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08-19-2012, 02:43 PM   #1
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Novice lens/exposure/sharpness question

Hi all,

Just this morning I picked up the Tamron 28-75, f2.8, after reading all the great reviews (and the couple worrisome posts about trouble with focus on certain copies).

Not wanting to fall into worrying about whether or not something is wrong with this brand new lens, I'm trying just to learn more about the craft, so I'm hoping someone can give an explanation of the two following images. The first of my daughter, taken at 75mm, f2.8, 1/40, ISO200, under bright natural light indoors. The second of a small yellow and purple flower outdoors at 55mm, f2.8, 1/400, ISO 200.

I understand that f2.8 provides a very shallow depth of field and the chance of missing the focus is higher....but the picture of my daughter appears sharp across her face, while the picture of the flower (and the flower itself, the one at the top of the three in foreground, was in the dead center of the frame, with single point focus) appears out of focus. Why would that be?

And if I'm allowed a second question, though my sample size is very small, this lens seems to have much more trouble finding focus under less than ideal light. I guess that's to be expected?

In advance, thanks for any help/suggestions you can offer (in case it's helpful to know, lens is mounted on a K-x).

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08-19-2012, 03:27 PM   #2
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Great pic of your daughter!! I can't wait to move pass the kit lens!!
08-19-2012, 04:34 PM   #3
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QuoteOriginally posted by anselesn Quote
Why would that be
I personally have neither camera or lens, but I do believe that the second shot might be the result of incorrect focus lock. Ie if you use the entire range of AF sensors then it will pick which ever one the camera sees as the best. I suggest that you switch to using centre point AF and try that out. I personally leave my camera on this setting and only might change it to selective point AF for tripod work, but most Tripod work is manual focus.
08-19-2012, 05:31 PM   #4
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Chuck....Thank you.

Scott, unfortunately, I already use center point focus, and I didn't move the camera (at least not intentionally) once I focused on the flower in the middle of the frame. Still, thanks for the thought.

08-19-2012, 06:01 PM   #5
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I have this particular lens. Does have trouble with low light at times. However:

Portrait: I think it could be sharper. Kids dont stay still and even if they do with shutter speed of 1/40s, it would be difficult to get a sharp picture and could be the issue.

The flower shot: flowers sometimes don't come out sharp because of wind. The shutter speed seem pretty decent so that is not likelythe issue. If you want to shoot flowers at f2.8 and you shoot this wide and the background is that close to the flower, it is going to look blurry sometimes because of the composition. Try to shoot a flower with nothing that close to it and it will look sharper. Also the camera is less likely to focus on something in the background.
08-19-2012, 06:33 PM   #6
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Thanks for the tip about the flowers shot Doc. So even with camera set to center point focus, the camera may get "distracted" by something in the background?
08-19-2012, 06:54 PM   #7
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The center AF point cover a rather large area, far beyond the little red mark in the viewfinder. In you flower picture, all 3 flowers will be more or less in the field of view for the center AF point. As such it is very challenging for the AF system to isolate the exact point you would like to focus. To me, the left flower and the leaf at the left are in better focus. Many lenses also suffer from front/back focus and may need adjustment in the camera, although the portrait photo is very sharp suggesting that the lens can focus fine in optimal conditions such as focusing on a face that covers all the center area.

Do you recall if the flower you wanted to be in focus was actually in focus in the viewfinder? You can also try to focus manually or use Live View, which is a far more accurate auto focusing method, and compare the results. Or try several times, refocusing on every attempt. One of the pictures may have what you are looking for.

For critical focusing with narrow depth of field, there is no substitute to manual focus or Live View.
08-19-2012, 08:41 PM   #8
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Another issue your example shows is overexposure in the red channel. The flowers are so small in the sample, it's hard to tell how much this affects the detail. It is very common for the meter to see a lot of darker green foliage and set the exposure for that. The sensor has twice as many green sensors as red or blue, so the red or blue channels are vulnerable to overexposure. When the red channel is overexposed, it loses detail. That might look like a loss of sharpness. It'll be easier to see in the original, and a histogram that shows the color channels individually will show what's happened.

08-20-2012, 06:19 PM   #9
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A problem I have with flowers is that if there is wind and the depth of field is thin, the flower can move out of focus before the shot is fired off. Not sure it applies here, but worth considering.
08-21-2012, 07:10 PM   #10
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The image of you daughter, besides being lovely, looks like it is in focus. Besides giving a shallow depth of field, shooting wide open is almost never the sharpest aperture for any lens. The photo is very workable and could be sharpened if you think it's necessary. I don't. The soft focus of the lens makes it nice for portraits. I have the similar Tamron 17-50 lens and probably wouldn't shoot flowers with it at F/2.8. The lens sharpens up a lot at f/4.

The flower photo is only sharp in the lower left and on the leaves behind the flower. Even with center spot focusing, you camera may simply miss something as small as the flower. Any lens will sometimes miss focus on a small target, especially if there is any movement. 1/400 is certainly fast enough to stop motion, but shutter speed doesn't affect focus.
08-22-2012, 04:58 AM   #11
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Thank you Michael.

And to everyone else, thank you for the feedback. It is much appreciated. Clearly, still much to learn, and also sometimes, important just not to think too much and enjoy the pictures. Thanks again all.
08-22-2012, 10:58 AM   #12
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I would be very chuffed to have taken the photo of your daughter, I wouldn't change it
08-22-2012, 11:26 AM   #13
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not sure about the shot of your daughter, but looking at the flower, and the grass in front of it, the grass is sharper than the flower! I think your focus point was off for some reason.
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