optimal focussing point - this is why it is slower than the phase-difference AF detectors that are normally used when liveview isn't being used.

Andy
 
The first image looks soft because it is - by that I mean the contrast could be stepped up a bit.

Also what lens did you use - to get ultimate sharpness you really need a very high quality lens - it looks as if the lens/sensor combination can't resolve detail to the degree you want.

The second image looks sharper because you were much closer and the lens/sensor combination can give adequate sharpness.

I also have 450D and I have also wondered about the sharpness but my conclusion is that the focus is quite all right but, like you, my expectations have been coloured by seeing ultra sharp pictures taken with FF cameras using high quality lenses.
 
trencheel303 said:
Hmm, Peter, I'm not quite seeing your logic with your answrs and I'm not entirely sure you were understanding my OP... :thinking: let me clarify

I probably didn't make this clear in my OP but I have narrowed down my lack of ability to get tack sharp images to only when I am shooting scenes like the first photo I took. The focus system quite often gets it wrong (manually or auto focusm - regardless, it will tell me I am in focus when I am not quite) regardless of the lens, but if I fire off shots while gradually moving through the focus range I *will* get at least one which is tack sharp. If I stop down to say, f/8, it gives me more DOF so the shot is more likely to be in focus. It's almost as if the body is back/front focussing ONLY when shooting bland, flat objects, which as suggested may well just be a design limitation.

I honestly don't think clamping on a thousand pound lens is going to make that much of a difference - an incorrect focus is an incorrect focus, regardless. I've probably confused the hell out of you, and at this point I am pretty much just going on assumptions - so I will post sample images tonight to show you what I mean!
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No i wasn't confused, just trying to give you some options, and you didn't mention that you get sharp images while gradually moving through the focus range.

And in many cases a high quality lens can often make a difference because the AF may find it easier to focus properly with the best optics.

Unfortunately AF can still give the wrong focus in the wrong circumstances.

I'm afraid all we can do is accept that nothing is perfect or change the camera/lens combinations trying to find the perfect one.

Which can cost a fortune.
 
The second has blur from camera shake which was caused by the slow shutter speed. You said that you used f/8 for the first photo, even if the camera focused on the fence the DOF will extend to infinity.

The photo that you posted is too small to be able to identify any problems with sharpness. It would be good if you could link to the full size photo, leave the EXIF intact and state whether or not a tripod was used. It would be very helpful if you could do the same for the other images that you promised to post tonight.
 
to me your pixel peeping to much to be honest... just get out there and take some cracking images... with what is a great camera for the cash... my wife has one...

all cameras have their wee weeknesses... youve found one of the 450d's... I would not worry about it too much... just get on and get out there and take some nice pics!
 
So you expect the AF to struggle along in poor light and then complain when it finds it hard to cope, a bit like asking you to drive full pelt down a winding country lane at night using sidelights!
 
There was a post on here a short while back specifically on Canon AF, would well be worth a looksee...
 
trencheel303 said:
It should be noted that the effect is more noticeable at lower F/ numbers, and seems to be regardless of the light levels.

What do you guys make of it?
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Nothing wrong with your lens - just tolerances adding up and not cancelling out I would think - see previous post.
 
trencheel303 said:
last time I checked this wasn't poor light:

OldFerryInnStCatherines.jpg
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Ooh...very poor... it's up North by the look of it - everyone knows that you lose a third of a stop every 15 miles north of London... no wonder your camera's struggling...
 
Normal focus (phase detection) vs Live view (Contrast detection) are totally different.

Contrast detection, providing your subject is static and has reasonable contrast, will be better- particularly in low light.

Your photos of the poster are a classic example of when live view will yield a better result.

If your subject was in daylight outdoors, the phase detect would probably be better. If your subject was a child running around in doors in the gloom, phase detect would be better but neither probably would be up to much unless you had some decent top end glass and body (eg a 200mm f2 and a D3S)
 
I'd say that the shots you've shown still are dimly lit, in bright conditions the 450D can track pretty accurately with a USM lens.

Here's an example, a shot of Robin Soderling taken from the penultimate row of Wimbledon's centre court, i.e. miles away!

3679101525_09697e3220.jpg


I've shot a few tennis events and the 450D keeps up well in bright light.
 
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