"clean" isos and focal length applications

[Josh Bass]

Interesting. We may be able to borrow a camera, but the lenses I have available through girlfriend are a 1.8 50mm prime, an f4 24-105 zoom, and a (presumably f4, haven't even messed with this one yet) 75-300 zoom.

I was planning on using the prime for a few outdoor night shots, and the 24-105 everywhere else (night interiors, with lighting) unless for some reason it doesn't cut it.

PS my girlfriend does not do video, she does stills, and shoots exclusively in RAW. Do these noise issues/ISO settings apply in that world or is it a video thing?

[Daniel Browning]

Originally Posted by Kris Koster

I'm afraid the multiples appear to be correct and someone has done the tests to prove it:

I've done my own tests as I shoot a lot of night video and the chart appears spot on to me.

You and AdamJ must have made a mistake in your tests, because that chart is way off. While it's true that 160 has less noise than ISO 100 in the shadows, the reason is not "because the sensor chip design was optimised for these settings and will produce the least noise" as you guessed in your blog, the reason is that three of Canon's design flaws happen to cancel eachother out at that setting. As I explained above, one flaw is digital scaling before writing the raw data, the second is too great of bit depth (canceling out posterization of scaling), the third is failing to use the correct white point at each ISO setting, which cancels out the 1/3 stop of dynamic range that would have been lost (so that 160 uses the white point that 200 should have been using).

Originally Posted by Kris Koster

I find it fascinating that ISO 640 has the lowest noise and far less noise than ISO 100.

It might be fascinating if it were true, but it isn't. 640 does not have the lowest noise.

[Daniel Browning]

Originally Posted by Josh Bass

Do these noise issues/ISO settings apply in that world or is it a video thing?

Some things apply, but most don't. For raw stills, it's this:

• ISO 100, 200, 400, 800, 1600 are fine.
• ISO 160, 320, 640, 1250 are fine too (but not better).
• ISO 125, 250, 500, 1000 are slightly worse (1/3 stop less headroom, i.e. dynamic range).
• Underexpose ISO 1600 rather than increase ISO above that (same noise but more headroom).

[Steve Oakley]

Originally Posted by Perrone Ford

This is a widely held myth. A GOOD F4 piece of glass should be just as sharp. The problem is a lot of F4 lenses are crap. So yea, the faster and more high quality lens will look better stopped down.

its not a myth. in fact I'm getting ready a lens review which will clearly show it. there must be another 30+ years worth of lens reviews in Pop or Modern Photography with charts showing the same. very few ( read very expensive ) lenses have very good performance wide open. close them down even one stop, never mind two and you are at the sweet spot of the lens's performance.

Now all of that said, shooting video on a dslr requires a lot less of a lens then does shooting stills. Some fairly poor still lenses will work just fine for video purposes because all the data being so down sampled.

MY personal quest right now is to find a lens I want on the camera all the time, to cover 60% of my shooting needs. I just got a Vivitar 35-105 3.5, ( there is a lot to like about this lens ) but now this lens has me seriously considering an olynpus OM 35-105 2.8. The OM is just the right specs, but the only one I've seen is with a dealer in hong kong for $1200. I'd certainly want to try it out first before....

of course new modern zooms generally do better then older ones. The kit 18-55 canon every one picks on, while slow, it still a very decent performer optically. thank you modern coatings and glass formulations.

[Daniel Browning]

Originally Posted by Steve Oakley

its not a myth.

It is -- I agree with Perrone Ford.

Originally Posted by Steve Oakley

in fact I'm getting ready a lens review which will clearly show it.

Here are a few examples where fast lenses stopped down are the same or worse compared to slow lenses wide open (or not stopped down as much), sometimes even when comparing huge price differences.

50mm f/1.2 ($1,500) @ f/2.8 vs 50mm f/1.8 ($100) @ f/2.8

16-35mm f/2.8 ($1500) @ f/5.6 vs 17-40 f/4 ($700) @ f/5.6

28mm f/1.8 @ f/2.8 vs 28mm f/2.8 wide open

100mm f/2 @ f/2.8 vs 100mm f/2.8 wide open

Originally Posted by Steve Oakley

there must be another 30+ years worth of lens reviews in Pop or Modern Photography with charts showing the same. very few ( read very expensive ) lenses have very good performance wide open.

That does not necessarily conflict with what Perrone is saying. For the same cost as a fast lens that gets sharp when you stop down 1-2 stops, you can make one that is 1-2 stops slower that has even higher quality wide open.

