Non Expert Photography for Non Expert Photographers

A fact that beginners and perhaps even some more advanced photographers may not be aware of is that the sharpness of any lens will not be the same at all aperture settings. The Diffraction factor has been discussed on a different page here so the focus (get it?) of this discussion will be on factors other than Diffraction which affect lens sharpness.

The general and reliable presumption is that the higher the quality of the lens (and yes, that generally means the higher the price) the sharper the lens can expect to be. Though sharpness can be a rather subjective concept (what looks sharp for one person may be seen as less than sharp by someone else) there are ways to measure the sharpness of a lens. This is a very good way to compare one lens to another. For most of us though, the only way we can tell how sharp a lens is is to use it and judge for ourselves, or to read the reports and reviews on that lens. Even by reading reviews and opinion reports, we may still be left with questions. The ultimate decision maker for us as the users is whether or not the pictures we shoot turn meet our expectations. Do they look good to us?

That is a fine criteria to use for a lens I already have, but what if I am contemplating buying one. Until I use it, I cannot know if the pictures I shoot with it will live up to expectations or not. We need to have something to use as a guideline. We need to know what we can expect of the basics and that is what I am going to attempt to layout here on this page.

Remember when we discussed lens speed, we talked about the aperture which is adjustable so that it can open to various sizes. The measurement of the size of the aperture opening is called the f-stop. The larger the aperture opening, the more light is allowed to enter the camera and the smaller the aperture opening, the less light is allowed to enter. If you do not remember this information or you have not read that page yet, now is a good time for the information presented there will make more sense of what we are going to be discussing here. Lens Aperture.

The general understanding is that no lens is as sharp wide open as it is closed down more. Let me restate the range of aperture settings using the term f-stops. The lower numbers represent the lens wide open and as the numbers increase, the size of the aperture decreases. Seems somewhat backwards, doesn't it? So if we have a lens with an f-stop range from f/2.8 to f/32, at f/2.8, the aperture is wide open. It is open to its maximum width. At f/32, the aperture is closed down so that the size of the opening is as small as it can become.

The better the quality of the lens, the sharper will be the image produced at all aperture settings. In another set of pages here we said that due to Diffraction there is a point where making the aperture size smaller the sharpness of the picture begins to fall off and the picture becomes "softer" or less sharp. You can see that discussion on the pages dealing with Diffraction Limited Aperture. Even the best of lenses will not be quite as sharp in the wide open position as they are a few f-stops away from wide open. In other words, at f/2.8, though the lens may be very sharp at this aperture setting, it will not be as sharp as it is if we close the aperture down maybe two or three f-stops. So if we shoot at f/4, for example, rather than at f/2.8 the image will be a little sharper. How much sharper depends on the lens. A very high quality lens will still be very sharp wide open. A cheap lens (and I mean more in terms of lower quality rather than simply price) will have a noticeable difference in sharpness wide open.

The fact that a lens may not be at its sharpest wide open is not a reason to avoid that setting. Because we may be shooting in lower light conditions, or if we are looking for a more shallow depth of field (DOF) we may want to use the lowest f-stop setting. The better the quality of the lens, the better will be the sharpness at that aperture setting and thus it will most often be quite ok to use the more open aperture positions.

Lenses have what is generally called the "sweet spot". That is not just one aperture setting but rather a range of aperture settings where that lens is the sharpest. Often that will be in the range of f-4 to f-11. The higher end may be impacted by Diffraction Limited Aperture so in reality it may be a smaller range. Common values are f-5.6 to f-8. That does not mean the lens is no good away from those aperture settings .It simply means that at those settings that lens is at its sharpest. A very good lens is generally at least as sharp at its weakest point, that is with the aperture wide open, than most lesser lenses are at their sweet spots so if we are using a very good lens, we really don't need to be afraid to use the lens wide open.

Below I have created a graphic which consists of a series of shots taken with one of my lenses. That lens is the Tamron 17-50mm f/2.8 LD XR Di II. This is my "standard lens" by that I mean the "other than the telephoto or macro" lens. I am never hesitant to use this lens wide open if I need to for it is very sharp at that setting even though it is at its sharpest a few f-stops above that setting.

<< Previous f-stop | Next f-stop >>

Take your time and scroll forward and backwards through the various f-stops. Look at the blocks and the numbers shown. Where they are sharpest, you will be able to see individual lines making up the blocks and numbers. As the image softens, the blocks and numbers will look less clearly defined and you will no longer be able to see the individual lines making them. See if you can find the "sweet spot" for this lens. Remember that is the area where one or more f-stops adjacent to each other look the sharpest. Once the aperture size drops to a particular size, the image begins to soften a little. That size is the Diffraction Limited Aperture (DLA) and as the aperture continues to become smaller, that softness increases. Now we are no longer dealing with just the lack of lens sharpness for this affect is not the fault of the lens. We can imagine though that if the image is softened by the DLA, that it is certainly not going to help the quality of the image if the lens is also not the sharpest. So we're magnifying the problem and making a bad situation worse. In the graphic I only went to f/22 even though that lens goes all the way to f/32. I saw no reason to carry it that far for this illustration.

If we are using a lesser quality lens, we will be more limited to the range of aperture settings which will give us a good, clear and sharp picture. If we are using such a lens, we will want to be careful to avoid, as much as possible, using it wide open and certainly we will not want to go far beyond the DLA point. Remember, what determines the DLA point is the camera and not the lens.

On another page we discussed the speed factor of lenses. Some are faster than others meaning they have a larger maximum aperture size allowing more light to enter the camera to be collected on the sensor. This lens is considered fast for it has a maximum aperture size of f/2.8. Many lenses have a maximum aperture size of f/3.5 or even smaller. My telephoto lens, the Canon 100-400 mm f/4.5-5.6 L IS USM lens, for example has a aperture range of f/4.5 to f/5.6. That means that at its shortest focal length, (100 mm) the maximum aperture size is f/4.5. At its maximum focal length of 400 mm, the maximum aperture size is f/5.6. It is not because it is an inexpensive lens. It was by no means an inexpensive lens. It is one of Canon's high end lenses. It is simply that in the world of telephoto lenses, it is very difficult and incredibly expensive to build a long focal length lens which is very fast. The good side is that since it is a very high quality lens meaning that the quality of the glass elements and other components and the precision of design is such that even wide open, the lens is very, very sharp so using it at the larger aperture sizes is never a concern or problem. That lens carries the "L" designation by Canon which is reserved for their high end lenses.

Lesser lenses which often in the telephoto class run 70 to 300 mm in focal length have an aperture range of f/3.5 to f/5.6. Since they are not as sharp wide open, even at f/5.6 which would be wide open for the lens at its maximum focal length will be noticeably softer than it will be at a couple of f-stops down. So such a lens generally will be much sharper at roughly f/8 to f/10 or there about. By the time the aperture becomes smaller where the actual optics of the lens might be even better, we have crossed the DLA point and now diffraction becomes the limiting factor making the lens even softer. So a modest lens though it may be acceptably sharp where it is going to be sharp, has a much narrower range of usable aperture settings.

A modest lens can certainly produce very good photography but we have to be sure we understand the limits of what that lens can do and try to avoid its weaknesses. More about good shooting technique and ways to maximize sharpness and over all image quality is discussed in the sections dealing with focusing and technique.

Home | Christian Ministry | Home Business | Contact Us | Links | Photography