Focal length and full sensor vrs.crop sensor

uplander

TPF Noob!
Joined
Mar 22, 2008
Messages
536
Reaction score
0
Location
S.E. WI.
Can others edit my Photos
Photos OK to edit
I've been thinking about this for some time. This statement is out there all the time, "A 400mm lens on a 1.6 crop factor sensor is equivalent to a 640mm on a full size sensor".

I believe this is a true statement when you are comparing field of view which is expressed in angle. So what you have is a lens that projects the same size image but because the sensor sizes are different the smaller sensor captures a smaller portion of the image which results in a narrower FOV. The image isn't any different it's just that the smaller sensor captures less of it, hence the narrower FOV.

I believe stating that a 400 mm lens on a 1.6 crop sensor becomes a 640mm lens is misleading. One tends to think it makes the lens more powerfull in magnification which isn't true. I would think the magnification is the same unless there is some difference in the distance from rear element to the sensor.

So if that distance is the same ( I would tend to think it is ...I may be wrong though) Then the only difference is the amount of the image projected by the lens each respective sensor catches. So then the area of the image each sensor captures are exactly the same in image size and quality except the crop sensor records less of it ( IE the bordering edges)

Hence I think that a 400mm lens is a 400mm lens despite what body it is on. Or in other words saying it is the equivalent of a 640mm on a 1.6 crop factor sensor is a bunch of hooey! A 400mm lens on a crop body does not get you any closer to your subject than a full size body

Unless.....you start looking at the amount of pixels in each sensor or lets just say does having more pixels/cm2 will give you better resolution when you crop and in effect give you more magnification which is a whole another can of worms. So maybe we should just stay and debate the first part of this post.

I got a feeling this should be interesting if you really put some thought into it.

Discuss!!!:mrgreen:
 
Last edited:

Jaszek

No longer a newbie, moving up!
Joined
Oct 7, 2008
Messages
2,177
Reaction score
36
Location
Brooklyn, NY
Website
michalphoto.tk
Can others edit my Photos
Photos OK to edit
The lens is the same (no duh!) it's just the field of view taht's different and aperture. Also if you look at P&S cameras the focal lenghts are like 6mm and the aperture is 2.8 since the sensor is so small. You also don't get such a shallow DOF on a P&S than on a FF body (I'm just using that example since there is a huge difference due to sensor size)
 

AlexColeman

TPF Noob!
Joined
Dec 13, 2008
Messages
1,732
Reaction score
1
Can others edit my Photos
Photos OK to edit
I don't really see your point. Obviously it is the same lens, however those equivalencies are just to give one standard measurement. Thats how everyone knows 28 equiv. is a little wide, while 400 equiv. is tele.
 

anubis404

TPF Noob!
Joined
Apr 5, 2008
Messages
955
Reaction score
0
Can others edit my Photos
Photos OK to edit
I'm not quite sure what you are saying. A 400mm is in fact a 600mm equivalent on a 1.5x body. No one ever said the focal length became 600mm. Crop factor is almost like cropping the image and blowing it up without losing any quality. While not being the same as focal length, it looks very similar. If you don't believe me I'll show you pictures taken with a 50mm DX lens and a 50mm FX lens.
 
OP
uplander

uplander

TPF Noob!
Joined
Mar 22, 2008
Messages
536
Reaction score
0
Location
S.E. WI.
Can others edit my Photos
Photos OK to edit
I don't really see your point. Obviously it is the same lens, however those equivalencies are just to give one standard measurement. Thats how everyone knows 28 equiv. is a little wide, while 400 equiv. is tele.


Alex ..your missing it completely.. A 50mm on crop body gives the same DOF as on a full frame body ..It's just the FOV is different.


A 28mm on a full frame body is a hell of a lot wider than on a crop body but the the DOF is the same. get it?

The lens sees the same and in the same aspect regardless of the body. The crop body just cuts the edges off.
 
OP
uplander

uplander

TPF Noob!
Joined
Mar 22, 2008
Messages
536
Reaction score
0
Location
S.E. WI.
Can others edit my Photos
Photos OK to edit
I'm not quite sure what you are saying. A 400mm is in fact a 600mm equivalent on a 1.5x body. No one ever said the focal length became 600mm. Crop factor is almost like cropping the image and blowing it up without losing any quality. While not being the same as focal length, it looks very similar. If you don't believe me I'll show you pictures taken with a 50mm DX lens and a 50mm FX lens.


But the image recorded is identical except the edges are cut off.

And yes there is a difference, An image taken with a 400 mm lens has a different DOF then one taken on a 600mm.

Put a subject in the foreground with either lens keeping the foreground subjects the same size and the background is dramatically different between the 400 and the 600.
 

Dao

No longer a newbie, moving up!
Joined
Mar 24, 2008
Messages
6,423
Reaction score
462
Location
St. Louis
This is what I believe.

A 100mm focal length lens on a 1.6x crop, is only a 100mm focal length lens with a "Field of View" equivalent to a 160mm focal length lens on a 35mm film SLR camera or a full frame sensor DSLR camera. That is. nothing more.

It should not be the same as you have a 160mm lens.

Let take an exmaple.


B----S--------------------------------C

Where B is the background. S is the subject and C is the camera.
"----" line is the distance.


Take a photo (PHOTO1) with a 100mm lens in a crop body.

