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KNOWLEDGE:- What Is The Best Aperture To Use? [Article]


Optimum Apertures

As we all know, not all lenses are made equally. There are some good and not so good lenses in most lens ranges, and some types and brands of lenses are generally better than others.

For instance, the 'large format' lenses made by Schneider and Rodenstock are generally considered to be the 'sharpest' lenses made. In terms of clarity and resolution from corner to corner, nothing comes close in the wide-angle range, but the competition is closer with telephoto lenses.

Next in clarity are the medium format lenses (the new digital designs), followed by a bevy of DSLR lenses. In fact, it's interesting to note some old 'German' names and lens designs being championed by some of the Japanese DSLR manufacturers. Certainly these lenses are extremely good, based on what I have seen.

At the bottom of the pack are the consumer and kit lenses, lenses which are manufactured to a price.

The Importance of Aperture

However, it is interesting that in some situations even an inexpensive kit lens can compete with an expensive professional model.

So what are those situations?

When we look at lens performance, there are many aspects to consider, including distortions, flare, colour and contrast. The latter has a lot to do with how 'sharp' we think the lens is, but of course sharpness also has to do with the lens's resolving power. When it comes to resolving power (the ability to create a clear, sharply focused image), there are two issues to consider.

First, is the image quality as good near the edges and in the corners as it is in the middle of the image?

Second, at what aperture is lens performance at its best? As well as controlling exposure and depth-of-field, apertures have an impact on the image quality produced by the lens.

So, in some situations a kit lens can possibly match a more expensive lens because at mid-apertures and in the centre of the image, it is relatively easy to produce a clear, sharp image. Problems arise, however, once you look at the edges of the image and when you use wider or narrower apertures.

When you use the maximum aperture available (such as f2.8), the image is being recorded by all the glass in the lens elements. When you use a smaller aperture, you're only using the middle area of the lens elements. It is easier, I am told, to make a small lens element optically perfect than a large lens element, and similarly, it is easier to get the middle area of each lens element perfect than it is the areas around the sides. Now you understand one of the reasons that lens quality suffers towards the edges of your frame.

However, modern lens design is pretty good and another reason the edges of your photos can appear blurred is field curvature. In a perfect world, the lens focuses the image in a flat plane, but the natural tendency of many optical designs is to focus to a slightly curved plane, which doesn't work so well with a perfectly flat sensor. It isn't such an issue with film because film had a somewhat three-dimensional quality (i.e. the thickness of the emulsion) which could hide some field curvature issues.

Depth-of-field (and depth-of-focus which is similar except it happens at the sensor plane) can also hide some of these issues, but depth-of-field itself doesn't really correct the focus. Simplistically speaking, depth-of-field means the human eye can't see the lack of focus for a given enlargement size.

So, every time you attach your lens and choose an aperture, there are a whole lot of issues swinging around that are going to affect the quality of your capture and, given we generally want the landscape to have has much detail as possible, it's important we know what to do.

Optimum Aperture

With a wide-open aperture, it is difficult to make a landscape appear in focus from foreground to background. It is also difficult for the lens designer to produce perfect image quality from centre to edge. So generally speaking, we don't use the widest aperture.

Some landscape photographers automatically go to the other end of the aperture scale, selecting f22 or f32 (or f45 or f64 with large format). This, they correctly believe, produces the greatest amount of depth-of-field. Unfortunately, when the aperture is really small, the light waves diffract around the iris blades as they pass through the aperture and you get a loss of clarity in the image. So while depth-of-field improves, image sharpness overall decreases.

Somewhere in the middle of your aperture range you are most likely to find the 'optimum' aperture, the point where the image is clearest and sharpest from one edge to the other. The aperture will be different for every lens, sometimes even within the same focal length and model.

I use a Mamiya 300mm f4.5 APO lens, but I only use it at f8. At any other aperture, it just isn't as sharp. Other lenses I can't tell the difference in quality between f5.6 up to f16, so I have a choice of apertures that I can use.

Does this mean I never shoot with my lenses at the other apertures? No. I'm talking about the ultimate in quality, so a photograph of horse jumping might not need the same level of quality as a classic landscape, especially for a smaller enlargement. We need to keep things in perspective.

However, as landscape photographers, we also need how to achieve the optimum image clarity when we need to, even if sometimes it is not possible to do so. Sometimes I know the 300mm needs to be set at f8, but I will shoot at f4.5 simply because I need a faster shutter speed to freeze the action (e.g. wind in the trees).

Testing Your Lens

The beauty of digital photography is that it is very easy to test your lenses. Pick a subject at a distance, with a strong pattern design. A city skyline or a row of houses is good. Set your camera up with a sturdy tripod, lock the mirror up and use a cable release to fire the shutter. If you don't have a mirror lock-up facility on your camera, make sure you're shooting in bright light so the shutter speeds are always 1/60 second or faster (because mirror bounce can impact the sharpness of your image even more than the aperture you select).

Shoot at each aperture, then open the files on your computer. If you process your raw files out, make sure you apply the same amount of sharpening to all of them. Now compare the images taken at different apertures at 100% on your screen. If you're struggling to see any difference, then you have a great lens (assuming the images are clear and sharp).

Now compare lens performance in the centre with the edges. To check this photograph a subject with it positioned in the middle of the viewfinder, and then again with it positioned at the edge of the viewfinder or in a corner. When you compare the two files (making sure they were taken at the same aperture this time), you will normally see the subject from the middle of the frame is much clearer. Again, if you don't see much difference, you have a very good lens. Repeat this exercise for each aperture and you will begin to understand your lens a lot better.

Keep a note of the best aperture or apertures. Now you know how to optimise image quality using the aperture.

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