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LENSES: Facts and Fallacies - Part XI
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PostPosted: Sun Apr 20, 2014 6:32 pm    Post subject: Reply with quote
Gerald wrote:

The MTF tells a lot about a lens. Including CA!
The figure below from a Zeiss article comments how the lateral CA affects the tangential (meridional) MTF:


It tells that lateral CA affects the tangential MTF. So it only tells that CA affects MTF. As other factors could affect the tangential MTF as well it doesn't speak about CA.
The cause -> effect link is CA -> MTF (as I can read in last rows of your MTF example).
The revers is not always true. You can't always draw a conclusion about CA by reading the MTF.


PostPosted: Sun Apr 20, 2014 7:03 pm    Post subject: Reply with quote
dan_ wrote:
Gerald wrote:

The MTF tells a lot about a lens. Including CA!
The figure below from a Zeiss article comments how the lateral CA affects the tangential (meridional) MTF:


It tells that lateral CA affects the tangential MTF. So it only tells that CA affects MTF. As other factors could affect the tangential MTF as well it doesn't speak about CA.
The cause -> effect link is CA -> MTF (as I can read in last rows of your MTF example).
The revers is not always true. You can't always draw a conclusion about CA by reading the MTF.


Yes, astigmatism also produces separation of the sagittal and meridional curves. However, it is possible to distinguish whether the separation is produced by lateral CA or astigmatism.

If the meridional MTF curve practically does not change with aperture then the predominant off-axis aberration is lateral CA. This is the case of the Canon 300mm F4:
http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup/ef_300mm_f_4l_is_usm

Note that out of the central zone, the black and blue dashed lines are very close one from the other.


PostPosted: Sun Apr 20, 2014 9:49 pm    Post subject: Reply with quote
"Prime lenses give sharper images. This is because zoom lenses have much more glass for the light to travel through in order to reach the sensor; extra elements are required to zoom in and out, and even more are needed to correct the aberrations created by all the extra elements. With the light altered so many times, it is bound to degrade. A prime lens, on the other hand, preserves the integrity of the image by its simple design. Having only one focal length and fewer moving parts, it can be much more precisely calibrated for maximum sharpness."


PostPosted: Sun Apr 20, 2014 10:31 pm    Post subject: Reply with quote
Gerald wrote:

If the meridional MTF curve practically does not change with aperture then the predominant off-axis aberration is lateral CA. This is the case of the Canon 300mm F4:...

That's correct, in some particular cases some conclusions can be drawn, as in your example. But not in all cases. My assumption remains valid : "You can't always draw a conclusion about CA by reading the MTF."

In fact that was not my point. My point was that your statement "... today the best zooms produce image as good as the primes. ..." is not valid simply because a "good image" can't be defined in such a manner that everybody will accept it. A good image is a image good for something - landscape, portrait, close up etc. There is not such a thing as a generic "good image".
A lens is only a tool and a specialized tool (mostly a prime) is, in most cases, a better tool than a generic one (mostly a zoom) in producing an image good for something .

Last edited by dan_ on Sun Apr 20, 2014 11:06 pm; edited 2 times in total


PostPosted: Sun Apr 20, 2014 10:35 pm    Post subject: Reply with quote
And Gerald, as an example the Canon EF 70-200mm f/2.8L IS II USM has just a messured transmission of 3.4T, while the Canon EF 200mm f/2.8L II USM has a transmission of 3.1T. Theoretical numbers are the same (f/2.8 ) but messured light transmission is much better thru a prime lens.


PostPosted: Sun Apr 20, 2014 10:41 pm    Post subject: Reply with quote
Quote:
From what has been said, it is clear that neither resolution, sharpness,graininess, nor tone reproduction is of itself the determining factor for good definition in a photograph. In order to combine these properties in the simplest way, it has recently become customary to express the performance of a lens by its modulation transfer function (MTF), on axis and at several points in the field.

As its name implies, MTF is a measure of the ability of the lens to form an image that is an accurate reproduction of an object


I grabbed this quote from Rudolf Kingslakes book "Optics in Photography". I added the bold.

If MTF is just about showing how a lens reproduces an image by expressing it in a chart format, that can be easily read and understood by lens technicians to help them on how a lens will perform, or if it is worth while going ahead with a lens design...it is only a small drop in the bucket as to why we use a lens. I believe we enjoy seeing the result of a particular lens regardless of what the MTF charts say,there are so many factors an MTF chart can not cover.

