What is mFT FoV?

Started Jan 27, 2013 | Questions
gollywop
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What is mFT FoV?
Jan 27, 2013

If one has, say, a 12mm lens on an mFT camera, it presumably implies some FoV. However, these days a large number of mFT lenses do software correction for lens distortions, and the image as rendered in, say, ACR is significantly different from the uncorrected version of the same image.

My question: is the 12mm FoV relevant to the FoV of the original image or the FoV of the corrected image, which is, typically, significantly smaller than that of the original?

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Anders W
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Re: What is mFT FoV?
In reply to gollywop, Jan 27, 2013

gollywop wrote:

If one has, say, a 12mm lens on an mFT camera, it presumably implies some FoV. However, these days a large number of mFT lenses do software correction for lens distortions, and the image as rendered in, say, ACR is significantly different from the uncorrected version of the same image.

My question: is the 12mm FoV relevant to the FoV of the original image or the FoV of the corrected image, which is, typically, significantly smaller than that of the original?

The diagonal FoV in the official specs is the one you get after correction. This follows from the fact that the FoV specs for MFT lenses are the same as for other lenses with the same EFL (e.g., 84 degrees for the 12/2) as well as from the formula for calculating diagonal FoV based on the FL for a rectilinear lens described here.

The FL and the horizontal and vertical FoVs are the same before and after correction. Geometric distortion implies a departure from strict rectilinearity that affects the diagonal FoV only.

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s_grins
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Re: What is mFT FoV?
In reply to gollywop, Jan 27, 2013

gollywop wrote:

If one has, say, a 12mm lens on an mFT camera, it presumably implies some FoV. However, these days a large number of mFT lenses do software correction for lens distortions, and the image as rendered in, say, ACR is significantly different from the uncorrected version of the same image.

My question: is the 12mm FoV relevant to the FoV of the original image or the FoV of the corrected image, which is, typically, significantly smaller than that of the original?

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gollywop

"is the 12mm FoV relevant to: the FoV of the original image

or

the FoV of the corrected image, which is, typically, significantly smaller than that of the original?"

What do you call "original" and "corrected"?

Why do you think that corrected image is SIGNIFICANTLY SMALLER than original?

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Guy Parsons
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Re: What is mFT FoV?
In reply to gollywop, Jan 27, 2013

Q. How long is a piece of string?

A. It's long enough if it does the job you need to do.

The M4/3 field of view varies depending whether you use jpeg or RAW so further complicates the issue. Many RAW converters can deliver right out to the edge where the default jpeg trims a bit, can be significant with some cameras and particularly with wide angle lenses.

Anyway, I think it's way past the time where any M4/3 user should be still thinking in archaic 35mm film terms, I know that I have moved on and just think in terms of what lens I have on the Pen at the time. (9mm really wide, 14mm wide enough for most things etc etc).

Regards..... Guy

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gollywop
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Re: What is mFT FoV?
In reply to Anders W, Jan 27, 2013

Anders W wrote:

gollywop wrote:

If one has, say, a 12mm lens on an mFT camera, it presumably implies some FoV. However, these days a large number of mFT lenses do software correction for lens distortions, and the image as rendered in, say, ACR is significantly different from the uncorrected version of the same image.

My question: is the 12mm FoV relevant to the FoV of the original image or the FoV of the corrected image, which is, typically, significantly smaller than that of the original?

The diagonal FoV in the official specs is the one you get after correction. This follows from the fact that the FoV specs for MFT lenses are the same as for other lenses with the same EFL (e.g., 84 degrees for the 12/2) as well as from the formula for calculating diagonal FoV based on the FL for a rectilinear lens described here.

The FL and the horizontal and vertical FoVs are the same before and after correction. Geometric distortion implies a departure from strict rectilinearity that affects the diagonal FoV only.

I'm not so sure, Anders; but thanks, by the way, for a very thoughtful answer. Here is an image taken with my 12-35 at 12mm. The first image is the dng as rendered in ACR, which does the lens corrections automatically. The second is as rendered in Photo Ninja, which takes the full raw data and makes no lens corrections. (The PN image is identical in size and distortion to the rendering obtained with RPP, which also makes no lens corrections.)





You can see that the FoV as seen in the PN rendering is both wider and taller. It's not just the diagonal that's affected. Once the lens-correction crop has been made by ACR, the FoV is less in every direction.

My question is whether it is the cropped FoV from ACR that corresponds to what should be the angle of view appropriate to a focal length of 12mm on a mFT camera -- or is it the FoV as seen in the PN image that does.

