Back focusing, lens or body problem?

Mark G

Mark G

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Mark
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Hi all.

I noticed that some of my shots are focusing a little behind where I'd like them to be. Is this a problem with the body or lens. I'm using a D90 with Nikkor 17-55mm F/2.8.

Can this be rectified and does anyone know of anywhere in Bristol I can have this done?
 
Mark G said:
Hi all.

I noticed that some of my shots are focusing a little behind where I'd like them to be. Is this a problem with the body or lens. I'm using a D90 with Nikkor 17-55mm F/2.8.

Can this be rectified and does anyone know of anywhere in Bristol I can have this done?
Click to expand...

The possibilities are lens, body, and user error. It could be any one, or even all of those things, but it has to be said the usual culprit is the latter ;)

Nikon D90 doesn't have AF microadjust, so test to make absolutely sure what the problem is, then return to Nikon for correction.
 
Thanks
I'm hoping its user error. I've a wedding to do in a little over a months time and I don't fancy sending my camera away just in case its not back in time. Also, I don't fancy shooting the wedding in mf.

The images below are just 2 of the oof shots from yesterday.

The first one you can see that they are soft in the image, however the flowers between their heads are pretty sharp. I'm hoping that the focus point was between them and on the flowers.

DSC_0007.jpg


This one the image is sharper on the carpet behind the subjects head. Again, I hope I focused incorrectly. Seems strange though as I don't normally miss.
DSC_0006.jpg
 
Easy to see that it could be user error there, but those are not difficult subjects to get right - centre AF point, standard focus-recompose technique.

If it's out, it will be out with every shot by an equal amount.
 
Assuming you were using a single centre AF point, you can see what the probable problem is with this one....


AF Point by tonky8203, on Flickr

You definitely need to eliminate all possibility of user error before you begin to suspect a problem with AF adjustment.
 
Yep, I'm using single point but I normally focus and recompose. I've checked and the camera is set to AF-S.

I just checked the images in the Nikon software and can see the focus point is away from the subject in hand. However I guess it just highlights the selected focus point rather than the item that was selected, so i'm still none the wiser as to if its user error or not.
 
Yes, it just shows which point was used, and with focus-recompsoe by definition that won't be much help.

But as I said before, if it's out, it will always be out - by the same amount, in the same direction, every time. Try a couple of test shots. I just posted a suggested test target on another thread, here post #5 http://www.talkphotography.co.uk/forums/showthread.php?t=340760
 
Focus and recompose = recipe for disaster IMVHO.

What you are basically doing is a sort of poor mans hit or miss hyperfocal IMVHO.

In the days of film this was ok as we mostly only ended up with small prints and frankly everything could have been out of focus, and probably was... and no one would notice but these days people look at images on computer screens at 100% and print bigger than they ever did so sloppy practice or poor mental arithmetic errors will show up.

My advice is to use another focus point which is over your intended point of focus or manually focus.
 
woof woof said:
Focus and recompose = recipe for disaster IMVHO.

What you are basically doing is a sort of poor mans hit or miss hyperfocal IMVHO.

In the days of film this was ok as we mostly only ended up with small prints and frankly everything could have been out of focus, and probably was... and no one would notice but these days people look at images on computer screens at 100% and print bigger than they ever did so sloppy practice or poor mental arithmetic errors will show up.

My advice is to use another focus point which is over your intended point of focus or manually focus.
Click to expand...

There's no problem with focus-recompose technique for those images - none at all, even if the subject was well away from the chosen focus point, which actually it isn't in this case.

Focus-recompose is only a problem at very close distances and very low f/numbers. At normal range, it is better because the centre AF point is more sensitive.

What I suspect has happened here is that the AF point wasn't actually over the subject when the focus was taken, and has accidentally picked up on the background which is immediately adjacent. The same danger would exist even if another focus point was used.
 
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HoppyUK said:
There's no problem with focus-recompose technique for those images - none at all,
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Well, quite obviously something has gone wrong.

Hoppy, love you to death honey buns but it's clear to me that we'll never agree on anything :lol:

When you focus on a thing... lets imagine a cereal box... you're focusing on something that's a fixed distance away and if you then move the camera and point it at something else the distance between the camera and the original target will almost certainly alter and then the thing almost certainly isn't in critical focus any more but if you're lucky it'll be in the murky world of DoF. Maybe. Maybe not.

