Can a lens's bokeh be predicted by reading an MTF chart?

Question

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?)

Asked by mattdm

Answer

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.

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

Answered by Matt Grum

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What are my options for an FX wide-angle prime to suit the D800's successor?

Question

I have a D700 which I'm not planning to replace at this point in Nikon's product cycle. I'll porbably buy the D800's successor. Not because I don't like the D800 (especially its usability improvements around focus mode switching and Live View, which don't seem to have attracted much attention) but because I can't justify the cost when there are too few things the D800 does that the D700 doesn't.

Anyway, although I'm keeping the D700 for now, I'm planning to buy a wide-angle prime so that I end up with this in my bag: D700, SB-800, 105mm micro, 50mm, wide-angle.

I need to choose a wide-angle lens. The 14-24 is a great lens, but too big for me to routinely carry it. So I'm going to buy a wide-angle prime. However, I don't want to buy a prime now that turns out to be disappointing on the replacement body I eventually buy.

As for focal length, the 35mm is a lens I could like but it's too close to the 50mm for me to seriously consider buying it. Hence I'm looking at the 20mm - 28mm focal length range. Which Nikon primes have sufficient micro-contrast to work well with bodies with a finer pixel pitch at full frame (I'm going to assume for the sake of the discussion that that is what the D800 successor will be like: FX, high resolution)?

I'm going to assume that the 24mm f/1.4 is going to be among the suggestions, and in fact Nikon points it out in the technical guide for the D800 as being suitable for use with the D800E. It's heavy and expensive so I worry that in practice I'd leave it behind with the 14-24. So I'm interested in my other likely options.

Budget is important for recommending the right choice. But I'm pretty flexible for a reason: new lenses will be launched between now and the launch of the D800's successor. So maybe the right approach is to buy something less expensive now and upgrade later to some not-yet-existing lens. The principle I'm going to operate on is that I'm OK with buying a lens now, that's actually not suitable for the D800's successor at up to €400. I'd just sell if that seems like the right thing to do, and not worry about losing some of its value on the re-sale. On the other hand, if I'm going to spend more than €500 I don't want to plan to replace the lens.

My planned uses are principally travel photography, interior shots, architecture, some environmental portraits.

What do you say?

Asked by James Youngman

Answer

My suggestion would be that you hold off on a wide prime if you're buying it for the future. Any of them are okay if you're looking to match them with the D700's sensor, but the D800 (and, one would assume, its successor models) has about the same pixel density on the sensor as the D7000, and the Nikkor full-frame wide angles are showing their age on that model (and not well, either). They're not particularly sharp at the corners, and display significant vignetting already -- and that doesn't account for performance at the edges of a full-frame sensor. If you can, try to get some feedback from people who've tried them on a D3X, but I wouldn't expect any rave reviews.

Since Nikon has decided to step into the miniature medium format world -- using a sensor that's verging on the very edge of even theoretically perfect lens performance -- and their stable of full-frame wide-angle primes is getting a little long in the tooth, I'd expect to see some newer design trickling in over the next few years. So buy for the short term rather than for the future if you're waiting for a successor to a model that isn't actually shipping yet -- something that might show its age on a D800 could be the best thing that ever happened to your D700. And get the focal length you need rather than worrying about which one is "best" -- the best lens is the one that lets you take the pictures you want to take, not the one with the best specs, build quality or handling.

Answered by Stan Rogers

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How can I compare the MTF results on Photozone from the Pentax SMC DA* 300mm with similar lenses from Canon and Nikon?

Question

I have been researching the Pentax SMC DA* 300mm f/4 ED [IF] SDM as a possible wildlife lens to use with my K-5. There are only a limited number of reviews of the lens from reputable sources and even fewer with MTF data.

I have been checking the result of this lens in combination with the apsc K-5 at photozone, however my understanding of MTF data is limited. What also important is that I can compare this data to the Canon and Nikon 300/F4 equivalents, but it is simply to complex to get my head around it.

Photozone MTF for the Pentax

Your help in comparing and understanding the data is much appreciated.

Answer

Well looking at the data for the Pentax lens, vignetting (very little and less than the Canon), distortion (virtually none) and chromatic aberration (minimal) are nothing to worry about.

The main thing to compare is the resolution. They are measuring this in Line Widths per Picture Height (LW/PH). Think of this as measuring the limit the lens can resolve (distinguish) very narrow lines close together before it just becomes a grey blur.

The Pentax lens maxes at around 2400 (LW/PH) in the center, the Canon at 3300. The figure of 2400 seems to be an very good result, as I see sharp macro lenses (Nikon 105mm for example) in that same range. The Pentax is almost as sharp in the corners, whereas the Canon is softer in the corners than in the center.

So the Canon lens appears to be able to resolve more detail (more distinct lines, 3300 vs 2400). However Photozone also say the "visually relevant" range is up to 2250 LW/PH . By that I take it that beyone 2250 the lens will resolve more than the sensor can record, or more than the human eye can detect in normal use. In fact the Nikon 300mm f/4 has a LW/PH value that maxes out around 2100, and it is called "very good to excellent"

I'm not an expert on MTF charts but it looks like optically the Pentax lens is very good. I guess you'd want to also evaluate the build quality, weather resistance, auto focus speed.

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