[Perrone Ford]

Daniel, I wasn't going to bother. But thanks.

[Steve Oakley]

Originally Posted by Daniel Browning

It is -- I agree with Perrone Ford.
Here are a few examples where fast lenses stopped down are the same or worse compared to slow lenses wide open (or not stopped down as much), sometimes even when comparing huge price differences.

50mm f/1.2 ($1,500) @ f/2.8 vs 50mm f/1.8 ($100) @ f/2.8

16-35mm f/2.8 ($1500) @ f/5.6 vs 17-40 f/4 ($700) @ f/5.6

28mm f/1.8 @ f/2.8 vs 28mm f/2.8 wide open

100mm f/2 @ f/2.8 vs 100mm f/2.8 wide open
That does not necessarily conflict with what Perrone is saying. For the same cost as a fast lens that gets sharp when you stop down 1-2 stops, you can make one that is 1-2 stops slower that has even higher quality wide open.

Huh ????? I'd looked at all those examples, welcoming to be proven wrong, but that simply wasn't the case. every single one of those lenses was softer wide open then closed down 2 stops. the 100 2.8 came the closest to being **almost** as sharp wide open as closed down, but there still was a difference, wider being softer. its certainly a very good lens overall. the 50 1.2 was particularly soft wide open, and didn't look very good until 2.0.

as for your last comment about making a sharper but slower lens, sorry by physics is in the way. when you close the iris down about 2 stops, it blocks stray light from passing thru the lens ( basic explanation ) resulting in increased contrast & resolution. sure there are some really expensive examples that might break this basic rule, but they are few and far between, and none of the lenses shown here. with very careful lens & coating formulation, you might get close, but physics is going to win in the end. the 100 2.8L was probably about the closest to having no major practical shooting difference wide open to stopped down a bit, but thats one lens. A long lens that wouldn't be your day to day shooting glass for most video purposes.

Most folks want a zoom for their goto lens that stays on the camera most of the time, which will cover a decent amount of their shooting needs. If you are really concerned with the sharpest lens, then use all primes. primes will perform better then zooms because you are again going head to head with physics that you can't cheat on. I'm not saying that modern zooms ( basically anything in the AF era ) isn't a much better zoom then the older glass, it almost always is. However, modern primes have also benefited from better glass & coating formulation. Where you might find a zoom out performing a prime is if you take a modern zoom against an older prime - a MF one.

if you want a lens sharper, you have to make it bigger. the **general** rule is that two lenses of otherwise the same specs ( focal length / stop ), the one with the larger front element will perform better. this is again all based on physics. can apo / FL glass help ? sure, but again I'd say compare 2 APO glass lenses side by side. of course the bigger the glass elements, the more they cost, so lens makers pick their compromises carefully.

most glass performs best starting at 2 stops closed down, thru about F8. Once you get smaller then F8 you start to get a pinhole lens effect from the iris again starting to soften the image. some glass may hold out to F11, but by F16 it starts to become apparent.

with all of that said, sharper isn't always best for video purposes with these cameras. the slightly ( sometimes not so slightly ) softer older glass still makes a nice video image, because the softer image produces less moire / aliasing. so there is clearly a trade off point here. in practical shooting terms though, wide open may produce visible softness that 2 stops in won't, regardless of lens, which is visible when shooting video. with all things being equal in that respect, a faster lens ( 2.8 @ 4 ) will consistently be sharper then a F4 lens wide open due to the basic physics involved with how lenses work, assuming reasonable equal glass - you aren't comparing a $5k or more lens to a consumer / pro photography lens for $500 or even less.oh.... and just for the record some one has done lens tests between cine style glass and still photography glass and basically the quick answer is, the cine glass wasn't any sharper then the still lenses. However, the cine lenses have much better mechicals for shooting purposes.

[Daniel Browning]

Originally Posted by Steve Oakley

Huh ????? I'd looked at all those examples, welcoming to be proven wrong, but that simply wasn't the case. every single one of those lenses was softer wide open then closed down 2 stops.

We may have a failure to communicate. :) It probably didn't help that I posted the examples without any explanation. Let me try to rectify that now by explaing what I meant the examples to show, and you can tell me if it helped or not:

When you open the first link, it shows three images from the $1,500 lens (Canon EF 50mm f/1.2 L) stopped down to f/2.8:



If you mouse-over the images, it replaces them from three images taken from a different lens, the Canon EF 50mm f/1.8, which only costs $100:



Do you agree that this image is sharper? To me it is. And it is from a lens that costs over an order of magnitude less, and it's only stopped down by 1.3 stops -- much closer to wide open than the $1500 lens which is stopped down 2.3 stops.

