Why is this happening please?

[LouiseTopp]

I have tried taking pictures of jewellery on a necklace, but sometimes the camera won't focus on the object. I have a Pentax k30 and the focus sign keeps flicking. How do i get it right please?

I have been shooting in F11 F16, and F22.





Thanks very much.

[Manfred M]

Autofocus relies on the camera detecting something easy to focus on, i.e. usually something contrastly with strong lines. Here the auutofocus mechanism is finding that the watch and is focusing on that rather than the watch face, that you want focus on.

Switch to manual focus...

[Black Pearl]

Could it just be that you're too close?

Also

Which focus points are active? Switch it to a single point and make sure that is on the watch face....or use manual.
By the way the K30 has focus peaking in live view. Great way to see what is and what isn't sharp.

[Mike Buckley]

It's also possible that the near part of the watch is closer to the lens than its closest focusing distance. If that's the case, manual focusing won't help.

[Inkanyezi]

I think that Mike Buckley hit the head of the nail regarding the first image. Then the second is taken from farther away, and it wouldn't have the problem of too close distance, but it seems as the camera has focused on the far side of the watch bracelet.

So first, you must be no closer than the minimum focus distance of the lens, 0.3 m (approximately 1') from the sensor plane, but also you need to choose the focus point that corresponds to the watch face. The camera might use the centre spot, which in the second image is not where the watch face is. According to the manual, there is a "Select" option for AF point, and you should use this mode and the arrows to choose the most suitable one.

A still better option might be to use live view and thoroughly check that you place your focus right. It is nigh to impossible to see whether the image is sharp when you look in the viewfinder, and the live view on screen may sometimes be a better indicator.

[AB26]

Could it just be that you're too close?

Agreed, I think that is most likely the cause.

[arith]

I was waiting for somebody answer. Best I can do is it looks dark, if you have an on camera flash or Speedlight, it doesn't work close up in the dark. So use a powerful torch or switch the lights on then off for the photo, flip the switch to manual OR, use Live View and move the camera backwards and forward to get focus.

[Stagecoach]

Hi Louise,

In addition to the comments already made regarding focus method/mode with a shot like this the choice of where within the depth of it that you focus on should also take account of the DoF (Depth of Field) that is going to be determined by the aperture you use and also the size of the subject within the frame.

The DoF is going to extend somewhere between 1/3 in front to 2/3 behind, to, 1/2 in front to 1/2 behind depending upon lens and distance from the subject.

Grahame

[LouiseTopp]

Thanks for your replies, so I need to be further away? I need to learn how to use my settings a bit better.

[Inkanyezi]

Thanks for your replies, so I need to be further away?

You need not be further away than in the second shot. The critical distance is the minimum focusing distance, one foot, thirty centimetres.

I need to learn how to use my settings a bit better.

Yes, particularly regarding "Focus point", that can be chosen, by setting to "Select" and then move between focus points in the viewfinder till you hit one that is on the watch face. It seems as the camera has chosen the centre spot in the second image, but you should be aware that the focus spot that lights up is the one where the camera focuses. If it is the wrong one, you must use single focus point and Select to get the right one. At very close distances, you cannot just use the centre spot and then reorient, but of course you can use the centre one if you have your object of interest in dead centre of the image.

It might also be worth the while to practise with Live View. Then you lock focus at a close distance, and move the camera into position while scrutinising the image on screen, to get what you want. Likewise, you can move the subject rather than the camera.

[Lancer]

I think your DoF is very narrow, you can change another aperture.

[jcuknz]

When I go after a small object and the focus light blinks at me I pull back a bit and try again until focus locks .... but I would be using a single focus point targeted on the point I want to be sharp. I never use manual focus but make sure the camera is focusing where I want it to.

There is no way the camera can know what you want to be sharp so you have to tell it by carefully placing the target area on that spot.

[Masq]

You're too close to the subject. If the cameras AF doesn't work - try to manually set the focus

[LouiseTopp]

I will experiement and try again with other objects. Anyone see the documentary about Tom Woods last night on BBC4?

[Donald]

Anyone see the documentary about Tom Woods last night on BBC4?

Got the end of it. Going to watch it on the BBC iPlayer.

[Inkanyezi]

I was playing around a bit with the theme of the thread start, to see if I could get the whole watch within depth of field, but stopping down did little at such short distance. However, it is wise to stop down when taking very close, to at least get some depth of field.

The image I post here cannot be taken unless you have access to special technique. I have tilted the lens forward 8 degrees to get the far side of the bracelet in focus at the same time as the watch face. However, with the watch at so close distance, the depth of field vertically does not cover the whole clock face. When I set focus on the centre of the clock, the crown would not fall within depth of field, so I adjusted focus a bit higher, and the lowest part of the clock face is not within depth of field.

I think it is reasonable to let the far part of the bracelet fall outside depth of field, but focus on the watch face. So the spot for focusing should be chosen at the watch face, and the lens should be stopped down, perhaps as much as possible.

This one was taken from 1' distance with a Schneider Xenar 50/2.8 stopped down to f/16 and tilted forward eight degrees.

[arith]

Try to get a little further away.

[Inkanyezi]

Try to get a little further away.

While further away will certainly mean more depth of field, the purpose is defeated by the shrinking size of the subject. In practice, this would mean that the image will be cropped, and thus its depth of field becomes shallower, as the format diagonal is smaller than the entire frame of the camera. It is akin to using a camera with a smaller sensor.

While it would make sense to use such a camera, it defeats the purpose of using an APS-C camera, regarding image quality.

A better solution is to back off and use a longer focal length, but at a smaller aperture. That would increase depth of field, so an ideal lens might be one of the macro lenses of around 100 mm focal length. This is a logical and sound solution to the problem, but my take is that we should not first-hand recommend buying new costly gear. A 100 mm macro lens might stop down to f/32 and would take the watch from about 60 cm distance with the same reproduction scale, hence use the whole sensor.

So the calculation with the DOF calculator becomes wrong, because one factor is changed, image size, which automatically demands a smaller circle of confusion. At the same aperture, depth of field will remain constant regardless of distance, if the object, on screen or printed, should be the same size in the final image. A crop will make the effective sensor size smaller than the DOF calculator was set for. At twice the distance, the circle of confusion should be half the diameter.

The tilt method does solve the problem, provided the lens can be stopped down sufficiently, but it takes new expensive gear to accomplish, and so does backing off to get larger depth of field, because you will then need a longer lens. It makes perfect sense to back off to about 60 cm and use a twice as long lens, stopped down further, to f/32.

For the thread starter, Louise Topp, this is not much comfort, because she needs a way to get the watch tack sharp with her standard zoom that zooms to 55 mm and stops down to f/16 and she needs to be about 30 cm from the subject. In that case it makes sense to let the far side of the bracelet fall outside depth of field, but concentrate on the watch face.