[Brightcolours]

I already wrote "(or angle of view, more precisely)".

 

Then, if the labeled 12mm FL for the wide end of the 12-40mm lens is the true FL at the wide end, then after the distortion correction the diagonal view angle of the image would be that of FL like 13mm?

No, it will be 12mm. Like I said, uncorrected lenses or under corrected lenses give a wider view for the same focal length. So what you end up with a similar view as any other well corrected 12mm lens, after correction of the barrel distortion.

The native distortion at 12mm is very large, about 8% according to the test. On the other hand, it seems that the labeled FL at the two ends of a zoom lens is often not quite accurate.

 

My Nikor 16-35mm also has a high distortion at 16mm, about 5% I guess. Each time after I correct the distortion afterwards I clearly see the change in angle of view which I don't like. So I only correct the distortion when it is necessary.

Same story. The 16-35mm you have is not well corrected at 16mm, and gives a wider view than a better corrected lens at 16mm. When you correct the barrel distortion, you end up with a similar width as with the well corrected lens.

[davidmanze]

      I think it's just a way of improving certain parameters of  lenses at the expense of others, other manufacturers are improving parameters without resorting to heavy distortions, Personally, I prefer the latter!  

[Guest]

No, it will be 12mm. Like I said, uncorrected lenses or under corrected lenses give a wider view for the same focal length. So what you end up with a similar view as any other well corrected 12mm lens, after correction of the barrel distortion.

Same story. The 16-35mm you have is not well corrected at 16mm, and gives a wider view than a better corrected lens at 16mm. When you correct the barrel distortion, you end up with a similar width as with the well corrected lens.

 

Yes, we know that 12mm is 12mm in theory. However, we know the companies fudge (i.e. a 300mm lens may be 280 mm). So, did they design the lens as an 12mm lens, with the uncorrected view like an 10 or 11mm field of view, and after processing you get 12mm? Or did they fudge, and the unprocessed is 12mm, while processed is only 14mm? 

 

If one calculates back, one could determine the actual field of view, but did anybody do that?

 

As a matter of fact, what about normal dSLR lenses, say a normal 18-55 kit lens that has barrel distortion that doesn't get automatically corrected. Is the field of view at 18mm the uncorrected or corrected one? Presumably the uncorrected one.

[frank]

Yes, we know that 12mm is 12mm in theory. However, we know the companies fudge (i.e. a 300mm lens may be 280 mm). So, did they design the lens as an 12mm lens, with the uncorrected view like an 10 or 11mm field of view, and after processing you get 12mm? Or did they fudge, and the unprocessed is 12mm, while processed is only 14mm? 

 

If one calculates back, one could determine the actual field of view, but did anybody do that?

 

As a matter of fact, what about normal dSLR lenses, say a normal 18-55 kit lens that has barrel distortion that doesn't get automatically corrected. Is the field of view at 18mm the uncorrected or corrected one? Presumably the uncorrected one.

 

Yes, that is what I think too. I don't believe I get a 15mm view at the 16mm of the 16-35mm lens without distortion correction in post processing. Otherwise I will be very happy with the high distortion.

[Guest]

Yes, that is what I think too. I don't believe I get a 15mm view at the 16mm of the 16-35mm lens without distortion correction in post processing. Otherwise I will be very happy with the high distortion.

 

popphoto determines focal length (somehow). For the Canon 18-55 IS kit, it gives the focal length as 18.61mm tested. That at 0.55% barrel (at Dxomark it's about 0.9% max, at slrgear it's 0.9% max, and about 0.55 % average, ok, seems it's all consistent based on Dxomark). This is very different from photozone barrel at 3.2% though?? The grid does sort of look similar though. Just some different way of measuring then?

 

Anyway, seems there are nice curves at Dxomark that show the distortion as focal length across the radial field

 

So, the 18mm goes from 18mm in the center to 17.2 at the edge.

 

 

for the 16-35 mark I, you get 16.6 mm in the center, and 15.7 towards the edge.

http://www.dxomark.com/Lenses/Canon/Cano...m-F28L-USM

 

check under measurement, distortion, profile.

 

Hmm, it does look like m43 takes into account correction.

this is for the Panasonic vario 14-42 lens 

http://www.dxomark.com/Lenses/Panasonic/...842#tabs-2

Center is 13.8mm, edge is 12.3mm

 

And the Olympus Ed 14-42, is 13.1 to 12.1mm.  

http://www.dxomark.com/Lenses/Olympus/Ol...era%29/842

 

So, seems no cheating   ;-)

[Klaus]

I think what matters is the final output regardless of the correction that happens before the RAW file is written on the SD card.

We always hear that such corrections affect the MFT results.

However, let's consider the review of the Olympus 12-40 which is heavily corrected for barrel distortion at 12mm. According to the whooping > 8% distortion at 12mm, one would assume it doesn't perform well at such focal length, especially in the corners. When looking at the resolution figures it doesn't appear to be the case at all. How do you explain this? Perhaps the figures would be even higher without correction?

Personally, I couldn't care less whether a lens is optically corrected or whether it's done in post processing. What counts is the en result: the resolution the whole system achieves. On the plus side, post capture corrections allows designers to create smaller lenses which is a good thing IMO.

 

As others noted the distortion correction is accompanied by cropping which basically means that the "borders" are shifted towards the center. Thus there are two opposing forces at work here - the negative one from the interpolation and the positive one from the shift towards the higher quality inner image zone. 

Whether this works out Okay is a matter of design. However, a fully corrected lens designed with the same efforts will always be better (albeit also bigger - which is the reason for underdesigning lenses - besides cost cutting).