Backfocus / Frontfocus info needed

09/12/2011 12:03:38 PM · #1

Hello,

I've been testing my 80-200 ED Nikkor for a few days. It is focusing behind my focus point in manual and auto focus. However, when looking through the viewfinder, my focus point looks "in focus".

Is this how back/front focus works? (Looks good to my eye but misses focus on caputre?

Yo-spiff gave me a good repair center but want to make sure about my diagnosis before sending.

Thanks,

EDIT: here are some 100% crops unedited along with a 20% view from photoshop. Can you tell me if these are acceptable as far as focus/sharpness go?
These were shot at f2.8 fully zoomed to 200mm on a tripod.

They may have lost a SLIGHT bit of focus saving to web but I really had to compare closely...so the 100% crops are pretty much what I see in photoshop.

Thanks

KS

Message edited by author 2011-09-12 20:35:18.

09/12/2011 08:36:54 PM · #2

Bump - I added test shots. I really don't want to send this lens in if I don't have too.

On another note. When I go the to minimum focus distance with the lens and zoom to 200mm, I really have trouble getting test subjects like coins and nails in focus.

Any advice?

09/12/2011 08:51:15 PM · #3

My first explanation wasn't too good-
Backfocus typically refers to your AF having backfocus, meaning it is autofocusing to someplace slightly behind your subject. You can also get backfocus from improper alignment of your sensor, but if you're only experiencing it on this one lens, it's more likely to be in the AF. Keep in mind all lenses have variation in terms of where they focus, and the same is true of bodies. As a result, some combinations of lenses and bodies do not work very well together. It does happen. One of the big rental places had a really good discussion of this but I don't remember where.
Ensure your diopter is set correctly. Also the D700 does feature microadjustment of lenses. Refer to your manual for this. To evaluate your lens, I find this focus chart useful.
Also, remember, shooting 2.8 and 200 is going to have the shallowest depth of field and be the most unforgiving you can get. Maybe your eye is off?

I looked through your shots, and most are not good indicators of front/backfocus, just missed focus, for whatever reason. There are many causes that can cause this. A better depiction would be a scene with numerous objects at varying distance from you with one object that you specifically try to focus upon. Also, the test chart I provided would show things better.
ETA: Clarity.

Message edited by author 2011-09-12 21:02:47.

09/12/2011 09:09:08 PM · #4

Thanks ! So by looking at my test shots, you would say "it's me"? That would be welcome news!

I'll check the chart out.

Thanks for helping me.

Originally posted by spiritualspatula:

09/12/2011 09:23:39 PM · #5

It's difficult to say if your camera was actually focusing on the wire or the post because of their proximity and you didn't note which specifically you focused on. Depending upon how your camera is evaluating things and where your focal point was, it may or may not be the lens, it could just be you getting accustomed to the DoF and how it evaluates the scene. This may not have been an issue up until now because it was never so unforgiving.
That blog has a ton of info on AF though and in general how it works. Also, we can't be sure because you didn't list whether this was on a tripod or what shutter speed you were using, which can cause soft shots separate from backfocus issues. The most common problem with focusing is user error, so I suggest that as a first cause.
As I said, however, you can micro-adjust your focus for individual lenses on the D700 somewhat, and it's a pretty easy process so just check your manual in regards to that. There is an off chance that that lens hypothetically backfocuses somewhat and your body hypothetically frontfocuses somewhat and the two errors together lead to incompatibility. As I said, there's a large rental place that started realizing this after running through a ton of bodies/lens copies, but evaluate the other two avenues first.

ETA question: When you say you get it with manual focus, are you using your lens on a tripod? Minor changes to distance can definitely throw your focus off when you're shooting 200 @ 2.8

Message edited by author 2011-09-12 21:24:52.

09/12/2011 10:03:22 PM · #6

Thanks...I'm checking out the site you posted as well as others.

I'll give you more info when I complete some tests!

Thanks again

Originally posted by spiritualspatula:

It depends-
What I mean is that it's hard to tell from your test shots. Scenes that have a more clearly defined subject with areas to see distance are the most effective to evaluate this. Also, you'll have to note for us where you were focusing.
Take this shot-
It's difficult to say if your camera was actually focusing on the wire or the post because of their proximity and you didn't note which specifically you focused on. Depending upon how your camera is evaluating things and where your focal point was, it may or may not be the lens, it could just be you getting accustomed to the DoF and how it evaluates the scene. This may not have been an issue up until now because it was never so unforgiving.
That blog has a ton of info on AF though and in general how it works. Also, we can't be sure because you didn't list whether this was on a tripod or what shutter speed you were using, which can cause soft shots separate from backfocus issues. The most common problem with focusing is user error, so I suggest that as a first cause.
As I said, however, you can micro-adjust your focus for individual lenses on the D700 somewhat, and it's a pretty easy process so just check your manual in regards to that. There is an off chance that that lens hypothetically backfocuses somewhat and your body hypothetically frontfocuses somewhat and the two errors together lead to incompatibility. As I said, there's a large rental place that started realizing this after running through a ton of bodies/lens copies, but evaluate the other two avenues first.

