-
Welcome to the new forums! Please read this first. For known issues we are working to resolve, click here.
D80 Backfocus
- Thread starter PhotoLeon1935
- Start date
Back focus really means that your camrea will focus beyond your set target points.
A very simple way of saying. You have two trees. "A" tree is closer to you and "B" tree is behind "A" tree. When you point your focus point to "A" tree and lock focus. But the result is that the "B" tree is sharp focus then "A" Tree
That means you have a back focus issue.
Did i confuse you?
You are seeing from my eyes!!!
Derek....
Nikon D80, MDD-80
Olympus SP350 w/0.7x / FL-40 Flash Gun
Panasonic FZ 2 w/Wcon~07, FX-7
Sony DSC_P1~717
Fujifilm F601
A very simple way of saying. You have two trees. "A" tree is closer to you and "B" tree is behind "A" tree. When you point your focus point to "A" tree and lock focus. But the result is that the "B" tree is sharp focus then "A" Tree
That means you have a back focus issue.
Did i confuse you?
--
You are seeing from my eyes!!!
Derek....
Nikon D80, MDD-80
Olympus SP350 w/0.7x / FL-40 Flash Gun
Panasonic FZ 2 w/Wcon~07, FX-7
Sony DSC_P1~717
Fujifilm F601
Confirmed that the second D80 I got also backfocuses slightly. The image below consists of 100% center crops from two lenses.
Top row shows the results from the 18-135 lens. Left most square is on autofocus. Each square to the right is a crop from image taken when I moved the camera away from the target about 1/4 inch and shot without refocusing.
Middle row is the same test with the 18-200 VR at 135mm.
Third row is at 200mm
The fourth square turned out the sharpest as shown by the highest standard deviation measured in the histogram in photoshop. It is also visually the sharpest. The results are consistent b/w the two lenses.
The right-most column shows the comparison in sharpness at three other focal lengths, the top is the 18-135. Again it shows the standard deviation - the hgher the better. This column is irrelevant to this test but shows how the two lenses compare in terms of center sharpness at these other focal lengths. Basically, the 18-135 is sharper at wide and telephoto and the 18-200 is sharper at mid telephoto (50-70mm measured only).
I can fix this backfocus, but I am probably not going to do it today as I might need to exchange this D80 - its 4-way pad is not working well (too easy to press in two directions, fine in the other two resulting in erratic behavior unless pressed very carefully, for which I have no time when in a rush to change settings).
Top row shows the results from the 18-135 lens. Left most square is on autofocus. Each square to the right is a crop from image taken when I moved the camera away from the target about 1/4 inch and shot without refocusing.
Middle row is the same test with the 18-200 VR at 135mm.
Third row is at 200mm
The fourth square turned out the sharpest as shown by the highest standard deviation measured in the histogram in photoshop. It is also visually the sharpest. The results are consistent b/w the two lenses.
The right-most column shows the comparison in sharpness at three other focal lengths, the top is the 18-135. Again it shows the standard deviation - the hgher the better. This column is irrelevant to this test but shows how the two lenses compare in terms of center sharpness at these other focal lengths. Basically, the 18-135 is sharper at wide and telephoto and the 18-200 is sharper at mid telephoto (50-70mm measured only).
I can fix this backfocus, but I am probably not going to do it today as I might need to exchange this D80 - its 4-way pad is not working well (too easy to press in two directions, fine in the other two resulting in erratic behavior unless pressed very carefully, for which I have no time when in a rush to change settings).
Yes!
Once you know what to do, this is no more difficult than changing a lens. You just need to be VERY careful not to touch the mirror or sensor at all. Plus, if you remember the initial position of the hex screw, it is fully reversible in case you do not like the results.
larrytusaz
Senior Member
Frankly, if you ask me, "backfocus" is people thinking that their cameras can't focus, never mind that we've had autofocus for over 20 years in zillions of cameras of all price levels.
Sorry, I'm with Phil Askey--when he's tested cameras for backfocus, typically he's found that there is no issue with any models. Yet, it always happens--people swear that their cameras can't focus properly, that their camera has an issue. Sorry, I don't believe any of it.
