Lens Distortion

[mrbutterworth]

Is lens distortion more noticeable in different focal lengths? Take

the Noctilux and 35mm Summilux ASPH as examples. Both have relatively

high distortion, but I have never noticed it in the Noctilux photos

that I have seen. However, I have noticed it in the Summilux photos.

I don't own either lens, so my sample size is admittedly small.

<p>

I know that it is harder to correct for distortion in wider lenses,

but is it also more noticeable when it is there?

<p>

Thoughts, comments?

<p>

Nick

[shambrick007]

"Both have relatively high distortion" <p> What does that mean? Pincushion, barrel, stretched objects towards the edges?

[frank uhlig]

Spherical aberration, color fringes, ... just naming a few more candidates for your post. Just try to state clearly, please, what you are talking about here.

 

As a matter of fact, the stretching by rectilinear wide angle lenses one observes near the edge of a picture is NOT distortion, but geometrically so. Look at maps of the earth, they also distort exactly the same way if you take a huge swath of land/ocean into one map = wide angle shot!

 

So, what distorts (term properly used) in your view at 50mm, 35mm?

[mrbutterworth]

According to the graphs on Leica's website, they both have barrel distortion of about 1.5%.

[geoff_portas]

Well don't take pictures of barrels!

[Rob F.]

Wide angle lenses must in some cases be made retrofocus, meaning they have a longer back focus in order to clear an SLR mirror, or in the case of the Leica M, to clear the light path for metering. Retrofocus lenses cannot be made without some degree of distortion. In a well designed lens, the distortion is on the order of one or two percent, just barely visible.

 

Wide angle retrofocus zooms may have somewhat greater distortion, sometimes as high as five percent. I think the early 28-70 Vario-Elmar is in that category, at the 28mm end. However, any distortion owing to retrofocus design is not exacerbated, IMHO, as focal length decreases.

 

What does increase with decreasing focal length is the exaggeration of foreground space, and the stretching of round and spherical objects into the corners. The corner of a building placed too near the corner of the photo becomes morphed into an unnaturally acute angle. These effects are viewed as distortion by some, because the photo is no longer a faithful replica of the subject. However, If you will view the photo from a closer than normal distance, until the angle the picture forms in front of your eye matches that of the original subject in front of the camera lens, the image now appears normal! Try it, it works!

 

So what this means is that when the taking lens angle of coverage is much different than the eventual angle taken in by the viewer, one perceives that the perspective is unnatural. Is that distortion? It's a matter of how we define distortion. I would say the picture is distorted when the lens permanantly alters the accuracy of the image. But when the accuracy of perspective just depends on viewing the photo from an appropriate distance, I'd say that's not distortion.

 

The distortion induced by retrofocus design or other lens faults should not be confused with perspective alterations, becasue they are different phenomena.

[bill_marshall1]

Nicely stated. Thanks for the explanation, Rob.

[mrbutterworth]

Yes Rob, thank you! That is exactly the answer that I was looking for.

Take care, Nick

[marc_lieberman1]

Wow, Rob. You deserve another "hero" globe for that one!

[MTC Photography]

Nick, what you refered to is called 'perspective distortion" not optical distortion<P>

Perspective distortion is caused by improper viewing distance<p>

To eliminate perspective distortion, the proper viewing distance of

a picture = magnification factor x focal length of lens<p>

 

For example, the proper viewingt distance for a 4x6" print from

a 35mm focal length lens is 4x 35mm = 14 cm. But people normally

view print at 25 cm, hence there is a perspective difference which

causes perpsective distortion (round object stretched at edges)<P>

 

Try to view such a picture at 14cm, the distortion will disapear

instantly<p>

 

For a 8x12" enlargement from a 35mm focal length lens, the proper

viewing distance = 8 x 3.5 cm = 28 cm, which is the norminal

viewing distance, there will be no perpective distortion. In other words, if you feel a 4x6 "picture from your 35mm Summilux has distortion, enlarge it to 8x12", the distortion will disappear<p>

 

For 4x6" print from a 50mm lens, the proper viewing distance is

4x 50= 20cm, when watched at 20 cm, there is no perpspective distortionm when view at 25cm, there is slight perspective distortion<p>

 

Tip: it is best to view a picture with one eye open, one eye closed.

 

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