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

deshojo

Well-Known Member
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
New Post:
> 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

New Post

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

New Post
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.

New Post

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

New Post


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

New Member
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

Well-Known Member
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

Well-Known Member
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

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

wang

Well-Known Member
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

Active Member
> 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

Well-Known Member
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

Well-Known Member
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

Active Member
> 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).

>
 

deshojo

Well-Known Member
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.
 

blumesan

Well-Known Member
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
 
S

spluff

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
 

wang

Well-Known Member
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.
 

blumesan

Well-Known Member
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:

Please, Log in or Register to view URLs content!



“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.
 

med007

Active Member
> 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
 

blumesan

Well-Known Member
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.
 

blumesan

Well-Known Member
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.
 

wang

Well-Known Member
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.
 

fotografz

Well-Known Member
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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