Why autofocus does not always suit macro.

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DaveW
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Joined: Thu Dec 20, 2012 2:36 pm
Location: Nottingham, England/UK

Why autofocus does not always suit macro.

Post by DaveW »

For higher magnification macro shots autofocus does not always produce the results desired. For one thing the depth of field may be only a few millimetres and autofocus may not be accurate to a few millimetres, which may not matter in conventional photography where depth of field can be in meters and cover any slight miss focusing. Say we wanted a close up of the stamens and stigma lobes of a flower but need to use the small depth of field available to cover both. Autofocus needs something to focus on so it either has to focus on the stigma or a an anther on a stamen. The small depth of field we have though may mean that focusing on either of these could put the other out of focus, so the ideal place to focus may be midway between the two but there is nothing there for autofocus to focus on, as in the diagram below.
focus.jpg
focus.jpg (22.22 KiB) Viewed 2696 times
The recommendation in those cases is to switch to manual focus if you can and if you are using a DSLR learn to screen focus. Those who came up through the pre-autofocus film days will be used to using the focusing screen in the viewfinder for this purpose, but many new camera users have always relied on autofocus and never learned to manual focus. Screen focusing means you simply view the image on the ground glass focusing screen in the cameras viewfinder and focus the lens until you get best distribution of depth of field available on the screen. Many of the old pre-autofocus screens used to have focusing aids like split image and microprisms in their centre, but these can be a hindrance rather than a help at higher magnifications since they like autofocus rely on something to focus on being at the point you need to focus.

If you wish to screen focus however on modern DSLR cameras you need to make sure the dioptre adjustment on your viewfinder is correctly set for your eyes. This is usually a small wheel or slider on the viewfinder and there is a proper way to set this. The wrong way is to focus on a normal scene and then just twiddle to knob until it looks sharp. When you screen focus your eye should be focused on the focussing screen itself, however make sure your camera has a battery in it and the camera is switched on as the LCD overlays on modern focusing screens do not clear properly unless under power. This is the proper way to do it, by adjusting the dioptre adjustment until the autofocus brackets etc on the focusing screen are at their sharpest then your eye is exactly focused on the screen itself where the ariel image will be displayed when focusing:-

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Modern DSLR's don't come set up for your eyesight straight out of the box and if you always have used autofocus you may not even have realised it was not set correctly for your vision. Adjusting the dioptre adjustment is one of the first things you need to do with a new DSLR. In the old camera days if your eyes were off you had to buy supplementary lenses that screwed into the eyepiece to correct your vision, but now a certain amount of adjustment can be dialled in so look in your manual to find where the dioptric adjustment is if you have not used it. We also tend to forget as we age our vision deteriorates which means we may periodically need to readjust the dioptre setting.

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One other thing is some authors talk about switching autofocus off and manual focusing, but then say use the focus confirmation light in the viewfinder. This is a lot of rubbish as the focus confirmation light works off the autofocus sensors, so if autofocus is inaccurate at higher magnifications so will be the confirmation light. Rely on your own eye on the viewfinder and ignore the light as modern technology has still to better the Mk 1 eyeball on a focusing screen!

If you are interested in going deeper as to how autofocus works see:-

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