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Author Topic: Front/Back Focus Test Result  (Read 5146 times)
AFairley
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« Reply #20 on: September 11, 2013, 09:58:06 PM »
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Just to chime in, I am finding that what works best for me (after using gizmos like the LensAlign, and software-driven solutions for a while), is to shoot that has good range of depth stuff wide open at the distance I shoot at, and fiddle with the AF adujustment so the focus is falling in the plane I'm focusing on.  With the D800E I can do the image review right on the LCD, and it gets dialed in pretty quickly.
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Fine_Art
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« Reply #21 on: September 12, 2013, 01:01:07 AM »
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I should have written can be a minefield. One little word makes a difference.  Grin

I'm not trying to imply that AF fine-tune is not a useful feature to have. Of course it is. There are some lenses on some bodies that are just a little bit out and a simple AF fine-tune adjustment results in a worthwhile improvement without complication.

Before the feature of AF fine-tune was provided on DSLRs, I would simply return a lens if it didn't autofocus accurately. Perhaps my frustration in this regard at the moment is due to my not having returned a recently bought lens as soon as I discovered it wasn't focussing properly on my D7100. I thought I'd fixed the problem with a fine-tune adjustment of +10, using a fairly close target, but later discovered that this fine prime lens, the Sigma Art 35/1.4, was producing abysmal results at infinity, on the D7100.

It required a fine-tune adjustment of the maximum of +20 for distant subjects. The difference between the +10 adjustment and the +20 adjustment at infinity is very obvious, particularly between the apertures of F1.4 to F4.

Now, the whole purpose of buying a lens like the Sigma Art is because it's ultra sharp and can produce a shallow DoF at F1.4. I can see no point in using a compromise AF fine-tune setting of +15, unless one is prepared to use F8.

When I use the lens for close-up work, I'll have to remember to check that the AF fine-tune is set on +10. When I use the lens for landscape, I'll have to check that it's set to +20, or use a tripod and LiveView. However, if I don't want to bother fussing around making changes to such adjustments according to the circumstances, I could just leave the setting on +15 and use F8 all the time.

Another complication I've noticed is the inherent inaccuracy of the autofocussing system regardless of AF fine-tune considerations. I use the AF-ON button and a single focussing square so that focussing is independent of metering. By pressing again the focussing button each time one takes a shot of the same scene, from the same position, focussing on the same spot, one gets an indication of the focussing inaccuracy of the system. It's as though one has altered the fine-tune setting for each shot. However, such differences are only apparent at wide apertures. At F8 there's no problem.


That is exactly the issue I had with mine which I returned immediately. It's a shame because I really want the lens. I will probably get another when work slows down.

I used af fine tune at a few feet to pick a setting. On landscapes I found infinity OOF even stopped down. The lens was great at close distances. If I am lucky it was a bad batch with a poor jig that has been fixed.

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Ray
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« Reply #22 on: September 12, 2013, 10:27:07 AM »
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That is exactly the issue I had with mine which I returned immediately. It's a shame because I really want the lens. I will probably get another when work slows down.

I used af fine tune at a few feet to pick a setting. On landscapes I found infinity OOF even stopped down. The lens was great at close distances. If I am lucky it was a bad batch with a poor jig that has been fixed.


I sort of wish I'd never bought the lens because I've spent so much time stuffing around trying to get the AF right. Nevertheless, I have got a sort of workable solution. Re-adjusting AF fine-tune on the menu is not particularly troublesome, although one might occasionally miss a good shot whilst doing so. I guess this lens is only going to be useful for certain specialized, preplanned, situations.
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bjanes
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« Reply #23 on: September 12, 2013, 11:05:11 AM »
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Hello,

I've read about front/back focus problems and, having recently acquired a used DSLR, I decided to test it and my favorite lens with the slanted ruler method.


If your camera has live view, you can use the method described by Roger Clark to check focus. Auto focus in live view is more accurate since it uses the image on the plane of the sensor as Roger describes.

