[IchabodCrane]

"Of course" because it was obvious what he was referring to.

Equivalent systems are a totally different question. The question is how the same lens performs on different formats. You equivalence guys seem to think that "equivalent" photos are the only valid comparison one can ever make when different-sized sensors are involved. But I don't have two equivalent systems (why would I?) and don't care about that. But I might want to know how a particular lens that I actually have performs on each type of sensor. If I have a lens that is a good performer on APS-C, I might be curious to know that I would not like it so much on an FF because of too soft edges -- edges I never see on APS-C. Or vice-versa, maybe APS-C is "stressing" the lens too much and FF is more suitable. Those are the interesting questions I am interested in -- I don't give a crap about equivalence (which often involves fantasy lenses that don't even exist) and it is annoying to have all discussions twisted to fit into that box...

I'm not sure I'm grasping what you're saying. When I think of edge/corner sharpness, I'm thinking of what an actual subject on the photo looks like. To get that same subject, I need a different lens for a different size sensor. How can I really compare what two images look like compared to each other if I don't have the same FOV? Seems to me I would need to compare a 50 or 55 mm FF lens on a FF camera to a 35 mm lens on an APS-C camera to truly make a valid comparison.

Am I missing something here?

[vonBaloney]

I'm not sure I'm grasping what you're saying. When I think of edge/corner sharpness, I'm thinking of what an actual subject on the photo looks like. To get that same subject, I need a different lens for a different size sensor. How can I really compare what two images look like compared to each other if I don't have the same FOV? Seems to me I would need to compare a 50 or 55 mm FF lens on a FF camera to a 35 mm lens on an APS-C camera to truly make a valid comparison.

Am I missing something here?

That's true, if what you are interested is in comparing "equivalent" images from different systems (which necessarily means different lenses). That's fine, it just isn't what the guys above were talking about before side-tracked into equivalence stuff, which was about the performance of the same lens on different sensors. But anytime anybody tries to talk about that (same lens, different format) someone always comes along and says that only "equivalent" comparisons are valid or interesting, and it gets old, because as a practical matter some of us are interested in how actual existing equipment we might have performs under differing conditions instead of theoretical equivalence comparisons. So if I've got current APS-C equipment and a boatload of lenses and thinking about getting an FF (or whatever), I'm not necessarily only interested in comparing in whether I'll get a better image with a 35 on the APS-C or a 55 on a FF -- I might also simply be wondering: What does this 35 look like on a FF? Is the CA and sharpness on the edges of the image circle (that I've never seen because they've been cropped out on my APS-C) really bad so maybe I wouldn't want to use it at all? Or will my 200mm that is mediocre actually look better on the FF because the view will be wider and the flaws won't be as visible with typical image sizes? As soon as anybody starts talking about anything like that, somebody comes and tells them they shouldn't be and should be talking "equivalent" images, even though equivalent images aren't always even possible or desirable or even relevant...

[Class A]

I might also simply be wondering: What does this 35 look like on a FF?

That's a perfectly fine question (and I have similar ones myself).
I'd say the best way to find out is to try the lenses you are interested in on an FF; there are theoretical considerations about sharpness but also practical matters, such as incident levels that may cause colouration, rear elements causing reflections, etc.

The problem starts when someone makes a comparison between systems, even when it is an implicit one such as "FF corners are rubbish". The question then needs to be asked "in comparison to what?". If someone is comparing different sensor formats ("sweet spot", etc) then they should better have a way of making fair comparisons.

You can argue that the same lens on APS-C and FF has the shallower DOF on APS-C. This is a fact, but is it a useful one? Not to me, hence, I'm making comparisons on equivalent images. I don't see what the merit of comparing two completely different images (the same lens on APS-C and FF produces different AOVs) is, so I don't do it and sometimes I point out to others that they may want to do the same.

As for the "equivalence story getting old": At least it is a true story that's getting old. So many more times you read people state that an f/1.9 lens for the Q has more light gathering ability than a f/2.8 lens for an MF camera, that DOF and light gathering are two different things, that FF lens corners are rubbish, etc. This stuff gets really old as well and is wrong on top of it.

[spade111]

I like what you're cooking here Von Baloney.

[vonBaloney]

The problem starts when someone makes a comparison between systems, even when it is an implicit one such as "FF corners are rubbish". The question then needs to be asked "in comparison to what?". If someone is comparing different sensor formats ("sweet spot", etc) then they should better have a way of making fair comparisons.