ETA question: When you say you get it with manual focus, are you using your lens on a tripod? Minor changes to distance can definitely throw your focus off when you're shooting 200 @ 2.8

Samsung Galaxy S24, S24+, S24 Ultra

09/12/2011 11:20:34 PM · #7

Ok...here is an easy chart for the test. I followed instructions on building the chart and the tripod...setup...etc...

Both were at 2.8 1/400 4000iso. The bottom is manual focus. The top is auto.

The thick line area should be the clearest. If you get good focus behind or in front of the line without the line being in focus then you know what that means !

This being on f2.8 at 200mm at about 4 feet from the target leads me to believe that my lens in NOT back or front focusing...at least not bad.

It looks like I'm ok. What do you think?

Edit: I'll do the chart that spatula posted tomorrow night. They are basically the same...it is a 45 degree angle test.

Message edited by author 2011-09-12 23:38:44.

Bear_Music

Sony 24-70mm F4 Vario-Tessar T* FE OSS

09/12/2011 11:28:35 PM · #8

Yes, you seem to be well within acceptable limits :-)

R.

09/12/2011 11:31:46 PM · #9

Thanks for looking bear...I have another question.

Notice on the test chart where I actually focus. It is the top part that says "focus panel". You are instructed to focus on the text in that panel. Then you check your focus by the "lines". However, notice that the "text" on the panel is blurry. So am I missing something? What if I want that text to come out in focus? How would I focus?

Originally posted by Bear_Music:

Yes, you seem to be well within acceptable limits :-)

R.

09/13/2011 01:11:25 AM · #10

Looks fine to me, as well. It looks like your AF version is sharper, to me, but it also looks like the exposure is a bit different on the images.
Keep in mind, you're losing some resolution shooting at 4,000 iso (yeah yeah it's a D700 but still).
My first guess would be that the paper was not perfectly parallel to your sensor, since the bottom of the paper is in better focus than the top. This is normal and is simply an example of DoF in action when you have a very thin DoF. That's what I meant by it being very unforgiving. I did some band photography for a friend of mine and was forced to shoot at 1.4 with my 30 due to low light, and there were shots that looked good initially and weren't once I looked at them closer because of how relentless the DoF was and due to movement on the subject and my behalf.

bohemka

Sigma 12-24mm f/4.5-5.6 EX Aspherical DG HSM for Nikon

09/13/2011 04:36:52 AM · #11

That test isn't going to tell you much unless you use a tripod. As Mr. Spatula has said, the DoF is relatively unforgiving at that aperture.

HarveyG

Nikon D800

Sigma 120-300mm f/2.8 EX APO IF HSM for Nikon

09/13/2011 05:52:01 AM · #12

All good advice.
In your D700 Manual p347
Good reading:
//www.focustestchart.com/chart.html
//regex.info/blog/photo-tech/focus-chart#print

Bear_Music

Samsung Galaxy S24, S24+, S24 Ultra

Sony 24-70mm F4 Vario-Tessar T* FE OSS

09/13/2011 09:14:03 AM · #13

Originally posted by kenskid:

You're not quite parallel to that vertical card, is why that's happening...

R.

MelonMusketeer

Nikon D7000

Nikon MF 50mm f/1.4

09/13/2011 09:43:36 AM · #14

Like Dereck said, iso 4000 is not going to be as sharp as iso 200, no matter what else you do.
Take a look at one of the full reviews of the D700, and note the difference in sharpness in the cropped iso test images.

Try shooting your test card with direct flash instead of existing light to eliminate camera shake and you will see a big difference.
At 200mm a very tiny bit of shake will make a big blur in the images.

There was a thread about manual focusing techniques a while back, and Robert posted there. Maybe he can recall and post the link to it.
I tend to work my point of eye focus back and forth between the subject and the grid lines in my viewfinder, to help control the way my eye attempts to adjust for the image focus point being a little off. If you concentrate on the one point in the image that you want to focus on, your eyeball will work to resolve that point for your brain, though it may not be the actual focus point that the lens is casting on the sensor.
Manual focus is a skill that gets better with much practice, and wider aperture lenses are the most difficult to master. Although the image is much brighter in the viewfinder, the DOF is so thin at big apertures that it becomes much less forgiving if you miss by even a little.

