Focus Test
At the moment both my Leica M Monochrom and Lecia M9, along with three lenses are in Germany being calibrated so that each lens is in focus on both cameras. Hopefully it will be third time lucky and they will all be ‘spot on’ or at least acceptable when they come back.
You may be questioning why I have written ‘acceptable’ and think that all lenses no matter what SHOULD be spot on. In this digital age, that is not always the case. Sensors and lenses have developed amazingly to show increased definition and detail. And in short, the higher the quality lens, the more micro contrast there is, and the better the sensor is, the more the slightest flaw in detail or lens focus is going to show up and in particular when a lens is wide open. It’s for this reason, that if you have a fast lens say a F1.4 (even faster) or even F2 you would be wise to close the aperture down by one or two stops, besides the fact that with the lens wide open focusing is critical and harder to do (if done manually), which with a Leica M you have to do. Even with a DSLR and auto focus the same applies, you would be wise to check your lenses if you feel they are slightly out or just to know they are not.
How do you know when a lens is not focusing correctly or it could be the auto focus of the camera? May sound a stupid question, but not necessarily as often you don’t really notice it, unless it is very noticeable and as often as not this can be down to the type of photographer you are, and what area of photography you work in or do.
Along with being a PR Photographer, a Photojournalist and Street Photographer, I am also a Portrait Photographer, and as often as not the type of portraits I take are close up, and taken within 3.5 to 4.5 feet (1 meter to 1.30) of my subject, and I work with a wide aperture, and if my focus is off, be it the lens or human error it will show. (This is what happened when I first got my Leica M Monochrom. Read my Monochrom blogs to know more).
As often as not, when you are doing Photojournalism, and in particular Street Photography you are not working that close to your subject, using a semi wide angle lens and perhaps an aperture of between F5.6 and F8, so when reviewing your work any slight shift in focus will not show or more then likely you will not be aware of.
In my portrait case, I was shooting with a 75mm, F4 at about 4 feet away (1.20 meter), focused on the pupil of the eye of my subject, and found that the eye was ‘soft’ focus and the eyebrows in focus, at F2, the widest aperture of the lens, the eyes were out of focus, no way could you get away with saying they were ‘soft’. This showed that my 75mm lens was front focusing. To check my lenses properly I did a controlled focus test on all my Leica lenses, and the fixed (Prime) Nikon lenses that I have. In fact I did a few test to limit ‘human error’.
To show up focus errors you need to give your lens a hard time, that means testing it at the lens’ widest aperture and as close as possible, within the focus range of the lens. The following is a very easy test to do.
Set your camera with lens on a tripod, use and a cable release, remote or timer to avoid any movement or shake. Get five AA batteries, and place them on a 45-degree diagonal with a slight gap between each, as shown in the featured photo (above). Focus on the centre battery, if there is text on the battery focus on this. If you are not using a range finder camera you can use auto focus. The centre battery should be the sharpest area of focus if your auto focusing or manual focusing meaning your lens is correct. If the battery in front (second from right) is more in focus as the featured photograph, then your lens is front focusing, if your lens is back focusing, battery number two, the one behind the centre one, will be in focus, if it is correct, the centre battery will be the sharpest.
The featured photo (above) the lens is front focusing. The image was shot using a Leica M Monochrom camera with a Summicron 75mm F2 APO lens.
The photograph within this blog (below) shows a lens that is correctly focused. It was shot using a Summicron 50mm lens F2 on a Leica M Monochrom camera.
If you find that your lens is either front or back focusing, all is not lost, a few of the newer DSLR cameras (i.e. Nikon D800) have a focus correction facility within the menu to adjust this. Check your camera manual. 
If you would like to know more information on focus testing, please contact me.
Thank you for reading this blog.