A question about equivalent focal lengths

[Kermit power]

I have a Canon 450D. This has a 1.6x crop factor compared to a full sensor. I am therefore told that, for example, a 100mm lens gives me the equivalent of a 160mm lens on a full sensor camera.

My question is this. Assuming both cameras have the same resolution, let's say 10Mp for sake of argument, do I have same image quality as a full sensor camera would provide with a 160mm lens, or do I have the same image quality as a full sensor camera would provide once the image taken with a 100mm lens had been cropped to provide the same coverage as my camera gives?

I realise, of course, that there's far more difference between my 450D and a 5D, but let's set that aside for the moment!

[ThatPhilBrettGuy]

Kermit power said:

Assuming both cameras have the same resolution, let's say 10Mp for sake of argument

As the 1.6x crop body by definition has the photocells closer together/smaller the quality (assuming the sensors are the same technology) will be worse. The bigger the photocell, the better the noise properties etc etc.

[Kermit power]

Thus claiming "equivalent to a 400mm zoom" or whatever is a bit of a marketing con?

[_dobbo_]

It's not really a con. If you attach a 200mm lens to a crop factor camera, and a 300mm lens to a full frame camera, the resulting image will have the same field of view. However... On most zoom lenses this means you lose out at the wide end.

Edited by _dobbo_ on Tuesday 17th February 10:04

[C&C]

Neither.

In theory same as full frame and 160mm but...

With a 1.6x 10Mp crop camera and 100mm lens you've got the same resolution as a full frame 10Mp camera with 160mm lens (same number of pixels for same field of view). But (and assuming both lenses are of equal quality), the main difference in picture quality is likely to be that the full frame has the number of pixels spread over a physically larger area, so you are likely to get less noise and interference between the pixels leading to a "better quality" image.

At least that's how I understand it.

Having said that, there are so many variables - e.g. it is harder (more expensive) to make a 160mm lens for a full frame camera of equal quality to a 100mm for a smaller 1.6x crop camera....

[ThatPhilBrettGuy]

Kermit power said:

Thus claiming "equivalent to a 400mm zoom" or whatever is a bit of a marketing con?

I think it is. Most lenses are quoted as their focal length on a full frame body which is the correct way in my mind. Otherwise you could state :-

1200mm telephoto lens for 40D!if the picture is cropped in Photoshop by 400% of the full image size

This could be a 200mm(ish) lens in reality!

[Simpo Two]

'Image quality' is rather a broad brush. Instead of having your 10Mp spread over the area of a full frame sensor, they're in an area about 62% of the size. So pound for pound you'll get a higher resolution and longer reach, but the price is a smaller pixel size and various other bits. It all depends which parameters are most important for you. For many people small sensors are actually more appropriate.

[SamHH]

ThatPhilBrettGuy said:

1200mm telephoto lens for 40D!if the picture is cropped in Photoshop by 400% of the full image size

This could be a 200mm(ish) lens in reality!

That is not analogous because cropping loses resolution, using a small sensor camera you get the full resolution.

[ThatPhilBrettGuy]

SamHH said:

ThatPhilBrettGuy said:

1200mm telephoto lens for 40D!if the picture is cropped in Photoshop by 400% of the full image size

This could be a 200mm(ish) lens in reality!

That is not analogous because cropping loses resolution, using a small sensor camera you get the full resolution.

Yeah, that's true. I bet eBay is full of those statements though

[VxDuncan]

Assuming the 160mm lens is covering the whole of the FF sensor, and the 100mm the whole of the APS sensor then the angular resolution (arc/pixels) should be identical for the same resolution sensors (I think!). However, as pointed out above, the image is likely to be of lower quality due to the higher pixel density of the crop sensor, and the associated noise issues with having smaller photosites (less photons hitting each pixel).

Also worth pointing out that although the image will have the same field of view, it will not be the same image as the perspective and depth of field effects will be different due to the different focal lengths - ie I couldn't get a telephoto style perspective stacking effect by cropping an image from a fish eye lens.

[C&C]

VxDuncan said:

although the image will have the same field of view, it will not be the same image as the perspective and depth of field effects will be different due to the different focal lengths - ie I couldn't get a telephoto style perspective stacking effect by cropping an image from a fish eye lens.

You sure about that?

[VxDuncan]

[quote=C&C]

VxDuncan said:

although the image will have the same field of view, it will not be the same image as the perspective and depth of field effects will be different due to the different focal lengths - ie I couldn't get a telephoto style perspective stacking effect by cropping an image from a fish eye lens.

You sure about that?

Fairly... If you take a shot with a wide angle (say 17mm on APS-C) lens, then you get fairly extreme perspective - the close objects appear far larger than those a few more feet away. If you were to crop this image to say the FOV of an 100mm lens, then this effect would still be there, unlike if it was shot on a native lens.

[C&C]

I've been thinking about this one quite a bit, and I believe you are quite correct (although initially I doubted it). From what I can work out/google, the compression from a telephoto and the appearance of close objects being really big from a wide angle are the result of perspective distortion. This being caused by the actual relative distance between a near and far object. So in the case where a "normal" view of 2 objects (A)1 metre away and (B) 2 metres away, if you shot this with a wide angle making the near one (A) look 2 metres away, the far one (B) would still be twice the distance away, so would look like it was 4 metres away.
If you then moved closer so the near one was the same size as at the start in the image, your distance might be only 1/2 metre, but the far object would be 1.5 metres (3 times the distance). If you then cropped the shot to simulate a telephoto lens, the relative size of the rear object would still be far smaller than if you'd shot the 2 objects originally with the telephoto lens.
I think...
Thanks

[Major Bloodnok]

ThatPhilBrettGuy said:

Kermit power said:

Thus claiming "equivalent to a 400mm zoom" or whatever is a bit of a marketing con?

I think it is. Most lenses are quoted as their focal length on a full frame body which is the correct way in my mind. Otherwise you could state :-

1200mm telephoto lens for 40D!if the picture is cropped in Photoshop by 400% of the full image size

This could be a 200mm(ish) lens in reality!

Focal length is focal length. The only reason for quoting the "equivalent 35mm length" is that most photographers are used to the field of view that a given length produces with a 35mm camera. You could get exactly the same effect by cropping the 35mm frame to the APS-C size - the focal length is the same, just the area that is recorded differs.

VxDuncan said:

CnC said:

VxDuncan said:

although the image will have the same field of view, it will not be the same image as the perspective and depth of field effects will be different due to the different focal lengths - ie I couldn't get a telephoto style perspective stacking effect by cropping an image from a fish eye lens.

You sure about that?

Fairly... If you take a shot with a wide angle (say 17mm on APS-C) lens, then you get fairly extreme perspective - the close objects appear far larger than those a few more feet away. If you were to crop this image to say the FOV of an 100mm lens, then this effect would still be there, unlike if it was shot on a native lens.

Perspective is dependent only on the distances from your viewpoint to the objects in the view. The extreme effect comes from having to stand much closer in order to fill the frame, thus affecting the perspective.

Crop and expand the relevant section from a wide-angle image and it'll look the same as a telephoto image taken from the same position (loss of quality aside, of course).


Edited to fix formatting

Edited by Major Bloodnok on Tuesday 17th February 19:31