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About Magnification, angle of view and more...
I very curious about question how are correlated two fundamental opticalparameters such as Magnification ratio and Angle of view.It is looks very simple but....

Lets take for instance two Pentax lenses:.

Pentax SMC P-FA 35mm f/2 AL with Magn. Ratio 0.17x Closest Focus 30.5cmand.

Pentax SMC DA 35mm f/2.8 Macro Limited with Magn. Ratio 1.0x Closest Focus 13.9cm.Both have the same focal length corresponding ASP-C sensor eqvivalentThe same Diagonal Angle of View =44 deg.but different Closest Focus. 12"/5.47 or appr. 2/1.

Now Questions.1. Can macro lens be focused at infinity (20 feet for example)?.

2. If they can, sinse both lens have the same angle of view presumably both should capture the same object equally..

For instance, camera with non-macro 35mm lens produces test image on sensor from left to right edge,.

What happend if I swap for 35mm macro lens. Picture will be cropped accordingly magmification ratio?But what means angle of view in this case?So, where is my misunderstanding?.

Thank you all...

Comments (8)

AKozik wrote:.

I very curious about question how are correlated two fundamental opticalparameters such as Magnification ratio and Angle of view.It is looks very simple but....

Lets take for instance two Pentax lenses:.

Pentax SMC P-FA 35mm f/2 AL with Magn. Ratio 0.17x Closest Focus 30.5cmandPentax SMC DA 35mm f/2.8 Macro Limited with Magn. Ratio 1.0xClosest Focus 13.9cm..

13.9cm is 2 x 70mm. Distances are measured to the SENSOR in macro working..

Both have the same focal length corresponding ASP-C sensor equivalentThe same Diagonal Angle of View =44 deg..

Yes, but it is NOT a constant 44 degrees. The angle narrows at closer working distances. This happens with all lenses, but it happen most with macros because they focus closest..

So it is that 44 degrees is the angle of view at infinity, when the acceptance angle of any lens is at it's widest. However, when the macro lens is focused at 1:1 magnification, it's extension from the sensor will be increased from 1f to 2f, (from 35mm to 70mm) which will naturally halve it's angle of view to 22 degrees..

At the 2f setting of focus the subject will then be the same distance in front of the lens (another 2f) as the sensor is behind it....

....in effect it is 4f between sensor and subject, with the lens exactly in the middle... and all fixed focal length macro lenses are obliged to fulfil this condition when shooting at 1:1 ratio (same size as subject)..

In this case.......

35mm x 4 = 140 mm ......

(or call it 39.9mm as Pentax will have it, since there's only 1/10 of a millimetre's difference!!)).

But different Closest Focus. 12"/5.47 or appr. 2/1.

Sorry [??] I thought you said the macro was 1:1 and 39.9mm. Please clarify..

Now Questions.1. Can macro lens be focused at infinity (20 feet for example)?.

1) Yes. Macro lenses have focusing mechanisms that can pull focus very close, but they still focus at infinity in the normal way... a lot further than 20 feet, BTW!.

2. If they can, since both lens have the same angle of viewpresumably both should capture the same object equally..

2) Yes. At all points between infinity and the closest focusing distance of the non-macro (usually 10f or thereabouts) the images should be very similar indeed..

For instance, camera with non-macro 35mm lens produces test image onsensor from left to right edge,.

Errr... yes..... .

What happened if I swap for 35mm macro lens. Picture will be croppedaccordingly magnification ratio?.

No..

If the image is at the same distance, and in focus, then the image will be (very close to) same size as with the non-macro lens, as stated above..

It is only when you get CLOSER to the subject than the non-macro can focus, (as I said, usually about 10f) that the close focusing of the macro comes into effect, and provides the larger image on the sensor......

... just as things look bigger when you bring them closer to your eye. .

So magnification ratios are determined by distance, and then focusing sharply on that distance. You cannot select the magnification ratio independently of the focus, or the distance..

But what means angle of view in this case?So, where is my misunderstanding?.

I already explained, a reduction of viewing angle takes place as the lens is focused closer and therefore extended more out of the camera body..

You can demonstrate the effect for yourself by looking through a rectangular hole cut in a piece of paper. If you peep through the hole.... and then move it away from your eye by degrees.... the amount of the scene visible through the hole gets less and less with increasing distance between your eye and that hole......

....with the angle of view HALVING every time you double the distance..

Is this all clear now?Regards,Baz..

Comment #1

Thank you very match for detailed answers.I need fresh head and more time to fully understand what you saidbut for now my understanding is:.

Macro lens is some of the kind "zoom" lens where variable parameter is not a focal distance but magnification.Thanks again...

Comment #2

Macro lens is some of the kind "zoom" lens where variable parameteris not a focal distance but magnification.Thanks again..