This is a discussion that can get lost in mind numbingly small details....although the discussion can be interesting.


PostPosted: Sun Apr 20, 2014 11:41 pm    Post subject: Reply with quote
Nordentro wrote:
"Prime lenses give sharper images. This is because zoom lenses have much more glass for the light to travel through in order to reach the sensor; extra elements are required to zoom in and out, and even more are needed to correct the aberrations created by all the extra elements. With the light altered so many times, it is bound to degrade. A prime lens, on the other hand, preserves the integrity of the image by its simple design. Having only one focal length and fewer moving parts, it can be much more precisely calibrated for maximum sharpness."


Where did you get this from? And why should I believe it? It seems like the veriest nonsense.

Oh, and by the way, I have a few zooms of varying degrees of complexity and a few primes as well. My long Schneider zooms for my Beaulieus are marvels but within its range the 8.5-26.5/1.0 that's permanently attached to my Canon 310XL gives better footage. Good primes, e.g., 25/1.4 Cine Ektar II and 100/2 Canon TV-16, give better still.

But all this misses the point of zooms. Some of us have them because we're stuck with them. Get a 310XL, have a fast zoom, end of that discussion. Others choose to get zooms, e.g., my humble 35-70/3.5-4.5 Nikkor, because they fill a need. Ultimate sharpness isn't part of the need even though by MP's test my little piece of plastic is a decent lens.

I doiubt that many people buy lenses because they're best by some test or other. Seems silly.

dan_, I'll all for solidarity among Dans but please, sir, understand clearly that you don't speak for me. Its fine with me that there are many posts here about how various lenses render but rendition isn't very important to me. To the posters, yes, and I don't want them to stop caring about what's important to them, but to me, sorry, no.


PostPosted: Sun Apr 20, 2014 11:52 pm    Post subject: Reply with quote
danfromm wrote:

dan_, I'll all for solidarity among Dans ...

Very Happy


PostPosted: Mon Apr 21, 2014 12:27 am    Post subject: Reply with quote
From here:

http://www.picturecorrect.com/tips/5-reasons-why-prime-lenses-are-better-than-zoom-lenses/

It makes sence to me! The video with Kai is very entertaining too Very Happy

But seriously, those who knows me know that I don`t care about MTF charts and I have never bought a lens because of one either Wink
If if feel the need of a zoom for an occasion, I use a zoom too!

My point (in this discussion) was just, primes have better IQ in general because they have a few benefits but it doesn`t mean that you should avoid zooms or that there isn`t any good zooms. Wink


PostPosted: Mon Apr 21, 2014 3:17 am    Post subject: Reply with quote
Visualopsins and Nordentro are correct.

Here is why we use MTF today to judge lens quality instead of lp/mm: zoom lens makers realized that lp/mm ratings show that zoom lenses perform poorly. However, they can be made to have a modular transfer function that's kinda almost like an average prime. My FA Limited lenses test out at around 162-165 lp/mm at their best aperture. Even the old kit Pentax 50mm f2 tested out at around 160 lp/mm. The BEST Canon L series zoom at its best aperture comes in around 120 lp/mm.

For edification, do a quick Google search for lp/mm ratings on major maker zoom lenses. You either won't find them or it will be hard. Camera makers do what they can to keep lp/mm data quiet and the long-term anti-lp/mm campaign has been so successful that people now believe that MTF is actually a measure of lens quality.

For the record, if I remember correctly, at least one of those two is or used to be an optics engineer (correct me if I'm wrong.)


PostPosted: Mon Apr 21, 2014 4:07 am    Post subject: Reply with quote
That's great conspiracy theory. If a zoom looks like an average prime in terms of MTF, it will look like an average prime in terms of lp/mm too. In reality, MTF and lp/mm are just complimentary ways to measure the lens performance. For instance lp/mm are always measured at specific MTF, e.g. MTF 50 and MTF 20. All lens testing sites report lp/mm because they are easier to interpret for a layman.

Coming back to the topic, in my experience modern zooms still lose to primes (both modern and old) in performance, pop, speed, character, pretty much in everything except the convenience, but zooms has become good enough to make that convenience count and often be the main factor.


