new Rebel XT-strange blue color-defective??

[jlu1234]

Hi guys,

I received my new Rebel XT last night. Today I shot some images from the balcony and found strange blue color on the tree top, see below:



The 1st towards southeast and the 2nd southwest direction.

The pictures are taken at 2pm, with an UV filter. Blue color appears in both Auto setting, and Av mode with the aperture step down several clicks.

Is this a defective camera, or some other reasons.

Thanks,
Jinggang

[fractalgfx]

I've never seen anything like this, but if it was a bad camera I would expect the the same region to be affected in both pictures. This looks like some kind of very strange chromatic abberration. Can you post a full resolution crop of the tree branches?

[jlu1234]

I've never seen anything like this, but if it was a bad camera I would expect the the same region to be affected in both pictures. This looks like some kind of very strange chromatic abberration. Can you post a full resolution crop of the tree branches?

here is the 100% crop

[timmciglobal]

CA. (chromatic aberration) combined with dynamic range issues.

That's what it is.

You've got small details in very high contrast areas set against a blown out sky.

Try taking the shot again stopped down a lot and you'll see far less of an effect.

Tim

[24Peter]

First thing I'd do is take off that filter. IMO "UV" filters are of limited (OK, no) optical value. Then if the problem remains, I'd try another lens. If that doesn't solve the problem, I'd think about returning the camera.

[jlu1234]

First thing I'd do is take off that filter. IMO "UV" filters are of limited (OK, no) optical value. Then if the problem remains, I'd try another lens. If that doesn't solve the problem, I'd think about returning the camera.

At first, I found the blue tree-top issue when shoot without an UV filter. I thought the UV filter might help, so I put it on, and it doesn't help.

Right now, I only have the kid lens (18-55mm f/3.5-5.6 II), I thought it is probably the lens problem.

thanks

[jlu1234]

CA. (chromatic aberration) combined with dynamic range issues.

That's what it is.

You've got small details in very high contrast areas set against a blown out sky.

Try taking the shot again stopped down a lot and you'll see far less of an effect.

Tim

Thanks Tim, so I understand that CA is due to the poor quality kit lens. What is the dynamic range issue, does that means the camera might be not good quality?

[JTL]

Thanks Tim, so I understand that CA is due to the poor quality kit lens. What is the dynamic range issue, does that means the camera might be not good quality?There are many pieces involved with responding to these questions, so I will break them down separately.

1. The International Organization for Standardization (ISO) definition of dynamic range is: The ratio of the relative maximum luminance at which an digital representation can be captured (essentially white) to the luminance for which the camera provides a signal to noise ratio of 1 (essentially black). See ISO 15739-2003. The scene you photographed likely exceeded this ratio for your camera's sensor, thus, the camera's sensor could not properly record the scene.

A decent article on this in practice can be found here: http://www.cambridgeincolour.com/tutorials/dynamic-range.htm

2. The XT is capable of producing images of above average quality...and with average to above average dynamic range for it's class and release date, although that does not necessarily mean that your camera is unequivocally problem-free.

3. The situation was not helped by the fact that you were shooting into the sun (a sure way to exceed the dynamic range of your camera's sensor). It can be done, but the technique requires skill, experience and thorough knowledge of your equipment. You have to become expert at using the manual controls of your camera to do it with any hope of success. So, until you have total control over your exposures, don't shoot into the sun.

And, use a lens hood when shooting outdoors in sunlight.

4. To improve dynamic range, if you are shooting JPEG, lower the contrast setting (page 64 of your user guide). But, better yet, only shoot in RAW and you will have much greater dynamic range latitude.

[griptape]

As JTL said, there's nothing wrong, or inferior about the camera's ability to record an above average dynamic range. And there's nothing inherently wrong with the kit lens. I don't think any lens with the same settings would get this shot to look completely right. To be perfectly honest, the biggest problem with the shot is poor framing. You're kind of asking your camera to play "where's waldo", but substitute exposure for waldo.

With dynamic range, a digital SLR can record about 3 different levels of light (recording light is what photography is at it's core). So if you have a scene with a bright sky, a dark cave, and a moderately lit rock face around the cave...

1. You can expose for the inside of the cave to be seen, but you will blow out your highlights. Your sky will be white instead of blue, and your rock face will be brighter than it looks in real life. But you got your subject (inside of the cave) exposed properly.

2. You can expose for the rock face, and your rock face will be exposed properly, your sky will be slightly blown out, and your cave will be underexposed (too dark to really see).

3. You can expose for the sky, and you will have a natural blue sky, a slightly underexposed rock face, and a very, very dark cave.

That example is a case of light levels that are several stops apart (very bright, average lighting, very dark) not unlike the pictures you posted. If the lighting had a smaller range, everything would be relatively well exposed. The next topic would be flash (which can be just as important in daylight as it is in low light), but to put it simply, if you weren't already aware of/didn't already know what everything I just said means, then you're better off blaming yourself than the camera or the lens. Study, study, and then study some more, then practice twice as much as you studied, and you can get those exceptional shots you expect out of yourself.

[coldrain]

http://www.dcresource.com/forums/showpost.php?p=267968&postcount=9

You should not make two same threads... but anyway.

What you see is not really CA, it just has to do with that the branches are not really in focus, so they blur... and the dark of the branches blurs into the light, meaning the light will be less light... and more blue.

Not a photo that will be easy to make into something nice because of the wrong light conditions... a sharper lens and having the focus on the branches will help a lot here.

[jlu1234]

Hello, I understand the explanation about sky-rock-cave, that is why the sky is white. I also agree that the blue tree-top is due to out-of focus, plus strong contrast. Right now, I am trying to figure out why it turns into blue, but not gray or green etc, considering the branches are nothing near blue.

I first posted the thread on the Canon forum, no reply after half hour, I am nerves whether I should return the camera, so I posted it again on another forum. Sorry, I'll not do it again.

I was really "blew away" by the initial blue-tree images. Now I believe both the camera and the lens are good, I am very happy with my new camera. Thanks again for all the replies.

Jinggang