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gollywop
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Re: What is mFT FoV?
In reply to s_grins, Jan 27, 2013

s_grins wrote:

gollywop wrote:

If one has, say, a 12mm lens on an mFT camera, it presumably implies some FoV. However, these days a large number of mFT lenses do software correction for lens distortions, and the image as rendered in, say, ACR is significantly different from the uncorrected version of the same image.

My question: is the 12mm FoV relevant to the FoV of the original image or the FoV of the corrected image, which is, typically, significantly smaller than that of the original?

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gollywop

"is the 12mm FoV relevant to: the FoV of the original image

or

the FoV of the corrected image, which is, typically, significantly smaller than that of the original?"

What do you call "original" and "corrected"?

Why do you think that corrected image is SIGNIFICANTLY SMALLER than original?

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See my answer to Anders above.

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gollywop
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Re: What is mFT FoV?
In reply to Guy Parsons, Jan 27, 2013

Guy Parsons wrote:

Q. How long is a piece of string?

A. It's long enough if it does the job you need to do.

The M4/3 field of view varies depending whether you use jpeg or RAW so further complicates the issue. Many RAW converters can deliver right out to the edge where the default jpeg trims a bit, can be significant with some cameras and particularly with wide angle lenses.

Anyway, I think it's way past the time where any M4/3 user should be still thinking in archaic 35mm film terms, I know that I have moved on and just think in terms of what lens I have on the Pen at the time. (9mm really wide, 14mm wide enough for most things etc etc).

Regards..... Guy

Gee, Guy, you surprise me.  This comment misses the point in just about every conceivable way.

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Anders W
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Re: What is mFT FoV?
In reply to gollywop, Jan 27, 2013

gollywop wrote:

Anders W wrote:

gollywop wrote:

If one has, say, a 12mm lens on an mFT camera, it presumably implies some FoV. However, these days a large number of mFT lenses do software correction for lens distortions, and the image as rendered in, say, ACR is significantly different from the uncorrected version of the same image.

My question: is the 12mm FoV relevant to the FoV of the original image or the FoV of the corrected image, which is, typically, significantly smaller than that of the original?

The diagonal FoV in the official specs is the one you get after correction. This follows from the fact that the FoV specs for MFT lenses are the same as for other lenses with the same EFL (e.g., 84 degrees for the 12/2) as well as from the formula for calculating diagonal FoV based on the FL for a rectilinear lens described here.

The FL and the horizontal and vertical FoVs are the same before and after correction. Geometric distortion implies a departure from strict rectilinearity that affects the diagonal FoV only.

I'm not so sure, Anders; but thanks, by the way, for a very thoughtful answer. Here is an image taken with my 12-35 at 12mm. The first image is the dng as rendered in ACR, which does the lens corrections automatically. The second is as rendered in Photo Ninja, which takes the full raw data and makes no lens corrections. (The PN image is identical in size and distortion to the rendering obtained with RPP, which also makes no lens corrections.)





You can see that the FoV as seen in the PN rendering is both wider and taller. It's not just the diagonal that's affected. Once the lens-correction crop has been made by ACR, the FoV is less in every way.

Yes. They crop away more than required in this case, perhaps to bring the FoV in line with the official specs.

My question is whether it is the cropped FoV from ACR that corresponds to what should be the angle of view appropriate to a focal length of 12mm on a mFT camera -- or is it the FoV as seen in the PN image that does.

In case of doubt, you can use the method described here to measure the FoV yourself. I tried it with the 20/1.7 (after correction) and found that the FoV was exactly what it was specified to be.

Obviously, any given lens can depart from the FoV specs in the one or other direction. That's nothing new. But such deviations is one thing and the systematic issue you are talking about (before versus after correction) another.

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gollywop
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Re: What is mFT FoV?
In reply to Anders W, Jan 27, 2013

Anders W wrote:

gollywop wrote:

Anders W wrote:

gollywop wrote:

If one has, say, a 12mm lens on an mFT camera, it presumably implies some FoV. However, these days a large number of mFT lenses do software correction for lens distortions, and the image as rendered in, say, ACR is significantly different from the uncorrected version of the same image.

My question: is the 12mm FoV relevant to the FoV of the original image or the FoV of the corrected image, which is, typically, significantly smaller than that of the original?

