I've heard people say that everything meaningful about a lens's optical performance can be determined simply by looking at the MTF charts — although nothing is mentioned of that under How do I interpret an MTF Chart? or for that matter What is bokeh, exactly?

I heard something about the distance between the lines on the chart, and their divergence or convergence.... Is it true that this can be used to predict whether a lens will render "good" bokeh or bad? (Note that I'm not talking about amount of depth of field, but rather the visual quality of the out-of-focus areas.)

The Pentax DA★ 200mm I'm trying out has generally nice bokeh (and gets lots of praise for that online) but in some situations I've noticed that it it's got a clear ring shape on highlights, rather than being perfectly smooth. Could I tell that from an MTF chart? And, if a "real" MTF isn't available, can I tell it from the MTF bar graphs given in some online reviews? (That is, do the line graphs show something these bars don't?)


1 Answer 1


Supposedly the deviation between the Saggital (solid) and Meridional (dashed) lines on the mtf chart can be used to judge the bokeh. If the lines deviate widely from left to right the bokeh can have undesirable qualities.

Saggital contrast is measured using a pattern of fine lines parallel to the image diagonal whereas Meridional lines are set at 90 degrees to the image diagonal.

You have to compare pairs that correspond to the same f-stop and contrast rating (i.e they are the same colour and thickness).

Here is an example of a good lens, although contrast varies the S and M lines stay reasonably close together.

(source: canon.com)

Here's an example of a less good lens (look closely at the thin blue lines):

(source: canon.com)


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