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Old Feb 5, 2006, 7:23 AM   #11
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Well, you're not going to find any L lenses from Canon in Minolta mount. ;-)

Again, you may not want/need top of the line glass, depending on what you're shooting. For one thing, it's very pricey (they may cost several times as much as lenses with equivalent optical qualiity).

Also, you may not want to lug around heavy lenses for everyday use. Most G lenses are designed with large available apertures. That makes your lenses larger and heavier for the same focal range.

Any lens choice is a compromise between things like size, weight, ergonomics, built quality, optical quality, etc.

I do not own any G lenses for my Maxxum 5D. So far, I've got the Minolta 28mm f/2, 50mm f/1.7, 100mm f/2, 135mm f/2.8 in primes; the MInolta 18-70mm f/3.5-5.6 (kit lens), 35-70mm f/4 and 24-85 f/3.5-4.5 in Minolta zooms; and also have the Tamron 20-40mm f/2.7-3.5 and 35-105mm f/2.8

I'll give you an example or two of why G is not always best. My Minolta 100mm f/2 is actually sharper at all apertures compared to the Minolta 85mm f/1.4G, according to the MTF tests at http://www.photodo.com (which is one source of lens ratings), not to mention that you'll have a shallower depth of field at larger apertures.

Yes, f/1.4 is nice to have (you could use shutter speeds twice as fast for the same lighting conditions and ISO speed compared to shooting at f/2.0). But, the 85mm f/1.4G is not that sharp when shooting wide open at f/1.4. I decided that it wasn't worth it to me to get the 85 just to get that extra stop. Ditto for the 50mm f/1.4 (it's a tad soft wide open, as you'll find the case to be with most lenses that start out that wide). I got the 50mm f/1.7 instead.

Lots of things come into play when lens shopping. For example, at one time, I was considering the Minolta 28-70mm f/2.8G. But, it doesn't focus as close as some of the similar lenses on the market, and it's a bit heavier (not to mention more expensive).

Most KM owners would probably buy the KM 28-75mm f/2.8 instead if they needed a bright lens with this focal range (which is an excellent lens by all accounts, with faster focusing and closer focusing compared to the G lens in this focal range).

Also, don't go by MTF tests alone. They never tell the entire story. You've got a lot of variables (resistance to flare, focus speed, etc.). You may see a lens that is razor sharp on tests. But, it may be slow to focus (because it's geared for more precise focusing) with out of focus areas (i.e., bokeh) that is not as soft..

1:1 Macro lenses usually fall into this category (extremely sharp, but often have slightly harsher out of focus errors, just because of their sharpeness, and are usually slow to focus because of the way they're geared for precision).

You've also got a lot of variation between ergonomics and design. For example, some lenses use an internal focus mechanism that keeps them shorter. You have lenses with and without rotating front elements (a concern to some usrs), different designs for focus rings, zoom rings, etc. You want to make sure you're comfortable with a lens design.

If you're just starting out and don't know a lot about lenses, my advise would be to buy the kit lens. It's got good focal range, gets you covered better on the wide end (becaue it's starts out 18mm, which is a 35mm equivalent angle of view of approximately 27mm after the 1.52x multiplier for the size of the DSLR sensor). It's also small and light for it's focal range.

Then, buy a 50mm f/1.7 to go with it for when you need something that lets you shoot in lower light without a flash (because it's sharp, bright and inexpensive).

After you've used the lenses for a while, you'll have a better idea of where you see limitations, based on the type of shooting you do most often, and can make a better informed decision for future purposes.


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Old Feb 5, 2006, 7:45 AM   #12
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P.S. -- I missed this comment:

" The discussion came up because I had to exchange my Tamron f/2.8, 28-75, because an uneven focus across the field of view."

You're probably seeing normal lens behavior. You'll have uneven focus across a lens plane (especially at closer ranges and wider apertures). Not everything in the field of view is the same distance from center. You just want to make sure the edges are equally soft when testing a lens.

Remember, you've got a much shallower depth of field with a DSLR compared to a point and shoot digicam. That takes some getting used to.

Also, you'll find that selecting a focus point often works better, too.

http://www.mhohner.de/recompose.php?lang=e

In addition, some lenses are actually sharper just outside of center, even if you use the center focus point and don't reframe at some apertures. My Tamron 35-105mm f/2.8 falls into that category.

If you look at it's MTF charts at http://www.photodo.com , you'll see that at some focal lengths and apertures, the curved lines comes up a bit from center (far left of the graph is center, and as you move right, you're getting further away from center).

At wider apertures (smaller f/stop numbers) and closer ranges, these design quirks are more obvious (because your depth of field is shallower at wider apertures and closer ranges).

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Old Feb 10, 2006, 12:47 AM   #13
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JimC wrote:
Quote:
P.S. -- I missed this comment:

" The discussion came up because I had to exchange my Tamron f/2.8, 28-75, because an uneven focus across the field of view."

You're probably seeing normal lens behavior. You'll have uneven focus across a lens plane (especially at closer ranges and wider apertures). Not everything in the field of view is the same distance from center. You just want to make sure the edges are equally soft when testing a lens.

