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Author Topic: MTF Charts vs. Photographs  (Read 13555 times)
Slough
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« Reply #40 on: September 04, 2008, 01:18:46 PM »
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Rest assurd...Nikon has the same rate of lens issues as Canon...all my Nikon shooting buddies attest to this, and if you look on Nikon sites, you too will see this as true.  But to be sure, I do believe that Nikon and Canon make great lenses, and often better then the best Germen models....ever try the Canon 35L?  Just one example of near perfection, and a lens that the Germen makes have little or nothing over.
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As I stated earlier, I have rarely seen discussion of issues with Nikon QC, whereas discussion of Canon QC issues is commonplace. Given that I usually look in Nikon forums, you would expect me to find Nikon issues.

I don't doubt the quality of many Canon lenses.
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schrodingerscat
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« Reply #41 on: September 04, 2008, 01:42:46 PM »
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Hi,

Anyone knows what "lens calibration" means?

Erik
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There's an encoder on the lens focus barrel that tells the camera where the lens is focused to. The wiper for this is sometimes adjustable, allowing the lens to be adjusted to a particular body. There are also electronic adjustments via EPROM by factory service software that are now being implemented in the new higher-end cameras via firmware. This will go a long way to overcoming the situation as it exists.

Due to manufacturing tolerances, there will be differences in the lens flange to sensor distance and the lens comes from the factory adjusted to a mean, which also has manufacturing tolerances. This is the main reason for variations between lens/body combos. The secret behind the focus accuracy of the Leica M series is instead of the lens flange being mounted on a sub-assembly attached to the main body, the body is cast as a single unit. In the case of the M3, the body was machined from a solid billet.

Designing auto focus lenses will always be a bit of a headache due to the necessity to have a certain  amount of slop in the tolerances to ensure the elements move freely. Add to this the front element wiggle inherent in a lot of zooms and the advent of image stabilization in the lens, and it's amazing that these things work as well as they do.

One thing I notice in most of these discussions is little or no mention of color and contrast, both of which, to me, are every bit as important as sharpness. I tried one of the Sigma 24/1.8 DG's, and while it was sharp enough, had flat color and harsh contrast characteristics that no amount of post-processing could overcome.

As to the original inquiry, I've never hung an MTF chart on the wall and admired it.
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ErikKaffehr
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« Reply #42 on: September 04, 2008, 05:35:59 PM »
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Thanks for good explanation. So calibration just fixes autofocus issues?

I'm actually somewhat confused. As far as I understand the camera is actually doing the focusing and the lens assembly just acts as a servo. Now I understand that servos can both under and overshoot, and that the amount of over/undershoot may be specific to each lens. Any error related to the flange distance should be specific to the camera body and not to the lens, in my view.

Best regards
Erik

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There's an encoder on the lens focus barrel that tells the camera where the lens is focused to. The wiper for this is sometimes adjustable, allowing the lens to be adjusted to a particular body. There are also electronic adjustments via EPROM by factory service software that are now being implemented in the new higher-end cameras via firmware. This will go a long way to overcoming the situation as it exists.

Due to manufacturing tolerances, there will be differences in the lens flange to sensor distance and the lens comes from the factory adjusted to a mean, which also has manufacturing tolerances. This is the main reason for variations between lens/body combos. The secret behind the focus accuracy of the Leica M series is instead of the lens flange being mounted on a sub-assembly attached to the main body, the body is cast as a single unit. In the case of the M3, the body was machined from a solid billet.

Designing auto focus lenses will always be a bit of a headache due to the necessity to have a certain  amount of slop in the tolerances to ensure the elements move freely. Add to this the front element wiggle inherent in a lot of zooms and the advent of image stabilization in the lens, and it's amazing that these things work as well as they do.

One thing I notice in most of these discussions is little or no mention of color and contrast, both of which, to me, are every bit as important as sharpness. I tried one of the Sigma 24/1.8 DG's, and while it was sharp enough, had flat color and harsh contrast characteristics that no amount of post-processing could overcome.

As to the original inquiry, I've never hung an MTF chart on the wall and admired it.
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Ray
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« Reply #43 on: September 04, 2008, 08:35:05 PM »
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Thanks for good explanation. So calibration just fixes autofocus issues?

I'm actually somewhat confused. As far as I understand the camera is actually doing the focusing and the lens assembly just acts as a servo. Now I understand that servos can both under and overshoot, and that the amount of over/undershoot may be specific to each lens. Any error related to the flange distance should be specific to the camera body and not to the lens, in my view.

Best regards
Erik
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Autofocussing accuracy is a major problem which Canon appears to be addressing by adding micro-autofocussing adjustments to their latest cameras. The recently announced 50D has such adjustment capability and any adjustments can be saved in relation to a specific lens.

