Brief Notes on Point of Focus / Even Plane of Focus Resolution Tests & Nikon Lens

27 January 2016

Tests and Charts

As the camera lenses get more specialised and differentiated for their characteristics it seems good idea to look at the test results in context of methodology that was used for obtaining the data. Here I sum up a few more or less known points about the topic.

The point of focus method tests for optimal resolution at points of focus. In order to record it, it refocuses the lens at different positions in the image field and measures resolution only at the points of focus. It shows maximum resolution for a given spot of the field the lens is capable of at the position. These values are then plotted into resolution map of a lens and can be compared to results of other lenses. Observations made during the test about impacts of aberrations such as more pronounced field curvatures are usually commented on separately by the tester.

With most other tests the point of focus is in the centre and these would be the standard measurement which maps aberrations as they develop with growing distance from the centre and combines it with a lens resolution. It gives us idea of resolution of the whole image and thus separates sharp plane of focus (and aberrations free areas) from the areas that are less sharp or downright soft. Comments on how the field curvatures observed during the testing impact the resolution are often made as the curvature isn't directly apparent.

Plots for field curvatures (Petzval and astigmatism curves) have to be measured and presented separately. That is common with the later but it is rare to see the charts for the former (AKA curvature of field).

Your Focus

Both methods show something different and it is not difficult to see why the ‘standard one’ would be considered a complete representation of the characteristic of the optic. But, and this is not surprising, the method has its limits or imperfections and those are addressed by the more specialised and resolution-biased ‘refocusing method’ which can often be more relevant for the real world photography, or, more precisely, for many kinds of it. Particularly so when you want to know what resolution at the point of focus you are likely to get which will also be related to the way the photographer is focusing, such as moving the camera focus point so that the focus is precisely on the subject at the time of the exposure with no concerns over what resolution in other parts of the frame, e.g. in corners, is achieved. The focusing methods also develop with camera technology which makes them more efficient and convenient to use. On the other hand, when we need to consider even plane of sharp focus we are looking to see how the lens does in this respect and standard measurement aims to cover just this. It gives us a more complete picture about the lens. Despite the fact that its limitations mean that establishing the precise point of focus resolution away from the centre is not possible.

Nikon has recently signaled prioritising lenses optimised for point of focus applications in connection with the common use of video and emphasis on their own way of expressive rendering. This is a welcome step for differentiation in lenses and their use today. Using today’s means for this design, it also deals with various aberrations (along with the other ways) by making them work towards the design goal which can even be cost and weight effective. As this approach represents more subjective renditions in the lens' output it is vital to get the cocktail of the aberrations balanced well and in line with the raising standards for quality of the photographic lenses. The differentiation means that lenses that are corrected (to some degree) for even plane resolution are also included for the variability of use and prioritised for suitable specifications and applications. It would otherwise mean limiting production to only a certain part of the market while including both design philosophies promises expanding production possibilities and serving even wider needs of Nikon users. And this is what we can see happening. As the lens designs are taking advantage of the technological advances, Nikon has also raised the bar for the range of the optimisations involved and in how specific (effective) they are in shaping the individual lens so that it channels the advantage into creating distinct and unique Nikkor lenses in the digital era. This transfers, along with commonly known progress in the sensor and digital imaging, into new meaningful impulse to photography, its use, potential and as the photography changes just as we do, its meanings.

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