Remember, you've got a much shallower depth of field with a DSLR compared to a point and shoot digicam. That takes some getting used to. .....
Quote:
At wider apertures (smaller f/stop numbers) and closer ranges, these design quirks are more obvious (because your depth of field is shallower at wider apertures and closer ranges).

Jim,

Thanks for the massive replies which I'm still mulling over.

Below is an example picture of the uneven focus that I encountered. The difference between the center and particularly the right side seemed excessive. Camera settings: no flash, ISO 640, Shutter 1/50, f/2.8, focal length 28 mm. I took other photos with a flash, similar uneven focus. Subjects were tired and not moving.



FYI: Picture is of a high school wrestling team. In the back row, far right, is Ben Petersen, Olympic Wrestling Gold and Silver medalist. My 11 year old son is at his feet.

Note the faint vertical lines that appear on the concrete wall, about one inch apart. They do not correspond to any existing features on the wall or nearby reflecting surface. Mild use of Unsharp Mask makes vertical lines appear on all subjects as white speckles. Some pictures look like people have striped pants.



I took printed samples with metadata to the camera dealer and asked if I was at fault or the camera. The dealer's GM performed test shots with a new lens, then a new body. The lens change made the most difference, but the dealer readily agreed that replacement of both provided the best picture! I'm guessing the focus to be a lens problem and the vertical white lines to be a camera problem.

You mentioned that the f/number affects the focus across the field of view. To see this first hand, plus checkout the replacement Tamron and KM, I made a series of test shots. I placed the camera on a tripod 10 feet from a wall map with lots of distinct lines and letters. I took shots varying the f/number in steps from f/2.8 to f/22. Then, repeated the process at 28, 45, and 75 mm focal lengths. In these conditions, I observed only slight differences in center, off-center, and edge focus.

However, I'm concerned that I might havealso have aproblem with the replacement camera or lens. Test shots revealed a unexpectedly large change in picture quality from f/2.8 to f/8. Sharpness of map lines increased over the entire image as the f/number increased from f/2.8, to f/4, to f/5.6, then a big improvement at f/8. From dullish to impressively sharp and vivid. Further increases in f/number didn't provide much improvement. The focal length was 45 mm, about middle of the zoom range.

The following pics were taken with the replacment camera body and lens. Here's the pic at F/2.8:





The pic below was taken at F/8.





I've heard that there would be improvement when the f/number was increased from the lowest value, but I'm hoping for some confirmation/feedback as to whether the degree ofdifference could suggest a problem… with the lens? I want to shoot in available light indoors, so low f/number sharpness is important. I'm wondering if a different lens would be better for me?

Thanks again,

Scott


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Old Feb 13, 2006, 6:12 PM   #14
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Scott, let me get this right....

The photo of the map was taken with a new camera body and a new lens, correct?

You were using a tripod, right? You had focus lock, right? You were within the rated focus distance for the lens, right?

If you have a focus calibration issue, it will be less noticeable at smaller apertures (because you have greater depth of field).

It's normal for a lens to be sharper as you use smaller apertures (usually about 2 stops down from wide open is pretty good). But, as soft as that map is at f/2.8, I'd think you misfocused (or have a focus calibration issue). It's hard to say without seeing the map, or taking photos with another camera using the same distance, etc. if it would be normal to see it that soft.

You do have the AF/MF switch on AF, making sure you have focus lock with a half press of the shutter button (prior to pressing it down the rest of the way) to test it, right? What focus point are you using? The center focus point is usually more sensitive.




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Old Feb 14, 2006, 12:43 AM   #15
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Answers to your questions:

The photo of the map was taken with a new camera body and a new lens, correct? Yes.

You were using a tripod, right? Yes. You had focus lock, right?Yes (Held, shutter release 1/2 way,until indication of focus, then press all the way.) You were within the rated focus distance for the lens, right? Tamron specs .33 m minimum focus distance. Pics taken at 8-10 feet from the map.

You do have the AF/MF switch on AF, making sure you have focus lock with a half press of the shutter button (prior to pressing it down the rest of the way) to test it, right? What focus point are you using? The center focus point is usually more sensitive.


Jim,

By "center focus point" I assume you refer to what KM callthe "AF area". This was set to Wide. I will change it to Spot AF area and retake the pictures. I will also take comparative pictures with my Fuji F10 and a friend's Canon A610.

I thought that if the F/number had this large of an effect on image quality that it might indicate poor lens quality.How likely is that? Wish I had the kit lense for comparison to the Tamron.

Thanks much!

Scott
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Old Feb 14, 2006, 8:05 AM   #16
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What you are seeing there is that it's not in focus at all.

If you are getting focus lock then there is probably something wrong with your camera.

With focus lock you should be able to get an in-focus picture at any aperture, your depth of field will be narrow with wide apertures but here the chart is clearly not inside your DOF at f2.8. If the camera told you that you had a focus lock on that chart it was wrong and is therefore faulty.
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Old Feb 14, 2006, 8:27 AM   #17
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Scott:

That looks pretty soft to me (so, it probably was not in focus).

