Focal length vs magnification

[TreeofLifeStairs]

Is there a correlation between focal length and magnification? I've seen lenses that say that they are 1:1 or 1:4 or so on but the focal lengths don't seem to be consistent. Am I not understanding how magnification works? I feel I have a pretty good grasp on how focal length effects a picture.

 

[480sparky38 38]

Magnification is the ratio between the subject's real size compared to it's projected size on the film/sensor.

A 10mm object that appears 5mm across on the film/sensor will have a 1:2 magnification ratio. If it is projected at 20mm on the film/sensor, it will have a 2:1 mag ratio.

 

[TreeofLifeStairs]

So for example if I have lens that is a 50mm 1:1 and one that is 200mm 1:1 then the subject would be the same size in the frame but allows the camera to be farther away from the subject? The focal length would just change the perspective then or no?

 

[480sparky38 38]

So for example if I have lens that is a 50mm 1:1 and one that is 200mm 1:1 then the subject would be the same size in the frame but allows the camera to be farther away from the subject? The focal length would just change the perspective then or no?

Not just the perspective. The DOF would change accordingly.

 

[KmH]

Understanding Camera Lenses

 

[Gavjenks]

Is there a correlation between focal length and magnification?

A CORRELATION? No, not at all.

A RELATIONSHIP? Yes. Obtaining a given magnification with a particular perspective (distance from the subject) is only possible with a single focal length. However, obtaining a given magnification simply by itself, with no other stipulations, is possible with any focal length, thus zero correlation between specifically those two variables alone.



For example, 50mm lens (with no extension tube) at a distance of 1 meter will have precisely the same magnification as a 100mm lens at a distance of 2 meters.
So you can get the same magnification with either focal length. But the images won't be identical. The stuff nearest the lens will be exaggerated in size in the 50mm as compared to the 100mm (such as a person's nose compared to their ears), due to geometric perspective.

Distance often matters. In the above example, it can matter by making people's proportions look closer to how we are more used to seeing them (further than a meter away for sure!) When taking photos of bugs or birds, distance for the same magnification is helpful to not scare them off, etc.

 

[JerryLove]

I'm guessing: but I suspect that the magnification (1:1, or 1:4, for example) is a function of focal length and minimum focus distance combined. The longer the focal length, the bigger the item as projected on the sensor. The closer you physically are: the larger the item as projected on the sensor.

What macro lenses have isn't so much high focal lengths (some do), but rather very short minimum focus distances.

 

[Gavjenks]

magnification (1:1, or 1:4, for example) is a function of focal length and minimum focus distance combined.

The MAXIMUM (in focus) magnification of the lens, yes. The actual magnification for a given shot changes depending on how far you are focusing at the moment.

For example, let's say a lens has a maximum magnification of 1:2 @50mm. That doesn't mean that whenever you're at 50mm you have 1:2 magnification. Only if you are currently for this particular shot aiming at something right at your minimum focal distance.

If you don't change any settings, but focus on something further away, you have a lower magnification.

 

[Overread14 2]

So for example if I have lens that is a 50mm 1:1 and one that is 200mm 1:1 then the subject would be the same size in the frame but allows the camera to be farther away from the subject? The focal length would just change the perspective then or no?

Not just the perspective. The DOF would change accordingly.

As far as I've ever worked out the depth of field remains identical.

A 50mm and a 200mm lens at the same magnification of 1:1 will give you the same frame content and the same depth of field. What will change is

1)The distance from the subject - the longer the focal length the further away (which tends to correspond to a greater working distance)

2) The degree of background blurring. The longer the focal length the quicker the fall off into blur and the greater the degree of blurring. This gives the impression that shorter focal lengths give more depth of field because the drop off into blur is less noticeable then it is with a longer focal length macro lens.


Minimum focusing distance - distance from the sensor/film to the subject
Working distance - distance from the front of the lens to the subject. (typically only important and referenced in macro photography).

 

[Gavjenks]

As far as I've ever worked out the depth of field remains identical.

It's complicated. It is close to identical at higher magnifications. At lower ones, there are huge differences:
http://theonlinephotographer.typepad.com/.a/6a00df351e888f883401157149a571970b-800wi

Not to mention that DOF changes with aperture for example, while magnification does not.

 

[Overread14 2]

Aye for the human eye and in real world context its basically impossible to tell - the biggest visual difference you might get is if one lens happens to internally operate differently. Most macro lenses on the modern market reduce their focal length and aperture as they focus closer to 1:1. However from what I gather most land around a similar rough aperture value - I don't think there are any extreme options that land far wider or smaller than the others to really give a big difference.

 

[WayneF]

Is there a correlation between focal length and magnification?

A CORRELATION? No, not at all.

A RELATIONSHIP? Yes. Obtaining a given magnification with a particular perspective (distance from the subject) is only possible with a single focal length. However, obtaining a given magnification simply by itself, with no other stipulations, is possible with any focal length, thus zero correlation between specifically those two variables alone.



For example, 50mm lens (with no extension tube) at a distance of 1 meter will have precisely the same magnification as a 100mm lens at a distance of 2 meters.

There are a couple of ways to look at it, but you confused them. You referred to subject size, but you just showed that a 100mm lens has 2x the magnification of a 50mm lens.

Focal length is exactly magnification. That is the only reason we use telephoto, to magnify the subject size.
4x focal length will be a view 1/4 as wide, but the subject image size will be 4x larger. That is magnification.

However, macro distance does not compare two magnifications of image size, but instead compares image size to size on the sensor (reproduction ratio).
(Almost) any macro lens can do 1:1 reproduction, regardless of focal length. That is a matter of focusing closer.

 

[KmH]

2) The degree of background blurring. The longer the focal length the quicker the fall off into blur and the greater the degree of blurring.

The degree of background blurring also stays the same.
The longer focal length magnifies the blurred background elements (background compression) but does not blur the background more.
The longer focal length also has a narrower field-of-view so less of the blurred background is in the image frame.

 

[Derrel]

As far as I've ever worked out the depth of field remains identical.

It's complicated. It is close to identical at higher magnifications.

A number of years back, I shot 1:1 magnification images using my 60mm Micro-Nikkor and my Sigma 180 f/3.5 EX Macro...I got what appeared to me to be the same depth of field with the 180mm lens as I did with the 60mm, despite the 180mm lens being 3x farther away...it kind of made an impression on me...

 

[480sparky38 38]

Is there a correlation between focal length and magnification?

A CORRELATION? No, not at all.

A RELATIONSHIP? Yes. Obtaining a given magnification with a particular perspective (distance from the subject) is only possible with a single focal length. However, obtaining a given magnification simply by itself, with no other stipulations, is possible with any focal length, thus zero correlation between specifically those two variables alone.



For example, 50mm lens (with no extension tube) at a distance of 1 meter will have precisely the same magnification as a 100mm lens at a distance of 2 meters.

...... You referred to subject size, but you just showed that a 100mm lens has 2x the magnification of a 50mm lens...........

Not when you double the distance between the camera & subject.