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Author Topic: Front/Back Focus Test Result  (Read 7116 times)
xpatUSA
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« on: September 05, 2013, 08:20:49 PM »
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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.

I've edited the subsequent content out of this original post due to the peculiar response below. The content was apparently too technical [for kaelaria, see below] and not "real world" enough. Perhaps a moderator would kindly delete this whole topic.

Sorry for any inconvenience to more normal members. The original post showed a ruler shot and basically asked if it was acceptable but I showed too much information about ruler angle, divisions, and more.

Since then, I've found that test results can vary with the direction of the wind, can be inconsistent, etc. And that some folks care about front/back focus and some do not. And that the scene's subject matter matter.

The reason for the test was that a camera I bought seemed to be a bit soft - this from a camera with no AA filter and with both a kit zoom and with a good macro prime on it.
« Last Edit: September 11, 2013, 10:30:05 AM by xpatUSA » Logged

best regards,

Ted
kaelaria
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« Reply #1 on: September 05, 2013, 11:59:06 PM »
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If you never noticed it before doing the test - then it's perfectly fine.  If you only think you have a problem because you are data peeping then just stop ding that.  If you saw focus issues before hand, how does the data correlate to what you were experiencing?  Too many people get wrapped up in numbers with no real world relevance.
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xpatUSA
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« Reply #2 on: September 06, 2013, 12:39:08 AM »
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If you never noticed it before doing the test - then it's perfectly fine.  If you only think you have a problem because you are data peeping then just stop ding that.  If you saw focus issues before hand, how does the data correlate to what you were experiencing?  Too many people get wrapped up in numbers with no real world relevance.

I live in hope that this aggressive response is not typical of this forum.
« Last Edit: September 06, 2013, 12:46:55 AM by xpatUSA » Logged

best regards,

Ted
kaelaria
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« Reply #3 on: September 06, 2013, 12:43:35 AM »
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Truth is not aggressive.
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xpatUSA
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« Reply #4 on: September 06, 2013, 12:48:28 AM »
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Truth is not aggressive.

How cute.
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best regards,

Ted
Wayne Fox
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« Reply #5 on: September 10, 2013, 12:27:51 AM »
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If you never noticed it before doing the test - then it's perfectly fine.  If you only think you have a problem because you are data peeping then just stop ding that.  If you saw focus issues before hand, how does the data correlate to what you were experiencing?  Too many people get wrapped up in numbers with no real world relevance.
and how are you supposed to know if you have a problem if you don't peep?    I'm not sure what the original post was, but the fact is almost every dSLR has a back or front focus issue, there are just so many variables and tolerances are tight, and autofocus has a pretty large "tolerance" which often isn't that great.  A photographer who fails to recognize this and calibrate their equipment may be very disappointed when they suddenly find out that awesome image they want to print large won't hold up or when then need to do some aggressive cropping.

We calibrate cameras almost every day, and only about 5% are in focus with no adjustment.  You can only tell it if you "peep". To me what some label "peeping" is just standard practice in a quality workflow. In my workflow, every image at some point will be examined at 100% early to insure it will hold up to printing and often later as I fine tune creative sharpening. 

I understand the entire discussion about technical vs creative, but mastering the technical is an important part of not letting it get in your way when creating.  I agree too many get wrapped up in "numbers" (DxO comes to mind), but to me fine tuning your gear is important, and peeping is how you do that.
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stamper
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« Reply #6 on: September 10, 2013, 02:43:15 AM »
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I live in hope that this aggressive response is not typical of this forum.

If you think that is aggressive then you are in for a shock. If that is the worst that has happened to you then you are lucky.  Cheesy
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Ray
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« Reply #7 on: September 10, 2013, 03:33:49 AM »
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I get the impression this issue is a minefield. One carefully calibrates the autofocussing by shooting a nearby object like a slanted ruler, only to find that focussing at infinity is now worse. And this is for a fixed focal length lens.

With zooms, one may not only get a variance of front and back focussing depending on focussing distance, but also depending on the focal length setting.

Add to this the fact that autofocussing precision, whether or not the AF Fine Tuning has been used, is rarely exact, then that old advice of "F8 and be there" still holds.
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Rhossydd
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« Reply #8 on: September 10, 2013, 03:58:27 AM »
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I get the impression this issue is a minefield.
That's true to an extent. However automated programs like Focal Pro make investigation of the issues involved a pretty straight forward, once you know what all the actual errors are you can decide what, if any, changes to make.
Whilst it can be subtle, it can also make the difference between 'just about acceptable' and 'good'. I'm happy I've done the work to make focusing 'good' more often.

