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03-28-2007, 03:31 PM   #1
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wanting to understand- lens distortion and 1.5 crop factor

this is a question I have been pondering to ask for a while but always forget.
I realize a 50mm lens when put on a pentax DSLR turns into 75mm. but does it still have the distortions (wider faces/bodys in portraits, etc compared to say a 105mm SLR lens) of a 50mm lens?

any help and input much appreciated!!

thanks

randy

03-28-2007, 04:06 PM   #2
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Distortion depends less on lens angle of view and more on subject position. For example using a wide angle lens on film will distort features in a certain way. If you put that lens on a DSLR you have two choices.

1) you can stay in the same position relative to the subject and lose much of the background in the other shot. The perspective and distortion wouldn't change in this instance.

2) you can move your camera so that the subject is the same size relative to the full frame image, "cropping with your feet" as it were. Your position relative to the subject has changed. In this case the distortion would change.

Remember that actual perspective is independent of focal length or crop factor it is apparent perspective is what changes.

I don't know if this clarifies your question or not.
03-28-2007, 04:34 PM   #3
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pretty much like a 75mm lens

QuoteOriginally posted by slipchuck Quote
this is a question I have been pondering to ask for a while but always forget.
I realize a 50mm lens when put on a pentax DSLR turns into 75mm. but does it still have the distortions (wider faces/bodys in portraits, etc compared to say a 105mm SLR lens) of a 50mm lens?

any help and input much appreciated!!

thanks

randy
Hi Randy - the 50mm lens will only be using the central portion of the image circle, so the perspective distortions will be the same as those of a 75mm lens since they relate to the angle of view...
Or in other words you have to get further away from the subject to fit it in the same frame, so the distortions and perspective change accordingly as the subject is now, relatively speaking, closer to the background/further from the camera.
03-28-2007, 04:40 PM   #4
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thanks to both of you so far.

maybe I am using a wrong lens example to get what I am trying to say.
instead of 50mm, lets use 17mm. with a 17mm lens, you can get close as you would like and there is still definately distortion.... wider then normal looking subjects. now, if I exagerate the crop factor to x3 which would make a 17mm lens 52mm, would it still distort the subject like a 17mm lens?

thanks

randy

03-28-2007, 04:48 PM   #5
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QuoteOriginally posted by slipchuck Quote
thanks to both of you so far.

maybe I am using a wrong lens example to get what I am trying to say.
instead of 50mm, lets use 17mm. with a 17mm lens, you can get close as you would like and there is still definately distortion.... wider then normal looking subjects. now, if I exagerate the crop factor to x3 which would make a 17mm lens 52mm, would it still distort the subject like a 17mm lens?

thanks

randy
No because you have cropped it x3 It would look like a 52mm shot with no distortion but probably a wider angle of the crop (I think Now I'm confused)
03-28-2007, 04:50 PM   #6
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No

QuoteOriginally posted by slipchuck Quote
thanks to both of you so far.

maybe I am using a wrong lens example to get what I am trying to say.
instead of 50mm, lets use 17mm. with a 17mm lens, you can get close as you would like and there is still definately distortion.... wider then normal looking subjects. now, if I exagerate the crop factor to x3 which would make a 17mm lens 52mm, would it still distort the subject like a 17mm lens?

thanks

randy
It does not matter what numbers you use - it may be a 17mm lens but with a 3X crop you are only using 33% of the view field so its going to behave like a 52mm lens. All those distortions you talk about require the full angle of view...
03-28-2007, 05:27 PM   #7
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Think of it this way. A 50mm lens is still a 50mm lens, no matter what it's attached to, and it shines the same amount of light no matter what. What's different is the little rectangle that's going to record that light.

If you can imagine shining your lens on a wall, with no camera; what you would get is a round (or nearly so; the actual shape is a polygon determined by the number of aperture blades) image on a big field of black. Then imagine drawing a rectangle on that image to represent the portion of the lens's image that film will record, and a smaller rectangle inside that one representing what your DSLR will record.