Message edited by author 2011-09-13 09:45:54.

Canon EF 70-200mm f/2.8L IS USM II

09/13/2011 10:48:53 AM · #15

I know that 4000 iso is not a good setting for this but my two test charts said to do without flash....so....at 10pm in my kitchen I had to crank it up in order to use a fairly fast shutter even with the tripod. I'll re-do tonight with the "flat" chart with better lighting.

With that said...I'm pretty confident that I'm within the "zone" with that lens. Surely close enough for the auto fine tune to handle...when I figure that out.

What's puzzling my now is all the web talk about trying to focus on subjects at 45 degree angles. Is 45 some kind of focus poison?

Message edited by author 2011-09-13 10:49:05.

Silent-Shooter

Canon EOS-5D Mark III

09/13/2011 10:55:50 AM · #16

Originally posted by kenskid:

...snip... Surely close enough for the auto fine tune to handle...when I figure that out... snip.

Careful though... I have heard that zoom lenses can have different front / back focus issues at different focal lengths. If you are testing at 200mm make sure you also test at min, middle and max zoom for that lens and fine tune according to what length you shoot most... but also keeping in mind the results you get at all zoom lengths with that lens.
As for the 45 degree question - I am sure it is so that when you focus on the middle of the subject it will clearly highlight a front focus problem with the items in front of the middle being sharper then the middle where you focused initially and visa versa for a back focusing problem.

bohemka

Sigma 12-24mm f/4.5-5.6 EX Aspherical DG HSM for Nikon

09/13/2011 11:11:48 AM · #17

Originally posted by kenskid:

Are you sure you used a tripod? When I toggle back and forth between those two shots the images don't line up. I wouldn't worry about shutter speed when everything's stationary. Maybe drag a brighter light int here and lock everything down before you shoot it again.

09/13/2011 11:26:51 AM · #18

I had it on the tripod. I may have bumped it or even moved it a bit...I don't recall.

I'll give it another go tonight with better light.

Originally posted by bohemka:

Originally posted by kenskid:
I know that 4000 iso is not a good setting for this but my two test charts said to do without flash....so....at 10pm in my kitchen I had to crank it up in order to use a fairly fast shutter even with the tripod.

Are you sure you used a tripod? When I toggle back and forth between those two shots the images don't line up. I wouldn't worry about shutter speed when everything's stationary. Maybe drag a brighter light int here and lock everything down before you shoot it again.

09/13/2011 11:28:59 AM · #19

I have to say...as far as this back/front focus thing is concerned, I'm pretty much satisfied that it is not my lens that is out of whack.

I'd like to see some other members post some unedited 200mm shots at 100% crop to compare...anybody?

Originally posted by Silent-Shooter:

Originally posted by kenskid:
...snip... Surely close enough for the auto fine tune to handle...when I figure that out... snip.

Careful though... I have heard that zoom lenses can have different front / back focus issues at different focal lengths. If you are testing at 200mm make sure you also test at min, middle and max zoom for that lens and fine tune according to what length you shoot most... but also keeping in mind the results you get at all zoom lengths with that lens.
As for the 45 degree question - I am sure it is so that when you focus on the middle of the subject it will clearly highlight a front focus problem with the items in front of the middle being sharper then the middle where you focused initially and visa versa for a back focusing problem.

09/13/2011 05:01:22 PM · #20

Originally posted by kenskid:

I'll try and spare some time in the next couple days, but I'm in the process of moving so it won't be for a bit.

09/13/2011 06:06:35 PM · #21

Thanks

MelonMusketeer

Nikon D7000

Nikon MF 50mm f/1.4

09/13/2011 10:26:28 PM · #22

The D700 may also have the feature where you can review the camera's chosen focus points in the playback mode in the LCD. With the D7000, it puts little red boxes around the focus point or points. That may help you to determine exactly where the camera is trying to focus in the AF mode. When shooting your test shots, use the single focus, single point focus mode. With continuous focus, the camera may be searching for a focus points and not locking on the one you want.
The closest that I have to the 80-200 is a 70-210 f 3.5, so shooting wide open would be similar to your shooting about 1/2 stop down therefore DOF would be deeper.
The D700 is a full frame and my 7K is APC, so results would be somewhat different at the same focal length.
Here's a crop of a snapshot of a yard critter, shot at 150mm. The whole image is with this one in the "detail" box.

ETA, You can also set up with the tripod, and use live view, and zoom in to see the exact focus point in the screen. I do that when using telescopes with the 7k.

Message edited by author 2011-09-13 22:40:14.