some confusion about crop factor and mm...

[angelo_lightning]

I bought a 35mm dx lens for my d3300 today.

I know this means that I will get the same area in my shot as an equivalent 52mm fx lens on a full frame camera. I totally get that.

But what about distortion?

Am I getting the distortion of an fx 35mm or an fx 52mm?

 

[tirediron]

Focal length is focal length is focal length. Crop factor (for all practical purposes) only affects the field of view of the lens.

 

[KmH]

Am I getting the distortion of an fx 35mm or an fx 52mm?

A 35 mm lens. It doesn't matter if it's DX or FX.

 

[Derrel]

Since the sensor in a Nikon a D3300 is an APS-C sized sensor, the 35mm focal length will be, proportionately in relation to the diagonal of the sensor, a LONGER focal length on APS-C than it would be on FX, so the "lens rendering" will look more like that of a so-called normal focal length lens than a wide-angle lens. Yes, the len's measured focal length will be 35mm on the Nikon D3300, just as it would be a 35mm lens on any other Nikon camera, but the way a lens renders scenes depends on both the size of the film or sensor and the lens's actual focal length.

The term "distortion" has several meanings associated with it. If you mean distortion in the sense of describing the way the lens renders the size of close objects in relation to the size of background objects, and how the lens renders the width of the background, when used on an APS-C sized sensor, a 35mm lens is NOT the same "semi wide-angle" lens length that it is when used on a 24x36mm sized sensor camera.

 

[Alexr25]

If you are talking about perspective distortion rather than things like barrel/pincushion CA etc then the distortion will similar to a 50mm lens on a FX body. Distortion is governed by the angle of view and the lens to subject distance. Since angle of view and hence the subject lens distance of a 35mm lens on a DX body is similar to a 50mm lens on a FX body the perspective distortion will also be similar.

 

[angelo_lightning]

argh... this is why I'm confused.

All I'm worried about is the difference in magnification of closer and far objects. The 'long nose' effect. Perspective distortion. This is what I am talking about when I'm talking distortion. I took some pics today and to be honest... the noses looked a little long.

1. So what Tireiron says makes sense to me. The small crop sensor cuts off the edges, but the picture left behind has a 'long nose' factor of a 35mm regardless.

2. I don't understand Derrel. Why would the ratio of sensor diagonal to focal length have any effect on long-nose distortion? In my mind, the mental model I have created, its more of the ratio of the angle of the center light rays vs the angle of the edge light rays. The shorter the focal length the wider the angle of view, but the greater the spread in the middle, right?

3. Alexr25 - you seem to be saying that a 35mm on a crop sensor has the same perspective distortion as a 35mm on a full frame. BUT in order to fill the frame with the same picture, a person with a dx sensor has to be farther back. Therefore, this effectively makes the image... size for size... end up with the same perspective distortion as something like a 52mm. I think I can buy that.

 

[Alexr25]

3. Alexr25 - you seem to be saying that a 35mm on a crop sensor has the same perspective distortion as a 35mm on a full frame. BUT in order to fill the frame with the same picture, a person with a dx sensor has to be farther back. Therefore, this effectively makes the image... size for size... end up with the same perspective distortion as something like a 52mm. I think I can buy that.

No, I'm saying that a 35mm lens on a DX camera produces the same distortion as a 5omm lens on a FX camera because both lens/camera systems have the same angle of view and need the same lens to subject distance to fill the frame.
Perspective distortion is only effected by the angle of view and lens to subject distance. Lens focal length when combined with sensor size will determine the angle of view and lens to subject distance but focal length on its own will not tell you anything about distortion if you don't know the sensor or film size.

 

[angelo_lightning]

ok

I think I got it. 'Preciate it.

 

[soufiej]

argh... this is why I'm confused.

All I'm worried about is the difference in magnification of closer and far objects. The 'long nose' effect. Perspective distortion. This is what I am talking about when I'm talking distortion. I took some pics today and to be honest... the noses looked a little long.

Don't automatically blame the equipment. Very minor changes in the model's position can result in significant changes in your images. If you are using off camera lighting, this can make matters worse. Any make up artist understands how to shape a face to achieve a more desirable look.

Be sure you have every other issue dealt with before you begin thinking "bad gear".

 

[petrochemist]

.

All I'm worried about is the difference in magnification of closer and far objects. The 'long nose' effect. Perspective distortion. This is what I am talking about when I'm talking distortion. I took some pics today and to be honest... the noses looked a little long.

The only lenses that introduce severe distortions are fish eye lenses, which introduce a completely different pincussion distortion.

Perspective distortion that is attributed to wide angle lenses is purely a factor of being too close to the subject. - It's down to changes in the relative distances to different parts of the subject.

Looking at some extreme cases to illustrate - If your only an inch from someones nose, then the rest of their cheaks will be around three times as far away as the tip of the nose. With a super telephoto the nearest point on the subject might be 50' away, the extra 1-2" to the rest of their face is hardly noticable.

Try some experiments with any zoom (or pair of primes of different focal lengths). Take a photo with the longer focal length of a suitable subject, then another FROM THE SAME POINT using the widest setting. Crop the wide image to match the FOV of the zoomed in shot, and compare perspective - no change.

Using a SINGLE FOCAL LENGTH take two images of the same subject, one form as close as practical, the second from at least 4 times as far away. Crop the wide shot & compare - the closer shot will have more perspective. The effect is typically more noticable with wide angle lenses as it's more practical to get closer with them, but with extension tubes it can be shown easily enough with telephoto lenses too.

 

[angelo_lightning]

.

All I'm worried about is the difference in magnification of closer and far objects. The 'long nose' effect. Perspective distortion. This is what I am talking about when I'm talking distortion. I took some pics today and to be honest... the noses looked a little long.

The only lenses that introduce severe distortions are fish eye lenses, which introduce a completely different pincussion distortion.

Perspective distortion that is attributed to wide angle lenses is purely a factor of being too close to the subject. - It's down to changes in the relative distances to different parts of the subject.

Looking at some extreme cases to illustrate - If your only an inch from someones nose, then the rest of their cheaks will be around three times as far away as the tip of the nose. With a super telephoto the nearest point on the subject might be 50' away, the extra 1-2" to the rest of their face is hardly noticable.

Try some experiments with any zoom (or pair of primes of different focal lengths). Take a photo with the longer focal length of a suitable subject, then another FROM THE SAME POINT using the widest setting. Crop the wide image to match the FOV of the zoomed in shot, and compare perspective - no change.

Using a SINGLE FOCAL LENGTH take two images of the same subject, one form as close as practical, the second from at least 4 times as far away. Crop the wide shot & compare - the closer shot will have more perspective. The effect is typically more noticable with wide angle lenses as it's more practical to get closer with them, but with extension tubes it can be shown easily enough with telephoto lenses too.

I been doing this all day. Using my kit lens to go from 18-55. And yeah... you guys are totally right! I'm distorting my own pictures on lower focal lengths because I walk up to the subject to fill the frame. Whereas on higher focal lengths I'm standing way back. I guess I had it wrong in my mind. Thanks guys. So... this is how I'm thinking about it now.

First I try to imagine the scene with an 18mm and the distance between me and the subject. Then I mentally crop and zoom as I go up in focal lengths.