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

[deshojo]

The Story Sor Far...


85mm lens after conversion becomes 135mm, portrait taken with this will be "compressed" "flat" "lack of 3D feel". This is why 135mm is less popular than 85 or 90 mm. I would use a 50 or 60 mm lens,after conversion,will produce a better result for portrait. Chi
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> Chi, the perspective and feel of the 85mm lens on any camera is > identical. That is optical physics and not open to any other > interpretation. All one is doing is cropping the image circle. If you > could sneak into the back of the 10D camera and put in a full frame > 35mm sensor, the central image would not change in any way. The only > change would be the extra image real estate around the original > limited 10D sensor.

Now what I think really happens is that a photographer using the 85mm lens with a camera with a smaller sensor steps back to include the field of view he is used to seeing with a full frame 35mm camera. This changes the image appearance making it flatter, not the lens!!

Asher

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The focal length of a lens depends on,apart from the lens structure, the size of the negative or sensor. Changing the light sensitive area effectively changes the focal length. Another way of saying this is changing the size of the light sensing area effectively changes the angle of view of the lens.
Asher,you are telling me something I already know,am I telling you something that you don't know ? Chi

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Wang:
Sorry, you are dead wrong. The size of the negative or sensor has NO effect on the focal lenght of the lens, or on the angle of view of the lens. It is just as Asher describes, the size of the sensor simply limits (crops) the area of the image projected by the lens. Thinks about it: if I take a photo and crop the image in processing, does this change the angle of view?

It is a bit presumptious alleging that you are telling Asher something he doesn't know, when what you are telling is plain wrong.

Respectfully
Mike.

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Hi Mike,
My answer to your question,"Thinks ab... of veiw" is yes. The size of the sensor is an important determinant of the focal length.
Let us look at a diagram.85 mm lens has diagonal angular view of about 28 degrees,from one corner of the photo to the far corner is about 28 degrees. Images of all the objects within this angle reach the sensor. If the sensor is reduced in size,an object at the corner of the photo will not be detected by the new sensor,the angle of view is effectively reduced with the new sensor. If the conversion is 1.5,the new angle of view is about 18 degree. Even if you crop the image in processing,you effectively changes the angle of view from 28 to 18 degrees.
The new image has all the looks of the new focal length. The face looks less 3D and more compressed,there is also a decrease in the variation of the sizes of the objects in different positions from the camera.
Let us consider an extreme ex&le. With an image taken by a wide angle lens say 21mm,if you crops the images enough you can make it looks like an image from an 600mm lens,provide the first image has enough resolution for you to crop.
I am happy to go to any sites,read anything,listen to anyone if the contrary is explained to me,otherwise I am dead right and I hope that you believe I have written something which you don't know. Chi

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I would have to agree with Mike and Asher (and my earlier post about the 85mm of course).

The results are perhaps most apparent with my 21mm Distagon on 20D. The beautiful perspective and visual 'feel' of the lens is not altered in photographs; you just can't see the edges!

I think the ex&le of post-cropping a photo is the easiest way to appreciate it, like taking a 36x24" print and just cutting it down to the 22.5x15" central area.

Cheers,
Matt

 

[emmawelsh]

So much confusion . . . FIELD OF VIEW is not the same as FOCAL LENGTH!! Sub-35mm sensors crop FIELD OF VIEW, they do not alter focal length. Perspective effects are created by FOCAL LENGTH. Therefore relative distances remain the same with any lens of any given focal length. Cropping a 21mm shot to the same FOV as a 600mm will not look the same as a shot made with a 600mm lens - try it.

 

[wang]

Hi Matt,
Do you feel that the 21mm in the 20D produces images more like images of the 35 mm lens ? It loses the ultra wide angle effect and becomes less wide. You can only see this if you are familiar with the individual images of the 21mm and 35mm.
Hi Emma,
If you cut the photo of a 21mm lens and make it into many fragments,you will find the images of each fragments look very compressed,just like the look of the images made by the telephotos including a 600mm. Indeed, the smaller you cut,the more it looks like from the 600mm.
The wide angle effect can be produced by the integration of all these fragments.Athough each fragments is very compressed and lacks a spatial feel,once they all join together,the spatial feel returns.
Cheers!