Now, change the camera with a full frame sensor body. Take another photo. But this time, I want to take the photo (PHOTO2) so that it has the same Field of View of the 1st one (PHOTO1).
In order to do that, I need to walk closer to the subject. So it become

B----S-------------------------C


As we all know, now the distance between "S" and "C" is shorter.


Now when you compare PHOTO1 and PHOTO2. You will find the FOV on both photos are the same. The object size is the same. In other words, the crop body seems to have a better "zoom" power.

Does that mean 100mm = 160mm in the crop body? I will say no since the ratio of the distance between "B-S" and "S-C" is different, therefore the perspective distortion characteristic is going to be different. Is it noticeable? I do not know since I only have crop body cameras, so I cannot do any test. But I think if distance of "B-S" is roughly equal to "S-C" in PHOTO1, then it may be noticeable.


This is what I believe and please correct me if I am wrong.
 

Dao

No longer a newbie, moving up!
Joined
Mar 24, 2008
Messages
6,423
Reaction score
462
Location
St. Louis
Also, from the above example, the Angle of view is also different.
 

anubis404

TPF Noob!
Joined
Apr 5, 2008
Messages
955
Reaction score
0
Can others edit my Photos
Photos OK to edit
But the image recorded is identical except the edges are cut off.

Exactly, which gives the effect of it having a longer focal length.

And yes there is a difference, An image taken with a 400 mm lens has a different DOF then one taken on a 600mm.

I'm not talking about DOF, I'm talking about equivalence.
 

Village Idiot

No longer a newbie, moving up!
Joined
Mar 20, 2008
Messages
7,275
Reaction score
406
Location
Shepherdsturd, WV / Almost, MD
Can others edit my Photos
Photos NOT OK to edit
Alex ..your missing it completely.. A 50mm on crop body gives the same DOF as on a full frame body ..It's just the FOV is different.


A 28mm on a full frame body is a hell of a lot wider than on a crop body but the the DOF is the same. get it?

The lens sees the same and in the same aspect regardless of the body. The crop body just cuts the edges off.

This is wrong.

DOF calculator:
Online Depth of Field Calculator

try it out. Does the crop factor and everything for you.
 

Steph

No longer a newbie, moving up!
Joined
Oct 18, 2006
Messages
1,314
Reaction score
0
Location
Steventon, Oxfordshire, UK
Can others edit my Photos
Photos NOT OK to edit
A 28mm on a full frame body is a hell of a lot wider than on a crop body but the the DOF is the same. get it?

Well, this comment about DOF is not true. It is not that simple and when you put more thought into it, it gets horribly complicated. The DOF is the zone of a picture which is acceptably sharp. That will depend on the size of the final print, the viewing distance and the circle of confusion diameter limit, which depends on the size of the sensor/film you use. So your 28mm lens won't produce the same DOF on a full frame body and on an APS-c sized sensor.
 

Big Mike

I am Big, I am Mike
Staff member
Supporting Member
Joined
Dec 16, 2003
Messages
33,900
Reaction score
1,863
Location
Edmonton
Website
www.mikehodson.ca
Can others edit my Photos
Photos NOT OK to edit
I'm not talking about DOF, I'm talking about equivalence.
Equivalence of what?

The only think that really is equivalent (to something different) in this scenario is the FOV (as mentioned in the original post).

The DOF, the perspective, the magnification...these things do not change because of the 'crop factor' of the camera.
 
OP
uplander

uplander

TPF Noob!
Joined
Mar 22, 2008
Messages
536
Reaction score
0
Location
S.E. WI.
Can others edit my Photos
Photos OK to edit
Equivalence of what?

The only think that really is equivalent (to something different) in this scenario is the FOV (as mentioned in the original post).

The DOF, the perspective, the magnification...these things do not change because of the 'crop factor' of the camera.


That's my take on it also. A 50mm lens is supposed to see what the natural eye sees or as close to it aproxamately. Thats why a 50mm takes nice portraits. What the lens sees and is recorded is the same whether on a full frame body or a crop body. The crop bodies just cuts the edges off.
The perspective is the same.

A 35mm on a crop body is said to be a 50mm equivilent yet the perspective is not the same as a 50mm on a full frame. I hope I'm expressing in terms that make sense.
 

kundalini

Been spending a lot of time on here!
Joined
Jul 18, 2007
Messages
13,609
Reaction score
1,936
Location
State of Confusion
Can others edit my Photos
Photos NOT OK to edit
That's my understanding uplander, it's the FOV. You can't change the focal length. I've been wanting to test the theory, but we are having nasty weather this weekend. Maybe I can figure something out........ non-lab science, of course.
 

Dwig

TPF Noob!
Joined
Mar 11, 2009
Messages
1,261
Reaction score
0
Location
Key West FL
Website
happythursday.com
Can others edit my Photos
Photos NOT OK to edit
Equivalence of what?

The only think that really is equivalent (to something different) in this scenario is the FOV (as mentioned in the original post).

The DOF, the perspective, the magnification...these things do not change because of the 'crop factor' of the camera.

My thought exactly, "equivalence of what". The one and only attribute of a lens/body combination that is "equivalent" when applying the crop factor (e.g. 400mm lens on a APS-c 1.5 crop factor body is equivalent of a 600mm lens on a 24x36mm sensor) is the FOV. All other attributes (DOF, impact of adding a particular extension tube, ...) are affected by the actual FL of the lens in use. For all those other attributes, a 400mm lens is a 400mm lens, period.
 

Most reactions

New Topics

Top