---
LRH
http://www.pbase.com/larrytucaz
http://larrytxeast.smugmug.com/
PhotoLeon1935
Senior Member
How do you get a standard deviation from the histogram in Photoshop?
Do you have to crop out a tiny sample of the bars?
Do you do external calculations?
I never heard that Photoshop could do statistical analysis to measure sharpness.
The right-most column shows the comparison in sharpness at three
--
http://www.leongoodman.com/d70focusnew.html
http://www.pbase.com/photoleon
http://www.leongoodman.com
I used the ideas that Bill outlines in his post:
http://www.nikonians.org/dcforum/DCForumID86/15493.html
That refers to this technique for measuring sharpness:
http://home.comcast.net/~TomatoMan/Mirror/Cached/86_7071.htm
See also his web site for more info: http://home.comcast.net/~NikonD70/
To display the Histogram, I used Photoshop Elements 4, under the "Window" menu there is "Histogram". When you make a selection on the image, the histogram is displayed and one of the numbers under it is the standard deviation. If all is fuzzy, there is not much deviation from the mean, if things are sharp, there is more deviation, so it is a relative measure, as long as you can keep the selection constant approximately. I used your center bar and highlighted a 50x50 pixel selection of the second to the last finest set as displayed (it covers about half of it at my magnification). My measurements are not very precise as they vary by as much as 1.5 if I move the selection a little, but they are about average and relatively speaking it is easy to see which square is the sharpest since the difference b/w two adjacent squares was more than my typical error due to selection inconsistencies. Plus, I could see the same visually...
From Bill's explanation:
"Keeping lighting constant take a series of images of the test chart.
Vary only one parameter in your experiment.
I varied the distance from the film plane of the camera to the target.
In PhotoShop (or your program of choice) measure the standard deviation of the luminance channel of a selected area that is consistent from image to image.
The ideal selection has about equal amounts of black and white.
I chose a set of resolution bars on my test chart."
http://www.nikonians.org/dcforum/DCForumID86/15493.html
That refers to this technique for measuring sharpness:
http://home.comcast.net/~TomatoMan/Mirror/Cached/86_7071.htm
See also his web site for more info: http://home.comcast.net/~NikonD70/
To display the Histogram, I used Photoshop Elements 4, under the "Window" menu there is "Histogram". When you make a selection on the image, the histogram is displayed and one of the numbers under it is the standard deviation. If all is fuzzy, there is not much deviation from the mean, if things are sharp, there is more deviation, so it is a relative measure, as long as you can keep the selection constant approximately. I used your center bar and highlighted a 50x50 pixel selection of the second to the last finest set as displayed (it covers about half of it at my magnification). My measurements are not very precise as they vary by as much as 1.5 if I move the selection a little, but they are about average and relatively speaking it is easy to see which square is the sharpest since the difference b/w two adjacent squares was more than my typical error due to selection inconsistencies. Plus, I could see the same visually...
From Bill's explanation:
"Keeping lighting constant take a series of images of the test chart.
Vary only one parameter in your experiment.
I varied the distance from the film plane of the camera to the target.
In PhotoShop (or your program of choice) measure the standard deviation of the luminance channel of a selected area that is consistent from image to image.
The ideal selection has about equal amounts of black and white.
I chose a set of resolution bars on my test chart."
I could not resist and adjusted the focus this morning. Now both lenses as well as the 50mm AF-D F/1.4 focus better with my first or second shot being the sharpest compared to the fourth - so I am within less than 1/8 of an inch off compared to as much as 1/2 inch before (at the same distance of about 3 feet at 135mm).
You can read the rest of the thread for an explanation of my subject.
Here, suffice it to say that if one understands the workings of autofocus and ways of testing it, one can tell whether the problem is there or not.
Regards,
Kocho
Here, suffice it to say that if one understands the workings of autofocus and ways of testing it, one can tell whether the problem is there or not.
Regards,
Kocho
Frankly, if you ask me, "backfocus" is people thinking that their
cameras can't focus, never mind that we've had autofocus for over
20 years in zillions of cameras of all price levels.