Bill
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BartvanderWolf
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« Reply #24 on: September 12, 2013, 11:32:34 AM »
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If your camera has live view, you can use the method described by Roger Clark to check focus. Auto focus in live view is more accurate since it uses the image on the plane of the sensor as Roger describes.

Hi Bill,

Roger's method works best with long focal lengths, and for lenses with modest widest apertures. Lenses with an aperture wider than f/2.8 may suffer from flatness of field issues, and possibly focus shift when stopping down. It's hard to predict, but it wouldn't surprise me if e.g. Ray's lens exhibits some of that behavior.

The AF aperture is limited at approx. f/2.8, so wider apertures will not contribute to the AF solution, but they may influence the overall optical focus result that the sensor records. Since AF is determined wide open, it may differ from the stopped down focus plane.

Cheers,
Bart
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xpatUSA
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« Reply #25 on: September 12, 2013, 12:28:44 PM »
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Hi Bill,

Since AF is determined wide open, it may differ from the stopped down focus plane.

Cheers,
Bart

Was feeling good about my Panasonic m4/3 until I read that, Bart. Although in manual focus I get 10X live view momentarily, the f-number number, as we know, reverts for the shot to whatever has been selected in the cam. Sadly, in my cam, pushing the button to preview "DOF" cancels the 10X view.

So, with two better quality Panasonic zooms and their Leica 45mm macro, I'm hoping to not notice front/back focus effects!

« Last Edit: September 12, 2013, 02:39:03 PM by xpatUSA » Logged

best regards,

Ted
Vladimirovich
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« Reply #26 on: September 12, 2013, 12:45:12 PM »
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Was feeling good about my Panasonic m4/3 until I read that, Bart. Although in manual focus I get 10X live view momentarily, the f-number number, as we know, reverts for the shot to whatever has been selected in the cam. Sadly, in my cam, pushing the button to preview "DOF" cancels the 10X view.

So, with two better quality Panasonic zooms and their Leica 45mm macro, I'm hoping not notice front/back focus effects!



you shall not, speaking about 20/1.7, 25/1.4, 45/1.8, 45/2.8, 60/2.8, 75/1.8, 35-100/2.8 AF m43 lenses...
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Fine_Art
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« Reply #27 on: September 12, 2013, 04:02:38 PM »
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I sort of wish I'd never bought the lens because I've spent so much time stuffing around trying to get the AF right. Nevertheless, I have got a sort of workable solution. Re-adjusting AF fine-tune on the menu is not particularly troublesome, although one might occasionally miss a good shot whilst doing so. I guess this lens is only going to be useful for certain specialized, preplanned, situations.

Send Sigma an email explaining the problem, maybe they will take it under warranty service.
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Ray
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« Reply #28 on: September 13, 2013, 09:38:46 AM »
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Send Sigma an email explaining the problem, maybe they will take it under warranty service.

Yes, I could do that. However, I've already had an unsatisfactory experience doing something similar with Canon equipment a few years ago, so I'm a bit reluctant to risk wasting more of my time.

I was very impressed with the sharpness of my Canon EF-S 17-55/F2.8 zoom, until I discovered that it wouldn't accurately autofocus at close distances when set at F2.8 on my new Canon 40D. I sent both lens and body to Canon service under warranty, to get the problem fixed. They were unable to fix it and claimed that both body and lens were within specification.

The Canon 40D didn't have the AF fine-tune feature, so I was stuck. But within a short period, Canon released an upgrade which did boast the AF fine-tune feature, the 15mp 50D. So I bought it, not just for the AF fine-tune, but that was a major attraction.

The irony is, the 50D focused perfectly at maximum aperture with the EF-S 17-55/2.8, without any AF fine-tune adjustment at all. I can only assume that the AF fine-tune has a dual purpose. To compensate for any slight maladjustment of the lens, and/or to compensate for any slight maladjustment of the camera body.