All that's fine if you know what you're doing and have made a conscious decision, however, there are lots of people who seem to suffer focus errors and I personally think that a little more thought and conscious decision based on experience and knowledge is required before venturing beyond the basic concept that you should aim at something, focus on it and take the shot.
 
Steps in waving white flag... ;)

The thing I still don't like about focus/recompose is that once you've focused in single shot mode, the focus is locked all the while you're messing about recomposing, with all the potential for movement of both yourself and subject(s) to throw off the focus. I'm not saying there's anything wrong with it per say but it's something you need to be aware of and the longer you hold onto that focus lock position before pressing the shutter the greater the potential for problems.

I was really aware of this doing weddings with my old Nikon f4 which only sensed focus vertically. You could run the batteries flat before you'd focus on something like Venetian blinds. Often doing wedding posed shots I'd have to rotate the camera to portrait orientation - lock focus, rotate back to landscape orientation, recompose and take the shot. I was always very aware of everything that could go wrong while doing all this.

Thankfully we have cross type sensors these days, which eliminate a part of the problem, but I'm still very wary of the focus /recompose routine.

I'm not saying it's the cause of the problem with these particular shots., but MarkG the OP does say the sharpest part of the shot in the first image is the flowers between the two heads, but it's strange that the flowers on the other side of the walkway in the same plane certainly aren't sharp - in fact I'm inclined to think the sharpest part of the image is that window frame in the bg which is slap bang in the centre of the image.

I'm not saying we should draw any firm conclusions from this, but certainly focus/recompose shots aren't the best to try to resolve this. Mark needs to put that AF point over a spot, keep it there and take the shot - that way we'll have a better idea of if there are any AF issues.
 
I've done a few test shots in the garden this evening with a deck of cards spread on a table. It pretty much focuses on the card specified every time. I'm wondering if there was something wrong on Saturday with the shots.

I understand what woof woof is saying with the potential for error with focus recompose, but I'm moving the camera a matter of millimetres to recompose the image which wouldn't account for the 5ft of focus difference on a subject 15ft away. As Hoppy says, this would only be a problem at close range.
 
Well, at least you're thinking about it :)

I suppose something else to think about is what the camera is attempting to focus on. It is possible that the auto focus was having a problem locking onto the target because of a lack of detail or contrast or it's possible that there was something more attractive to the autofocus within the focus area.

Whenever I think that there might be a possibility that the auto focus might be confused I take more than one shot or switch to manual.
 
woof woof said:
Well, quite obviously something has gone wrong.

Hoppy, love you to death honey buns but it's clear to me that we'll never agree on anything :lol:

When you focus on a thing... lets imagine a cereal box... you're focusing on something that's a fixed distance away and if you then move the camera and point it at something else the distance between the camera and the original target will almost certainly alter and then the thing almost certainly isn't in critical focus any more but if you're lucky it'll be in the murky world of DoF. Maybe. Maybe not.

All that's fine if you know what you're doing and have made a conscious decision, however, there are lots of people who seem to suffer focus errors and I personally think that a little more thought and conscious decision based on experience and knowledge is required before venturing beyond the basic concept that you should aim at something, focus on it and take the shot.
Click to expand...

Alan, the potential problems of focus-recompose are well known, but must be kept in perspective, and it's more a shift in the plane of sharp focus than movement of the camera. See explanation/diagram here http://visual-vacations.com/Photography/focus-recompose_sucks.htm which shows it nicely, but I feel exaggerates the problem so I did some tests the other day following another thread, and posted this [edits].

"The problems of focus-recompose technique should not be exaggerated. I use it almost all the time, [as I think most people do, because it's fast and easy and the centre AF point is more sensitive.] It is only an issue if you are shooting both very close and with a very low f/number, like f/1.8. Example: a tight head shot, just chin to forehead in the frame, [focus on the nearest eye off-centre,] shot from under two feet - even then the shift in the focus plane is likely to be only about 1cm.

"That is enough to make a big difference in that situation, often compounded by movement by the subject and/or the photographer. But a more normal portrait, say head and shoulders from about four feet (crop format DSLR like the OP's 400D, 50mm lens at f/5.6) [not only is the angle of shift likely to be much less, but] the the depth of field rises to around 5in and any focus-recompose errors will be invisible."

The 'murky world of DoF' is quite extensive in practise.

In the case of the OP, the problem is compeletly unrelated to focus-recompose technique per se - the AF point has simply fallen on to the background immediately behind the subject. Basically, user error.
 
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