Blurry shots question

TiCoyote · Jul 21, 2010

I ran a test. I went outside with the 28-75 Tamron, I set it to 2.8, ISO 400, 70mm focal length in bright sun. Today is bright, the sky is a little hazy, but there is no visible ground haze or fog for over 300 feet.

There is a blue and white realtor's sign in front of my building. I set the camera to "one shot" AF, I held it as still as I could, and I set the AF points to center only. I aimed the center AF point at a white number on a blue background, so I had lots of contrast and clear lines. I took the same shot from 5 paces, 10 paces, 15 paces, 20 paces, and 30 paces. I did not use a tripod, because this does not look like camera shake, but I did use all the best handheld techniques.

I repeated the test with the following variations:
f/2.8 ISO 400 with cir-pol filter
f/2.8 ISO 400 without cir-pol filter
f/8 ISO 400 without cir-pol filter
Then I repeated the test with a Canon 50mm 1.8, no filter, with the following variations
f/1.8 ISO 200
f/2.8 ISO 400

Test Results:

Drum Roll Please

The Tammy wide open is soft. The left side is softer than the right. As I move away from the subject the softness becomes more apparent.

The Tammy at f/8 is sharp. It is slightly sharper on the right than the left, but this is only noticeable at a full crop. The left side is not noticeably soft at f/8. The filter did not noticeably affect the sharpness, though it did make the image seem darker and slightly lower contrast. (I normally only use this filter to enhance sky or water, though, and as it has a bit of a grey tint even when in the "non polarizing mode" this is to be expected).

The Canon is sharp at f/1.8 and f/2.8. One shot missed the focus. For some reason, it focused behind the sign, so the background is sharp, but the sign is blurry. I will attribute this to human error at this point.

Upon very close inspection, the left side of the Canon is also a bit softer than the right. I imagine there are 3 possibilities:
1. I have two lenses that are soft on the left.
2. Some part of the camera (mount, sensor) is falsely calibrated or even crooked.
3. I was standing to the left of the sign, because there was hosepipe in my way.

Under careful inspection and deliberation, I have chosen #3.

Here are my determinations:
1. The Tammy is soft wide open, and therefore has limited use as a portrait lens, and probably wouldn't be very good for a wedding or something dark and indoors.
2. The Tammy is reasonably sharp at f/8 so it's still a good general purpose lens.
3. Canon makes some hotass sharp lenses.
4. I bought the lens used for $350 (and he threw in 2 filters, including the cir-pol), and I could probably sell it for the same in a year or so, so I'm not really disappointed.
5. I'm happy to know the lens' limitations.
6. Field testing beats the pants off armchair conjecturing.
7. My test results are fairly similar to TheDigitalPicture
8. I have to figure out how to get my fiancee to let me drop the $3000 on the Canon 24-70 f/2.8 and the 70-200 f/4 IS.

Thank you everyone for this great information. I would post the test shots, but I don't want to fill up my flickr account. I'll accept suggestions for easy and free ways to post the test shots if anyone is really interested. Again, thank you all for your advice.