Both lens and body could be within specifications, that is, within the acceptable margin of error, but there could be situations where the errors in both body and lens are at the same end of the allowable spectrum of error, and have an additive effect.

Conversely, there could be other situations where the errors in both the lens and the body are at opposite ends of the spectrum and cancel each other out, creating the false impression that one is lucky to have an ideal camera body and lens without error.
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Isaac
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« Reply #29 on: September 13, 2013, 10:43:54 AM »
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Both lens and body could be within specifications, that is, within the acceptable margin of error, but there could be situations where the errors in both body and lens are at the same end of the allowable spectrum of error, and have an additive effect.

Conversely, there could be other situations where the errors in both the lens and the body are at opposite ends of the spectrum and cancel each other out, creating the false impression that one is lucky to have an ideal camera body and lens without error.

That's my understanding too.
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bjanes
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« Reply #30 on: September 13, 2013, 12:19:15 PM »
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Hi Bill,

Roger's method works best with long focal lengths, and for lenses with modest widest apertures. Lenses with an aperture wider than f/2.8 may suffer from flatness of field issues, and possibly focus shift when stopping down. It's hard to predict, but it wouldn't surprise me if e.g. Ray's lens exhibits some of that behavior.

The AF aperture is limited at approx. f/2.8, so wider apertures will not contribute to the AF solution, but they may influence the overall optical focus result that the sensor records. Since AF is determined wide open, it may differ from the stopped down focus plane.

Cheers,
Bart

Bart,

Your post brings up another problem with AF fine focus adjustment: many large aperture lenses exhibit focus shift when stopped down due to spherical aberration. This can be pronounced with f/1.2 lenses. The peripheral rays (those farthest from the optical axis) intersect closer to the lens along the optical axis than those closer to the optical axis, and the focus shifts further from the lens as the lens is stopped down. I'm not sure what you mean when you say AF is determined wide open. DigLloyd has a good section on focus shift in his Making Sharp Images (a pay site, but well worth the modest cost). He says that Canon AF sees the image effectively at f/5.6 and does not utilize the peripheral rays. He says Nikon is somewhat better in this regard, but does not specify by how much.

The result of these considerations is that if one uses AF to focus with the Canon on a subject at infinity using the 50 mm f/1.2 lens, the AF is effectively focusing at f/5.6. If one takes the shot at f/1.2 focus moves forward (front focus), and the result is quite blurred. One could use the AF fine adjustment to correct for this, but the correction would be valid only for one aperture. If one adjusts for f/5.6, the image at f/1.2 will be front focused (focused closer than the intended focus point).

He says that the f/1.2 lens is best focused by eye or with live view. If one is focusing by eye with the Canon f/1.4 lens when taking a portrait where the eyes should be in sharp focus and shooting at f/4, the focal point will move backward by an inch or thereabouts, so one could focus on the nose to obtain sharp eyes.

Your comments would be appreciated.

Regards,

Bill
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BartvanderWolf
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« Reply #31 on: September 13, 2013, 01:21:42 PM »
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Bart,

Your post brings up another problem with AF fine focus adjustment: many large aperture lenses exhibit focus shift when stopped down due to spherical aberration. This can be pronounced with f/1.2 lenses. The peripheral rays (those farthest from the optical axis) intersect closer to the lens along the optical axis than those closer to the optical axis, and the focus shifts further from the lens as the lens is stopped down. I'm not sure what you mean when you say AF is determined wide open.

Hi Bill,

What I mean is that the AF determination is done at the widest aperture of the lens, not at the aperture one has dialed in for the actual subsequent exposure.

The AF optics are a compound lens system, sampling the image with different apertures and hence more or less contribution from the more peripheral rays at the widest aperture, but limited to f2.8 maximum. Here's an old post on DPreview, therefore may be dated a bit, but it explains the principle well.

Here is a more recent post from the same poster, where he clearly states that there are basically 2 AF separator lens apertures in play, f2.8 and f5.6. I know him to be a very well informed person, he used to design imaging sytems for the UK military.

