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Does 16 crop factor on digital actually change focal length of lens

What he said.

It's the weekend; I think we should all go and take some photos now.

My head hurts.

Best wishes to all!
Cheers,
Matt.
 
Obviously I was pretty tired by the time I finished the above document. Let me correct the obvious errors:

The definition of grapefruit should read:
Grapefruit: Taking a photo from a fixed position with two lenses of different focal length and then enlarging and cropping the wide angle image to duplicate the FOV of the longer focal length lens.

My comments to Asher's posts should read:
Asher writes, very convincingly:
“One can't trick the optics by enlarging a head and shoulders to the size of a telephoto image occupying the whole frame. Why? Because the picture was taken at the same position from the subject and with a wide angle lens, the light was bent a lot to get all the subjects to the left and right, above and below the head and shoulders on the fixed sized sensor/film.”
<<both lenses bend the light to form the image. The wide angle lens simply takes in more real estate. When you enlarge & crop you end up duplicating the image taken by the tele.>>

“By contrast, taking an image with a telephoto lens from a distance such that the same field of view was obtained and the same objects are included to each side and above and below the head and shoulders, will yield a flatter image and although the same number of objects would be captured, objects, and parts of objects previously rendered clearly, would now be hardly visible or out of focus. The face now would be much flatter than the same composition taken close up with the wide angle lens.”
<<Ah, now here we get to the heart of the matter. In this ex&le we have moved the camera to get the same FOV with the tele as that captured by the wide angle lens. I am in full agreement with this conclusion.”

“So, my friend and patient reader, enlarging images from a wide angle lens will not reproduce the image taken with a telephoto lens. They are absolutely different.”
<< Asher my friend, assuming both images were made from the same distance, I believe you are wrong. You have changed the ground rules from your previous ex&le. It is my mission here to convince people that the two situations are categorically different in their effect on the image. >>

My apologies,
Mike Blume
 
Guys,

It appears that we have agreed that the image sensor does not change the focal length of the lens.

So, if I use the Sonnar 210mm f4 on my 645, it has an equivalent focal length in 35mm terms of around 150mm. But if the lens is put on an N1 is it a 150mm lens or a 210mm?

Cheers, Saras
 
Well,so many different opinions.

I wish to make one point clear with Mike. Mike wrote,''if all you are doing is cropping,the image information remains the same.'' Does image information include perspective and 3D effect ? I wish to tell everyone that cropping of a photo or an image alone changes its perspective and 3D effect.

Have you used a G2 rangefinder ? Have you changed the lens of the G2 ? What happened to the viewfinder when you change the lens ? If you change from 28mm to 45mm ( these lenses are used as ex&les only because I have these lenses).The camera has the mechanism of cropping and enlarging the view of the viewfinder. The 28 mm view of the viewfinder is cropped and enlarged to become the view of the 45mm lens. Only cropping and enlarging is involved,but the two views are different in perspective and the 3D effect,one is a 28mm view and the other is a 45mm view.
Other ex&le are the Leica M system. It has the mechanism of cropping the view of the viewfinder when narrower angled lens is used. This cropping also changes the perspective and 3D effect of the view of the viewfinder.

The angle of view of an image is the angle of view of the lens which produces the image. If one wish to know the angle of view of the image produced by 35mm distalgon,say,one can look up from the Zeiss lens data that the diagonal angle of view of that particular lens is 63 degrees. All the images produced by this lens have a diagonal angle of view 63 degrees.To go from the tip of one corner of the photo to the tip of the far corner has a certain distance as well as an angle of view,and the angle is 63 degree in this case. Zeiss also produce the vertical and horizontal angle of view for reference.
People will notice that the wide angle effect of a photo is most pronounced between the corners of the diagonal. This is because the diagonal angle of view is always greater than the horizontal and vertical angle of view.
As Mike has kindly agreed with me that the perspective and 3D effect of the image is determined by the angle of view of the lens,so is the angle of view of the image.
Go back to the image produced by the 35mm lens,if one goes from the midpoint of the image to one of the 4 corners,the angle of view of this distance is half of the diagonal angle of view,half of 63 is 31.5.
Cropping, in the way like the crop of the sensor,of this image reduces the size as well as all the angles of view of this image. As the perspective and 3D effect is determined by the angle of view of the image,cropping affects perspective and 3D effect.

Afterall,I just want to say 50mm or 60 mm lens is better than 85mm lens in 20D for portraits.
 
