35mm vs 50mm
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Scaglietti
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Perspective is all about relative sizes of objects and the resultant perceived distances as percieved by human eye...:dunno:
BC
gooseberry
Senior Member
Yes, and in the two images you posted above you are confusing image magnification with perspective. If you can see the "subject" (the small bush) in the 12mm image (perhaps by enlarging it) and compare it to the 550mm image, you can see that the relative size and position of the "subject" compared to the other objects around it, they will be the same in both images.
Guys, as a newbie, i find the explanantion on perspective in the link below makes the most sense..
In a nutshell it says that when u keep the subject at the same size, "The shorter the focal length, the more perspective you see in the background. And the longer the focal length, the closer the background seems to be to the subject."
http://www.canon.co.jp/Imaging/enjoydslr/p_2_013.html
Whdaya guys think?
In a nutshell it says that when u keep the subject at the same size, "The shorter the focal length, the more perspective you see in the background. And the longer the focal length, the closer the background seems to be to the subject."
http://www.canon.co.jp/Imaging/enjoydslr/p_2_013.html
Whdaya guys think?
Absolutely correct.
The animation of the camera which was moved back with increasing focal lenght to keep the subject at the same size. So the perspective did change. If all photos were taken at the same distance, all u will see with different focal lengths is a manifying or cropping effect. I think the definition of perspective is clear enough, it is the angle of view and distance from your subject, focal lenght only helps to include or exclude elements of the scene from your photo.
if dat's true, a 50mm will be the normal lens no matter whether its on FF or DX body... but u'll have to change ur distance to object to nullify the 1.5x crop.. is dat correct?
What is normal lens? :dunno:
The lens that is functioning normally and which u use normally when u are normal
Wah... so cheem!
Ack, that's why I said this is a really confused issue.
I guess Scaglietti is right that a lot of this will depend on the definition of perspective. But my understanding of it is pretty much that in the link above provided by kzone.On the other hand, gooseberry provides good evidence that for the same distance to subject, the "perspective" (To me: "apparent size of background relative to foreground subject") remains the same.
To Scaglietti: About the lens mount to film plane issue, I am referring to the use of 80mm lens as the normal for medium format and 50mm lens for 35mm SLRs. You mentioned that the "normal" perspective is that when the focal length is the same as the diagonal of the image frame. Quite true, though as I recall reading somewhere long ago, that it is actually the diameter of the image circle that should be the same as the focal length to be "normal" (and this stands to reason). This may be the reason why 50mm was chosen as the focal length, and not 40+mm, so that the image circle is slightly larger than the film frame to allow for light-falloff and other aberrations in those days. Hence, for a 50mm normal lens, the image circle is 50mm in diameter, while an 80mm normal lens has 80mm image circle (to fit the medium format frame). Now, imagine a simple single glass element (instead of a complex multi-element lens) of focal length 50mm and 80mm. When focused at infinity, the lens is 50mm away from the film plane, or 80mm away, respectively. But both lenses are giving "normal" perspective by the definition "focal length = image circle diameter or frame diagonal". By my understanding, this simplest case also applies to the distance between the optical centre of a camera lens and the film plane (ie. effective focal length of the camera lens).
Anyway, I could be wrong
and I suppose this is pretty much an academic discussion anyway. Any moment now, someone is going to say "go and shoot more, talk less!" :bsmilie: :devil: Still, I am trying to understand this better... Any optical engineers here?
longkangman
Senior Member
Hope this help http://en.wikipedia.org/wiki/Circle_of_confusion again I could be wrong. 
I always lost/confuse when come to this.
I always lost/confuse when come to this.
hmmm guys..how about this...what u see in with your eyes is abt the same size as what a 50mm can capture? 1.5x sensor has a cropped factor...so when u capture smt with a FF....u open it in your computer..then u cropped away the corners..now tt's the the picture u see in a cropped sensor...so i assume the size is the same...just that the corners are cut off...am i right to say this? sorry i'm still trying to understand.
gooseberry
Senior Member
Yes, the key to it here is what I have made bold in your summary - by keeping the subject the same size, you have to move the camera relative to the subject for the different focal lengths, and thus change the perspective because you are at different position (or viewpoint)
Scaglietti
New Member
Ack, that's why I said this is a really confused issue.