 

[deshojo]

OK, we've all seen the film 'Jaws' right?

Remember that scene of chief Brody sat in his chair and the dramatic change of perspective, as he seemed to still fill the frame in the same way, but the whole beach suddenly came into view?

That was achieved by zooming out from telephoto to wide angle, whilst moving the camera closer to him. He remained the same size in the frame but the background expanded hugely. That's perspective.

It's the relationship between the photographer, the subject and the background which only alters if you actually change focal length, not by cropping the image.
Matt.

 

[deshojo]

Of course that should have read..
"only alters if you actually change focal length and position of photographer relative to subject".
Cheers,
Matt

 

[wang]

Hi Matt,
Don't restricted your thought to only changing the focal length,this special effect can absolutely be done by cropping together with magnification.
If you wish to change from wide angle to tele,ie the expansion of the background. You start with the wide angle, you crop and magnify and you will produce exactly the same effect as zooming. Of couse,this will also make him gradually getting bigger,you have to move the camera away from him while doing this.
Thank you for giving me an ex&le to illustrate the point.

 

[med007]

> Chi, you have it backwards. Reread Matt's post!

We have gone through this enough. Try what Matt has suggested and reread what I have written.

Back to the C/Y adapter issue.

I believe that the Zeiss lenses are so good in color, contrast, clarity and something special that they render the wide end needed for Canon Cameras very well.

Everyone should at least keep a 28mm Distagon. There is nothing in the Canon lens inventory at that focal length. At 35mm, the Canon lenses are very very good and have autofocus.

The 21mm distagon is optically great even when it is tricked into shamefully sending it's light to a Canon CMOS sensor! I have only heard rave reviews.

Remember, for Pano shots (ie wide field of view...focal length stays the same

, one can use the Zoerk.com shift adapter with a 35mm Mamiya MF lens or a 45mm (or any other) 6x7 Pentax lens. The latter is my choice. There is also the Schneider 28mm shift lens (excellent), the Canon 24mm, 45mm and 90 mm shift lens (the latter a marvel) so there are lots of choices. If you are rich enough, the 15mm Z/C Distagon is a nice luxury.

I have these Z/C lenses which delight me: 18mm distagon, 28-85 Variosonnar, 50mm Planar.

Remember that the Canon 50mm 2.5 Macro is beyond superb and the L lenses from the 70-200 f4.0 and 2.8 upward are super! Even the el cheapo 85 1.8 is a very wonderful, hard to match fast focussing sharp lens.

I only use the Cameraquest.com adapters and don't ever remove them. One for each lens! I don't want to be fiddling with adapters when I have to change lenses on a shoot.

I have, like many of you obsessed about MTF's. However, practical use for what you need is the only way to finally decide.

Also reports from other photographers are highly influential for me.

Ultimately, all the lenses are capable. Here we are talking about finesses. The Z/C lenses I have metioned deserve your strong consideration.

The longer focal length too if you have the experience to use them and cant afford the really stellar Canon glass.

I still may get the Z/C f 1.4 85mm.

Asher!

 

[blumesan]

First I have to amend my remark in an earlier post to Wang. If you define the angle of view of a lens as the included angle subtended by lines drawn from the extreme left and right side of the imaged object to the film plane; you are quite right. The size of the sensor will certainly affect the angle of view. However this topic began with a discussion of the 3D rendering of the image and the question of change in perspective. These factors are directly dependent on the focal length of the lens which does not change with the size of the sensor.

Do the following experiment: Take two lenses, say a 35mm and an 85mm. Mount the longer focal length lens on your camera and take a photo (carefully noting objects at the extreme left and right in the viewfinder). Now mount the shorter focal length lens and take a hike. Walk up close enough to the object first photographed so that the same field of view is seen in the viewfinder and take another photo. Compare the two, you may be surprised at the difference. You could do the same test with a zoom lens.

I first tried this experiment after reading an article the claimed there was little need for a zoom lens (and by extension little need for primes of different focal length), one can always zoom with your feet. That assumes, of course, that you are not standing at the rim of the Grand Canyon.