Chi & others:

I quite agree that two photos taken from the same vantage point with a wide angle and a telephoto lens will have vastly different perspective. The WA photo contains much more information, particularly when it contains many objects in the foreground and background. But when you enlarge and crop that photo to produce the same FOV as captured by the tele, you discard all that information and end up with the same image as the tele.

The following is an excerpt from an excellent article by Alain Briot, a well respected professional photographer and teacher. I would highly recommend that the entire article be read at:

http://www.luminous-landscape.com/columns/composition-3.shtml


“Exercise C

Take a photograph of the same scene, without moving your camera and tripod,
with both a wide angle and a telephoto lens. When you return to your studio
crop and enlarge the photograph taken with the wide angle lens so that you get
the same composition as the image taken with the telephoto lens.
Compare the two telephoto images: the one created with a telephoto lens and
the one created by cropping the wide angle view. You will not see any
differences as far as composition and “compression” of the scene are
concerned.

Telephoto lenses do not compress distant scenery any more than wide-angle lenses distort scenery (except for fish-eye lenses). The perspective created by a telephoto lens is the same as that of a wide angle lens, except that only a fraction of the wide angle photograph is visible in a telephoto view.

Distant objects are naturally “compressed” because as objects recede in the distance (get further away from us) they appear to be closer and closer together. This effect is caused by perspective, not by lenses. Going from a wide angle to a telephoto lens or cropping a photo has the same effect.

Theoretically, provided we were able to take photographs with limitless resolution, we could always use a wide angle lens and crop our photographs as we see fit once we back in our studio. From this single image we could get all the croppings and compositions we want and each of these croppings would look as if they had been created by using different lenses. In practice, due the finite resolution of film and image sensors, this is not feasible.”

Mike.
 
> Mike

It is a little more complex than presented in the LL article.I agree =20 that if a picture is taken at the SAME position from the subject, the =20=

perspective is the same but the magnification may be different. =20 Perspective only depends on distance and the diameter of the subject.

So if the central image of the compressed image of a wide angle image =20=

taken from the same position is blown up to the size of the telephoto =20=

image,the blown up image will closely approximate geographically the =20 image with the telephoto lens, but the signals of 3 dimensionality, =20 such as depth of focus, texture etc may well be different.

This difference is aperture dependant and will be more important for =20 closer objects and for objects at infinity will be gone.

However, if one takes and image circle and crops it or any image and =20 crops it the perspective doesn't change, not even a smiggin! After all, =20=

how do you inform the image elements in the center of a large image =20 that a cm border all around has been removed? Those image details, like =20=

the focal length are impartial and fixed!

Also, whenever the lens to subject distance changes, the perspective =20 changes. Always!

Asher
 
Asher:

I think we are getting pretty close to complete agreement. When I think of perspective I believe it is determined by the entire content of the photo, and that in turn is determined by the AOV (focal length) of the lens and the distance from the object. In that regard a wide angle image will have more perspective "information" than a tele.

We certainly agree that a wide angle image, once enlarged and cropped to produce the same image as a tele (taken from the same position) will look very much the same if not identical. You mention differences in dof. An image taken with a 50mm lens must be enlarged 7.5x to reproduce an image made with a 135mm lens. In the process you will loose much of the dof advantage of the WA, not to mention the loss of resolution.

The take home lesson here is: if you are mainly interested in the central portion of a vista, use a telephoto lens. You certainly will end up with a better image than using a WA and enlarging. On the other hand, if perspective and 3D effect is your goal, put on the WA and your hiking boots and get a lot closer to the subject.

Cheers,
Mike.
 
Saras:

Your 210mm Sonnar has a focal length of 210mm no matter where you mount it.

On a 645 the lens has a diagonal angle of view of 19.6 degrees, which is approximately the same as a 125mm lens with a film format of 24x36mm.

I don't know the size of the sensor in an N1, but if you tell me, I can calculate the AOV for that format.

Regards,
Mike.
 
I am in agreement with Mike now but not with Asher yet. Perhaps I just have to accept that people think in different ways. I have learned a lot from these discussions.

N1 is a 35mm film camera. N1 digital is full frame 35mm.
 
So, if I shoot a group of people with a 20mm and crop the far left side for ex&le to equal a 135mm tele effect, the distortion from the 20mm "pull" will suddenly disappear? : -)
 
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