As I understand, the alignment of the lens elements in their groups is fixed during manufactuire. If some minor misalgnment has taken place, resulting in the lens being less sharp than another copy from a different manufacturing batch, then nothing can be done later to correct this, so I believe.

This is why, in my view, it is always best to start off with a lens that is sharp. Why waste one's time testing autofocus and flare issues if you subsequently discover the lens is not sharp. If the lens is not sharp, there's little you can do about it, except perhaps always use it at F11. In the case of some ultra-wide angle lenses, they are sometimes not even sharp at F16, in the corners on full frame.

Flare and a lack of precise autofocussing, are definitely concerns. However, it is possible to get around them. If calibration doesn't fix the autofocussing problem, Live View allows for great accuracy of manual focus. Flare can often be stopped by holding a card or hat at the edge of the lenshood, blocking the sun's rays.

My Sigma 15-300 has a repution for being susceptible to flare because of its bulbous, protruding front element. It's the nature of the design. I think the Nikon 14-24 also suffers from a similar problem for the same reasons, but perhaps less intrusive due to better internal coating. Sometimes just holding the palm of one's hand between the direct rays of the sun and the edge of the lenshood is sufficient.

I also wonder if such attributes (apart from autofocus accuracy) which are not reflected in MTF charts, have the same degree of QC variability as resolution.

If a lens has a reputation for bad flare, how would you test different copies of the same model in order to select the one which produced the least flare? Has anyone done such a test and found that there is significant variation in proneness to flare amongst different copies of the same model?
« Last Edit: September 04, 2008, 09:09:01 PM by Ray » Logged
schrodingerscat
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« Reply #44 on: September 04, 2008, 10:14:31 PM »
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Thanks for good explanation. So calibration just fixes autofocus issues?

I'm actually somewhat confused. As far as I understand the camera is actually doing the focusing and the lens assembly just acts as a servo. Now I understand that servos can both under and overshoot, and that the amount of over/undershoot may be specific to each lens. Any error related to the flange distance should be specific to the camera body and not to the lens, in my view.

Best regards
Erik
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With Canon and all the other lenses with built in motors, all moving parts are in the lens, which is what does the focusing. The body just contains the sensors to corroborate with the lens when focus has been obtained. Both the lens and body contain processors and the processor on the main circuit board does the heavy lifting. As with all mass produced products, there will be a certain amount of variance from example to example. Both the body and lens are adjusted to a mean, so how the body/lens combo performs will depend on which end of the tolerance levels both the body and lens are.  Also, any anomalies in the communication lines can cause grief, including an inability for the lens and camera to work together. Stabilization lenses have another motor to add to the mix.

Without the lens, flange to focal plane distance errors are moot. It's a symbiotic relationship.

In the never ending quest to out hype the other guy, I think we may be approaching the complexity barrier insofar as current technology is concerned. Like the Mhz, MP, and FPS wars, it may be time to back off on the whiz bang and concentrate on making the current stuff work better. Unfortunately the consumer has been conditioned to constantly chase their tail so the cycle continues.
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Fine_Art
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« Reply #45 on: September 05, 2008, 12:39:36 AM »
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You can save a lot of money with this poster from Edmunds optics. $18 plus shipping.



This has the old USAF resolution test pattern in various colors and angles all the way out to the corners. When you get a new lens you take a shot at a few different f stops. Pixel peep the raws for resolution. You will know in seconds if you have a dud lens.

Edit: thats a crop from the center. I didn't put in a pic of the whole thing for copyright reasons. You can also read the text in the crop.
« Last Edit: September 05, 2008, 12:41:20 AM by Fine_Art » Logged
Ray
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« Reply #46 on: September 05, 2008, 08:17:27 AM »
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This has the old USAF resolution test pattern in various colors and angles all the way out to the corners. When you get a new lens you take a shot at a few different f stops. Pixel peep the raws for resolution. You will know in seconds if you have a dud lens.
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Yes. You can do that. There are also free charts available at Norman Koren's site, and of course newspapers are always readily available.

However, as slough has already pointed out, performance at infinity is not always indicative of performance at close distances. When I tested my 400/5.6 prime on real world targets, I tested it at infinity, close to infinity and at around 30m distance. I didn't test it at a distance of 3 or 4 metres because that wasn't the typical distance from which I figured I'd be using the lens.

For all I know, that lens might have been sharper than my 100-400 zoom at a distance of 4 metres from the target.

The issue is really a matter of efficiency and how much you value your time. If you enjoy testing lenses, then you would probably be against the idea of the manufacturer providing you with a complete set of MTF charts at various frequencies, various apertures and various focussing distances.

After all, you might hit the jackpot and find that a lens you've just bought at a regular price has absolutely stellar performance, way above the average, yet you've paid only an average price for it.

