Here's a weird one for you guys and gals. My D70 had severe backfocus problems, so I did some adjusting (Instructions on the www) and things were better. I then managed to scrape up the cash to buy a used 80-200 f2.8 lens. Great stuff! - however at close range the lens appeared to be soft when I shoot wide open. At further distances it is nice and sharp.
So - with the devil in me I decided to shoot some focus test shots to see where the problem was. The results were interesting to say the least. The question now is quite simply: where is the problem?? Camera or lenses?
And before you ask - I shot the tests twice, and both times the results were the same
Sigma 90mm macro - spot on centre
18-70mm Kit lens - 10 - 15mm backfocus
Nikon 70-210 f4-5.6 - Front focus
Nikon 80-200 f2.8 - back focus
Nikon 70-300 f4-5.6 - spot on centre (no picture)
Is your issue at distance or at f 2.8? None of the 80-200 / 70-200 are dead nuts sharp until you hit f4 IMHO....they are ok at 2.9 but when you start doing focus tests on angled charts which Nikon clearly says will fool your AF sustem at times.....are you really comparing and seeing this issue in real situation shooting?
Yes, it happens in "real" shooting as well - I was trying to get a shot of a small bird from a close distance, and the twigs behind his head were in focus, and not the bird itself. The weird thing is that the sigma lens is dead in focus where the camera focussed, the other lenses exhibit back focus.
The way that you have presented the pictures and test here suggestst that not all lenses we at the same setting, in terms of either focal length or F Stop. If this is the case then your test is not very meangingful.
Secondly how have you got the AF system set up, this will affect its performance. In particular since focus is based on contrast it is very easy in real life to be focusing on an object only to find that the camera locks on to somthing else that is of higher contrast. This can happen in any mode eg in single mode when the object you are focusing on is smaller than the focus reticule or in dynamic/single mode whenthe object moves outside the focus reticule for an istant (long instant if you use a D70 :-))
.
Note looking at the focus point in NC after the fact is meaningless, this just tells you which reticule was selected by you, not where the focus point actually is.
Note looking at the focus point in NC after the fact is meaningless, this just tells you which reticule was selected by you, not where the focus point actually is.
Are you sure about this. I'm on the road and can't verify but it would seem counterproductive to have a focus point indication and it not be the focus point...
Hi Marc. George is right. The focus indication only indicates which focus point you selected. If you use AF-S, focus and then recompose, the focus indication will be totally misleading.
Just to clear a few things up:
The tests were shot at minimum focus distance plus six to eight inches to make sure that the chart was not too close to focus.
Camera was mounted on a tripod, test chart was propped up on a table and held in place with cellotape and Prestik. Lighting was 150 watt halogen light, two feet from the chart. AF - S, centre sensor in all cases.
The pics that you are seeing is a 100% crop of the left side of the chart (to save bandwidth).
The point I am trying to make is that focus is not consistent across a range of lenses! The only lens that was spot on was the Sigma. Do the lenses need calibration? - Or is the camera focussing inaccurate and inconsistent?
If I had the lenses calibrated, and I bought a second D70, will they focus correctly on the other camera body?
Makes you think, doesn't it?
Maybe someone else feels like doing the same test with different lenses and post the results.
Louw
Edited by Lowkey on Aug 12, 2005 at 10:14 AM GMT (Reason: correction -AF - S)
mkonik wrote:
Note looking at the focus point in NC after the fact is meaningless, this just tells you which reticule was selected by you, not where the focus point actually is.
Are you sure about this. I'm on the road and can't verify but it would seem counterproductive to have a focus point indication and it not be the focus point...
Camera was mounted on a tripod, test chart was propped up on a table and held in place with cellotape and Prestik. Lighting was 150 watt halogen light, two feet from the chart. AF -C, centre sensor in all cases.
