Difficulty with auto focus.

[CQGems]

Hi all.

I have a bit of a weird problem.

I am trying to photograph a facetted gemstone and for some reason it will not autofocus onto the stone.

I am using a Canon 400D with a 100mm 2.8L series lens, using spot focus, and shooting Raw in either AV or P mode. I can get a great shot with manual focus so I am not too close, however, even when I manually focus on the stone and revert to auto focus, it will simply blow the focus right out. Have tried various Iso's, f settings, shutter speeds, etc.

Is it because of the clarity of the stone not enabling focus, or the reflections off the facet faces, or what.

Got me a little bushed I must say.

Any ideas?

[nardes]

I think that AF systems perform best when the AF sensor finds a sharp, contrasty edge. Some are optimised for horizontal or vertical edges, whilst others can utilise both horizontal and vertical edges.

If you place an AF sensor over a monotone subject, such as a blank, monotone wall or a featureless object such as a clear blue sky, there is insufficient contrast/edges for the AF to work.

So, in your case, it might be that the faces of the gem cannot provide adequate contrast, but I would have expected that an AF point straddling an edge would AF okay?

Cheers

Dennis

[Mark L]

Think Dennis is on the right track.
I tried auto focusing on some patterns in a mirror today. Didn't seem to have anything with enough contrast to work. Had to use manual focus.
You could try different AF points maybe.

[ricktas]

Yes Dennis is correct. AF systems work by detecting contrast and 'edges'. This is the reason you cannot sometimes focus on clear blue sky, but if you put the focus point on a cloud it can detect it.

Perhaps use manual focus and live view?

[arthurking83]

And to add to what's already posted, if my understanding of the 100/2.8 L lens, is a macro lens. So it begs the question were you focusing whilst the subject was very close to macro range?

For best macro focusing, AF is not really an ideal method.
Does the lens have a focus limiting switch? If so use it too.

Usually it's best to manually focus to a certain point that looks pretty much focused, and then try to AF from there.

Nikon's 105VR lens also annoys with similar focusing histrionics, even with the focus limiting switch set to use close up range too.
(the options are something like '0.5m-infinity' and 'full' .. where the close range option is to use the full setting!)

But in general, I tend not to use AF for macro(that is really close up - real macro range.
From about 1:3 and lower magnifications, AF is usually temperamental, but can work.

[CQGems]

Yep. Makes sense. I am trying to centre the focus on the table of the gem (the flat top bit), where there is no vertical or horizontal line. The only other problem with manual focus, the way I am doing it, is that I use an additional hand held light to try to get the best refraction coming out of the stone. It is a matter of fractions and I have found that using a moving light rather than moving the object or the camera is a lot easier.

They reckon that photographing faceted stones is about the hardest thing to do; I am tending to agree.

Thanks everyone. At least it is not only me, phew. Back to the practice session in the morning. Will try a different metering to see if it will pick up the edges without picking up the background.

Got to get it right for this weeks members challenge too.

[ricktas]

Yep. Makes sense. I am trying to centre the focus on the table of the gem (the flat top bit), where there is no vertical or horizontal line. The only other problem with manual focus, the way I am doing it, is that I use an additional hand held light to try to get the best refraction coming out of the stone. It is a matter of fractions and I have found that using a moving light rather than moving the object or the camera is a lot easier.

They reckon that photographing faceted stones is about the hardest thing to do; I am tending to agree.

Thanks everyone. At least it is not only me, phew. Back to the practice session in the morning. Will try a different metering to see if it will pick up the edges without picking up the background.

Got to get it right for this weeks members challenge too.

Are you using a tripod? If not, do so! Put the camera on the tripod. Manually focus on the gem, and then play with your light angles. Focus doesn't change cause light angles do. Shooting macro requires great precision in actual distance to subject and shooting hand held will make this almost impossible, especially if you are looking through the viewfinder and then using the other hand to manipulate your light.

Use a tripod, use live-view, lock focus on the gem, then play with the light, and if possible use a remote shutter release and even mirror lock-up mode. These will get you the very best results.

[nardes]

I think that the brains or smarts that manage the AF in the camera (via the viewfinder) are a different set of sensors and electronics to those that measure and manage the scene’s exposure measurements via shutter speed, aperture and ISO, so it might be best to decouple these things when trying to understand the AF problems.

On my Canon camera the centre AF point is the most accurate and the best one to use for low light situations.

An AF area (red square) in the viewfinder is a small group of sensors and you need to place the AF point over a (high) contrast region of the scene in order for it to auto focus.

Also, when using multiple AF points, if you happen to have a more contrasty object in the scene that is closer to the camera than the subject you want to focus on, the AF would normally select the closest contrasty object unless you force it to AF on the desired object by selecting a single AF point and placing it on the desired object.

On full auto, the camera tries its best but it may not realise that this might be different to what you actually want to do.

