Surprised: 70-200 F4L Best Wide Open

[tangram]

As many of you have probably done, I decided to make some test shots of one of my lenses yesterday. One reason was to "remind" myself how good this lens is (and to discourage myself from lusting over the 70-200 F2.8L Mk II any further - big time waster!) Setup: 70-200 F4L with lens hood, no filter, 20D, ISO 100, tripod mounted, cable release. RAW, processed exactly the same for each image (minor sharpening, highlight, shadow, saturation and brightness adjustments). Focal length: 140 MM effective. Took a series of photos of a lovely can of WD-40.

The biggest surprise to me was that the best IQ was wide open, at F4. I was expecting the best IQ to be at anywhere from F8-F11. At F16, diffraction began to play a significant role in IQ degradation.

I can report that while my camera is showing its age, the IQ of this lens is very high quality and, let's face it, an absolute steal at $625. Considering its light weight, there is rarely a time when I feel like I can't take it with me when heading out on a hike. Something to think about!

F4.0









F8









F16

 

[Peter 13]

I am surprised that you were surprised. Lenses of this class peak at f/5.6 or so. Check photozone.de, for example.

 

[Ken Phillips]

... at the maximum aperture! In practice, not so much, due to compromises in design.
KP
--
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'The urge to save humanity is always a false front for the urge to rule it.', H. L. Mencken

 

[tangram]

Thanks for that. I guess anectdotally most shooters seem to not get their best results wide open, hence my surprise. Maybe using a crop camera has some impact on the outcome. Anyway, call me naive, uninformed, whatever, it was a pleasant surprise pour moi.

 

[tangram]

Thanks for that Peter. I had a look at the resolution test results using an 8 mp camera (same res as 20D) and F5.6 was indeed the best result for that copy. Mine was best at F4 but that is really just splitting hairs.

F4 100% crop





F5.6 100% crop

 

[selwynbr]

I have no hesitation using this lens at F4. Without pixel peeping, cannot tell if any sharper at 5.6.

 

[gurbally]

This lens is just about as sharp at f4 as at f5.6.

 

[Fantasy Photo]

Any lens will work best at its maximum aperture, but only if the manufacturer does a nearly perfect job of grinding all lens surfaces. Since it costs more money to manufacture a lens with nearly flawless surfaces on all lens elements, most lenses are built to a less than perfect standard and will be a bit fizzy when shot wide open. When such a lens is stopped down, the imperfections in the surfaces of the lens elements contribute less to the formation of the image and the lens sharpens up. As you stop down farther, diffraction takes over and the lens gets fuzzy again. Most pro lenses are built as well as the manufacturer can profitably make them, so they work well wide open. What a lot of people don't realize is that all lenses work pretty much the same when stopped down due to diffraction becoming the limiting factor, so a landscape shooter who always stops down to f/22 or smaller to get maximum depth-of-field can use just about any lens and get decent results. Only people who shoot wide open a lot (sports shooters, for example) will benefit from using expensive pro lenses.
--
http://www.fantasy-photo.com

 

[Dale Buhanan]

That shows you got a very fine copy of the lens. Good for you! I'm envious.
--
kind regards
Dale

 

[Peter 13]

Any lens will work best at its maximum aperture, but only if the manufacturer does a nearly perfect job of grinding all lens surfaces.

No, that is wrong. Look at the MTF chart of (almost any) fast lens.

Since it costs more money to manufacture a lens with nearly flawless surfaces on all lens elements, most lenses are built to a less than perfect standard and will be a bit fizzy when shot wide open. When such a lens is stopped down, the imperfections in the surfaces of the lens elements contribute less to the formation of the image and the lens sharpens up.

Misallignment of the lens elements is far more serious factor than the alleged imperfections of the surface. But that is a different conversation.

The softness wide open of fast lenses is not due to imperfections; it is due to the fact that you cannot achieve theoretically perfect focus at all FL and all points on your focus plane, even if you ignore the wave nature of light. The faster the lens - the farther away from the perfection, even if the lens is build perfectly.

 

[Rich Dykmans]

That shows you got a very fine copy of the lens. Good for you! I'm envious.
--
kind regards
Dale

Yep I agree, hold on to that one. I owned 2 different copies of that lens and they both sucked.

 

[tangram]

Thanks Dale, that was my conclusion. In reading the many 70-200 F2.8L Mk II threads, one thing that seems to be an issue is the weight of this lens. My F4 non IS is very light - in fact it is half the weight of the Mk II (705 g vs 1,490 g) which isn't a big deal if you are stationary but if you are like myself and take photos while hiking, that weight savings is pretty nice to have.

