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Archive 2012 · Can someone explain to me lens magnification?
  
 
RogerC11
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p.1 #1 · p.1 #1 · Can someone explain to me lens magnification?


Is the MM number an indication of how big the subject can get in the frame regardless of focal length? Does the focal length also come into play? In other words, if lens A has a MM of .23 and lens B has MM of .28, but lens A has a longer focal length, will the subject always appear smaller in lens A at minimum focus distance then compared with lens B also at minimum focus distance?

The reason I ask is because the 400mm 5.6 seems to be a great lens for birds but with such small subjects, it would be ideal to get close as possible. Comparing it with the 70-200 II, which has a higher max mag, if one is not restricted to subject to camera distance allowing minimum focus distance in both lenses, which lens will give a larger subject on the sensor? Hope this makes sense.



Aug 24, 2012 at 04:00 AM
arbitrage
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p.1 #2 · p.1 #2 · Can someone explain to me lens magnification?


Im fairly sure the MM value is a function of the MFD and the focal length. Therefore no more math is required. Only other factor is the sensor size if you had access to a crop and a FF. MM value is ment to be compared directly. .23 will be less mag than .28


Aug 24, 2012 at 04:21 AM
Bob Latham
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p.1 #3 · p.1 #3 · Can someone explain to me lens magnification?


The maximum magnification is the image size (on your sensor) divided by the object/subject size.

Theoretically, the longer focal length would have a higher magnification if you could focus at the same distance. The 70-200 will focus much more closely than the 400/5.6 and hence has the higher native magnification.

Bob



Aug 24, 2012 at 05:04 AM
wickerprints
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p.1 #4 · p.1 #4 · Can someone explain to me lens magnification?


The maximum magnification of a lens is precisely that--the largest possible size of the projected image of a given subject that can be produced with that lens. As such, it is independent of focal length or the sensor size.

To use your example, at MM = 0.28, Lens B is capable of producing a slightly more magnified image than Lens A. However, because Lens A has a longer focal length, you will need to be closer to the subject with Lens B in order to achieve that magnification, because the magnification of a typical lens increases with decreasing subject distance (hence MM occurs at minimum focusing distance, or MFD).

Rarely is it the case that subject distance is completely flexible. You may find that the MFD of the lens with greater MM may be too small, thus making it impractical to get the desired magnification. Or you may have the opposite problem--the MFD is too large, forcing you to stand unnecessarily far away.

That said, as long as you have sufficient flexibility in choosing your subject distance, the lens that has the greater maximum magnification value is the lens that will be able to provide the larger projection onto the sensor, regardless of any other properties.



Aug 24, 2012 at 05:08 AM
AmbientMike
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p.1 #5 · p.1 #5 · Can someone explain to me lens magnification?


Get an extension tube for the 400 for birds. Answer to first paragraph yes.

Let's say mm=.25
1/.25=4
So your in focus area will be 4x your sensor/film size.
Aps sensor 14x22 so 56x88
Ff 24x36 so 96x144



Aug 24, 2012 at 01:24 PM
verbiage
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p.1 #6 · p.1 #6 · Can someone explain to me lens magnification?


AmbientMike wrote:
Get an extension tube for the 400 for birds.


Does this work? Can you still use a lens with extension tubes to focus beyond a couple of feet? I haven't used extension tubes with the 400 F/5.6, but with other lenses it's not just infinity focus that goes... So I guess my question is can you still do, say, 10 - 100 ft with the 400mm and an extension tube?

I don't have the lens, so cannot check myself.



Aug 24, 2012 at 03:00 PM
 

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Bob Kane
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p.1 #7 · p.1 #7 · Can someone explain to me lens magnification?


verbiage wrote:
Does this work? Can you still use a lens with extension tubes to focus beyond a couple of feet? I haven't used extension tubes with the 400 F/5.6, but with other lenses it's not just infinity focus that goes... So I guess my question is can you still do, say, 10 - 100 ft with the 400mm and an extension tube?

I don't have the lens, so cannot check myself.


Don't hold me to these exact numbers, but my recollection is that I can focus between about 6 and 15 feet with the 400/5.6 and 37mm of extension.



Aug 24, 2012 at 03:20 PM
Don Clary
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p.1 #8 · p.1 #8 · Can someone explain to me lens magnification?


