Infinity focus position

[Robert Camfield]

A Nikon 24mm f2.8 AIS, in virtually new condition, underperforms expectations...simply not as resolute as expected at distances greater than about 30 meters, at all apertures. Holding image size constant, comparative tests (35s, 28s) confirmed underperformance. And the issue did not seem to be an matter of field curvature, where the center image is in focus but the edge is somewhat soft...or vice versa. Well then, perhaps the lens is a poor sample, understanding that realized performance is randomly distributed across lenses of a common specification (24s, 28s, etc.). 

A while back, I recall Richard Haw's comments about the importance of calibrating the position of the CRC mechanics in a post about 28mm f2.8s (AIS)...something like "one can have high resolution at infinity or close in, but not both". This caused me to test the lens slightly off the infinity hard stop...perhaps no more than 0.2-0.3mm of rotation of the focusing grip/helicoid (the focus throw is very short on the AIS 24 2.8s) off the stop for maximum focusing distance. Image quality markedly improved, both center and edge.

Any thoughts you may have are appreciated.

Robert     

[pluton]

Is it possible that the precise match of marked infinity to optical infinity is off on the 28/2.8 AiS?
If Richard's statement is true, maybe it'd be worth it to have two copies of the 28/2.8 AiS, with one each calibrated for short and long distances? 
I am investigating similar anomalies with a Zeiss "Classic" ZF.2 25mm f/2 lens---a lens that has a 'reputation' for being sharper close up than at infinity.

[Ian Watson]

Is it possible that the precise match of marked infinity to optical infinity is off?

That was my thought too. My 24mm f/2.8 AI has the same behaviour. Is it perhaps intentional to allow for production tolerances? A feature rather than a bug?

[MEPER]

Is the F-lens used on a F-body?

I have observed that when a F/AI/AIS lens is used via a F-Z adapter on a Z-body it could look like that the adapter is just a bit "short" so F-lens goes past infinity at infinity but used on a F-body with split image screen the infinity mark fits quite well with split image?

[Roland Vink]

A Nikon 24mm f2.8 AIS, in virtually new condition, underperforms expectations...simply not as resolute as expected at distances greater than about 30 meters, at all apertures. Holding image size constant, comparative tests (35s, 28s) confirmed underperformance. And the issue did not seem to be an matter of field curvature, where the center image is in focus but the edge is somewhat soft...or vice versa. Well then, perhaps the lens is a poor sample, understanding that realized performance is randomly distributed across lenses of a common specification (24s, 28s, etc.). 

A while back, I recall Richard Haw's comments about the importance of calibrating the position of the CRC mechanics in a post about 28mm f2.8s (AIS)...something like "one can have high resolution at infinity or close in, but not both". This caused me to test the lens slightly off the infinity hard stop...perhaps no more than 0.2-0.3mm of rotation of the focusing grip/helicoid (the focus throw is very short on the AIS 24 2.8s) off the stop for maximum focusing distance. Image quality markedly improved, both center and edge.

You said your lens underperforms at distances greater than about 30 meters - is that measured on the focus ring or as focused through the viewfinder? If the infinity hard-stop is not calibrated correctly, it could cause problems when focusing at far distances. for example, if the lens cannot focus to infinity then subjects at far distances will never be fully sharp, although stopping down may increase the DoF to cover infinity to give adequate sharpness for far subjects.

If the hard stop is a little beyond infinity, that is less of a problem. The temptation is to assume the hard-stop is at infinity and not check that the lens is correctly focused in the viewfinder, resulting in out of focus images. Backing off infinity, as you tried, should solve that problem in which case you should be able to get sharp images for subjects beyond 30m. But you need to carefully check the focus in the viewfinder to ensure the image is sharp and not rely on the infinity hard stop.

The AI version of the 24/2.8 has a longer focus throw than the AIS version, which might help to focus more accurately. The optics are the same although coatings on the later AIS lenses are probably a little better than earlier AI and AIS lenses.

The whole point of CRC is that the lens should perform well at infinity and at close range, although I imagine that some lenses still perform better at one end of the range than others.

[Robert Camfield]

Roland,
MEPER,

Thanks for the comments...response as follows:

1) The over 30 meters test distance refers to my judgement...In this case, the classic test brick wall was approximately 100 feet which, with the focus set at the infinity post, resulted in an "in focus" signal in the viewfinder (D600). Yes, the infinity position is perhaps not properly calibrated, or the CRC set point (calibration via internal screw adjustment, I think) is incorrect. I also would have expected, as you indicate Roland, that DOF would negate the issue...even wide open, DOF of 24s is substantial, particularly at longer distances. I should add that, through the viewfinder, I don't notice much difference in resolution between the "at the infinity post" and the "adjusted focus ring position (by 0.2mm-0.3mm of throw)", even at 100% crop. However, the resolution difference is quite evident at 200% crop; I wonder if I'm inadvertently exploring things beyond Nikon's tolerances/standards for lens assembly. Per Roland's "Nikon specifications" web site, this particular 24mm lens is a pre-2006 series vintage...perhaps '95 or so.

2) My stash of lenses includes the 24mm AI which, as you indicate, has the significantly longer throw. In contrast, the 24mm AIS version reaches minimum focus of <1 foot in about 90 degree of focus ring rotation.

I intend to turn back to this issue and take another look.

Thanks again, Robert