Yosemite Valley, USA. Phase One XF 150MP with Schneider Kreuznach LS 110mm lens, f4 @ 1/2000 second, ISO 400. It would be a tragedy not to have a scene like this correctly focused! Fortunately this is!
Does your camera focus accurately? How do you know? And if it does, why do the better cameras provide some form of focus correction? That’s right, autofocus isn’t perfect and when it comes to high resolution photography, you better be sure your camera is focusing accurately!
So, how do you check? Some cameras automate the process for you, but let’s do it manually so we understand what we’re doing. Set up your camera on a sturdy tripod and point it at a subject with sharp, contrasty edges, such as a city skyline or a sheet of newsprint. Set the aperture to the widest setting. This aperture might not be the sharpest, but it will have the least depth-of-field, making it easier to work out whether the lens is focusing correctly or not.
A fast shutter speed is best to ensure we’re looking at focusing issues and not camera shake. Using the autofocus system, focus on your subject and take a photograph. Now switch to manual focus and live view at 100% magnification. Manually focus the lens as precisely as you can. Take a photo.
Now open the two files in your image editing software, enlarge them to 100% and compare them side-by-side. Check that the second (manually focused) photo is sharp – it should be perfect if you have focused properly.
If it is focused properly, compare it with the autofocus shot – both should be as sharp as each other! If the autofocus shot isn’t as sharp, this indicates your autofocus system needs adjusting. Check your camera manual for how you do this, but essentially, there will be a focus setting you can increase or decrease. If in doubt, increase it a little and take another photo. If it’s sharper, keep going; if it’s not sharper, go the other way. By a process of testing, backwards and forwards, you can work out the correct focus setting.
The Phase One XF has a wonderful system that does all this testing for you automatically. And all cameras these days will save separate settings for each lens you use as the adjustments required will vary from lens to lens.
Modern cameras are very good and you may not need to adjust your autofocus system to give you better results. On the other hand, if you’ve invested in a high resolution sensor, it simply makes sense to do this simple test for each lens you own. That’s why the technology is there – to give you better results.