The diagonal FoV in the official specs is the one you get after correction. This follows from the fact that the FoV specs for MFT lenses are the same as for other lenses with the same EFL (e.g., 84 degrees for the 12/2) as well as from the formula for calculating diagonal FoV based on the FL for a rectilinear lens described here.

The FL and the horizontal and vertical FoVs are the same before and after correction. Geometric distortion implies a departure from strict rectilinearity that affects the diagonal FoV only.

I'm not so sure, Anders; but thanks, by the way, for a very thoughtful answer. Here is an image taken with my 12-35 at 12mm. The first image is the dng as rendered in ACR, which does the lens corrections automatically. The second is as rendered in Photo Ninja, which takes the full raw data and makes no lens corrections. (The PN image is identical in size and distortion to the rendering obtained with RPP, which also makes no lens corrections.)





You can see that the FoV as seen in the PN rendering is both wider and taller. It's not just the diagonal that's affected. Once the lens-correction crop has been made by ACR, the FoV is less in every way.

Yes. They crop away more than required in this case, perhaps to bring the FoV in line with the official specs.

That makes sense.  And that's what I suspected was the case.  It seems problematic for a lens manufacturer whose design requires software distortion correction to specify a focal length that corresponds to a FoV only found in the uncorrected image.  But stranger things have happened.

My question is whether it is the cropped FoV from ACR that corresponds to what should be the angle of view appropriate to a focal length of 12mm on a mFT camera -- or is it the FoV as seen in the PN image that does.

In case of doubt, you can use the method described here to measure the FoV yourself. I tried it with the 20/1.7 (after correction) and found that the FoV was exactly what it was specified to be.

I believe you, but thanks for the link.  I'm going to give it a try.

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gollywop
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Anders . .
In reply to Anders W, Jan 27, 2013

Whoop, Anders, your link to the means for testing angle of view doesn't work.  Can you try again?

thanks,

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Anders W
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Re: Anders . .
In reply to gollywop, Jan 27, 2013

gollywop wrote:

Whoop, Anders, your link to the means for testing angle of view doesn't work. Can you try again?

thanks,

Don't know what went wrong with that. But I corrected the link by editing my previous post. It should work better know (at least it did when I tried) but here it is again just in case:

http://www.panohelp.com/lensfov.html

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Guy Parsons
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String theory
In reply to gollywop, Jan 27, 2013

gollywop wrote: Gee, Guy, you surprise me. This comment misses the point in just about every conceivable way.

My excuse that it was way too early here and the brain hadn't got out of bed yet.

Anyway the piece of string argument still applies to FoV arguments, it's as wide as you like corrected or not depending on lens and where you stand.

Your question was.....

My question: is the 12mm FoV relevant to the FoV of the original image or the FoV of the corrected image, which is, typically, significantly smaller than that of the original?

FoV is always deemed to be the diagonal of the frame and we have to assume refers to the FoV of the in-camera corrected frame, which you see in live view. Again I assume that would be the diagonal at 4:3 ratio to get the best number for marketing reasons.

But here's where the string reappears, there is so much slop in quoting focal lengths on lenses and variations due to aspect ratio choices that it really only a matter for arguing about and not there to be considered for any practical reason in the real world when we take photographs.

As I said, the view gets bigger anyway when you use a RAW converter that does the corrections but also allows the edge pixels to be included. Often the elbow just chopped off in a jpeg can sneak back into a RAW full frame extraction. So the FoV result can vary quite a bit for the one lens and may or may not actually be truly a 12mm lens due to the way the focal length can vary with initial design (is it really 12mm or 11.8mm or 12.2mm?) and with internal focus lenses as they focus closer.

I stand by my string argument.

Regards...... Guy

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gollywop
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Re: String theory
In reply to Guy Parsons, Jan 27, 2013

Guy Parsons wrote:

gollywop wrote: Gee, Guy, you surprise me. This comment misses the point in just about every conceivable way.

My excuse that it was way too early here and the brain hadn't got out of bed yet.

I can relate to that.

Anyway the piece of string argument still applies to FoV arguments, it's as wide as you like corrected or not depending on lens and where you stand.

Indeed, but I want to measure from where I am standing, not where I could be. You see, that's what zooms bring to the party.

Your question was.....

My question: is the 12mm FoV relevant to the FoV of the original image or the FoV of the corrected image, which is, typically, significantly smaller than that of the original?

FoV is always deemed to be the diagonal of the frame and we have to assume refers to the FoV of the in-camera corrected frame, which you see in live view. Again I assume that would be the diagonal at 4:3 ratio to get the best number for marketing reasons.