Now let's look at the second example:

Here is the $1,500 Canon 16-35 f/2.8 II at 16mm f/5.6 (stopped down 2 stops) - mid-frame:



Compare that with the $700 17-40 f/4 at 17mm f/5.6 (stopped down just 1 stop) - mid-frame:



Do you see how the slower lens is sharper? This despite the fact that it's half the price and is one stop closer to being wide open.

I hope that helps to explain why I think the examples help support the position I've taken. In any case, I can at least say that we agree on this much:

• Usually, 1-2 stops down is sharper than wide open.

I think this is what we disagree about:

• A fast lens stopped down by two stops is sharper than a slow lens stopped down only 1 stop.

Originally Posted by Steve Oakley

the 100 2.8 came the closest to being **almost** as sharp wide open as closed down, but there still was a difference, wider being softer.

Yes, and I agree, because I agree that stopping down makes lenses sharper. But my point is something different -- that a slow lens can be (and often is) sharper than a fast lens stopped down to the same f-stop.

Originally Posted by Steve Oakley

as for your last comment about making a sharper but slower lens, sorry by physics is in the way.

One of my pet peeves is when people throw the word "physics" around incorrectly. If you look at angular resolution, sure, fast f-numbers have more angular resolution than slow ones -- but that that's only in the object plane, which is useful to astrophotographers and the like with their wide converters ("field flatteners"), but not the image plane, which is how most folks think.

For a given cost, you can polish a slow lens to a higher Strehl ratio than a fast lens, if not for any other reason then at least because the faster lens is going to have far more area, and lens polishing cost scales exponentially with area.

Originally Posted by Steve Oakley

when you close the iris down about 2 stops, it blocks stray light from passing thru the lens ( basic explanation ) resulting in increased contrast & resolution.

Well, to my understanding that would only explain why it has less flare when stopping down -- not better sharpness or resolution (I think those are improved for entirely different reasons).

I think that you get less flare from not having the glass there in the first place than by putting an aperture stop to block the glass (though not all of it's reflections).

Originally Posted by Steve Oakley

with very careful lens & coating formulation, you might get close, but physics is going to win in the end.

Would it be alright if we left physics out of it? At least until you have a specific physical law that you want to use to support your position?

Originally Posted by Steve Oakley

if you want a lens sharper, you have to make it bigger.

No. Again, that only applies to angular resolution, i.e. resolution in object space. That is how astronomers and astrophotographers such as myself think about it. But photographers and videographers aren't interested in arbitrary angles of view -- they want specific wide, normal, and telephoto fields to go with their accustomed perspectives. And even if they did know about angular resolution, they don't have unlimited flexibility in backfocus distance (39mm -- it's the law!), so they couldn't convert object space resolution into image space with a wide converter anyway, because then it wouldn't focus on the camera.

Originally Posted by Steve Oakley

the **general** rule is that two lenses of otherwise the same specs ( focal length / stop ), the one with the larger front element will perform better.

That is not a general rule. Take one of the sharpest photographic lenses in existence: Coastal Optics 60mm f/4 Macro. You can take any 50mm or 60mm lens you want from f/1.0 on up, stop it down to f/4, and it's still not going to bear the performance of that lens, despite the fact that the Coastal Optics front element is much smaller. With the $4,000 cost of the lens, the lens designer, Brian Caldwell (brilliant fellow by the way), had all the freedom he wanted to make it an f/1.4, f/1.2, or f/1.0, but he didn't because he put that cost into the sharpness instead.

Originally Posted by Steve Oakley

Once you get smaller then F8 you start to get a pinhole lens effect from the iris again starting to soften the image.

Agreed. (BTW, the proper term is diffraction.)

Originally Posted by Steve Oakley

some glass may hold out to F11, but by F16 it starts to become apparent.

Not so. Diffraction affects all lenses equally - no lens can "hold out" any better than another.

Originally Posted by Steve Oakley

with all of that said, sharper isn't always best for video purposes with these cameras. the slightly ( sometimes not so slightly ) softer older glass still makes a nice video image, because the softer image produces less moire / aliasing.

Agreed.

Originally Posted by Steve Oakley

and just for the record some one has done lens tests between cine style glass and still photography glass and basically the quick answer is, the cine glass wasn't any sharper then the still lenses. However, the cine lenses have much better mechicals for shooting purposes.