Quote
DigLloyd has a good section on focus shift in his Making Sharp Images (a pay site, but well worth the modest cost). He says that Canon AF sees the image effectively at f/5.6 and does not utilize the peripheral rays. He says Nikon is somewhat better in this regard, but does not specify by how much.

Well, that contradicts what the other poster said, f2.8 is most certainly also involved on Canons, and it apparently has been for a long time ... I'm pretty sure that "Its RKM" has a better understanding of the technical principles involved. DigLloyd is known to be a Nikon/Leica/Zeiss lover, so I'm not sure how unbiased his comments are about other brands.

Cheers,
Bart
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PhotoEcosse
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« Reply #32 on: September 13, 2013, 01:33:51 PM »
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It seems that the days are long gone when photography was a visual art, dependent largely upon soul and intuition.

All this technobabble really makes me sad.
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bjanes
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« Reply #33 on: September 13, 2013, 02:24:47 PM »
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Hi Bill,

What I mean is that the AF determination is done at the widest aperture of the lens, not at the aperture one has dialed in for the actual subsequent exposure.

The AF optics are a compound lens system, sampling the image with different apertures and hence more or less contribution from the more peripheral rays at the widest aperture, but limited to f2.8 maximum. Here's an old post on DPreview, therefore may be dated a bit, but it explains the principle well.

Here is a more recent post from the same poster, where he clearly states that there are basically 2 AF separator lens apertures in play, f2.8 and f5.6. I know him to be a very well informed person, he used to design imaging sytems for the UK military.

Well, that contradicts what the other poster said, f2.8 is most certainly also involved on Canons, and it apparently has been for a long time ... I'm pretty sure that "Its RKM" has a better understanding of the technical principles involved. DigLloyd is known to be a Nikon/Leica/Zeiss lover, so I'm not sure how unbiased his comments are about other brands.

Cheers,
Bart

Bart,

Good information, as usual. I looked at the DPReview thread you referenced, and the information does seem valid and the article by Doug Kerr referenced in that thread also mentions that Canon uses two apertures for auto focus. If I understand this correctly, auto focus with his f/1.2 lens would use the f/2.8 set and there would still be some focus shift, but not as severe as would occur at f/5.6. I sent him an e-mail referencing this thread in case he would like to comment.

In any event, these complications should be of interest to readers of this forum.

Best regards,

Bill
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Rhossydd
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« Reply #34 on: September 13, 2013, 04:48:23 PM »
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It seems that the days are long gone when photography was a visual art, dependent largely upon soul and intuition.
All this technobabble really makes me sad.
Photography has always had a technical aspect since photographers had to coat their own plates. There's never between a time when creating high quality photographs didn't need an understanding of the technology involved.

If you'd prefer technology free photography go and post to Instagram.
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Fine_Art
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« Reply #35 on: September 13, 2013, 08:36:14 PM »
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Yes, I could do that. However, I've already had an unsatisfactory experience doing something similar with Canon equipment a few years ago, so I'm a bit reluctant to risk wasting more of my time.

I was very impressed with the sharpness of my Canon EF-S 17-55/F2.8 zoom, until I discovered that it wouldn't accurately autofocus at close distances when set at F2.8 on my new Canon 40D. I sent both lens and body to Canon service under warranty, to get the problem fixed. They were unable to fix it and claimed that both body and lens were within specification.

The Canon 40D didn't have the AF fine-tune feature, so I was stuck. But within a short period, Canon released an upgrade which did boast the AF fine-tune feature, the 15mp 50D. So I bought it, not just for the AF fine-tune, but that was a major attraction.

The irony is, the 50D focused perfectly at maximum aperture with the EF-S 17-55/2.8, without any AF fine-tune adjustment at all. I can only assume that the AF fine-tune has a dual purpose. To compensate for any slight maladjustment of the lens, and/or to compensate for any slight maladjustment of the camera body.