But here's where the string reappears, there is so much slop in quoting focal lengths on lenses and variations due to aspect ratio choices that it really only a matter for arguing about and not there to be considered for any practical reason in the real world when we take photographs.

As I said, the view gets bigger anyway when you use a RAW converter that does the corrections but also allows the edge pixels to be included.

Problem there is that, if there is substantial rectilinear correction required, the converters producing the "bigger" images are producing distorted images. In the example I gave above to Anders, that distortion is obvious in the PN rendering, particularly to one who knows what the scene looks like. The telephone pole, for example, in the PN image looks fairly straight and nearly vertical. It is, in fact, quite curved and out of plumb, just as depicted in the ACR conversion.  The brick walk simply doesn't "sit" like that.

Often the elbow just chopped off in a jpeg can sneak back into a RAW full frame extraction. So the FoV result can vary quite a bit for the one lens and may or may not actually be truly a 12mm lens due to the way the focal length can vary with initial design (is it really 12mm or 11.8mm or 12.2mm?) and with internal focus lenses as they focus closer.

I stand by my string argument.

I stand where I'm standing, and that's where I want my angle of view.

take care,

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gollywop
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Re: Anders . .
In reply to Anders W, Jan 27, 2013

Anders W wrote:

gollywop wrote:

Whoop, Anders, your link to the means for testing angle of view doesn't work. Can you try again?

thanks,

Don't know what went wrong with that. But I corrected the link by editing my previous post. It should work better know (at least it did when I tried) but here it is again just in case:

http://www.panohelp.com/lensfov.html

That one works.

thanks,

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Detail Man
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Re: What is mFT FoV ?
In reply to gollywop, Jan 27, 2013

gollywop wrote:

If one has, say, a 12mm lens on an mFT camera, it presumably implies some FoV. However, these days a large number of mFT lenses do software correction for lens distortions, and the image as rendered in, say, ACR is significantly different from the uncorrected version of the same image.

My question: is the 12mm FoV relevant to the FoV of the original image or the FoV of the corrected image, which is, typically, significantly smaller than that of the original?

I agree that the diagonal FOV (thus the 35mm equivalent focal length when the crop factor is considered) of the default (rectilinear distortion corrrected) 4608x3456 pixel size is designed to be relatively close to the intended specification.

There do exist additional photo-sites on M43 image-sensors. They are in the horizontal sections. Some of those photo-sites are unusable - because they are masked-off, and used in order to report black-level offset information only. There are only a few extra photo-sites in the vertical dimension (but they are apparently set aside for use in implementing rectilinear distortion corrections).

The image-file meta-data for your E-M5 (4:3 AR) only reports "Image Dimensions" the calculated diagonal FOV of which is only very marginally different from the default value of 4608x3456. Those pixel-dimensions (for the 3.73 Micron pixel-pitch of the E-M5's image-sensor) calculate out to be only very marginally "longer" (in 35mm equivalent focal length), meaning "narrower" (in FOV) than the specification value.

See: http://forums.dpreview.com/forums/post/50738679

However, it is clear (from your posted example as well as another poster's example) that the E-M5 image-sensor does have some extra (horizontal) photo-sites that are not reported in the E-M5's image-file meta-data.

GH2 RW2 (4:3 AR) image-file meta-data reports "Sensor Dimensions" of 4760x3472.

See: http://forums.dpreview.com/forums/post/50738731

DxO Optics Pro 7.23 (only when automatic distortion corrections are enabled, and "Keep Aspect Ratio" is unchecked) widens the horizontal field (only), and produces output images that are 4668x3457 (as compared to it's default output image pixel-size of 4609x3457). The wider horizontal pixel-dimension yields a diagonal dimension that is 0.821% larger than default diagonal pixel-dimension.

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Vlad S
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practical measurement with 20mm
In reply to gollywop, Jan 28, 2013

gollywop wrote:

If one has, say, a 12mm lens on an mFT camera, it presumably implies some FoV. However, these days a large number of mFT lenses do software correction for lens distortions, and the image as rendered in, say, ACR is significantly different from the uncorrected version of the same image.

Measured to the best of my ability with a 20mm lens (because it has fairly strong distortion correction):

Distance from the sensor plane to the horizontal measuring tape: 20 in.

horizontal field of view: 19 7/32 in - 3 7/32 in = 16 7/32 in = 16.21875 in.

diagonal field of view: 16.21875 / 4 * 5 = 20.2734375 in.

