Can we judge a lense based on the MTF only?

Hi everybody,

Yesterday somebody posted a link to a Nikon Japanese site

 

http://www.nikon-image.com/jpn/products/af_nikkor/index.htm that

shows the MTF curves for many Nikkor lenses.

 

The gentleman was claiming that the 18-35 3.5-4.5 is sharper than

the 17-35 / 2.8. I own the 17-35 but I've never tried the 18-35 and

I was a bit shocked about this statement. I printed then the two

curves and placed them one on top of the other. Effectively the

curves for the 18-35 were better than those of the 17-35. Is is the

same for the curves of the 80-400VR which seem to be better

(sharper) than the 80-200ED. Does it make any sense? Here comes the

stupid question:

 

I'm not an expert in determining the exact meaning of the MTF

curves.

 

Are these curves the absolute and best method to determine the

optical quality of a lens? (in terms of sharpness). Can we base our

lens evaluation only on that, or are there any other factors?

 

I cannot believe that the only reason for such a big price

difference between the 17-35 and the 18-35 would be just for the 2.8

opening and the AF-S. I like the 17-35 but I have to admit that it

is very sharp in the center and not that sharp at edges. Is it

really worst than the 18-35? Maybe I�m wrong. I know all the 17-35

owners will say: Oh I love my lens, and the 18-35 owners will say I

love mine too. But let's be objectives for once.

 

with regards,

Titus

First off, I looked at the curves and noticed that it didn't state (I couldn't find) what f-stop the lenses were tested at. This can have a major effect on the numbers. Also, MTF curves give information on resolution and contrast, but those are not the only important optical characteristics of a lens. In the end, if you love your lens, that's all that really counts.

At best, they capture information about the sharpness at

the tested aperture and working distance. Extrapolating

the information to other apertures and working distances is

dicey.

<p>

They don't capture info on flare, distortion, light falloff,

bokeh, mechanical build quality, size/weight, presence and readability

of DoF scale etc. Sometimes, these other factors can make a

slightly less sharp lens stand out over a sharper cousin when

both are "good enough" as far as sharpness goes.

Yes, MTF info is important for sharpness and resolution. Obviously it should be done very systematically for all apertures or at least three representative ones (fully open, mid, closed). But I agree with Richard that there are also many other parameters. For me, an important one (not yet mentioned) is colour rendition (I mainly use slide film -- if you mainly shoot negs, this might be of less importance). Some lenses do have significant colour shifts, some reproduce colours which somehow look "technical". If you have to use a set of different lenses with different colour characteristics within one series (e.g. on a trip), this can be very disturbing. And this is difficult to compensate with filters: typically you loose in brilliance.

MTF is often abused to rate one lens over another.

Effectively, a single curve means absolutely nothing. And putting a single number score on a lens based on that curve like some sites do means even less.

 

In the end the only thing that counts is the photographical output, the image on film (or CCD) and that is made up of a lot more than just the one or two factors used to determine an MTF curve.

No, MTF alone is insufficient for assessing the quality of a lens. Such things as geometric distortion and colour aberrations are not measured by MTF. MTFs however do give a pretty good idea of several other aberrations such as coma and astigmatism. Widely separated tangential and radial responses are symptomatic of optics that suffer such aberrations. Lens makers do not like to talk about such as this is tantamount to drying your lundry in public view. The best tonal or dynamic ranges are achieved by lenses that have closely matching MTFs which are high up in the chart. According to that criteria, the 18 mm lens is better at the shortest focal length than the 17mm, while at 35mm it is a toss up. Remember that these MTFs are very low tech stuff. As other poster pointed out, the aperture at which measured is not given, and the spatial frequencies are only given to 30 L/mm. I suspect that at 40 L/mm, at which Zeiss gives its MTFs, the results were too embarrassing for Nikon to bear. Just the same, Nikon should be embarrassed by not providing the apertures at which measured and then MTFs at only 30 L/mm. In other words, these MTFs are merely token stuff to pacify the masses, not technically respectable scientific data. As such, I would not give them much credibility.