It occurs to me that the counterpart of this experiment is to do it the same way using one lens and two cameras with different size image sensors. Here I believe you will see no difference in the images.

Cheers,
Mike

 

[wang]

I have to correct some of what I wrote yesterday, the focal length is fixed and does not change with the sensor size, cropping only changes the angle of view.

My message to Matt is still the same, zooming from wide angle to telephoto has exactly the same effect as cropping and magnification. What actually happens in zooming is a gradual reduction of the angle of view. Cropping also reduces the angle of view. In fact, the amount of cropping can also be expressed by the angle of crop.To every bit of area in the photo,there is a corresponding bit of the angle of view. You could crop a photo so badly that the angle of crop is very much larger than the angle of the small remain.This is the situation when the photo with original 91 degree is cropped to a small angle of only a few degree. (Yesterday's 21mm to 600mm crop).Changing the focal length,changing the angle of view and cropping are equivalent.

Back to the origin of this topics, it is right to say cropping does not change the focal length, but it does change the angle of view. The conversion of 85mm lens in 20D, what actually happens is there is no change in focal length of the 85mm lens, but cropping reduces the angle of view. The new angle is the same as the angle of the camera in full frame using a 135mm lens.

Hi Mr Blume, we learn from each other and after disagreeing, one of us will change and agree with the other. Here come again the situation that I do not agree with you.
The 3D rendering of an image and the change in perspective depends on the angle of view. The angle of view depends on both the focal length and the sensor size,therefore the 3D rendering and the change in perspective change with focal length and the sensor size.
The experiment you did yields different looks in the photo,I have to say this is because you are using lenses with different angles of view, rather than the difference in focal length.

To us as photographers, it is more useful to talk about the angle of view, rather than the focal length. Why do you care where the image is formed when the object is at infinity, only the lens designer will need to know that. It make a lot more sense if we talk about the angle.
Let us go to a hypothetical situation. What will happen if we increase the sensor size and keeps the focal length constant.Let us use a 35mm lens. What will happen to the angle of view ? This situation is similar, but not equivalent,
to the situation of 38mm Biogon in medium format. In this situation,the light sensing area is increased, the focal length
is similar to our 35mm lens,but the angle is increased from 63 degrees to about 90 degrees, an angle similar to our familiar 21mm distalgon. Therefore, increasing the sensor size will increase the angle of view when the focal length is constant.

Hi ladies and gens,do you all agree with me ? If you people do we can all go taking photos. Cheers.

 

[med007]

> Chi, Hi again!

Glad you now are OK with the idea that the lens focal length is constant as long as you don't take the optic apart and rearrange the glass, LOL!

Still, like Paul on the road to Tarsus, maybe a light will shine in your direction and you might venture one step further.

Unfortunately, angle of view and field of view are total different. We are concerned here by the arrangement of objects in 3D space rendered by a lens in a 2D plane of focus. The exact arrangement, or particular distortion to go from 3D to 2D is dependent on three variables. These are: focal length of the optic, distance of the subjects from the lens and the aperture. The latter, with the focal length and distance to the subjects will define the depth of focus**(see footnote)

So not only is the angular rendering of the image (ie perspective) different with different, focal lengths, but also the depth of field and therefore the rendering of even the nose and eyes versus the ears.

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. It was like the view of an ant looking at a mouse! Or else, like the nose jutting into the lens of a camera with a wide angle taken too close. Enlarging or contracting that image cannot alter the distortion originally made to grasp all the light from left and right and squeeze it together to form a tiny image on your sensor. Such images are, however, very or even too three dimensional.

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.

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.

Also, each image will ONLY become optimally real" or "alive" when the viewing angle matches the capturing angle, so you can't fool physics or mother nature. Well, at least not in this case.

This is a three credit course, you will be charged with heresy if you don't get with the program and "see the light"!

Good luck,

Asher

** Depth of focus, ie the range of distances in front and behind the declared distance to the subject that other objects are still in focus. (The aperture will also define the resolution of the lens which paradoxically is greatest at greatest aperture for those objects in the center which are in optimal focus).

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