But no one did that -- no need to drag the conversation into the equivalence domain when it isn't about that. A poster above said he put what was supposedly an FF lens on an FF camera only to discover the edges were crap. End of story -- equivalence has nothing to do with it.

I'm not disputing the info, the facts or the usefulness of it -- it is all good stuff, but it is not the only thing to talk about. And also, the way it is presented I feel is often very confusing because of the assumption that the only "valid" comparisons are between images on theoretically equivalent systems so that part is often left out as a given and statements like "FF has thinner DoF" are often made that make no sense without that assumption.

[Clavius]

Why "of course"?

Say you have FF corners that resolve as much as APS-C centres. Why then care that your FF centres are even better?

Because I don't have FF corners that resolve s much as APSC centres.

Centres will always perform better than corners; the interesting question is how what you've cropping away with a smaller sensor performs with a bigger sensor in comparison to equivalent image areas captured by the smaller sensor.

True... I think. Because that was my point. Falconeye says that the socalled sweetspot doesn't exist. I wasn't comparing systems, I was just trying to understand why it doesn't exist, and what is causing the bad borders and corners on my FF system with FF lenses.

[bossa]

I like the lines per millimeter measurement format as it doesn't matter what format you're looking at - it's more of an absolute value than the LW/PH format. Lenstip is pretty good at reviewing lenses in my book and they use that particular method.

The resolving power of an optical system (telescope) is usually measured as an angle (arc seconds) and the sensor pixel pitch/density of a DSLR also has an important role to play in the overall resolution of a camera system in addition to the optics. But to say that a FF system which resolves "n" LW/PH (or "x" lines/mm) is of a higher resolution than the same lens on an APS-C sensor of equal pixel density (also measuring "x" lines/mm) is frankly BS as the resolution of the 'data' has nothing to do with the size of the sensor. Obviously, when you output an image to a 'standard' format the APS-C is enlarged more and there is an apparent improved resolution of the FF. But it's only apparent due to the output being altered to facilitate viewing. The K-5 has a slightly more dense sensor than the D800 and thus that camera is in fact a higher resolution camera as a result - It just doesn't capture the larger area of the FF sensor. LW/PH is a phoney resolution format as it also takes into account the sensor size and disregards the absolute resolving power of a system.

PS: Perhaps the term resolution needs a more specific definition when applied to DSLR's for any arguments to have a common value for all users.

Regarding 'sweet spots': Some lenses definitely have them, the Sigma 50 F1.4 being one of them.

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Last edited by bossa; 11-14-2012 at 02:59 AM.

[gazonk]

"Of course" because it was obvious what he was referring to.

Equivalent systems are a totally different question. The question is how the same lens performs on different formats. .

Hear, hear!

Many of those who dream of FF are planning to use their Pentax K lenses on a Pentax FF camera, and then the quality of those lenses is what counts, not the quality of imaginary equivalent lenses.

[Clavius]

I like the lines per millimeter measurement format. It doesn't matter what format you're looking at as it's more of an absolute value than the LW/PH format. Lenstip is pretty good at reviewing lenses in my book and they use that particular method.

The resolving power of a lens (telescope) is usually measured as an angle in arc seconds and the sensor pixel pitch/density has an equally important role to play in the overall system),but to say that a FF system which resolves 4000 LW/PH is of a higher resolution than the same lens on an APS-C sensor of equal pixel density is frankly BS as the resolution of the 'data' has nothing to do with the size of the sensor. Obviously, when you output an image to a 'standard' format the APS-C is enlarged more and there is an apparent improved resolution of the FF. But it's only apparent due to the output being altered to facilitate viewing. The K-5 has a slightly more dense sensor than the D800 and thus that camera is in fact a higher resolution camera as a result. It just doesn't capture the larger area of the FF sensor.

Regarding 'sweet spots': Some lenses definitely have them, the Sigma 50 F1.4 being one of them.

Thank you very much for this explanation. I thought I was going crazy, seeing non-existent "sweetspots" and/or corner/border softness.

Some people don't care about the borders. Sadly, I do care a lot. I never had the pleasure of using FF on film, so I guess APSC + FF lenses spoiled me with good uniform corner to corner resolution.

..I've been mounting some of my 645 lenses on the 5D though, those produce excellent results. LOL!

[bossa]

I edited my previous post to try and clarify it a bit...

[Class A]

A poster above said he put what was supposedly an FF lens on an FF camera only to discover the edges were crap. End of story -- equivalence has nothing to do with it.

It would have been "end of story" if the poster had simply written "this lens has weak corners". Many lenses have weak corners on all sorts of formats...

