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Having problems with a Sigma DG 28-300mm 3.5-6.3 lens - just not quite right in focus.

ChrisMadog
Contributor

He everyone,

 

I have a Canon 450D (Rebel I think in the US) dating back to 2009.
The Canon 18-55mm lens won't focus and it looks like the Flex Cable has died so I've got one on order.

 

I have a Sigma DG 28-300mm 3.5-6.3 lens and while it works, the image quality just isn't 'quite right'. It always seems to be that little bit fuzzy.

Has anyone found some little bit of 'magic' to get it right ?

 

Kind regards,

Chris

22 REPLIES 22

kvbarkley
VIP
VIP

You have to determine whether it is focus or camera/subject movement.

 

You might ask Sigma whether there is a firmware update to the lens.

Thanks for the suggestions.

I have determined it is the lens. On 28mm and comparing it to a canon 18-55mm lens, there is quite a difference.


Where I can view the picture and zoom in well on the Canon lens, I cannot come anywhere even close with the Sigma.

It doesn't have Image Stabilization.

Even using a tripod and remote trigger doesn't improve the image.

 

I'm at a loss as to what to do.

 

Regards,

Chris


@ChrisMadog wrote:

Thanks for the suggestions.

I have determined it is the lens. On 28mm and comparing it to a canon 18-55mm lens, there is quite a difference.


Where I can view the picture and zoom in well on the Canon lens, I cannot come anywhere even close with the Sigma.

It doesn't have Image Stabilization.

Even using a tripod and remote trigger doesn't improve the image.

 

I'm at a loss as to what to do.

 

Regards,

Chris


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.

--------------------------------------------------------
"The right mouse button is your friend."


@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.


@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.

--------------------------------------------------------
"The right mouse button is your friend."

Thank you for your reply 🙂

I now understand Front/Back focus and I will check mine tomorrow. I suspect it is doing one or the other and now I'm itching to find out which 🙂

 

I do understand Depth of Field and I often use ADF in landscape shots.Other times I go manual to get the result I want.

However, thank you for the in-depth explanation.
While I am a photo enthusiest, I certainly need to learn more 🙂

 

Lind regards,

Chris

"I now understand Front/Back focus and I will check mine tomorrow."

 

Please, let me know how this goes.

EB
EOS 1DX and 1D Mk IV and less lenses then before!

 

@ebiggs1 wrote:

"I now understand Front/Back focus and I will check mine tomorrow."

 

Please, let me know how this goes.


I checked the focus this morning and found that the lense focusses in front of the focus point, so front-focusing

 

I've checked the camera settings and threre is no way to adjust or compensate for it apart from manually focusing.

 

At least I now know what is happening. Thank you.

 

Kind regards.


@ChrisMadog wrote:

 

@ebiggs1 wrote:

"I now understand Front/Back focus and I will check mine tomorrow."

 

Please, let me know how this goes.


I checked the focus this morning and found that the lense focusses in front of the focus point, so front-focusing

 

I've checked the camera settings and threre is no way to adjust or compensate for it apart from manually focusing.

 

At least I now know what is happening. Thank you.

 

Kind regards.


How many test shots did you take?  I suggest taking at least a dozen.  If you took the test shots indoors, try taking another set of test shots under different lighting conditions, preferably sunlight.  

 

What did you use for a test target?  Did you use a tripod?  What focal length and aperture did you use for the tests?  At what distance was the test target?

 

I ask all of these questions because it is all too easy to get inconsistent results.  You want to take several test shots, and take an average of all of the results.  Hopefully, the various test shots are somewhat consistent, and not all over the map, sometimes in front, sometimes in back, sometimes dead accurate.

--------------------------------------------------------
"The right mouse button is your friend."
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