Focus Shift on Stopping Down

Is this why some carefully focussed shots at f/8 - f/16 don't come out as sharp as you expected them to.

Sorry for using a Nikoone lens (looks an excellent one as well) it was the only one I could find using the latest test method from f/1.4 to f/8

Link

Go to the focus chart (bottom of page)and mouseover f/1.4 the focus is at 100mm move the mouse to f/8 and the focus has moved back to 85mm.... sort of makes focusing a bit of a joke.

And this is on a very good lens look at the example shot at f/8 of the tree post in the shed (2nd down left column) you can read the lettering in pencil.

That was the first lens I bought for my D700 and did a day in York with it followed by a day at Whitby. I have nothing but praise for it. Not noticed any focus shift but an interesting article. I also have the Pentax 50 f/1.4 and also have nothing but praise for it.

Not a lot else one can say really.

Regards

RSA (residual spherical aberration)Is quite common on older Nikon and Canon AF lenses, I've never seen it reported on Pentax lenses but I wouldn't rule it out altogether.

John

generally, the extra depth of field gained from stopping down more than compensates for the focus shift.

but it might be an issue for ultra-wide aperture lenses. i can see that it might affect a f/1.4 lens being shot at f/2, and this is certainly the case with the samyang 85/1.4.

generally the depth of field in front of the plane of focus is less than the depth of field behind the plane of focus, particularly when close to the subject. i think this is at play to a large extent in that link that you posted.

At less than 1m which this is the DOF to the back and front
is identical.

i don't think the focusing distance is 1m. i reckon 1m is the focusing mark, but that doesn't mean the camera is 1m away.

edit: but clearly i should take the reviewer's (and your) word for it (sorry for being stubborn just now) since they mention the focus shift. still, note that the 100mm point, even though further from the field of optimum focus, is still very much sharper at f/8 than it is at f/1.4, so i don't think it's focus shift that is causing soft results at small apertures.

I've often been surprised when testing a lens (at 25x) that
the f/5.6 result is better than the f/8 result and it
shouldn't be? I usually take 4 shots at each aperture
and pick the best... refocusing for each shot. At f/1.4
to f/4 only 1 shot might be good. At f/8 all 4 are
usually identical due to DOF, but it could be identicaly
off on each shot.

Next time I'll try shifting the camera back and forth
+-15mm at f/8 (on a Novoflex rack)to see if it's
any better.

There is no golden rule that says a lens will be at its best at f8. High quality modern primes may well be at their best at f5.6, or even f4. In theory, lenses should perform best at open aperture but we know they don't - this is because no lens is a "perfect lens" so there are compromises in its design and construction.

yeah, and the performance of the lens is inextricably linked to sensors as well. diffraction issues become more apparent with sensors with a high pixel density, and i think this increased sensitivity to diffraction means that it is sometimes better to use a lens a bit wider open on modern sensors.

but it really is nitpicking. if a shot at f/16 isn't sharp enough for you because of diffraction, you really need to think about how sharp you need your pictures to be, or a bigger sensor.

i digress. sorry.

Absolutely. All those 100mm macro lenses we've been buying are superb at f11 in particular, and any aperture is totally useable. If it's slightly softer wide open (haven't tested that yet) then great for portraits as well.

Lens "defects" are exploitable.