Interesting.

[Kris Koster]

Originally Posted by Daniel Browning

You and AdamJ must have made a mistake in your tests, because that chart is way off. While it's true that 160 has less noise than ISO 100 in the shadows, the reason is not "because the sensor chip design was optimised for these settings and will produce the least noise" as you guessed in your blog, the reason is that three of Canon's design flaws happen to cancel eachother out at that setting. As I explained above, one flaw is digital scaling before writing the raw data, the second is too great of bit depth (canceling out posterization of scaling), the third is failing to use the correct white point at each ISO setting, which cancels out the 1/3 stop of dynamic range that would have been lost (so that 160 uses the white point that 200 should have been using)..

It's not entirely true that I guessed it. I was trying to listen to the Canon rep who was explaining the reasons to us, likely I misunderstood that explanation. I'm not a sensor expert. Although Canon already acknowledge that lower noise levels can be found at higher ISO numbers. I just couldn't find another source anywhere else online (other than that chart) that could break it down properly.

My own tests found that the graph appeared to be telling the truth, although I admit my tests were mere casual observations of footage taken at various ISO levels.

I am more than happy to change the information in the blog to factual analysis. If you could point me in the right direction as to where you acquire that information from, I'd be very grateful. As a filmmaker, I would be particularly keen to learn from you what the most ideal ISO vs Noise levels are.

Kris

[Josh Bass]

ARe we saying the whole thing is nonsense? Or just that the chart is inaccurate?

[Steve Oakley]

Originally Posted by Daniel Browning

We may have a failure to communicate. :) It probably didn't help that I posted the examples without any explanation. Let me try to rectify that now by explaing what I meant the examples to show, and you can tell me if it helped or not:

When you open the first link, it shows three images from the $1,500 lens (Canon EF 50mm f/1.2 L) stopped down to f/2.8:


If you mouse-over the images, it replaces them from three images taken from a different lens, the Canon EF 50mm f/1.8, which only costs $100:


Do you agree that this image is sharper? To me it is. And it is from a lens that costs over an order of magnitude less, and it's only stopped down by 1.3 stops -- much closer to wide open than the $1500 lens which is stopped down 2.3 stops.

comparing the 2 @ 2.8, yes the $100 is sharper, but at the same time, the 1.2 is sharper wide open then the 1.8 - to be expected for the price difference. really the 1.2 doesn't seem to be that great a lens, despite its price, across the range. if you compare the 1.4, its clearly the best of the 3 that canon makes

now if you want a comparison that makes my point, look at the 1.4 @ 2, and the 1.8 @ 2... which is sharper :) then compare both at 2.8.

to go further, if you look at the Nikon 50 1.4, its just plain nasty loaded with purple fringing, but at 2.8 looks very very good.

so lens design, glass & coating formulation all play roles. a lens builder can pick where they want their lens to be best, at the compromise of where it does less then great - wide open or stopped down a bit.

Originally Posted by Daniel Browning;1568994

Now let's look at the second example:

Here is the $1,500 Canon 16-35 f/2.8 II at 16mm f/5.6 (stopped down 2 stops) - mid-frame:
[img

http://thebrownings.name/misc/2010/2010-09-13-lens-comparison/2009-04-09_07-55-42.jpg[/img]

Compare that with the $700 17-40 f/4 at 17mm f/5.6 (stopped down just 1 stop) - mid-frame:


Do you see how the slower lens is sharper? This despite the fact that it's half the price and is one stop closer to being wide open.

I hope that helps to explain why I think the examples help support the position I've taken. In any case, I can at least say that we agree on this much:

well if you look at the both wide open, 2.8 v 4 @ 16 and 17mm, the 17 F4 is sharper... but stop the 16 2.8 to 4, and now its better based purely on F stop. then at 5.6, you win because you may well be out of the faster lens's sweet spot for optical performance. I'm sure there are other lenses out there that would do better or worse, because in absolute terms the 17 F4 is better, but they are both generally close enough that no one would ever turn up either lens as bad.

