Check out any photographic internet forum, or for that matter any Photographic Magazine’s letters pages and you will inevitably come across posts discussing focusing. The terms ‘Back focus’ or ‘Front focus’ crop up regularly, and often it is the lens that is blamed. But is it always the case? We decided to look a little further.
To expect any lens and camera combination to produce absolutely sharp images with every single example is a little optimistic. Yes, highly corrected lenses in specialised application can be expected to produce such results, but at a cost. In the real world there are the constrictions of price and versatility to consider.
Mass produced cameras, fitted with mass produced lenses, cannot be expected to yield perfectly sharp images, but is that what we expect. What is the expectancy of the average photographer. How close to perfect do you expect an image to be.
There is, of course, a limit to how sharp the human eye can see. But let’s not go there just yet. A few years ago, when film was the King and digital was only embryonic, we looked at our images through a loupe, or magnifying device that utilised one eye and brought them up large enough to assess whether they were sharp or not. Then we printed them or projected them onto a screen that we looked at from a distance, both methods needing the use of another lens, be that enlarger or projector.
Today, with high quality digital files, we load them onto a computer and, through the editing software, we promptly examine them at 100% or more to assess sharpness. and that is on a 17-20″ or more screen from a distance of about two feet. (Sorry about all these imperial measurements, but screen sizes still seem to be understood better that way!) That is far more critical than we ever were with the old loupe! Mostly, we only printed up to around the 10×8″ mark (more Imperial) or viewed the projected images from a minimum of 3 metres away (metric at last)!
So, what effect has this had on modern lenses? Well, they have got better, mainly because they have had to. Needs must, and all that. Yes, manufacturing methods, aided by CNC tools, have helped, but that is a universal phenomenon. It has helped with the manufacture of cameras also.
But wait, I’m wandering off the subject. What has all this got to do with focusing? There are a lot of things that can affect the focus of an image, but the ability of a camera and lens combination to apply the gathered light onto the sensor at exactly the right spot is paramount.
Despite all of the advances, the lens is still mounted on the front of the camera, and the sensor on the back. And the camera is still, essentially, a hollow box!
This can allow variations in the placement of the sensor, and, although these variations may only be tiny, the nanometres we are talking about can have an effect. ( A nanometre is forty-billionths of and inch, so we’ll stick to metric!!)
Circles of Confusion
How sharp is sharp? In any image, points are recorded as tiny discs, or circles. The human eye that I said we would come back to, also ‘sees’ things as tiny discs and as long as the discs in the image are no larger than those the eye can see, everything will look sharp. We have all seen out-of-focus highlights causing rings of light, and the shape of these is determined by the lens aperture. (It is often called ‘Bokeh’ nowadays).
As soon as these rings become large enough for the eye to detect, things start to look out of focus. The larger the ring or disc, the more out-of-focus the image will seem. Are you all with me so far?
The size of these discs is referred to in photography as the Circle of Confusion and it has a relationship with a number of areas, not the least of which is depth-of-field. But we will leave that area out for the moment and save it for another day. Lets concentrate on focus. To be ‘in focus’ the image, or the area of the image we are interested in needs to have discs so small that the eye thinks they are points and if it does, the image will look sharp. To do that, the lens has to direct the light so that it converges at exactly the right point on the sensor.
If the sensor is not exactly where the lens expects it to be, then the lens will ‘miss’ and the image will become out-of-focus by the amount the lens has missed by. If the lens is further away from the sensor, it gets called front focus, and if the sensor is closer to the lens, it gets called ‘back focus’.
For the most part, camera manufacturers manage to get this distance, between the lens mount and the sensor, fairly consistant, although a camera body that has been dropped even lightly, can be slightly displaced. When a camera auto-focusses, it does so at the widest aperture that the lens that is fitted can manage. If everything is perfectly lined up, this gives the best focus, but if something in the system is a couple of those nano-whatsits out, then things start to go awry.
So is it the camera. or is it the lens? How can we tell?
Well, the first thing to do if at all possible, is to try the lens on another camera. It can be that old film camera that hasn’t seen the light of day for eons! This is totally different than trying another lens on the same camera, especially if the ‘faulty’ lens has a wider maximum aperture. If there is any difference in the results, the problem is almost certainly the camera.
Things that could be wrong with the camera are sensor mis-alignment, either in manufature or post manufacture with things like knocks or moisture absorbtion of the expoxy the sensor is mounted on. Mirror mis-alignment causing the AF system to malfunction. AF points not picking up the correct area, etc, etc.
Fear not though. The expense of having the camera body rebuilt can be avoided in much the same way that an out of tune piano can be re-tuned. The piano gets it’s strings re-tensioned, whereas the camera has the lens re-calibrated to suit the individual camera.
Because the problem can be overcome by the lens manufacturer more easily than the camera manufacturer, do not be fooled into believing it is the lens at fault when there is, in fact, nothing wrong with the lens at all.
Be warned though, that when you change your camera body for the latest must-have, you might have to go through the whole procedure again.