On the other hand, gooseberry provides good evidence that for the same distance to subject, the "perspective" (To me: "apparent size of background relative to foreground subject") remains the same.
To Scaglietti: About the lens mount to film plane issue, I am referring to the use of 80mm lens as the normal for medium format and 50mm lens for 35mm SLRs. You mentioned that the "normal" perspective is that when the focal length is the same as the diagonal of the image frame. Quite true, though as I recall reading somewhere long ago, that it is actually the diameter of the image circle that should be the same as the focal length to be "normal" (and this stands to reason). This may be the reason why 50mm was chosen as the focal length, and not 40+mm, so that the image circle is slightly larger than the film frame to allow for light-falloff and other aberrations in those days. Hence, for a 50mm normal lens, the image circle is 50mm in diameter, while an 80mm normal lens has 80mm image circle (to fit the medium format frame). Now, imagine a simple single glass element (instead of a complex multi-element lens) of focal length 50mm and 80mm. When focused at infinity, the lens is 50mm away from the film plane, or 80mm away, respectively. But both lenses are giving "normal" perspective by the definition "focal length = image circle diameter or frame diagonal". By my understanding, this simplest case also applies to the distance between the optical centre of a camera lens and the film plane (ie. effective focal length of the camera lens).
Anyway, I could be wrong
Still, I am trying to understand this better... Any optical engineers here?
I am alright with the lens and camera design part. But the basic idea behind normal lens is to try to replicate what the human eye see. Due to the crop factor of the DSLR, the image appear magnified. Therefore, a shorter lens is needed to achieve the same magnification. When you look through the viewfinder while using a normal lens, you will find the image roughly of the same magnification as your naked eye.
Hmmm... magnification probably explain better in this case than perspective.:think:
BC
Yes, I think that is the crux of the matter, you were describing how an image looks relative to the human eye (magnification and sort of "angle of view"), while I was trying to describe the size/appearance of the background relative to the foreground.
Gooseberry's explanation (distance to subject determines perspective) looks to be the most accurate so far for what I was thinking about (and clearly more correct than my own flawed explanation that focal length determines perspective), while the effect shown in the canon link provided by kzone is what I was trying to explain, and I think what Scaglietti was referring to as well.
Good discussion, a lot clearer now for me. Perhaps one day, if I can borrow a FF Nikon (Kodak? Future D3x?) DSLR, I'll try to shoot a complete comparison for some evidence-based conclusions, and to see the effect viz-a-viz perspective, DOF, etc between 35mm and 50mm normal lenses.
Scaglietti
New Member
You can do a simulation on the cropped sensor, but by further 1.5x cropping on the photo. It might not replicate exactly, but it will give you the idea.
BC
In real world usage, a 30 or 35mm lens on a 1.5x crop DSLR would function similarly to how you would use a 50mm on a film SLR. You would stand in the same spot to take the picture and get a generally similar image (subject magnification, and angle of view). Relatively small differences would be a slightly different perspective (apparent distance to background behind the subject), and more DOF for the same f-stop.
Other small differences would be due to the lens design and quality, a 35mm prime lens may be less sharp by a tiny margin and possibly has more barrel distortion, than a 50mm prime.
So... if you are used to using a 50mm on film SLR, and want to continue to shoot with that familiarity of equipment, then get a 35mm lens. Otherwise, it will just depend on personal preference of how close you like to be to the subject and how tight you usually frame it.
gooseberry
Senior Member
If you want the equivalent FoV and perspective of a 50mm lens on 35FF camera on a 1.5x crop body, then you get the 35mm lens (that would be the closest approx. since you actually need something like a 33mm lens) - that is to say, if you were standing at the same position and you wanted to frame the subject the same way as a 50mm lens on a 35FF camera, you would need a 35mm lens to get approx. the same on a 1.5x crop camera.
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