@ChrisMadog wrote:
@Waddizzle wrote:
Not so fast. Use a simple focus target. The lens could be back focusing, or front focusing. Also, what aperture setting are you using? I suspect the lens may be front or back focusing. Well, at least I am hopeful that's all it is.
Basically, go through all of the motions of making test shots for a "Dot Tune" AFMA adjustment. If the images are still soft when shooting a target at a distance of 50x the focal length, while mounted on a tripod and an aperture of f/8, then it can be concluded the lens is soft.
Back focussing / front focussing - Sorry, I don't get it.
I have some camera knowledge and I suspect I am going to add to it 🙂
Also, what is a " "Dot Tune" AFMA adjustment" ? How does one do it ?
I've capmpared the two lenses to each other and would loke the Sigma to be sharper - expected that it should be much sharper that it is. It is very poor compared to the Canon 18-55mm lense even without IS being turned on.
I can zoom in on the cannon lens generated image far more that I can zoom in on the Sigma generated image.
That includes on a tripod as well as hand help.
The ccanon lens needs the auto-focus flex cable replaced and I'm waiting on it's arrival and in the meantime I'm using the Sigma DG lens.
Thanks for the reply 🙂 Kind regards.
Do a web search for the phrases you don't understand. You can begin with " depth of field ', for a camera and lens. Basically, when a lens focuses on a subject, the creates what is known as a "plane of focus". Imagine an invisible wall at a fixed distance from the lens.
Objects that are at a distance from the lens that is equal to the plane of focus will appear razor sharp to the camera. As you move the object away from the plane of focus, either towards or away from the camera, objects will gradually fall out of focus. There is a zone beginning in front of the plane of focus and extending beyond and behind the plane of focus where objects are not in perfect focus, but have what is described as acceptable focus. This zone is called depth of field.
The size of the zone of acceptable focus, depth of field or DOF, can vary widely depending upon the focal length being used and the distance to the invisible plane of focus. Depth of field is most strongly affected by your aperture setting. Do a web search for depth of field table. A narrow aperture [higher f/stop value] creates a wider DOF than wide aperture [lower f/stop value] would. This is why I asked what aperture value are you using when photos seem out of focus.
When an auto focus lens focus on a subject, it tries to adjust itself so that the plane of focus is at the precise distance to the subject that it is focusing on. Lenses are mechanical devices, and as such they are not always perfect. Sometimes a lens will lock focus with the plane exactly on a subject, and sometimes it will be just ever so slightly off. But, because of the nature of DOF, should a lens lock focus that is slightly off, we do not notice it. This type of behavior is normal for mechanical lenses.
However, sometimes a lens may lock focus and set the focus plane at a distance that is great enough for the subject to be at the edge of the DOF, or even outside of it. This results in a subject appearing out of focus, but other parts of your photo may appear to be in focus. When a lens consistently places the plane of focus behind the subject, or target, then this is described as back focusing. When a lens consistently places the plane of focus in front of the subject, then this is called front focusing. The keyword here is consistently. However rare, it is not uncommon for a lens to occasionally miss focus, especially inexpensive lenses. Temperature extremes can cause a lens to be slightly off when it focuses, for example.
I am not suggesting that your lens is not focusing properly. You can do a web search for a YouTube video about dot tune, which describes how to conduct focusing tests. What you really need to do is find a test target, and print it out. Even a yardstick might suffice for shorter focal lengths. Take pictures of the test target with the camera on a tripod, and the target slightly angled to the camera. This will allow you to see where the plane of focus is, and the extent of the DOF.
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"Enjoying photography since 1972."