I happen to value my time more highly, even though that might seem odd in view of the number of posts I have on LL. I'm willing to pay more for a lens if I can be assured of it's sharpness. Chromatic aberration, flare and bokeh I can deal with. Lack of sharpness I can't.
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ErikKaffehr
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« Reply #47 on: September 11, 2008, 07:52:11 AM »
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Hi,

I think that Ray has a point here. On the other hand I think that there are amny sets of MTF:s curves needed to characterize any lens. There would probably be sets of MTF diagrams for different distances and wavelenghts of light.

Erik

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I'm willing to pay more for a lens if I can be assured of it's sharpness. Chromatic aberration, flare and bokeh I can deal with. Lack of sharpness I can't.
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Ray
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« Reply #48 on: September 11, 2008, 08:50:32 AM »
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Hi,

I think that Ray has a point here. On the other hand I think that there are many sets of MTF:s curves needed to characterize any lens. There would probably be sets of MTF diagrams for different distances and wavelenghts of light.

Erik
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Only one?  

If one is going to devise a streamlined, efficient and automated system for putting each lens through an MTF test, one might as well take the opportunity to do a thorough test at various apertures and possibly different distances if there's any reason to suppose performance will vary with focussing distance.

If a job's worth doing, it's worth doing well   .
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lovell
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« Reply #49 on: September 11, 2008, 11:32:58 AM »
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Autofocussing accuracy is a major problem which Canon appears to be addressing by adding micro-autofocussing adjustments to their latest cameras. The recently announced 50D has such adjustment capability and any adjustments can be saved in relation to a specific lens.

As I understand, the alignment of the lens elements in their groups is fixed during manufactuire. If some minor misalgnment has taken place, resulting in the lens being less sharp than another copy from a different manufacturing batch, then nothing can be done later to correct this, so I believe.

This is why, in my view, it is always best to start off with a lens that is sharp. Why waste one's time testing autofocus and flare issues if you subsequently discover the lens is not sharp. If the lens is not sharp, there's little you can do about it, except perhaps always use it at F11. In the case of some ultra-wide angle lenses, they are sometimes not even sharp at F16, in the corners on full frame.

Flare and a lack of precise autofocussing, are definitely concerns. However, it is possible to get around them. If calibration doesn't fix the autofocussing problem, Live View allows for great accuracy of manual focus. Flare can often be stopped by holding a card or hat at the edge of the lenshood, blocking the sun's rays.

My Sigma 15-300 has a repution for being susceptible to flare because of its bulbous, protruding front element. It's the nature of the design. I think the Nikon 14-24 also suffers from a similar problem for the same reasons, but perhaps less intrusive due to better internal coating. Sometimes just holding the palm of one's hand between the direct rays of the sun and the edge of the lenshood is sufficient.

I also wonder if such attributes (apart from autofocus accuracy) which are not reflected in MTF charts, have the same degree of QC variability as resolution.

If a lens has a reputation for bad flare, how would you test different copies of the same model in order to select the one which produced the least flare? Has anyone done such a test and found that there is significant variation in proneness to flare amongst different copies of the same model?
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But don't the latest Nikon bodies offer micro adjustments too?!?
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After composition, everything else is secondary--Alfred Steiglitz, NYC, 1927.

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« Reply #50 on: September 11, 2008, 12:29:23 PM »
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But don't the latest Nikon bodies offer micro adjustments too?!?
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Yes, they do and Nikon systems have trouble with front- and back-focus too.

However, I've seen little information on how to calibrate one's lenses. Obviously, focusing bracketing is necessary and one selects the best focus. Small changes on focus are difficult to make with auto focus lenses as the focusing ring is relatively coarse.


An idea I had is to use a tethered setup with Nikon Control Pro II, which allows small changes in focus. One could use live view at high magnification to get an idea of focus and then confirm by actual photographs. Has anyone tried this approach?

Bill
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ErikKaffehr
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« Reply #51 on: September 11, 2008, 01:13:50 PM »
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Would it not be best to make shots with different calibartion settings, perhaps ten samples and using the setting that gives the best MTF?

Erik


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Yes, they do and Nikon systems have trouble with front- and back-focus too.

However, I've seen little information on how to calibrate one's lenses. Obviously, focusing bracketing is necessary and one selects the best focus. Small changes on focus are difficult to make with auto focus lenses as the focusing ring is relatively coarse.
An idea I had is to use a tethered setup with Nikon Control Pro II, which allows small changes in focus. One could use live view at high magnification to get an idea of focus and then confirm by actual photographs. Has anyone tried this approach?

Bill
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bjanes
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« Reply #52 on: September 11, 2008, 03:26:32 PM »
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Would it not be best to make shots with different calibartion settings, perhaps ten samples and using the setting that gives the best MTF?

Erik
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Yes, measuring MTF with Imatest or a similar method would be the most objective confirmation of focus. Multiple samples would be necessary to take non-repeatability and random error into account. With Imatest it is relatively easy to perform multiple analysis with a tripod mounted camera.

Bill
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