My backyard measurements:

The camera (full frame 5D) was mounted on a tripod and the horizontal (landscape) field of view was measured, looking at a horizontal ruler. The second column is closest focus distance, and the third column is horizontal field of view

400mm 11.5' 10.5" no tube
400mm 9.0' 8.0" EF 12
400mm 7.9' 6.5" EF 25
400mm 7.2' 5.5" EF 12 + 25




Aug 24, 2012 at 05:43 PM
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p.1 #9 · p.1 #9 · Can someone explain to me lens magnification?


verbiage wrote:
Does this work? Can you still use a lens with extension tubes to focus beyond a couple of feet? I haven't used extension tubes with the 400 F/5.6, but with other lenses it's not just infinity focus that goes... So I guess my question is can you still do, say, 10 - 100 ft with the 400mm and an extension tube?

I don't have the lens, so cannot check myself.

Depends on the size of the extension tube and all. Typically come in those three sizes, and it's a more dramatic effect on the lower focal length lenses than something like a 400mm. At 400mm, even the largest size extension tube (typically like an inch and a half long) isn't going to have as much effect as on a 100mm lens.

The extension tube ONLY helps you if you are getting too close to the bird for the lens to focus normally (11.5 feet for the 400mm f/5.6). It doesn't change the magnification of the lens at any given (fixed) distance from the bird. In other words, if you are 30 feet away from the bird, whether you use an extension tube or not, the bird will be the same size. Consider something like a 1.4x or 2x teleconverter instead.

Magnification numbers are used a lot in macro photography, a true macro lens has 1:1 (an object the size of the 36mm x 24mm sensor will fill the frame on a full-frame camera). I don't think the number is used much beyond that. If you are photographing a bird that is 15cm or 6 inches long, 0.20x magnification is more than sufficient to fill the frame with the bird.



Aug 24, 2012 at 09:26 PM
AmbientMike
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p.1 #10 · p.1 #10 · Can someone explain to me lens magnification?


The more extension tubes you use the closer you can focus. You'd probably need a 12 or 25 tube depending on how close. 1:2, 1:4 are used all the time. I doubt you will get to 100ft. with an extension tube on. But you cant get a good photo of a small bird at 100ft anyway.






Aug 25, 2012 at 12:14 AM
RogerC11
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p.1 #11 · p.1 #11 · Can someone explain to me lens magnification?


wickerprints wrote:
The maximum magnification of a lens is precisely that--the largest possible size of the projected image of a given subject that can be produced with that lens. As such, it is independent of focal length or the sensor size.

To use your example, at MM = 0.28, Lens B is capable of producing a slightly more magnified image than Lens A. However, because Lens A has a longer focal length, you will need to be closer to the subject with Lens B in order to achieve that magnification, because the magnification of a typical lens increases with decreasing subject distance (hence
...Show more
This explains it perfectly, thanks. As mentioned, extension tubes seem to be a workaround at least but limiting focus to closer distances.



Aug 25, 2012 at 01:28 AM
wickerprints
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p.1 #12 · p.1 #12 · Can someone explain to me lens magnification?


You're welcome.

Please note that extension tubes may adversely affect imaging quality, and/or autofocus performance, depending on the particular lens and the amount of extension.

When using an extension tube, you are operating the lens in a regime in which it is not designed to operate. Aberrations may increase, especially in the image periphery. This is mitigated by stopping down the aperture, or using the least amount of extension that is required.

Autofocus performance may be compromised when extending lenses with a slow aperture, because increasing the back focus distance makes the angle of incoming rays smaller, thus making phase detection more difficult. The decrease in incoming light corresponds to the effective f-number of the lens + extension, which for a lens whose maximum aperture is already f/5.6, may easily become f/8 or slower at magnifications it was not designed to achieve natively. Consequently, the AF system may hunt excessively, or worse, refuse to lock focus entirely, moving the focusing group back and forth rapidly about the position of sharpest focus. This is also in part due to the AF system not "knowing" that you have inserted extra distance between the lens and the camera--it can't move the focusing group in sufficiently fine increments.

In light of this, extension tubes should be considered a secondary resort when investigating options for achieving higher subject magnification. The ideal choice is to use a lens that natively possesses the desired magnification at the required subject distances. If this cannot be done, a teleconverter is often preferable, depending on the application. For birding, a teleconverter is almost always the better choice over an extension tube.



Aug 25, 2012 at 03:26 AM





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