Diagonal angle of view: 2 * arctan (20.2734375 / 2 / 20) = 53.7550599 degrees

Specified: 57 degrees

How significant is this deviation?

Working back from 57 degrees, the horizontal field of view at 20 in should be:

tan(57/2º)*20*2/5*4=17.374582388429978 in.

The deviation is

(17.374582388429978-16.21875)/17.374582388429978*100 = 6.7%

It should be taken into account that the 20mm lens exhibit observable "focus breathing," and the angle of view at infinity is somewhat larger. How significant it is is up to the user. I was not planning to use the camera for triangulation anyway.

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Some comparisons
In reply to gollywop, Jan 28, 2013

Just for fun I ran the same 9mm RAW image though Oly Viewer 2, with the first animated gif showing the difference between the default produced image and the image when manual distortion correction of 0 is applied and the lens information ignored, not as much change as I expected so I guess the Oly 9-18mm is not too wonky at 9mm.

Next the same RAW image through Silkypix Pro V5 and the default jpeg produced as image one and the maximum trim version resized to match this small frame as the second image of the pair in this animated gif. The default is from E-PL5 4608 x 3456 and the Silkypix maximum trim is 4640 x 3482.

Silkypix default and maximum trim

Not sure what it all proves but it does mean to me that quoted focal lengths and expected fields of view are all a bit nebulous.

Regards........ Guy

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s_grins
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Re: What is mFT FoV?
In reply to gollywop, Jan 28, 2013

gollywop wrote:

s_grins wrote:

gollywop wrote:

If one has, say, a 12mm lens on an mFT camera, it presumably implies some FoV. However, these days a large number of mFT lenses do software correction for lens distortions, and the image as rendered in, say, ACR is significantly different from the uncorrected version of the same image.

My question: is the 12mm FoV relevant to the FoV of the original image or the FoV of the corrected image, which is, typically, significantly smaller than that of the original?

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gollywop

"is the 12mm FoV relevant to: the FoV of the original image

or

the FoV of the corrected image, which is, typically, significantly smaller than that of the original?"

What do you call "original" and "corrected"?

Why do you think that corrected image is SIGNIFICANTLY SMALLER than original?

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See my answer to Anders above.

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gollywop

I've no problems with Anders.

I can't fathom your problem

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gollywop
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Re: What is mFT FoV?
In reply to s_grins, Jan 28, 2013

s_grins wrote:

gollywop wrote:

s_grins wrote:

gollywop wrote:

If one has, say, a 12mm lens on an mFT camera, it presumably implies some FoV. However, these days a large number of mFT lenses do software correction for lens distortions, and the image as rendered in, say, ACR is significantly different from the uncorrected version of the same image.

My question: is the 12mm FoV relevant to the FoV of the original image or the FoV of the corrected image, which is, typically, significantly smaller than that of the original?

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gollywop

"is the 12mm FoV relevant to: the FoV of the original image

or

the FoV of the corrected image, which is, typically, significantly smaller than that of the original?"

What do you call "original" and "corrected"?

Why do you think that corrected image is SIGNIFICANTLY SMALLER than original?

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Looking for equilibrium...

See my answer to Anders above.

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gollywop

I've no problems with Anders.

I can't fathom your problem

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Looking for equilibrium...

That appears to be your problem.  I, quite frankly, cannot see the relevance of anything you've said.

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Re: Some comparisons
In reply to Guy Parsons, Jan 28, 2013

Guy Parsons wrote:

Just for fun I ran the same 9mm RAW image though Oly Viewer 2, with the first animated gif showing the difference between the default produced image and the image when manual distortion correction of 0 is applied and the lens information ignored, not as much change as I expected so I guess the Oly 9-18mm is not too wonky at 9mm.

Next the same RAW image through Silkypix Pro V5 and the default jpeg produced as image one and the maximum trim version resized to match this small frame as the second image of the pair in this animated gif. The default is from E-PL5 4608 x 3456 and the Silkypix maximum trim is 4640 x 3482.

Silkypix default and maximum trim

Not sure what it all proves but it does mean to me that quoted focal lengths and expected fields of view are all a bit nebulous.

Regards........ Guy

Interesting, Guy. That's a heck of a lot less difference than I'm seeing with the 12-35 at 12mm.  And I'm sure you're right about nebulousness, but I was just trying to figure out of all that "added space" I was seeing with my 12-35 was being included in the 12mm focal length FoV.  According to Anders it's not, and I suspect he's correct.  I'll do some measurements when I get a chance.

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