With 35mm film, the native "light throw" (I'm sure there's a technical term) of the lens covers the entire film rectangle, and "wastes" some of the focused light around the edges. On a DSLR, the sensor is even smaller, and takes up less of the focused light from the lens, and "wastes" even more.

That's why it's called a "crop factor" and not a "magnification factor" or something like that. The image is the same, but with the DSLR you're cutting out, or cropping, a smaller rectangle, for your picture.

The focal properties of the lens are completely unchanged, however; the focal length of the lens, the amount of linear distortion, etc. etc. are all identical. What's changing is the field of view, which RESEMBLES the effect you get by changing to a longer lens on a film camera, but is NOT exactly the same. The image on the DSLR shows less width in the projected image, but the lens doesn't know what kind of a camera it's attached to!

Remember that most lenses get worse as you go out to the edge: fuzzier, more linear and chromatic distortion, and so on. With a DSLR, you're throwing more of the distorted parts around the edges away, which is good. You're getting the FIELD OF VIEW characteristics of a longer lens, but not the distortion characteristics (or lack of them).

So a 17mm lens attached to a film camera would show a wider view than it would on a DSLR, but it would show more edge distortion too -- edge distortion that still exists on the DSLR but is outside the edges of the sensor. The reason the new DA lenses won't work on a film camera isn't that they won't WORK; it's just that the circle they throw is too small to completely cover the film, and would cause really bad vignetting.

I hope that answers your question. In short, the answer is no: the crop factor means that the lens has the field of view of a 1.5x longer lens but not the distortion characteristics.

That's my understanding of crop factor at least. I am prepared to be embarrassed by an expert telling me I'm all wrong!
03-28-2007, 05:40 PM   #8
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That was a great way to explain it, Fnarf... Thank you!

03-28-2007, 07:01 PM   #9
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QuoteOriginally posted by slipchuck Quote
this is a question I have been pondering to ask for a while but always forget.
I realize a 50mm lens when put on a pentax DSLR turns into 75mm. but does it still have the distortions (wider faces/bodys in portraits, etc compared to say a 105mm SLR lens) of a 50mm lens?

any help and input much appreciated!!

thanks

randy

Hi Randy:

I thought I would muddle this up with a different "viewpoint" :-)

The distortion and perspective of a lens is based on the relative position of the subject to the camera.

I will use telephone poles along the side of the road for my first example.

Imagine each pole is 20 feet from the next pole.

If you are 20 feet from the first pole, then the next pole will be 40 feet (2X as far). Therefore, the second pole will be 1/2 the size of the first in the image.

If you are 100 feet from the first pole, then the second pole is only 1.2x as far, (relatively) and they will appear almost the same height.

If you are 1 mile away and have a really nifty telephoto, all the poles would be essentially the same size and would "stack up" in the image because they are all within about 1.00001X away.

Now back to your portrait question. Instead of the telephone poles, you have a nose, eyes and and ears.

If you frame the face to occupy, say, the central 1/2 of the frame with the 50mm on film, you will have to move back 1.5X the subject distance to keep the face the same size when the 50mm is on DSLR.

Because the relative distance to the nose, eyes and ears changes as you physically change the subject distance, when you move closer the features become more of a different size, and when you move back they tend to assume the same size.

Generally the "classic" focal length (film equivalent) is in the 70-105 range, which prevents either the "Alfred E. Newman" big ears from being too far away, or the "Jimmy Durante " nose from being too close. Your style may vary, however.

I hope that helps, or at least answers some other question.
03-28-2007, 08:46 PM   #10
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SpecialK, you explained what I was trying to explain far better then I could. Exelent examples
03-29-2007, 06:59 PM   #11
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Perspective is related to camera-subject distance. Period.

Lens angle of view is related to focal length and sensor size. Period.

Take a photo with a 200mm lens, stay in the same spot and take a photo pointing the same direction with a 20mm lens.

Crop the 20mm image down to equate the 200mm image. There'll be more grain, but the perspective, and the photo, in fact, will be the same.

This is how I work: stake out your perspective, choose a lens that will frame the shot the way you like it.

It's kind of funny to watch tourists zooming in and out to get the framing they want without moving their position. And they wonder why the photos all look the same!
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