Sorry, I'm with Phil Askey--when he's tested cameras for backfocus,
typically he's found that there is no issue with any models. Yet,
it always happens--people swear that their cameras can't focus
properly, that their camera has an issue. Sorry, I don't believe
any of it.
---
LRH
http://www.pbase.com/larrytucaz
http://larrytxeast.smugmug.com/
iseeu
Leading Member
Please your help Leon. Thanks in advance.
17-55mm at 50mm f5.6
35mm 2.0 at f5.6
50mm 1.4 at f5.6
17-55mm at 50mm f5.6
35mm 2.0 at f5.6
50mm 1.4 at f5.6
A little hard to tell from this size. Can you crop the center line with a little from the side and re-post at a larger size? Also, if you do the test at F/2.8 it may become more obvious.
I used F/5.6 and 6/3 on my longer lenses as they can't go to 2.8. But with the 50mm I tested at 2.0 and it is much more obvious when the focus is off with it.
Again, the 45 degree test is fine many times and especially for telephoto lenses. But until I do a parallel target test, I'm somehow not 100% satisfied by the validity of the results.
One reason for the 45 degree not being perfectly adequate is that most lenses have focus field curvature. I have observed this with many lenses - the center where it is in focus is not at the same distance to the camera as are the side areas that are also in focus. The focus field is not flat but like a part of a sphere...
This means that even if they are in focus in the center, the side lines of this chart may not show this properly as the field curves! They may show back or front focus while in fact you have good focus in the center (or the other way around). Soft corners are most often the result of such curvature.
You can cheat by focusing in the middle as you do per the instructions, then slide the target left or right without changing its distance to the camera, so that the intersection of the center line and the side line is in the middle of the frame. Or instead, put a ruller on top of the target in the middle after you focus. Then shoot without refocusing. This would eliminate the focus curvature error from the picture and you would see the DOF position more clearly.
I used F/5.6 and 6/3 on my longer lenses as they can't go to 2.8. But with the 50mm I tested at 2.0 and it is much more obvious when the focus is off with it.
Again, the 45 degree test is fine many times and especially for telephoto lenses. But until I do a parallel target test, I'm somehow not 100% satisfied by the validity of the results.
One reason for the 45 degree not being perfectly adequate is that most lenses have focus field curvature. I have observed this with many lenses - the center where it is in focus is not at the same distance to the camera as are the side areas that are also in focus. The focus field is not flat but like a part of a sphere...
This means that even if they are in focus in the center, the side lines of this chart may not show this properly as the field curves! They may show back or front focus while in fact you have good focus in the center (or the other way around). Soft corners are most often the result of such curvature.
You can cheat by focusing in the middle as you do per the instructions, then slide the target left or right without changing its distance to the camera, so that the intersection of the center line and the side line is in the middle of the frame. Or instead, put a ruller on top of the target in the middle after you focus. Then shoot without refocusing. This would eliminate the focus curvature error from the picture and you would see the DOF position more clearly.
iseeu
Leading Member
Kocho, thanks for your instructions I will try and post again with wider aperture and crop a bit more. Thanks again.
iseeu
Leading Member
Here is few more samples at 2.8:
17-55mm f2.8
35mm f2.8
50mm f2.0
17-55mm f2.8
35mm f2.8
50mm f2.0
iseeu
Leading Member
How come the 50mm came out so bad also the purple thing at the bottom, is your 50mm do the same thing? Thanks in advacne.
Yours are way overexposed with the 50mm at F/2. The 50mm is known to have CA and low contrast near wide open and I have seem similar behavior as your test. This is a difficult shot with high contrast for the relatively old design without ED glass...
I only did a parallel test with mine so I could not see what you see. I will try mine when I get home to compare.
PhotoLeon1935
Senior Member
Standard deviation is only available with Elements 3, 5 and 5, not with PSCS2. What a shame. I am going to develop a new target better suited for this method of detecting sharpness.
I hope you will agree to help me test it. I am going to create an new pattern that extends from top to bottom which will allow sliding a selection window up and down to find the perfect focus.
Email me so we can correspond off line.