Was the map reflective? Sometimes a shiny surface can fool AF systems.

I'd try it again using something with good contrast and no reflective surface.

You may also want to try turning Anti-shake off when on a tripod for testing. KM recommends disabling it on a tripod as you can sometimes get a feedback loop with zero motion (in theory, anyway).

My favorite test for AF accuracy is using something like books on a table stacked at close distances using a bright prime. Focus on the center one, then see if it's sharp and the ones in front of and behind it are not as sharp.

For example, this photo (a rather crude test, but I just wanted to do a quick one to check mine). You'll find more sophisticated tests for AF around the net.

http://www.pbase.com/jcockfield/image/51015183




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Old Feb 22, 2006, 1:35 AM   #18
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Belated thanks to peripateticand Jim- very helpfulfeedback,just what I was hoping for.

I had a former professional photographer help me set up some three different shooting tests and watch me take pictures withmyKM5D withTamron f/2.8, 28-75 mm.The camera and lensstill provides mostly poorly auto-focused pictures at F/2.8 and reasonable focus at F/8. With Manual Focus, the pictures wereimproved at F/2.8, with anoticeable but not large difference in sharpness between the F/2.8 and F/8.

My camera store suggested that I drop off the camera and their photographer would evaluate it, but now he is out of town for over a week. In the interim, the dealer suggested replacing the lens.The store previously replaced both the camera and lens, the camera due to vertical banding, and the lens due to blurriness in the lower right corner of the image.

A seemingly knowledegable guy at the storeclaimed they had another customerread on the Internetthat a bad batch of lenses of this modelwere made by Tamron and their store stock might be from that batch.I'mgoing to ask them to confirm that withthe distributior, cause that sounds likewhoey. To replace the Tamron, the dealer offered the same spec lens in KM.

Jim, you mentioned that Tamron makes the lens for KM, but I wonder ifmade to a different spec. Do you think this is a goodtest and a likely solution to my focusing problem?

I'm glad I read your comments in the KM DSLR forum about another Tamron lens which I was considering:

"The Tamron 35-105mm f/2.8 is probably my least favorite lens (although it's OK stopped down some in better light). It would probably be a good lens for head shots and it's longer end is a bit better.. My copy may not be as good as some, as I haven't been impressed, and it's AF is iffy.."
I can't get my books to look as sharp as those inyour AF test in the link you provided.

I feel as if I shouldask foradditional lens, one that is totally different, so Ican eliminatethe lens as the problem. They are out of stock of 50 mm F/1.7 or I'd buy that.

The result of all this is that I am learning alot from these tests and your comments, but wearing thin on time.

Your comments would be appreciated.

Thanks again,

Scott
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Old Feb 22, 2006, 6:51 AM   #19
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Quote:
I can't get my books to look as sharp as those inyour AF test in the link you provided.
That test was performed with a Minolta 100mm f/2, which is one of the sharpest lenses you can get your hands on. No zoom lens is not going to match it, especially at larger apertures. That lens (100mm f/2) even tests sharper at all available apertures compared to the Minolta 85mm f/1.4G, which is a "flagship" lens for Minolta. You'd be hard pressed to find a sharper lens.

Make sure it's not the camera body. Use a bright prime like a 50mm f/1.7 and see how it works. If it's backfocusing or front focusing with a good prime, it's the camera. If not, it's probably the Tamron.

Make sure you're seeing focus error versus softness, too. That's what the tests can tell you. But, at first glance, your map looked plain out of focus to me (versus just soft from shooting at wider apertures). How does it work with a different type of target, checking backfocus or frontfocus?

Virtually any lens is gong to be softer wide open versus stopped down a stop or two.

You don't see many reports of BF or FF with the 5D. But, it happens. There are some hex screws that adjust the AF sensor alignment under the metal plate around the tripod mount, and KM can adjust one to specs (they use specialized equipment, and a 50mm f/1.7 for aligning one). I have seen reports of users adjust their 5D themselves, though.

But, if it's a new camera, you should get your dealer to handle any problem like that by replacing the defective camera or lens. It's a shame you're having problems, but it happens from time to time with products with any manufacturer. I'd work through them until you get one that works right.

If your dealer won't work with you, send the camera back to KM for AF alignment if it's the camera (making sure with a good prime, checking for backfocus or frontfocus).

If it's the lens, send it to Tamron and let them check it out (they have a 6 year warranty). ;-)

Unfortunately, QC problems happen from time to time, from all of the manufacturers. It's possible that your dealer got a bad batch of cameras or lenses due to a production problem.

You'll find threads on backfocus or frontfocus problems from other camera brands, too (Canon, Nikon, etc.). None are immune to QC problems from time to time.

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Old Feb 22, 2006, 10:26 AM   #20
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JimC wrote:
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Make sure you're seeing focus error versus softness, too. That's what the tests can tell you. But, at first glance, your map looked plain out of focus to me (versus just soft from shooting at wider apertures). How does it work with a different type of target, checking backfocus or frontfocus?
Jim,

How does focus error differ from softness? What is backfocus and frontfocus?

Thanks,

Scott
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