Originally Posted by Daniel Browning

One of my pet peeves is when people throw the word "physics" around incorrectly. If you look at angular resolution, sure, fast f-numbers have more angular resolution than slow ones -- but that that's only in the object plane, which is useful to astrophotographers and the like with their wide converters ("field flatteners"), but not the image plane, which is how most folks think.

ok, its been a long time since I sat thru classes on this stuff, and google failed to quickly turn up what I was looking for ... for now ;)

Originally Posted by Daniel Browning

Well, to my understanding that would only explain why it has less flare when stopping down -- not better sharpness or resolution (I think those are improved for entirely different reasons).

correct, wide open, stray light comes in from the edges of the lens reducing contrast and causing aberrations that reduce sharpness / contrast. at about 2 stops down from wide open, the iris blocks out most of this stray light, until you get the iris small enough to start causing diffraction on its own. this is what I was talking about in general terms of physics of the lens. I wasn't going to start putting up the math... its been way too long for that.

Originally Posted by Daniel Browning

I think that you get less flare from not having the glass there in the first place than by putting an aperture stop to block the glass (though not all of it's reflections).
Would it be alright if we left physics out of it? At least until you have a specific physical law that you want to use to support your position?

well in general you are right, the more glass, the more chance for reflections between elements to reduce contrast. thats where lens coatings come into play to reduce these. so in general a lens with fewer elements should be sharper then one with more. however, with coatings, and even painting the edges of the glass black, it can change this so that a lens with more elements can perform better then one with less. perhaps this is part of how some slower / cheaper lens can sometimes be sharper - they are simply made with fewer elements to reduce contrast, where as the wider more expensive super fast glass may require more glass, or more radical shapes to make it work.

Originally Posted by Daniel Browning

That is not a general rule. Take one of the sharpest photographic lenses in existence: Coastal Optics 60mm f/4 Macro. You can take any 50mm or 60mm lens you want from f/1.0 on up, stop it down to f/4, and it's still not going to bear the performance of that lens, despite the fact that the Coastal Optics front element is much smaller. With the $4,000 cost of the lens, the lens designer, Brian Caldwell (brilliant fellow by the way), had all the freedom he wanted to make it an f/1.4, f/1.2, or f/1.0, but he didn't because he put that cost into the sharpness instead.

pity me never having heard of such lens. I think you can always find an exception to just about anything. a faster lens will let more stray light in reducing contrast, so you stop it down towards 4 / 5.6 which is where more lenses do best ( very generalized rule, plenty of exceptions of lenses that do better or worse ).

back to my original statement edited " a larger front element will result in a sharper lens "

Originally Posted by Daniel Browning

Not so. Diffraction affects all lenses equally - no lens can "hold out" any better than another.

quoting -------

Picture sharpness also varies with f-number. The optimal f-stop varies with the lens characteristics. For modern standard lenses having 6 or 7 elements, the sharpest image is often obtained around f/5.6–f/8, while for older standard lenses having only 4 elements (Tessar formula) stopping to f/11 will give the sharpest image. The reason the sharpness is best at medium f-numbers is that the sharpness at high f-numbers is constrained by diffraction,[4] whereas at low f-numbers limitations of the lens design known as aberrations will dominate. The larger number of elements in modern lenses allow the designer to compensate for aberrations, allowing the lens to give better pictures at lower f-stops. Light falloff is also sensitive to f-stop. Many wide-angle lenses will show a significant light falloff (vignetting) at the edges for large apertures. To measure the actual resolution of the lens at the different f-numbers it is necessary to use a standardized measurement chart like the 1951 USAF resolution test chart.

------------
F-number - Wikipedia, the free encyclopedia

I'll add that coatings, APO / FL / exotic glass, and the refractive index of the particular glass used can all come into play here. even the internal construction of the lens can have play here - there are some lenses that have internal baffles to control stray light, or at the rear exit. this all means its easy to find exceptions to general rules.

also, because of how F stops are calculated, a larger diameter lens should be able to top down more before diffraction thru the iris takes place... well that would be a very interesting one to figure out.

[Jon Fairhurst]

Often, the advantage of a fast lens stopped down over a slow lens opened wide is that the fast lens will have less falloff. This is especially true of lenses from off-brand manufacturers who want to offer high-end specs for a low-end price. To achieve that, they simply let you open the aperture wider on a relatively small lens than where Canon, Nikon, Zeiss, or Leica would ever allow their designers to go.

Personally, for video on a DvSLR, I'm more sensitive to unwanted falloff than softness. You can always vignette in post. Un-vignetting isn't as successful. As long as you really nail focus on your subject, the sharpness results are usually acceptable with any quality lens.

For 21MP photos and large prints, sharpness comparisons mean a whole lot more than for video.

[Daniel Browning]

Thanks for the response, Kris. I should not have guessed that you guessed it. :) Sorry about that.