[email protected]
--
http://www.leongoodman.com/d70focusnew.html
http://www.pbase.com/photoleon
http://www.leongoodman.com
I hope you will agree to help me test it. I am going to create an new pattern that extends from top to bottom which will allow sliding a selection window up and down to find the perfect focus.
Email me so we can correspond off line.
[email protected]
--
http://www.leongoodman.com/d70focusnew.html
http://www.pbase.com/photoleon
http://www.leongoodman.com
PhotoLeon1935
Senior Member
Looking forward at Back Focus –NEW
After publishing targets and information on Back Focus issues I kept running into people who, no matter how careful and meticulous they were kept getting unexplainable problems in focus.
Phil Askey says that back focus is way overblown. Cameras come out of the factory with reasonably correct focus. I agree with him. The methods used by Nikon and other manufacturers are able to let the computer on the factory test bench adjust the focus and the mirror stops are set at an arbitrary position to allow minor adjustments in the field if needed later.
However all test targets used by the consumer including my own are flawed. They do not account for the myriad of flaws, compromises and various astigmatisms that are in the lenses sold in today’s market. A lens that starts at 18 mm and goes to 70, 135 and 200 mm was unheard of just a few years ago. In designing this new generation of compact, wide lenses there had to be some shortcomings to accommodate the problems of “wide”.
A new form of astigmatism was created that as far as I know has not even been given a name yet. We know about cylinder astigmatism if we wear eyeglasses. This new astigmatism appears in different areas of the image and maintains geometry at the expense of focus.
I have created a new focus target and method which will allow you to see what is happening. It uses a new and accurate method to apply the statistical tools built into PhotoShop to measure Standard Deviation to an accuracy of three significant digits. There is now no guessing at what is sharp and what is not. You can see the numbers.
If you go to my site you will learn:
1) How the DSLR phase based focus works.
2) How to absolutely identify the sharpest point in an image
3) How to identify the flaws in a lens and map them
4) How tell whether the problem is with the camera or the lens
If you go to this site you will be there for a while. Don’t forget to leave a comment or the thread will roll off page one and I will have to shamelessly bump it. Sorry, Phil,
Go now to:
http://www.leongoodman.com/d70focuspart5.html
--
http://www.leongoodman.com/d70focusnew.html
http://www.pbase.com/photoleon
http://www.leongoodman.com
After publishing targets and information on Back Focus issues I kept running into people who, no matter how careful and meticulous they were kept getting unexplainable problems in focus.
Phil Askey says that back focus is way overblown. Cameras come out of the factory with reasonably correct focus. I agree with him. The methods used by Nikon and other manufacturers are able to let the computer on the factory test bench adjust the focus and the mirror stops are set at an arbitrary position to allow minor adjustments in the field if needed later.
However all test targets used by the consumer including my own are flawed. They do not account for the myriad of flaws, compromises and various astigmatisms that are in the lenses sold in today’s market. A lens that starts at 18 mm and goes to 70, 135 and 200 mm was unheard of just a few years ago. In designing this new generation of compact, wide lenses there had to be some shortcomings to accommodate the problems of “wide”.
A new form of astigmatism was created that as far as I know has not even been given a name yet. We know about cylinder astigmatism if we wear eyeglasses. This new astigmatism appears in different areas of the image and maintains geometry at the expense of focus.
I have created a new focus target and method which will allow you to see what is happening. It uses a new and accurate method to apply the statistical tools built into PhotoShop to measure Standard Deviation to an accuracy of three significant digits. There is now no guessing at what is sharp and what is not. You can see the numbers.
If you go to my site you will learn:
1) How the DSLR phase based focus works.
2) How to absolutely identify the sharpest point in an image
3) How to identify the flaws in a lens and map them
4) How tell whether the problem is with the camera or the lens
If you go to this site you will be there for a while. Don’t forget to leave a comment or the thread will roll off page one and I will have to shamelessly bump it. Sorry, Phil,
Go now to:
http://www.leongoodman.com/d70focuspart5.html
--
http://www.leongoodman.com/d70focusnew.html
http://www.pbase.com/photoleon
http://www.leongoodman.com
Similar threads
Keyboard shortcuts
- f
- Forum