Not quite. It is not a zoom lens as the focal length is fixed (in the examle you and Baz described). A macro lens allows you to magnify things more than a non-macro lens simply because it allows you to get much closer to the subject, and is optimised for close-up work. In *any* lens the closer you get to the subject, the bigger the image will be on the sensor: but you can get closer with a macro lens. That's all..

Best wishesMike..

Comment #3

AKozik wrote:.

Thank you very match for detailed answers.I need fresh head and more time to fully understand what you saidbut for now my understanding is:Macro lens is some of the kind "zoom" lens where variable parameteris not a focal distance but magnification..

Good gracious! NO!.

A macro lens is NOT a zoom lens. It is a perfectly normal fixed f-length lens..

It gets bigger pictures by going CLOSER to the subject. That's all!.

What's different about the macro type lens is that it has a special double-helix focusing mechanism that extends the lens enough to PERMIT getting sharp images when it is really close..

As I told you before, the magnification is NOT independant of the distance.... you have to go CLOSER to get the image BIGGER.... (meaning closer than ordinary lenses can go... closer than ordinary lenses can focus.).

I) I suggest you go to a camera shop and try a macro lens on your camera.... then the mode of operation should become obvious in seconds, I think..

Ii) I also suggest that 35mm is a little too short for convenient macro shooting.....

An f-length of around 80, or even 100mm approx will increase the stand-off distance from the subject at maximum magnification... which makes it less likely you will frighten way insect subjects, etc. It also easier for putting in suplementary lighting.Regards,Baz..

Comment #4

Thank you all.I got the answers on my questions although I am not fully agree with.

All statements. But you helped me resolve my doubts and I this is most important.Thanks again and I hope on your help in future...

Comment #5

AKozik wrote:.

Thank you all.I got the answers on my questions although I am not fully agree withall statements..

Well, if you tell us what it is you don't 'fully agree with'...we can sort it out... (errr.. maybe?).

But it also may be that you need to shoot with a proper macro lens yourself, before things become fully understood..

But you helped me resolve my doubts and I this ismost important.Thanks again and I hope on your help in future..

Okay. Bye bye. Good luck. Regards,Baz..

Comment #6

I am not a good "explaner" but you ask me..

My understanding of zoom lens is fact that optic system has variable focal distancethats means variabe angle of view.Another words it can change (read"magnify") size of object's image on sensor..

When I named Macro lens ""some of the KIND "zoom" lens"" I realy mean lens with variable magnification ratio, because it works pretty much like zoom, just Macro can get closer and probably used different mechanicks..

In opposite to macro lens my understanding of Prime lens that there is not Variation of Magnification at all. I have old Helios lens (if it could be named "Prime") And I belive there is very little changes of angle of view of that lens.Let me show you my calculation what can make you smile a lot.It is just for fan..

I took two Pentax lens because they both have 35mm but different close focus distance..

12"/5.47 or appr. ratio 2/1.

Sorry [??] I thought you said the macro was 1:1 and 39.9mm. Please clarify. (Your comment).

Lets discover out 35mm macro lens. At ifinity it has 44 deg on ASR-C sensor..

At Closest Focus Distance (when magnification ratio 1:1), optical lens center travel from 35mm to 139/2 mm what roughly equal 70mm (ASP-C eq.) and angle of view (AoV) will be 22deg (22.8 deg to be exact).

So, we have variation AoV from 44deg to 22deg. No matter how it works but if real or imaginary focus moves it looks like "ZOOM".Now according http://en.wikipedia.org/wiki/Angle_of_view22 deg corresponds to roughly 105-110 mm!!!!So, "in fact" Pentax SMC DA 35mm f/2.8 macro should be namedPentax SMC DA 35-105mm f/2.8 macro!How about that..

Did I make Math correct or I did mess?.

Please don't take my thought seriously but there is something what seems like "zoom".

And Yes. You absolutely right I have to try real Macro to get feeling how it works,But before bying I want to know more to make right purchase.Thats why I started this tread. Sorry for long post..

Thank you...

Comment #7

AKozik wrote:.

Snip..

Did I make Math correct or I did mess?.

Not exactly correct, but close enough..

Please don't take my thought seriously but there is something whatseems like "zoom".

It is as much *a zoom* as WALKING CLOSER to the subject is "a zoom". .

So, is walking closer "a zoom".. ???.

I don't think so. The mechanisms and operations are different. The effects are not the same, either..

And Yes. You absolutely right I have to try real Macro to get feelinghow it works..

It works the same as your Helios (no zoom) but focusing much closer. That's all..

But before buying I want to know more to make right purchase..

Buy a macro of about 80 or 100mm... easier to use than 35mm..

Thats why I started this tread. Sorry for long post.Thank you..

You're welcome.Regards,Baz..

Comment #8

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