Both lens and body could be within specifications, that is, within the acceptable margin of error, but there could be situations where the errors in both body and lens are at the same end of the allowable spectrum of error, and have an additive effect.

Conversely, there could be other situations where the errors in both the lens and the body are at opposite ends of the spectrum and cancel each other out, creating the false impression that one is lucky to have an ideal camera body and lens without error.

That's my understanding too.

Only regarding error on the mount. Remember that all those pieces of glass have to be set to very fine tolerances all of which have an error. More elements, more errors. If it is a high end lens with fine quality control more elements can be more corrections as mathematically designed. In the real world...

For example I bought an old lens cheap on ebay many years back that is officially a dud. A Minolta 135 2.8 It wont focus at infinity. It is absolutely marvelous for up close work with a depth like a Leica. My guess is the factory made a batch with testing on portrait distance subjects. There was a big oops. I can 'fix' it by moving the group held by a heavy tape inside. I fear I will lose the uniqueness of the output so I keep it for special uses. All my other lenses can take the job at infinity.
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Ray
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« Reply #36 on: September 13, 2013, 10:49:04 PM »
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Only regarding error on the mount.

Why do you presume that, Fine_Art? This is out of my area of expertise, but my general understanding is that the Phase Detection autofocus system does not measure the light at the plane of the sensor, but diverts a portion of the light which has reached the mirror, to separate AF sensors located elsewhere.

I would imagine that the precise position, orientation and distance of those AF sensors within the camera body could affect the accuracy of autofocusing. But I'm just making a reasonable assumption based on very limited knowledge of the process.
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BartvanderWolf
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« Reply #37 on: September 14, 2013, 01:59:26 AM »
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I would imagine that the precise position, orientation and distance of those AF sensors within the camera body could affect the accuracy of autofocusing.

Hi Ray,

That's one of the variables on the camera body side of the equation. The larger variable may well be the (secondary) mirror that deflects the AF portion of the incoming light. That mirror's position is determined by the main mirror, on which it hinges. Displacement of that mirror will lengthen/shorten the optical path to the AF sensor, and thus the apparent focus position for optimal phase contrast.

And then there is also a potential prism or mirror at the bottom of the mirror box which deflects the light further down the mirror box. There might even be some dust there, throwing the signal levels a bit off maximum achievable.

Lots of small variations/tolerances may add up and/or cancel each other out. And on the image side of the lens, small differences make big deviations.

Cheers,
Bart
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Ray
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« Reply #38 on: September 14, 2013, 06:19:29 AM »
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Hi Bart,
Thanks for confirming that. The fact that my Sigma 35mm doesn't require any AF fine-tune adjustment on the D800E, whether used close-up or at infinity, but does require a +20 adjustment on the D7100, implies there's something very different in the internal adjustments of the D7100 body, compared with the D800E

If the D7100 were to require such extensive adjustments with my other Nikkor lenses, then that would imply that the D7100 body were at fault, or outside the tolerance range. But the fact is, the other lenses I use, such as the Nikkor AF-S 80-400/F5.6 and the 24-120/F4 require very little adjustment on the D7100.

For example, I've got the AF fine-tune on the D7100 set to +5 for the the 80-400. On the D800E the 80-400 doesn't appear to require any AF fine-tune at all, at least at 400mm.

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alain
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« Reply #39 on: September 15, 2013, 05:28:18 AM »
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this takes very little time ... maybe 20 minutes?  even a complex system like a tech camera which requires manually calibrating each lens to the body/back combination takes about 15 minutes per lens.  This is once for each camera lens combination... certainly a pretty insignificant amount of time to invest in quality for something you will most likely use for quite some time (often years).  I don't think it will infringe on much shooting or family time.

And if you don't want to take the time, many good camera stores offer it as a service.  At my store, we will do it for you for $45 for the first lens, $25 for each lens done at the same time.  Just drop it off and pick it all up the next day.

Which technique do you advise to do the calibration?

Do you test at two distances?
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