Gear Talk > Lenses
Canon 17-85mm vs. Tamron 17-50mm vs. Tamron 28-75mm
pdirestajr:
I only have experience with the EF-S 17-85.
I bought it on CL for 300, played with it for a week and sold it for 300. There was serious distortion on the wide end- pretty much unusable. And with video, I'm not sure how easy it is to correct lens distortion.
The other 2 lenses usually get positive reviews. IS/ VC and a constant f2.8 are also great for video.
The good thing with lenses is that their value holds pretty well, so you can always sell/ trade it down the line.
neuroanatomist:
@elford, if you scroll down further on the page you link, Norman Koren (or whoever wrote that page) states, "Sensor response above the Nyquist frequency is garbage."
elflord:
--- Quote from: neuroanatomist on January 21, 2012, 06:34:41 PM ---@elford, if you scroll down further on the page you link, Norman Koren (or whoever wrote that page) states, "Sensor response above the Nyquist frequency is garbage."
--- End quote ---
I understand that the sensor can't resolve above its nyquist frequency, and that's the point that they are making in your quote. However, in the part I quoted, the imatest page is claiming that they can analyze higher spatial frequencies even if the sensor can't resolve them.
Here are a couple of questions I have:
(1) if you take a sharp lens and a soft lens, and shoot a pattern that has a higher spatial frequency than the nyquist frequency of the lens, will both lenses produce identical images ? If the lens faithfully reproduces the test pattern, you will get aliasing effects. Will these look different with the two lenses ?
(2) suppose i could do the following: take a picture, then move the camera 1/4 of a pixel, then take another, then combine the resulting images. Can I produce an image that resolves better than the sensor ?
(3) While it isn't practical to do (2), could I get a similar effect by carefully choosing the test pattern ?
Again, it's somewhat elementary that the sensor doesn't produce real signal above the nyquist frequency. I don't think it's anywhere nearly as obvious (I don't think it's even true though I'm not an expert on this) that you can't devise some test that can get around the the limitations of the sensor. The sensor by itself can't resolve this information, but if you have some knowledge about both the sensor and the spatial structure of the test pattern, I'd think you should be able to make a prediction of what the resulting image should look like.
neuroanatomist:
--- Quote from: elflord on January 21, 2012, 06:49:50 PM ---Here are a couple of questions I have:
(1) if you take a sharp lens and a soft lens, and shoot a pattern that has a higher spatial frequency than the nyquist frequency of the lens, will both lenses produce identical images ? If the lens faithfully reproduces the test pattern, you will get aliasing effects. Will these look different with the two lenses ?
(2) suppose i could do the following: take a picture, then move the camera 1/4 of a pixel, then take another, then combine the resulting images. Can I produce an image that resolves better than the sensor ?
(3) While it isn't practical to do (2), could I get a similar effect by carefully choosing the test pattern ?
--- End quote ---
I'll have to think on #1. I can say that while #2 isn't possible with Canon dSLRs, it is possible in other systems - the Zeiss AxioCam HRc cameras ($14K each) on several of my microscopes do just that. They can move the sensor (a 1.3 MP Sony 2/3" CCD) in one-pixel increments to sample each color in the Bayer mask and avoid interpolation for color, and they can also translate the sensor in sub-pixel increments to sample a 2x2 array for a 5 MP image or a 3x3 array for a 12 MP image. That's potentially real resolution. However, the basis for that is the pixel 'sweet spot' (photosensitive area smaller than the total pixel area) - microlenses obviate that approach, as do subjects that move.
The other issue is that of sensors out-resolving lenses. While we're not there yet for dSLRs, microscopy is a different story. For my higher power objective lenses, e.g. 100x 1.4NA Plan-Apo with a 1.4NA condenser focusing the transmitted light, even VGA resolution (640x480) is sufficient to resolve the full optical resolution of the system, and anything over 0.3 MP is just empty resolution.
Beautor:
This may be a slightly old thread, and the OP may have made his decision, but since I currently own the first two lenses I'll chime in with my personal experience. Disclaimer: I'm only an amature photographer who loves to take pictures.
I have the Tamron 17-50 f2.8 non VC. The image quality of this lens is great. The lens is light and easy to carry. The AF is quick but noisy, and in dim light it will often make 2-3 micro adjustments following the first autofocus movement. I've found that when using a flash the focus is sketchy, and the keeper rate goes down unless you're using only the center focus point. I don't like that the focus ring turns when autofocusing, and there is no full time manual focus. The zoom has just the right amount of dampening to make it smooth, without turning too easily. It has no gravity creep. Wide open you can get a pretty shallow depth of field, but the bokeh is not as smooth as high end zooms like the Canon 24-70. In general the lens is a great step up from a kit lens, and asside from a couple of quirks my wife and I have been quite happy with this lens.
The Canon 17-85 is a good lens, and was a huge step up from the 18-55 kit lens we had initially. On my 40D it works fine, but doesn't have nearly the low light capabilities of the Tamron. Its a good walk around lens, especially outdoors. I've found the autofocus with this lens to be fast, smooth and accurate. The only place that the AF searches is in really low light. Usually it will lock on, even in situations where its too dark to get a reasonable picture. I like that with the USM the AF ring doesn't turn while autofocusing. I have found that the zoom ring has loosened up quite a bit since we got the lens 2 years ago, to the point where it will gravity zoom in certain situations. This is my biggest compliant about my 17-85.
I don't have the Tamron 28-75, and have never used it, but a good friend of mine (who is a lot more serious photographer than me) had one for a while. He was always complaining about the autofocus of that lens. He said that his keeper rate for low light was well below 50%.
Between the two lenses that I have direct experience with, I think I would go with the Tamron. It is more flexible with the faster aperture, and I can overlook the loud AF. I think the image quality is better from the Tamron, and its slightly narrower and lighter to carry. I rarely miss the 50-85mm focal length, and I have a telephoto lens for those times when I want it. I hope this helps.
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