Originally Posted by Kris Koster

I am more than happy to change the information in the blog to factual analysis. If you could point me in the right direction as to where you acquire that information from, I'd be very grateful.

Well, it's easy to point out flaws in the work of someone else, but coming up with your own work is a lot more effort. :) The only comprehensive data sets I can find are for raw files, like the chart you linked to.

I'm sure there are some good ISO-vs-noise comparisons for 5D2 video out there somewhere, but the only ones I can find right now are the flawed ones, such as testing with the lens cap on, using the same exposure, and/or low-frequency flourescents, etc.

Compiling that sort of useful data for video is more difficult because it only applies to the specific dynamic range of the scene, color temperature of the light, in-camera settings (especially tone curve, contrast, and white balance), what tonal level of the image is analyzed, and how much detail it has.

For example, in a low dynamic range scene with magenta light, neutral white balance, and normal in-camera contrast settings looking at middle gray in a highly detailed section, ISO 800 looks great -- just a little bit of random noise, like grain -- and in fact a lot of people prefer the nice random "grain" of the higher ISO setting.

But in high dynamic range scene, with low-CRI tungsten light (e.g. 3000K), blue-ish white balance, low in-camera contrast settings, looking at deep shadows (e.g. 4 stops below middle gray), with HTP enabled, even ISO 160 will have the much disliked pattern noise (lines).

To make matters more complicated, in some cases, one setting may have less random noise, but more pattern noise. That's why most people prefer ISO 160 over ISO 100 -- the 160 actually has more random noise in the midtones and highlights, but it (sometimes) has less pattern noise in the deep shadows, making it preferred.

Sorry I can't be more helpful.

Originally Posted by Josh Bass

ARe we saying the whole thing is nonsense? Or just that the chart is inaccurate?

Just that chart.

[Daniel Browning]

Originally Posted by Steve Oakley

really the 1.2 doesn't seem to be that great a lens, despite its price, across the range.

I think it's because Canon put all their effort into creating fantastic bokeh -- sharpness wasn't a priority at all. I absolutely love that lens for environmental portraits, but I'd rather take the f/1.8 for landscapes.

Originally Posted by Steve Oakley

now if you want a comparison that makes my point, look at the 1.4 @ 2, and the 1.8 @ 2... which is sharper :) then compare both at 2.8.

You've got me there, but I would point out that there's still a four-fold difference in price. I maintain that if they quadrupled the cost of the 50mm f/1.8, they could make it even sharper than the 50mm f/1.4 when both are stopped down to the same f-stop.

Originally Posted by Steve Oakley

pity me never having heard of such lens. I think you can always find an exception to just about anything.

I don't think it's the exception, though. If you have two similar lenses, one fast and one slow, and stop them down to the same f-stop, I think the slower lens will either be sharper or cheaper (or both). In further effort of demonstrating this as a general rule, here are a few more examples:

Nikon 17-35 f/2.8 ($1800) @ f/4 vs 16-35 f/4 @ f/4 ($1200) - the slow lens is much much sharper and much cheaper.

70-200 f/2.8 L IS (Mark I for $1900) @ f/4 vs 70-200 f/4 L IS ($1200) at f/4 - the slow lens is slightly sharper and much cheaper.

70-200 f/2.8 L ($1300) @ f/4 vs 70-200 f/4 L ($700) @ f/4 - the slow lens is softer but much cheaper.

24mm f/1.4 L Mark I ($1200 used?) @ f/2.8 vs 24mm f/2.8 ($300) @ f/2.8 - the slow lens is a little sharper and much cheaper.

Originally Posted by Steve Oakley

back to my original statement edited " a larger front element will result in a sharper lens "
quoting [...] from "F-number - Wikipedia"

I don't think that quote supports your position.

Originally Posted by Steve Oakley

also, because of how F stops are calculated, a larger diameter lens should be able to top down more before diffraction thru the iris takes place... well that would be a very interesting one to figure out.

Let me illustrate it. f/10 on a 10mm optic is a very small aperture stop (1mm in diameter), while f/10 on a 2500mm optic like my telescope is much larger (250mm). The 10mm weighs 1 pound and the 2500mm weighs 50 pounds. But when they're both used on the same camera, the diffraction in image space (what photographers care about) is the exact same, because the f-number is the same.

[Daniel Browning]

Originally Posted by Jon Fairhurst

Often, the advantage of a fast lens stopped down over a slow lens opened wide is that the fast lens will have less falloff.

I agree. Even if you had automatic vignetting correction, the corners would have more noise, which might be worse than just being darker.