Barrel distortion question on a 20mm lens on a 1.6 crop camera?

ok here is a dumb question...

BUT.. if i have a 20mm lens ( which is a regular SLR lens ) on a canon APS-C sensor 1.6 crop camera.. it effectively makes the view 20x1.6 = 32mm??

And the Sigma 30mm f/1.4 EX DC HSM is a true 30mm on a 1.6 crop camera (the lens is designed to match the APS-C size image sensors of digital SLR cameras)

ok so question is... will the 20mm lens give me MORE barrel distortion? or less than the 30mm sigma lens?

meaning i know the 20mm lens will give me more wide angle distortion.. BUT since its being pushed out to 32mm is it still going to have the same barrel distortion as the 20mm or will it now have the dist of a 32mm lens?

And does anyone know how the dist on the 30mm siggy 1.4 is?

The sigma is effectively a 48mm on a crop camera, the crop factor still applies even though it is specifically designed for APS-C cameras. As for the barrel distortion, I can't say I've noticed any on mine.

Mr B Snappy wrote in post #5863846
The sigma is effectively a 48mm on a crop camera, the crop factor still applies even though it is specifically designed for APS-C cameras. As for the barrel distortion, I can't say I've noticed any on mine.

oh really? well what do they mean by it has been designed for the aps-c camera then?

It's not being "pushed out"... you're just only seeing the centre part of the projected image circle.

That's also pretty dependent on the individual lenses - take a look at a variety of wide angle lenses of any particular focal length and you'll see just as much variety in distortion.

There ought to be a Sigma 30mm 1.4 archive thread around here somewhere... have you checked for that yet?

Jim G wrote in post #5863853
It's not being "pushed out"... you're just only seeing the centre part of the projected image circle.

That's also pretty dependent on the individual lenses - take a look at a variety of wide angle lenses of any particular focal length and you'll see just as much variety in distortion.

There ought to be a Sigma 30mm 1.4 archive thread around here somewhere... have you checked for that yet?

yup ive looked at tons of threads concerning the 30mm siggy.. guess i didnt pick up that its only seeing the center of the lens making it effectively 48mm, i like the images that it produces its a pretty nice lens.. just getting schooled on lens 101 here.. just call me Mr n00b!

bocaj wrote in post #5863851
oh really? well what do they mean by it has been designed for the aps-c camera then?

That it's been specifically designed for the smaller sensor of an aps-c camera and has various coatings to reduce internal reflections etc. It's still a 30mm lens though, effectively giving you a "standard" lens for a crop camera. Which I believe is it's reason for existing.

ahh ok so it seems it does make it an effective 48mm.. which is actually a good thing... 30mm would have been too wide... i actually wanted a 50 mm lens on a FF so i might just get the 30mm sig so it will give me just about out to 50mm on a crop sensor.

Made for APS-C means that it has a smaller image circle that won't cover the full 36x24mm frame of full-frame. Designing lenses that make that smaller image circle uses similar design techniques as their equivalent FOV lenses for full frame. So, making a 17-50 for APS-C is much more like designing a 27-80mm lens for full frame than it is designing a 17-50mm lens for full frame. Likewise, the same design compromises are to be made.

So, all things being equal (which is almost never the case), a 30mm for APS-C should have similar distortion to a 50mm lens on full frame. Actual lens design will vary between lenses, however.

Jman13 wrote in post #5863907
Made for APS-C means that it has a smaller image circle that won't cover the full 36x24mm frame of full-frame. Designing lenses that make that smaller image circle uses similar design techniques as their equivalent FOV lenses for full frame. So, making a 17-50 for APS-C is much more like designing a 27-80mm lens for full frame than it is designing a 17-50mm lens for full frame. Likewise, the same design compromises are to be made.

So, all things being equal (which is almost never the case), a 30mm for APS-C should have similar distortion to a 50mm lens on full frame. Actual lens design will vary between lenses, however.

so the 20mm which was designed for a FF will have more dist then a 20mm designed for an APS-C camera?

and do some lenses just have more dist than others even though they are the same mm?

ugh stick with me guys i will get this soon

bocaj wrote in post #5863822
ok here is a dumb question...

BUT.. if i have a 20mm lens ( which is a regular SLR lens ) on a canon APS-C sensor 1.6 crop camera.. it effectively makes the view 20x1.6 = 32mm??

And the Sigma 30mm f/1.4 EX DC HSM is a true 30mm on a 1.6 crop camera (the lens is designed to match the APS-C size image sensors of digital SLR cameras)

ok so question is... will the 20mm lens give me MORE barrel distortion? or less than the 30mm sigma lens?

meaning i know the 20mm lens will give me more wide angle distortion.. BUT since its being pushed out to 32mm is it still going to have the same barrel distortion as the 20mm or will it now have the dist of a 32mm lens?

And does anyone know how the dist on the 30mm siggy 1.4 is?

bocaj wrote in post #5863915
so the 20mm which was designed for a FF will have more dist then a 20mm designed for an APS-C camera?

and do some lenses just have more dist than others even though they are the same mm?

ugh stick with me guys i will get this soon

First off, please read this thread from the forum "stickies" about "crop factor" issues to get a better understanding about the "crop factor". The key thing you need to understand is that focal length is focal length, regardless of the lens' design. The "effective" focal length thing is merely a comparison to the field (angle) of view with the same lens on a 35mm film (or a so-called "full-frame" DSLR). The format (size of the film frame or digital sensor) of a camera does not change the focal length of a lens attached to it. What I highlighted in blue in your quote above shows that you have (or had) a total misunderstanding of the "crop factor" issues.

To your second post that I quoted above: Even Canon's EF-S lenses, which cannot be mounted on any cameras that are not designed to accept them, are no different from any other lens when it comes to the focal length. A 50mm EF-mount lens and an EF-S zoom lens set to 50mm will produce precisely the same field (angle) of view on a 40D, for example.

Now to the "distortion" issues....

All lenses have some level of inherent pincushion or barrel distortion. However, most decent lenses are good enough that it would take a laboratory setup to see the distortion.

What most folks see in their images and call "distortion" is really perspective distortion and does not have anything to do with the quality of a lens. It has to do with the distance between the camera and the various elements in a scene.

If you are too close to a person and use a wide-angle lens to include more of the person in an image, you will see apparently mis-shapen parts of the person's body - the typical "wide-angle distortion" that you referred to. This is not "barrel distortion" and has nothing to do with the quality of the lens used.

If you are taking a photo of a nearby building the camera is not held with the film (sensor) plane parallel to the surface of the building, you will see additional types of "distortion" in the image (the sides of the building not being parallel, for example) which, again, have nothing to do with the quality of the lens used.

Control of perspective in photographs is important when you are trying to control the composition of your images. You need to know how to "see" the image and what effect changing your position will have on the resulting image.