Back focus when manual focusing ?

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Xjacktar

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David
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Serious (though possibly stupid) question. I really want to know.

I always believed that, with an SLR, what you saw through the viewfinder was what you got in the photo. Now I'm not so sure.

The light path of object-lens-sensor is very different to object-lens-mirror-prism-viewfinder.

Any clever folks out there who know the answer to this ?
 
The focussing screen (object-lens-mirror-focussing screen) is the same distance from the lens as the sensor. The prism just lets you look at it from the back of the camera instead of from the top.
 
The focusing screen you look at is in the same simulated focal plane as the sensor.

Unless there's something seriously wrong, ie camera taken a wallop, it's unlikely to be out.
 
and don't forget to focus the eyepiece to your eye. It can make a difference between what you see and what the sensor sees.

Thanks,
Rick
 
Similar to the above, check you haven't accidentally knocked the dioptre adjustment wheel.
 
Thanks all for the response, and especially Stratman and HoppyUK for the info.

It's something I've wondered about ever since I became aware of the 'back-focus' business, but not really worried.

Eyepiece diopter is set up OK, and I can achieve what looks like sharp to me. I don't find manual focus easy though. (It is always wide aperture as well, to defeat zoo fences.)

Seems this old geezer's eyes are going up the creek as fast as all the other bits. All the best.
 
I've got to disagree slightly with some of the posts above. I'll explain from a Canon perspective but I've no reason to suggest that Nikon do it any differently.

The focussing screen is generally shimmed to place it the same "light path distance" from the lens mount that the sensor is. Standard shimming aims to be correct within 1/3 of the single sided depth of focus (a figure also used by Canon's micro-adjust feature). Using a fast lens, wide open and closely focussed ( in the high magnification zone) will possibly show some front or back focus....I've tried this on several bodies in the past and purchased a variety of shims to calibrate and overcome the error (critical with the MP-E65 at the tight end).
Canon's 1srs bodies have a tighter spec and there are no shims available but errors can still be detected at high magnification as both my 1D MkIV's prove.

A second possible problem is the focus shift exhibited by some wide aperture lenses.....f/1.2 and maybe f/1.4 lenses to a lesser degree. The focus is performed at f/1.2 but in stopping down for the exposure, the focus will shift backwards due to the optical properties of the lens....usually most evident at f/1.4 to f/1.8 at distances close to the MFD. Whilst this was also a problem for the AF sensors for many years, it appears that a small offset is now applied in the focus algorithm when the body detects these specific conditions.

Bob
 
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Interesting comments Bob :thumbs:
 
The slight calibration error on the 1DMkIV's is very easy to see with the MP-E at 5x magnification. Using one of my split prism screens to manually focus and then flicking into Liveview shows that the focussing screen is clearly wrong....you'd never detect it in normal usage though.

Bob
 
Interestingly the Zeiss 85mm f/1.4 apparently exhibits notable focus shift and given that it is MF only I'm not sure how you work around it aside from Live View (& then stopped down - dof preview?? - I can't remember if Live View stops down anyway!).

Phil
 
Phil.....You can focus through the viewfinder with DoF preview selected if you'er in the danger zone....assuming that the focus screen calibration is nuts on!

Bob
 
This is good stuff Canon Bob, a good kick-start to the thread. Learning with every post. Only wish you could give us an insight into Nikon gear.

I wonder if the D7000 has that clever sensing of focus-shift. Of course it's always at wide apertures and near MFD when this is a worry.
 
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