Louw
Not sure why you would use AF-C in a static test. I am sure that you have been very accurate in ensuring that the photos are taken in such a way as to make comparison valid i.e (Same FOV, same Fstop etc), but with AF-C and the tiny depth of field in this test even the slightest movement in the camera could bring about different results.
I guess what I am saying is that I have had a number of focusing issues with a D2X when shooting sports, and in each case it was my error not the camera's. The results of your test make me first suspect the test, not the camera.
Hi Lowkey, I'd like to come in on your side on this one - I have exactly the same problem but with my Nikon D100. I posted a similar question to this forum best part of a year ago and also received the same chorus of total disbelief from most readers.
In my case I have six lenses, and although I have not done the tests as rigorously as you this is what I came up with:
Tamron 90mm f2.8 macro: spot on, sharpest of all
Nikon 17-35 AF-S: spot on
Nikon 28-70 AF-S: very small amount of back focus, noticeably softer than above two
Nikon 80-200 f2.8: serious back focus (~15mm), unusable at close range below f16
Nikon 300 f4: as near as damn it spot on, perhaps slight back focus
Nikon 70-210: pretty close, much better than the 80-200
Based on my informal tests (conducted in a similar way to what you described, which BTW I think is useful and meaningful) I concluded that my real problem was with the 80-200 lens rather than the camera so I sent it off to Nikon for maintenance. They declared there was something wrong with it and £250 later I got back the "fixed" lens. It is now much better, but still exhibits slight back focus - enough to make me wary of using it wide open at close range.
Maybe I should do a similar more rigorous set of tests. If I can find the time I'll post the results here, but no promises. ;-)
I think there is definitely something funny with the D100 (and D70 by your account) back focusing, particularly with some 80-200 lenses. And, yes, I hear loads of people already saying "Mine is perfect, I don't see the problem." - You make me so jealous :-O because I and a few others don't have the same experience. :-[
I would agree with Lowkey and David. I am also doing some tests with my lenses (or some I can borrow). When results are not "in line" with what most people think, many tend to blame the test. In fact many tests like those are valid, but in a certain context (valid only for a specific copy of the lens, or a certain usage). I also did some tests with focusing and had problems with my Sigma 70-200 f2.8 wide open at 200mm (sharpness or focus issue I wasn't sure). Thanks to those tests I sent it back and got my lens "fixed". It is now incredibly sharp all over the range and focus is where I want it. I will redo the tests with my other lenses but if I remember well they were extremely consistent. Even moving the tripod and changing distance the very slight back-focusing I remember on another lens was systematic and reproducible. That is the reason why (consistency) I consider those tests as perfectly valid. If not there would have been some differences in the results. Just my opinion.
There is a known sharpness problem with the 80-200 2.8 when used wide open and focused under 8 feet, this was discovered when the 80-200 with the tripod mount was first introduced.
Lowkey, I ran a few tests with a whole range of lenses (mine + borrowed) on two different bodies - my D100 and a F50 film body. I have not got the film back from processing yet, but here's a summary of my D100 results:
17-35 f2.8 AF-S, slight back focus at 17mm, spot on at 35
28-70 f2.8 AF-S, spot on
90mm f2.8 Tamron macro, spot on
300 f4 AF-D, spot on
80-200 f2.8 AF-D, slight back focus at 80, significant back focus at 200
28-105 f2.8-f4 Sigma, significant back focus at 28, slight front focus at 105
70-210 f4-f5.6 Nikon, slight back focus at 70, slight front focus at 210
The Sigma 28-105 is pretty poor, but the one that annoys me is the 80-200 f2.8. Even after £250 recalibration by Nikon it is still not good enough (though much better than when I sent it off). At 200 the back focus error is nearly 15mm at closest focus - although as noted above the lens is very poor optically at this distance. If I manually focus the focus point is correct, so it is the AF that is the problem.
Even though I measured slight back focus on the 17-35 at the wide end I have never noticed this in practice. This lens gives me the sharpest image (subjectively judged) on my D100.