PostPosted: Mon Apr 21, 2014 7:58 am    Post subject: Reply with quote
I have a few high quality zooms and they perform amazingly well, but I still prefer to use primes because they tend
to be faster, smaller, lighter, and even higher quality image.


PostPosted: Mon Apr 21, 2014 1:03 pm    Post subject: Reply with quote
mo wrote:
Quote:
From what has been said, it is clear that neither resolution, sharpness,graininess, nor tone reproduction is of itself the determining factor for good definition in a photograph. In order to combine these properties in the simplest way, it has recently become customary to express the performance of a lens by its modulation transfer function (MTF), on axis and at several points in the field.

As its name implies, MTF is a measure of the ability of the lens to form an image that is an accurate reproduction of an object


I grabbed this quote from Rudolf Kingslakes book "Optics in Photography". I added the bold.

If MTF is just about showing how a lens reproduces an image by expressing it in a chart format, that can be easily read and understood by lens technicians to help them on how a lens will perform, or if it is worth while going ahead with a lens design...it is only a small drop in the bucket as to why we use a lens. I believe we enjoy seeing the result of a particular lens regardless of what the MTF charts say,there are so many factors an MTF chart can not cover.

This is a discussion that can get lost in mind numbingly small details....although the discussion can be interesting.



Excellent quotation! Kingslakes was one of the greatest experts in photographic optics in the world.

MTF is a fundamental tool for analysis and design of lenses. That's why MTF is so used by optical engineers and technicians in their work.

A photographer concerned only with the artistic aspects of photography does not need to know anything about MTF, but as you know, there are many photographers who are very interested in the technical aspects of lenses, cameras, sensors, etc. This explains why the lens manufacturers usually provide information on the number of optical components of a lens, type of coating, MTF, etc. This information is usually ultra simplified and not always technically true, but serve the purpose of creating a fantasy in the minds of many photographers. In fact, the technical information provided by lens manufacturers is basically an advertising element of their products.


PostPosted: Mon Apr 21, 2014 1:15 pm    Post subject: Reply with quote
Nordentro wrote:
"... zoom lenses have much more glass for the light to travel through in order to reach the sensor; extra elements are required to zoom in and out, and even more are needed to correct the aberrations created by all the extra elements. With the light altered so many times, it is bound to degrade..."


Oh, my... People have a bad experience with cheap filters and then come to bizarre conclusions...

By that "logic", a Domiplan with 3 elements is a better lens than a Zeiss Otus with 12 elements. Or that a $11,799 Canon EF 200-400mm f/4L with built-in 1.4X extender is a crappy zoom. After all, the Canon zoom has 33 elements!


PostPosted: Mon Apr 21, 2014 2:52 pm    Post subject: Reply with quote
I speak in general. The Canon 200-400mm f/4 you refer to is only a meassured 6.9 lens.


PostPosted: Mon Apr 21, 2014 3:25 pm    Post subject: Reply with quote
Nordentro wrote:
I speak in general. The Canon 200-400mm f/4 you refer to is only a meassured 6.9 lens.


I think you are talking about the T-number of the Canon 200-400mm F4. Note that the T-number 6.9 was measured with the extender ON!

The F-number is F5.6, so the loss is little more than 1/2 stops (0.6 stops to be exact). In comparison, the prime 50mm F1.4 loses 0.39 stops. That is, the zoom loss is about 0.2 stops higher than the prime. Not bad for a zoom with 33 elements, compared with a prime with only 7 elements.


PostPosted: Mon Apr 21, 2014 11:07 pm    Post subject: Reply with quote
Yes, mesured is always T stops and F stops is the theoretical speed. As you say, with the extender is 6.9T and 4.7T without. I have no doubt about this being a fantastic lens.


PostPosted: Thu Apr 24, 2014 1:44 am    Post subject: Reply with quote
Okay, so we have the fantastic Canon 70-200/2.8 with nice sweet zoom spot at 200mm -- i.e. designed to be best at maximum zoom, where most users will set zoom...and... the fantastic Sony 14-24mm, along with something or two from Zeiss. What other newer zoom lenses are there which are better than primes.

I will concede that a few modern zoom lenses give as good or better performance at a sweet zoom spot than corresponding prime, but that leaves the entire remaining zoom range...can we say that Canon 70-200mm/2.8 gives as good or better performance at 135mm, at 85mm? No.
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