Quote
"F8 and be there"
Fine if you want everything at f8 and have enough light for it.
The other side of testing is that you might find that f8 isn't the best stop to use anyway. A lot of my lenses perform better at wider apertures than f8, testing has ensured that I nail the focus more reliably at those wider apertures too.

Don't knock technologies like AFMA until you've exploited them. It can all be part of the small differences that separate the ordinary from the excellent.
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BartvanderWolf
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« Reply #9 on: September 10, 2013, 04:26:33 AM »
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Don't knock technologies like AFMA until you've exploited them. It can all be part of the small differences that separate the ordinary from the excellent.

Hi,

I agree. Not only do my lenses improve their focus acuuracy over the entire focus range, but I also get the impression that focusing is faster. One typically calibrates for a commonly used distance anyway, so even if the lens requires different settings for different distances, chance has it that the adjustments are not too different from the most common case.

Zoom lenses may require a compromise, but even there one can usually find an average adjustment that improves focusing across the range. Even with lenses that exhibit focus shift when changing the aperture, one can often find an improvement around the more commonly used aperture setting, an adjustment which will differ from the wide open AF optimum.

Cheers,
Bart
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PhotoEcosse
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« Reply #10 on: September 10, 2013, 04:43:08 AM »
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and how are you supposed to know if you have a problem if you don't peep?  

What a strange point of view. (in a very strange thread)

To my way of thinking, photography is a visual art - so any "fault" that cannot be visually discerned in the normal output is, from any practical point of view, totally irrelevant.

Now, of course, the key words in that statement are "normal output" and that may be different for different photographers. The output of a landscape photographer is different from that of a portrait photographer or a wildlife photographer or a product photographer or a macro photographer...etc., etc. So a fault that may matter to one may be completely inconsequential to another.

But to go looking beyond your own normal output for faults, real or imaginary, is a pretty pointless exercise.

So, I don't regard the second post on this thread as in any way "aggressive" but, rather, as a perfectly reasonable response to the problem (or non-problem) of the OP.
« Last Edit: September 10, 2013, 04:45:00 AM by PhotoEcosse » Logged

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Rhossydd
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« Reply #11 on: September 10, 2013, 04:53:55 AM »
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any "fault" that cannot be visually discerned in the normal output is, from any practical point of view, totally irrelevant
What's 'normal output' ?
I think a lot of people can be aware something is wrong with an image without quite knowing what the problem is. Looking closely often reveals the problem, if you can then fix it and things improve, great.

If pictures aren't as sharp as you'd like, investigate.
Was the best focus where you'd expected ? If not, was it user error or technical fault ? If it was a technical fault can I improve it ?
Would it be sharper if you'd used a different aperture ?
Is the lens just not very good ?

There's nothing wrong with trying to get the best out of kit, it would be crazy not to.
Some people do obsess about it, but most of us test once and get on with shooting the best pictures our kit will allow. If you're happy with sub-optimal results, that's up to you, but don't complain when some people strive for excellence.
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Wayne Fox
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« Reply #12 on: September 10, 2013, 02:27:25 PM »
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What a strange point of view. (in a very strange thread)

To my way of thinking, photography is a visual art - so any "fault" that cannot be visually discerned in the normal output is, from any practical point of view, totally irrelevant.

Now, of course, the key words in that statement are "normal output" and that may be different for different photographers. The output of a landscape photographer is different from that of a portrait photographer or a wildlife photographer or a product photographer or a macro photographer...etc., etc. So a fault that may matter to one may be completely inconsequential to another.

But to go looking beyond your own normal output for faults, real or imaginary, is a pretty pointless exercise.

So, I don't regard the second post on this thread as in any way "aggressive" but, rather, as a perfectly reasonable response to the problem (or non-problem) of the OP.

I guess we disagree, because I think your response is  odd . Making sure your equipment is performing at it's optimum before using it is pretty standard practice for anyone seeking maximum quality. Many of us test a lens before we buy it, or shoot with a system before we invest in it - a common practice for decades (nothing to do with digital)l. I completely agree that many get way too wrapped up in the technical, but this issue doesn't really belong in that discussion.  

Very few photographers, especially those who enjoy being creative and shooting all types of photography, have a predetermined "normal".  I hear this all the time, and sorry, just don't buy it. It's the old "if your really an artist you can't let the technical get in your way".   I sit here watching customers come in all day (some of them pros and semi pros) asking me to get more out of an image than I can because they didn't use a tripod or used a poor lens - or it is just not quite as sharp as it could have been.  It's so easy to calibrate your system, why wouldn't anyone not want to do it?  Once you do it, your done till you buy a new body or lens.


I get the impression this issue is a minefield. One carefully calibrates the autofocussing by shooting a nearby object like a slanted ruler, only to find that focussing at infinity is now worse. And this is for a fixed focal length lens.

