[jboyde]

I have my guess but I need backup.

We all know that putting a 50mm lens on a Pentax body will give you the view area of 75-80mm because of the 1.6x crop factor...
but......
does the crop factor only affect your view area or does it change the image properties as well?

As an example, you wouldnt take a closeup head and shoulders portrait of someone at 18mm unless you want the distortion to happen (big nose). But if you put the 18mm lens on a crop body (now 35mm equiv) will you still get the same level of distortion as if it was 18mm? My guess is, yes. I figure that the wide angle distortion won't change, you're just cropping down the resulting image.

I would love to test this out but I lent out my 18-55 and thats my only wide angle so far.

[wallyb]

All it does is crop. Period. 1.5x.

Nothing else. How could it? A "distorted" 18mm image isn't distorted in the middle, only to the sides, so the cropped image will "appear" less distorted.

But there's nothing changed, anywhere, just a crop. No need to test anything. You can do the same thing in photoshop or paint with 35mm scans.

[Marc Sabatella]

The distortion you get from shooting a head and shoulder portrait at 18mm is *not* because of any distortion in the lens - it is simply because you are standing so close your subject that their nose is noticably closer to you (and ears noticeably further away) than the rest of the face. The crop factor means you will stand further back to take the same picture, and *that's* what will reduce the distortion. You'd get *exactly* the same reduction in distortion if you simply stood further back with that 18mm lens on the film body. In short, perspective is *not* a function of focal length - it is a function of distance to subject, period. This has been a commonly misunderstood subejct for years because most people never thought about using different camera systems where the same focal length provided different fields of view, so people were lulled into thinking all sorts of things were specific to focal length that really weren't.

So the answer to your question is yes, a crop is just a crop - but it's a much more significant thing that you might think, because really, most of the things you might attribute to "focal length" (such as distortion, minimum shutter speed for handholding, etc) are really not functions of focal length at all, but of field of view. DOF is sort of a special case.

BTW, the crop factor Pentax DSLR's is 1.5, not 1.6. Canon uses that slightly smaller smaller sensor, but pretty much everyone else uses one with a 1.5 crop factor.

I've moved this to the Beginner's forum, becuase it is kind of a common area of confusion for newcomers to the world of APS-C.

[Marc Sabatella]

Nothing else. How could it? A "distorted" 18mm image isn't distorted in the middle, only to the sides

That's not true. If you shoot a head and shoulders portrait at 18mm on a 35mm camera, the nose - which will be more or less dead center - is often going to be the single most obviously distorted aspect of the picture.

[ytterbium]

It depends on what do you mean by distortion. Perspective distortion is related to field of view, so a 35mm Lens being equivalent to 50mm lens on film body will show the same perspective.

For a long time i was confused by this. I thought that larger format cameras produce more correct perspective just because they had the correct actual focal length. I was wrong.

As you said "I figure that the wide angle distortion won't change...", but if you put a 28mm FF wide angle on a APS-C camera, it's not a wide angle any more. It is 42mm equivalent, and produces the exact same perspective as a 42mm lens on FF.

You can do a simple test with two different lenses and cropping in post processing. If all other parameters are equal, you should get the same image (except for lens quality and resolution). Or you can test this by drawing 1D projections on paper trough imaginary pinhole.
Just make sure to crop the Center of image. Otherwise you'd get a shift lens equivalent .

Another things is actual lens ideality. Obviously you can make better 35mm f4.5 lens for a medium format camera, than 18mm f3.2 for APS-C. Especially when one costs 1000$ and the other one is 100$. You need very high precision to correctly squeeze an image on those 1/2.5" P&S sensors. With larger pixels you can have larger tolerances as long as you cover the whole sensor.

[jboyde]

The distortion you get from shooting a head and shoulder portrait at 18mm is *not* because of any distortion in the lens - it is simply because you are standing so close your subject that their nose is noticably closer to you (and ears noticeably further away) than the rest of the face. The crop factor means you will stand further back to take the same picture, and *that's* what will reduce the distortion. You'd get *exactly* the same reduction in distortion if you simply stood further back with that 18mm lens on the film body. In short, perspective is *not* a function of focal length - it is a function of distance to subject, period. This has been a commonly misunderstood subejct for years because most people never thought about using different camera systems where the same focal length provided different fields of view, so people were lulled into thinking all sorts of things were specific to focal length that really weren't.

So the answer to your question is yes, a crop is just a crop - but it's a much more significant thing that you might think, because really, most of the things you might attribute to "focal length" (such as distortion, minimum shutter speed for handholding, etc) are really not functions of focal length at all, but of field of view. DOF is sort of a special case.

BTW, the crop factor Pentax DSLR's is 1.5, not 1.6. Canon uses that slightly smaller smaller sensor, but pretty much everyone else uses one with a 1.5 crop factor.

I've moved this to the Beginner's forum, becuase it is kind of a common area of confusion for newcomers to the world of APS-C.

WOW! I've been demoted to "beginner"!!!!
No problem tho. I've had that question for years. Thanks for making it clear.

[Marc Sabatella]

Yeah, I tried to word my comment in a non-confrontational way: beginner in the world of APS-C. Like I said, until you are forced to deal with multiple systems, it's pretty common to not be clear on this sort of stuff (and some manage to remain unclear for quite a while even after using both systems).

Yeah, I tried to word my comment in a non-confrontational way: beginner in the world of APS-C. Like I said, until you are forced to deal with multiple systems, it's pretty common to not be clear on this sort of stuff (and some manage to remain unclear for quite a while even after using both systems).

I found that the easiest thing to do was to forget about crop factors. All the rules and techniques are the same regardless so you might as well concentrate on those and enjoy taking pictures.

[Wheatfield]

I have my guess but I need backup.

We all know that putting a 50mm lens on a Pentax body will give you the view area of 75-80mm because of the 1.6x crop factor...
but......

No, it doesn't. It gives you an angle of view of a 50mm lens.
Why don't we just settle on it giving the same angle of view as a 150mm lens?
Or a 100mm lens?

It makes as much sense to compare it to a 4x5 or 6x7 format as anything else.
The sooner you give up the "crop factor" idiocy, the better off you will be.