Focusing problems

[TobyMac]

Maybe someone out there can explain to me why my camera refuses to focus on any dark green or black surfaces?
For example: My girlfriend has a pitch black cat and when I try taking pictures of her on grass or tiles the camera refuses to focus on her although her deep yellow eyes stand out and there are sharp lines outlining her from the surface she is on.
A similar thing happens when I try to take pictures of dark green surfaces.
Any ideas????

[Rayne Storm]

Are you using auto focus, or manual? I have only recently started using manual focus, and it has solved a lot of the problems I have had. Not too sure what is happening with what you are experiencing, so will leave that up to the pro's to answer.

[TobyMac]

with autofocus if I switch to manual I can focus.

[Donald]

Tobias

In your comments about the cat being distinguished from its background, you are obviously aware of the problem of trying to focus on one block of colour. You suggest that you are not doing this, but are you sure. Remember, the camera will be attemting to focus grom the centre focus spot, not the whole frame. So, if the 'pitch black' is covering the whole of the centre area, then the camera is likely to be unable to autofocus.

Is this a possible explanation?

[TobyMac]

I thought that was the problem at first but then I tried focusing on the eyes, which stand out a lot, or where the cat meets the background and it still doesn't work.

[inkista]

Well, low contrast is one possibility (i.e., the ideal target is a sharp edge between black and white). Also, if you're shooting in low light, that's another issue. Maybe using AF assist on an external flash unit could help.

Being inside the minimum focus distance of the lens is another. Are you using a telephoto zoom? Does it have a focus limiter switch on it?

And (sorry, has to be asked), are you sure the lens AF/MF switch is set correctly?

And last, but not least, what AF mode are you using? Tracking modes tend not to work so well on still subjects and vice versa. And the modes that autoswitch between the two modes can be notoriously inaccurate.

[krispix]

Hi Tobias,
You need to look at the way autofocus works. In many respects it is similar in function to the exposure metering. It will focus the lens to the point where it achieves maximum illuminance (highest reading) because anything less than sharp focus will give a diffuse scattering of the light being measured and a consequential lowering of the light value being measured. In instances such as black cats, dark green jackets etc. the system will have very little light to play with and will be unable to distinguish between sharp focus and out-of-focus. If you have nothing bright and contrasty that you can focus on, you will be best off with manual focus, which is no bad thing IMO

[shreds]

Hi,

Can you move the focus spot around in the viewfinder as some cameras can. That way you can deliberately set it on a point of high contrast. If not, then manual focus is no bad thing, we all use it from time to time for such as seascapes, sand or similar non contrasting scenes, or for macro close up where better accuracy can be achieved. It happens to us all, so dont panic. Once upon a time, I recall manually focussing everything!

[TobyMac]

Hi Ian, no I can not move the focusing points around, I'v been practicing with manual focus for a while now so it's not a problem, I was just a bit curious as to why it struggles so much only with those two colors.
But thanks for all the pointers they do kinda make sense

[krispix]

Hi,

Can you move the focus spot around in the viewfinder as some cameras can. That way you can deliberately set it on a point of high contrast. If not, then manual focus is no bad thing, we all use it from time to time for such as seascapes, sand or similar non contrasting scenes, or for macro close up where better accuracy can be achieved. It happens to us all, so dont panic. Once upon a time, I recall manually focussing everything!

Hi Ian,

If you can remember manually focusing, did you also have to hold a light meter up and transfer the data to the aperture ring and speed dial on your camera. If so you must be as old as me!

[Dave Humphries]

Maybe someone out there can explain to me why my camera refuses to focus on any dark green or black surfaces?

Hi Tobias,

This is actually a good question, in fact the more I think about it, the better it gets.

A surface, of any given color, is insufficient for AF to focus upon, just because you know it is there and a certain distance away doesn't help an AF system. If the surface has no resolvable texture and hence no contrast edges for the AF to discriminate with, it cannot achieve a focus lock, that's just because of the way it works.

If I shoot say, a brick wall, it will be seeing the edges between bricks and mortar and focusing with those.
If I shoot say, a plain emulsion painted wall inside a house, that has no features, the AF is going to struggle.

AF needs edges to work, the better the contrast (difference in brightness) between those edges, the better and quicker, it will be.

Those two colours are actually relevant too, because the green channel contributes the most to an image, it will be looking for contrast differences between green and black, so if all you give it is dark green, it just sees varying dim shades of green, not enough contrast - result, it doesn't work.

Also, black isn't a color, it is a lack of light (reflected), so no wonder it struggles - only if the black is say gloss paint with some highlights on, will you get a good lock and that's because it is working with those white highlights, not the black surface.

If what I am saying still doesn't make sense, let me know and I'll post some example pictures to demonstrate.

Here's a tutorial that may help on other aspects of how AF works.

Hope that helps,

[Peter Ryan]

While you are aiming the centre point in your viewfinder onthe area you want to focus on you need to make sure your cameras Focus Mode isset to Single Point and not multi point focus. If you have it set to multipoint focus then the camera selects ‘nearest-to-the-camera-subject-priority’and if there is no specific broad contrast then it will have trouble auto focusing.If you set your Focus Mode to Single Point and then direct your focus area towhere the contrast is – the eye, then it will focus.

[TobyMac]

Thanks for all those tips guys, but where I struggle to understand why it doesn't focus is when I have my focusing point set on the black cats eyes and it still won't focus. The eyes stand out a lot, I mean the cat is pitch black and its eyes are a bright yellow, even if the entire frame is filled with the pitch black cat except for the bright yellow eyes standing out and my focus point on the eyes it will not focus.

@Dave, what you say about the flat single color surfaces not having any points to focus on makes sense and I just had a complete "wow that's so obvious" moment

[Peter Ryan]

Having the focus point set on the cat's eye is one thing. Having your focus mode set to single point is another. If you have your focus mode set to multi-point you still only see one focus point in the viewfinder but you have 9 or 52 active points. While in this mode your camera works on closest to the camera subject priority (i.e a global view) - even though you only see one focus point in the viewfinder.

Tell me what Focus Mode your camera is set on - not what you are pointing your focus point in the viewfinder at?

[TobyMac]

I am pointing the focus point I set onto the the cats eyes.

[Peter Ryan]

Hi Tobias. Sorry I didn't have access to the web over the weekend.

I understand what you are tellng me but while I was away recently I changed the Focus Mode (in the menue) from Single point to multi point. It still gave me just one focus point in the viewfinder to point at the subject but by changing the Focus Mode in the menu from Single area to multi area you change the way the camera actually focuses. When choosing single point it acts only on the focal point you see in the viewfinder - in your case the cats eye. When choosing multi point it just takes the nearest to camera subject priority even though you are pointing at the eye. You need to change the Focus Mode in your menu then try again.