I guess when it comes down to it, lenses mean different things to different people. My lens, which at F4 is capable of, dare I say, images that are very, very close in IQ to a lens that is, gulp, 4X the price. Yes, yes, I know, some posters here would rather eat cat food for the rest of their lives if it meant being able to buy the Mk II. But for me, paying 25% of the price for the same image quality (granted, under easy shooting conditions) that is hal the weight is quite satisfying.

 

[rebel99]

Thanks Dale, that was my conclusion. In reading the many 70-200 F2.8L Mk II threads, one thing that seems to be an issue is the weight of this lens. My F4 non IS is very light - in fact it is half the weight of the Mk II (705 g vs 1,490 g) which isn't a big deal if you are stationary but if you are like myself and take photos while hiking, that weight savings is pretty nice to have.

I guess when it comes down to it, lenses mean different things to different people. My lens, which at F4 is capable of, dare I say, images that are very, very close in IQ

how dare ya, don't let Rich Dykman know that, he'll hound you for the rest of your life just for pointing out that fact and being logical; that will be an insult to his newly aquired f2.8 IS II gearheads will always be 1st to get the new toys and impress their neighbors!

to a lens that is, gulp, 4X the price. Yes, yes, I know, some posters here would rather eat cat food for the rest of their lives if it meant being able to buy the Mk II. But for me, paying 25% of the price for the same image quality (granted, under easy shooting conditions) that is hal the weight is quite satisfying.

 

[Fantasy Photo]

Actual lens MTFs are not relavent to this discussion because we are talking about theorectcally perfect lenses; MTFs measure (or are calculated for) real world glass (which doesn't focus all light wavelengths "perfectly" and measure other degradation issues related to attempts to maintain lens performance over a wide focus range, etc. Nor is "lens alignment" a factor since this discussion applies equally well to telescope optics in which there is usually only a single mirror or lens element.

Lens resolution is limited by the size of the Airy disc. The wider the aperture, the smaller the Airy disc, and therefore the higher the resolution. But any imperfections in grinding the surface(s) of the optic will cause aberrations that degrade the lens resolution. If those aberrations are eliminated by grinding a perfect lens, then the lens will give its best performance at its widest aperture due to the small Airy disc. Aberrations degrade resolution the most for fast f-stop lenses. For example, if you grind two 6-inch diameter mirrors for use in a telescope, one is f/5.6 and the other is f/10, and each mirror has the same 1-wavelength surface imperfection in a 1 square inch area, the f/5.6 will suffer the most resolution degradation from that imperfection. That's why it's easier to grind a good 6-inch f/10 mirror than it is to grind a 6-inch f/5.6 mirror.

Lens manufacturers could design lenses that approach the theoretical limit for resolution when shot wide open if they wanted to spend the money to grind and test each lens element surface to perfection. They actually do that for some pro lenses that they know will be used at their maximum aperture. Just look at Canon's 200 mm f/2 and 300 f/2.8 lenses. But manufactures also have other design issues they have to deal with. The glass they use doesn't focus all wavelengths equally and that requires that more lens elements be used (which increases the chance of one surface not being ground perfectly). That also makes it difficult to maintain lens resolution performance while focusing at different distances. Then there are zoom lenses which add to the complexity. Plus the manufacturer wants a flat field lens, one that doesn't vignette, etc., etc.

The bottom line is that a lens theoretically gives it's best resolution when shot wide open and manufacturers can and will manufacture such lenses that approach that thoretical resolution limit if they think we end users can pay the cost of what it takes to manufacture such lenses. I stand by what I said earlier.

Any lens will work best at its maximum aperture, but only if the manufacturer does a nearly perfect job of grinding all lens surfaces.

No, that is wrong. Look at the MTF chart of (almost any) fast lens.

Since it costs more money to manufacture a lens with nearly flawless surfaces on all lens elements, most lenses are built to a less than perfect standard and will be a bit fizzy when shot wide open. When such a lens is stopped down, the imperfections in the surfaces of the lens elements contribute less to the formation of the image and the lens sharpens up.

Misallignment of the lens elements is far more serious factor than the alleged imperfections of the surface. But that is a different conversation.