Cheers

Dennis

[arthurking83]

......

Use a tripod, use live-view, lock focus on the gem, .....

I'm not sure that the 400D has liveview!

But! .. Rick has made a very good point.
if you are trying to get high-ish magnification, and you aren't using either a supremely solid tripod, or are trying handheld(even if the lens has IS) .. then you are really wasting your time.

You will have trouble trying to differentiate between camera shake, and focusing accuracy.

So, hopefully you are using at least some of the methods that Rick had mentioned .. ie. tripod, remote(of any kind) .. and if you can't get mirror lockup or exposure delay then a higher shutter speed(1/100s preferably more to be sure).

With focusing inaccuracies, you can actually account for them by focus bracketing.

That is, try to manually focus to where you think you should, then study the image on a large screen for where the focus plane actually lies.

Firstly you will know where you want the point(or plane) of focus to be.
On reviewing the images on a larger screen you will clearly see if you have focused either forward or behind the point you want.
Adjust focus manually by a miniscule amount to suit .. repeating the review process.


Here's a couple of my images of jewelry from a million years ago:







These are all the same gemstone thingy(I'm not a jewelry person, this is my daughter's pendant) .. so I understand your interest in trying to inject various lighting qualities too.
They were all manually focused .... although using liveview, which makes it immensely easier .. but as already said, using focus bracketing you should be able to do it without needing liveview.
(it's just more time consuming/bothersome)

But!! ... don't make things harder for yourself by trying to do too much at one time.
(ie. hopefully not handholding camera AND handholding lights and so on).

[CQGems]

Thanks all.

Am using a tripod. Camera shake does not seem to be the problem. When I do get a shot in, it is crisp as.

Live view I am not sure of, am going over the book, there is video out plug on the camera, so maybe there is. Been too busy to do much the last couple of days.

When I do get a good shot, there is the added problem of the shot being so clean that minute bits of a fingerprint are showing up. Going to have to get them ultrasound cleaned and use tools to pick them up in future.

Have had a bit of success in moving back about 4-5 inches and then cropping the image, but that is cheating.

Thanks again for all the help.

[Mark L]

Have had a bit of success in moving back about 4-5 inches and then cropping the image, but that is cheating.

It's using the tools we have available to us.
If you get the exposure slightly wrong and brighten or darken it, is that cheating?
If it works then use it.
I had many years between taking film photos and then using digital. Started by thinking SOOC was the only way to go. Rubbish. Use the tools digital gives us (and I still dislike the PP required). Cropping is a good simple one.

[arthurking83]

Thanks all.

Am using a tripod. Camera shake does not seem to be the problem. When I do get a shot in, it is crisp as.

.... .

You say that "when I do get a shot in ... "

But when you say this, do you refer to the point that in a sequence of shots, you get some sharp, and you sometimes don't get them sharp?

if so, then part of the problem is camera shake.

What sort of shutter speeds are you shooting at?

as for cropping, my preference is not too.
Usually, if I make even a very slight error in framing, I'd rather do it again with the correct framing, rather than crop the image I already have.


Also another aspect I didn't consider originally, and is dependent on the lens ... some lenses can focus closer manually, than they can automatically!
One Nikon lens I used to own(80-200/2.8) has this behaviour.

And from memory, the Nikon 105VR can also focus more closely manually than it can automatically(but this I haven't proved yet).
This is a macro lens, and should be able to focus as close as 1:1, but you can focus a bit beyond 1:1 with this lens. It's not by much(much less than the old 80-200's difference), but if you're getting down to MFD with your lens, it may have a bearing on your results.

post some pics too.

[SteveInNZ]

The 450D was the first model in that series to have liveview so no liveview on the 400D.
A technique that may help is to use "back button focus". I'm sure there will be a thread on that somewhere. It removes the focus action from the shutter button so that when you half press the shutter, it doesn't try to refocus. Auto focus is activated by the "*" button on the back of the camera so once you have it, it will stay focused while you mess around with exposures, etc.
You might also try adding another light source so that you get a specular reflection (a sparkle) that is contrasty enough for the autofocus. Once you have focus, turn that light off.

Steve.

[CQGems]

Thank you very much for the great suggestions. Got some great ideas on how to go next, as soon as I can get away from work and sit down with it all.

In regard lighting, I am finding that by using natural sunlight, I am getting the truest colour of the stone, by using a simple lightbox made out of a translucent water container, rather than a commercial lightbox and lights. Will keep practicing and post results when I have it sorted.

In regard to the comment that I get a sharp image at times and it looks great, usually it is of the pavillion of the stone (the bottom section) where there seem to be more edges that the focus can pick up on.

I feel that part of my problem, when looking back on the shots, is that I am trying to focus on the table, the flat section on the very top of the stone, rather than trying for a facet edge, slightly off-centering the stone.

All these things that will be tried shortly.

Thanks again for the inputs.