With zooms, one may not only get a variance of front and back focussing depending on focussing distance, but also depending on the focal length setting.

Add to this the fact that autofocussing precision, whether or not the AF Fine Tuning has been used, is rarely exact, then that old advice of "F8 and be there" still holds.

logic is sound, and yes even this isn't perfect but what you are compensating for the the autofocus precision issue you mentioned ... the relationship of what the focusing sensors think is sharp vs what is sharp on the sensor itself.    No matter how tight you try and manufacture the tolerances, this is just asking too much to get the distances of the two paths exactly the same, and to be out of focus doesn't take much.   So the offset compensates for this issue on a camera by camera/lens by lens basis.

Regarding zooms, yes still an issue, although if you calibrate a zoom you will see improvements in the entire range. the 5D 3 now has the ability to correct both ends of the zoom range.  For those needing perfect focus, live view is still by far the best option.
« Last Edit: September 24, 2013, 05:16:12 PM by Wayne Fox » Logged

Ray
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« Reply #13 on: September 10, 2013, 11:10:29 PM »
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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.
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Rhossydd
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« Reply #14 on: September 11, 2013, 03:31:42 AM »
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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.
Yes, from your description you should have just returned it as faulty.
This is one advantage of being able to test and quantify problems, retailers won't argue if you can prove faults with kit. You should never have to live with kit that not fit for purpose.

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hjulenissen
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« Reply #15 on: September 11, 2013, 05:14:02 AM »
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Measuring something that can otherwise _never_ be detected in "real-world" use may be counter-productive.

Measuring something that can otherwise only be detected once every 2 years, when you decide to print that special image big, sounds like a sensible thing to do.

If you do not know beforehand how big you are going to print (or how much cropping, shadow push etc), then it seems reasonable to do the appropriate measurements (or read about other peoples measurements) and apply some margin. The upside is that your images may have some (potentially very small) improvement in quality. The downside is that you have to spend time and resources that could otherwise be spent taking images or with your family. Choose accordingly.

-h
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ErikKaffehr
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« Reply #16 on: September 11, 2013, 07:27:11 AM »
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Hi,

Than you get a new camera that is propely adjusted, and now you realize that all of your images of yore are subpar...

Best regards
Erik


What a strange point of view. (in a very strange thread)

To my way of thinking, photography is a visual art - so any "fault" that cannot be visually discerned in the normal output is, from any practical point of view, totally irrelevant.

Now, of course, the key words in that statement are "normal output" and that may be different for different photographers. The output of a landscape photographer is different from that of a portrait photographer or a wildlife photographer or a product photographer or a macro photographer...etc., etc. So a fault that may matter to one may be completely inconsequential to another.

But to go looking beyond your own normal output for faults, real or imaginary, is a pretty pointless exercise.

So, I don't regard the second post on this thread as in any way "aggressive" but, rather, as a perfectly reasonable response to the problem (or non-problem) of the OP.
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Ray
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« Reply #17 on: September 11, 2013, 09:43:37 AM »
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Yes, from your description you should have just returned it as faulty.
This is one advantage of being able to test and quantify problems, retailers won't argue if you can prove faults with kit. You should never have to live with kit that not fit for purpose.


I left it too late. I assumed I could fix any focusing problems with AF fine-tune, and in a sense I can, provided I take the time to readjust the fine-tune setting according to the conditions when using the D7100. At least the maximum setting of +20 on the D7100 is sufficient to get a sharp image at infinity.

Another issue which muddies the waters, is determining whether it's actually the camera body that's at fault, rather than the lens. This Sigma lens doesn't require any AF fine-tune on my D800E, although there are still issues with the lens on the D800E, such as uneven sharpness in the four corners at wide apertures.

However, the one solid improvement of this Sigma prime compared with my only other Nikkor lens of the same focal length (the 24-120 zoom set at 35mm), is the edge and corner sharpness at apertures between F5.6 and F11. It really is noticeably sharper in the corners than the zoom is at those f/stops, on the D800E. That's worth something.
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xpatUSA
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« Reply #18 on: September 11, 2013, 10:33:16 AM »
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Hello all,

I'm the perp of the original post.

Since this thread seems to be ongoing, I edited the OP again and put some explanation so as to make the thread a little less "strange".

Again, apologies for any confusion . . .

P.S. I've just noticed at bottom left of the page "remove topic" which I could have used when my contretemps with kaelaria got personal. Can Original Posters really wipe out an entire thread?

« Last Edit: September 11, 2013, 10:37:09 AM by xpatUSA » Logged

best regards,

Ted
Wayne Fox
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« Reply #19 on: September 11, 2013, 09:23:19 PM »
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The downside is that you have to spend time and resources that could otherwise be spent taking images or with your family.

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.
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