By expressing that this or that lens has weak corners on an FF camera, the poster is implicitly making a comparison to corner performance on a crop camera. This raises the question of what a good way of ascertaining whether corners are really weak (compared to what).

Anyway, I'm out of this discussion.

[ElJamoquio]

LW/PH is a phoney resolution format as it also takes into account the sensor size and disregards the absolute resolving power of a system.

LW/PH describes the maximum resolution of the picture.

LW/mm describes the maximum resolving power of the lens.

Both are completely valid. LW/PH is the better metric for people taking pictures IMO.

[vonBaloney]

By expressing that this or that lens has weak corners on an FF camera, the poster is implicitly making a comparison to corner performance on a crop camera.

No he wasn't. See you've got "equivalence" on the brain so you naturally go there. He wasn't comparing the corners to ANYTHING because he was saying he *has never seen the corners* before (because they've been cropped out on APS-C) -- he was talking about the image circle of a particular lens not equivalent images. He was I suppose implicitly comparing the APS-C corners (the ones he gets) with the corners he gets from the same lens on FF, but those are very definitely not equivalent images (knowingly) -- one is just a crop from the other which is what the "of course" was all about since that should have been obvious to anyone not suffering from equivalenceitis.

Ok, that settled, Falk Lumo did in fact bring up an interesting point about the sweet spot not existing, which was again confusing because of his assumption that everyone will immediately think that any comparison will of course be from theoretically equivalent systems. Of course "sweet spots" exist on particular lenses, which you've said and everyone agrees. Centers are better than corners on most lenses, therefore the sweet spot exists. But what Lumo was getting at was that he doesn't think it really exists between equivalent images from different systems, right? Totally different discussion, but probably worth having. So what he was saying (I think) is that if you take equivalent pictures (different sized sensors using different lenses to get same image), then the FF image will still end up superior (even to the corners) even though the APS-C system used the "sweet spot" of its lens and the FF system used most of its image circle, and that's because the APS-C version also has to be magnified more, etc etc to make it equivalent, eh? Is that the gist of it? So he was not denying that sweet spots exist on lenses, just there is no ultimate advantage to that if you think that you're getting better quality on APS-C because of it, and that's because again if you take the equivalent image on FF (with a different lens) those advantages are nullified and still even exceeded by the advantages of equivalent FF. Have I got that at least approximately right?

Still, how can that statement be made as a blanket statement? Since equivalent images are necessarily made with different lenses, if you use a total piece of crap lens on FF and the best thing Zeiss has to offer on the APS-C, isn't the latter going to end up superior? Or maybe I totally missed his point?

Anyway, the original discussion was very definitely not about that, but about what is now seen on FF that wasn't seen on APS-C with the same lens (simply because it was cropped out), i.e. would I rather use ONLY the sweet spot of this lens (on APS-C) or is it still good enough to use on FF once I uncover the edges of its image circle? (On at least one lens of the poster that made the above comment about his FF lenses on a Canon, he seemed to indicate a definite no to the latter -- the lens performs ok on APS-C but not on FF.)

[ElJamoquio]

Since equivalent images are necessarily made with different lenses, if you use a total piece of crap lens on FF and the best thing Zeiss has to offer on the APS-C, isn't the latter going to end up superior?

I know this was a rhetorical question - but in all probability the total piece of crap lens on FF will outperform the Zeiss on APS-C in terms of resolution. That's the point - that the change in format is a huge factor in terms of resolution per picture.

I say this both as an owner of a Zeiss lens and a FF camera (but primarily just understanding the optics/tests involved).

I'm sure someone could dig out a 1963 JC Penney lens that's been scratched to hell... but in terms of new lenses sold today, 95% of FF lenses out-resolve the best 5% of APS-C lenses.

Whether any of this matters to you...?

[vonBaloney]

I know this was a rhetorical question - but in all probability the total piece of crap lens on FF will outperform the Zeiss on APS-C in terms of resolution. That's the point - that the change in format is a huge factor in terms of resolution per picture.

I say this both as an owner of a Zeiss lens and a FF camera (but primarily just understanding the optics/tests involved).

I'm sure someone could dig out a 1963 JC Penney lens that's been scratched to hell... but in terms of new lenses sold today, 95% of FF lenses out-resolve the best 5% of APS-C lenses.

Whether any of this matters to you...?

Well yeah, but a soft lens is a soft lens, ain't it? And it does matter because all the FF lenses I have are decades old manual K-mount and m42 glass and I'm wondering how they will perform when I get that FF body sometime not too far in the future...