The softness wide open of fast lenses is not due to imperfections; it is due to the fact that you cannot achieve theoretically perfect focus at all FL and all points on your focus plane, even if you ignore the wave nature of light. The faster the lens - the farther away from the perfection, even if the lens is build perfectly.

--
http://www.fantasy-photo.com

 

[Rich Dykmans]

Read my post above moron I already told him to hold onto it.

You know what though Rebel? I'm surprised you were stupid enough to spend twice as much on an f4 L IS when you could have had the same wide open IQ this guy's getting out of the non IS f4 L for a lot less money!

So what makes your situation and lens so special?

Like I mentioned above I've owned 2 of the f4 L's (Hey I tried), the f4 L IS, both the 70-200 f2.8 L non IS and f2.8 L IS version I as well as the new version II. but WTF would I know about the performance differences?

And I swear if you come back and post an f4 L vs f4 L IS analysis and why it's worth double the price of the non IS I think I'll vomit!

Thanks Dale, that was my conclusion. In reading the many 70-200 F2.8L Mk II threads, one thing that seems to be an issue is the weight of this lens. My F4 non IS is very light - in fact it is half the weight of the Mk II (705 g vs 1,490 g) which isn't a big deal if you are stationary but if you are like myself and take photos while hiking, that weight savings is pretty nice to have.

I guess when it comes down to it, lenses mean different things to different people. My lens, which at F4 is capable of, dare I say, images that are very, very close in IQ

how dare ya, don't let Rich Dykman know that, he'll hound you for the rest of your life just for pointing out that fact and being logical; that will be an insult to his newly aquired f2.8 IS II gearheads will always be 1st to get the new toys and impress their neighbors!

to a lens that is, gulp, 4X the price. Yes, yes, I know, some posters here would rather eat cat food for the rest of their lives if it meant being able to buy the Mk II. But for me, paying 25% of the price for the same image quality (granted, under easy shooting conditions) that is hal the weight is quite satisfying.

 

[Peter 13]

Actual lens MTFs are not relavent to this discussion because we are talking about theorectcally perfect lenses; MTFs measure (or are calculated for) real world glass (which doesn't focus all light wavelengths "perfectly" and measure other degradation issues related to attempts to maintain lens performance over a wide focus range, etc.

I lost you here. MTF charts are based on math models.

Nor is "lens alignment" a factor since this discussion applies equally well to telescope optics in which there is usually only a single mirror or lens element.

It is, believe me. I wish it were not, this would have saved me a lot of frustration.

Lens resolution is limited by the size of the Airy disc.

If you mean "only by [that]", then this is an unproven statement.

The wider the aperture, the smaller the Airy disc, and therefore the higher the resolution.

Wrong. You are assuming that the lens theoretically can focus perfectly.

But any imperfections in grinding the surface(s) of the optic will cause aberrations that degrade the lens resolution. If those aberrations are eliminated by grinding a perfect lens, then the lens will give its best performance at its widest aperture due to the small Airy disc.

Again a statement based on a false assumption.

Aberrations degrade resolution the most for fast f-stop lenses. For example, if you grind two 6-inch diameter mirrors for use in a telescope, one is f/5.6 and the other is f/10, and each mirror has the same 1-wavelength surface imperfection in a 1 square inch area, the f/5.6 will suffer the most resolution degradation from that imperfection. That's why it's easier to grind a good 6-inch f/10 mirror than it is to grind a 6-inch f/5.6 mirror.

Telescopes are cases where some parameters are close to infinity. What is typical there does not need to be typical for a generic photo lens.

Lens manufacturers could design lenses that approach the theoretical limit for resolution

There is a limit now? Does it depend on the aperture?

when shot wide open if they wanted to spend the money to grind and test each lens element surface to perfection. They actually do that for some pro lenses that they know will be used at their maximum aperture. Just look at Canon's 200 mm f/2 and 300 f/2.8 lenses.

They are still better stopped down a bit. Regardless of whether that is true or not, there is nothing that suggests that they are great lenses because their glass is grinded better. Why would not Canon grind its wide primes to the same perfection?

The bottom line is that a lens theoretically gives it's best resolution when shot wide open

You keep repeating that if it were true. How do you know this?

and manufacturers can and will manufacture such lenses that approach that thoretical resolution limit if they think we end users can pay the cost of what it takes to manufacture such lenses. I stand by what I said earlier.

Why does the performance of a lens gets worse in the corners? Sloppy grinding?

 

[mkerz]

I think I see dust spots in the f/16 one.
--
Tommy