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The Canon 50mm f1.8 MKII lens has a deserved reputation as a great available light lens. It's fast, with its widest aperture of f1.8, but even for normal conditions, it's fabulously sharp and does lovely things with the light that passes through it.
The only downside is the fixed focal length of this wonderful little prime lens, equivalent to 85mm on a 35mm film SLR with the digital sensor crop factor (x1.6 on the 300D) which is perhaps too long for many domestic environments, group shots and many landscapes and yet too short for many sport and outdoor activities.
But its real strength is with available light work, it enables photographers, me included, to get shots they couldn't even contemplate with any other lens, especially considering the modest investment required to own a copy, it really has to be bargain of the century for photographers. Every Canon SLR owner should consider getting a copy and I doubt anyone would regret the purchase.
But it often catches new users and inexperienced photographers unawares and photography forums are regularly witness to questions about the focusing ability of the lens and its sharpness. I think it's true to say that the modest cost of this lens perhaps leads to inconsistent quality control compared to more expensive glass, giving rise to occasional copies that give less satisfactory results (front and back focusing are often cited), but I think the vast majority of problems I've seen illustrated can be explained through a misunderstanding of the geometry involved, when the lens and camera actually perform in a perfectly predictable manner.
The great strength of the 50mm f1.8 that drives people to buy it in droves, is the speed of the glass with the very wide apertures it's capable of, along with its very evident image quality and sharpness. But that speed comes at a price - the geometry is very tricky to manage and really needs to be understood to get successful shots at the extremes - the very scenarios where this lens out-performs everything else in your camera bag are the ones that need greatest care.
When fully wide open at f1.8 (a low f stop opens the aperture of the lens wider, letting more light reach the sensor) and the scenarios in which you're most likely to use this lens, the depth of field (DOF; the amount of the scene from front to back that is of an acceptable level of sharpness) is very shallow indeed, much more so than with slower lenses.
A good rule of thumb to keep in mind which is easy to remember is that if you're at f1.8 and 1 meter from your subject/focus point, your DOF is under an inch - that's less than the depth of someone's nose. So if you focus on a pair of eyes from a meter away when wide open, chances are that the tip of your subject's nose might be beyond acceptable sharpness. That's not a fault or failing of the lens, just mathematics. Any lens capable of 50mm and f1.8 will give the same result.
That phenomena can lead to you being dissatisfied with your shots and to declare the lens soft or mis-focusing. But if you're aware of the reason for this result, you can take measures to improve the situation as you take the shot. Stopping the lens down a little (i.e. closing the aperture/hole in the lens; a bigger aperture number) increases DOF, as does moving away from your subject. So instead of shooting your loved one from a meter away at f1.8 and being unhappy with the results, take a few steps back and stop down the lens a little, available light permitting.
At 2m away from your subject and at an aperture of f2.8 - still jolly fast - your DOF now becomes over six inches, enough to get most of a head in focus. Stop down to f4 and you get a further 3" of DOF. So you can see how this is a numbers game and what seemed hopeless initially can be improved by working the geometry. So, if your DOF is too shallow; step back and/or stop down. It's really that simple.
If you really have no alternative but to shoot from close and wide, think carefully about exactly where you focus - your DOF, no matter how little, can be made to work better for you. Some of the DOF falls in front of where you actually focus and some of it behind that point. As a very generalised rule of thumb, about the proportions of 40% in front, 60% behind. So faced with a portrait to take and only an inch of DOF and when portraits work best with the eyes in focus, perhaps try focusing on the bridge of the nose rather than the eyes themselves - the eyes will still be sharp behind the point of focus and the nose in front. If you focus on the eyes, you may have to sacrifice the nose. If you have the opportunity, do a bit of trial and error and take several shots and remember to use the DOF preview button. See the illustrated samples below.
There's a second problem you might encounter with the shallow DOF at wide apertures. If you're even closer than we've already discussed - and the lens can focus from less than half a meter from the subject - your DOF can be so shallow (less than 10mm at f1.8) that there's no margin for error and if either your subject or yourself moves even the slightest amount between focusing the shot and it being recorded, your focus may well not be where you intended and lead you to think the lens isn't giving good results.
This is the reason for many shots I see where the photographer feels sure the lens isn't performing very well. See the shot above right, where my husband was laughing and moved just enough while I focused and composed to take the sharpest focus off his eyes, but the edges of the towel further forward than his eyes are in good focus.
The 50mm is a great little performer and the work it's capable of with available light makes it worth far more than Canon charge for it - but perhaps we should keep that between ourselves or they might decide that it's worth more!
But remember that the extremes this lens permits you to work at require careful thought and much of your success with it will depend on you mastering the geometry. It may be photography, but it's also a numbers game.
As mentioned above, the DOF - however little that may prove to be for your setting - extends approximately 40% in front of where you focus and about 60% behind. So if your shot will have a very shallow DOF, choosing your point of focus can make a huge difference to the perceived success of the image. In the little test scene below, two series of shots were taken with the camera focused on a different detail in the scene. The left hand frame, taken at f1.8 (all three frames were manually set to the same exposure) was focused on the blue torch at the front, the middle frame approximately 40% into the scene on the glass grapes and the right hand frame was focused on the lettering on the toy van.
Although the depth of field in each shot is only about 6 or 7mm, they vary enormously in how successful they appear, creative issues aside. In the left hand frame, you can see that some of the woodgrain of the surface is sharp in front of the torch - is that what you want at the expense of the scene behind? Likewise in the right hand frames, there is woodgrain in focus behind the toy vehicle, yet the foreground is uncomfortably out of focus.
In the centre frame, the focus was taken on one of the glass grapes in the middle of the scene and although the depth of field is exactly the same as the other two versions, it feels much more comfortable to view and is perceived as being sharper and in better focus, although it isn't actually, but it's much more pleasing on the eye. More of the objects of interest are in focus and none of your acceptable sharpness is wasted on less-important details projecting in front of and behind the elements in your scene. It's a natural instinct to focus on the front-most point in a scene, but when you're working a minimal DOF, you need to think more carefully about the impact of your choice of point of focus. It can make or break a shot for a few millimetres.
The second series underneath is the same exercise repeated with a slightly tighter aperture; f3.2. Although the shutter speed was slower and may necessitate a higher ISO where possible and neither may be practical, the middle scene where a combination of careful focusing and tighter aperture (the DOF is now about 10mm) gives an even more acceptable result. If you could step back and re-frame and perhaps crop the image later, this could be improved further still.
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