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Old Mar 2, 2008, 5:07 PM   #1
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I was looking over my Nikkor Lenses brochure. 1998-2006 edition. In the DX pages they spent a page with DX lens FAQ. Great idea for novices like myself.

One of the questions was "Does the focal length of a lens increase when the lens is attached to a DX format digital camera?". The answer was "No". So that sounds like they are saying that if you put a DX 12-24 on a DX body you're going to get the full 12-24 focal range. Yes?

I eman we know that if you put a FX lens on a FX body the full focal range is used.
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Old Mar 2, 2008, 5:18 PM   #2
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That is correct. The focal length doesn't change, and in fact cannot change...it is what it is. What does change is the field of view when mounted on a aps sensor sized DSLR. Because of the smaller sensor, you have a field of view that is narrower. So a 12-24 has the field of view of a lens that is 18-36. Of course this relationship is in comparison to and references focal length of a 35 mm SLR.
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Old Mar 2, 2008, 5:28 PM   #3
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DarkDTSHD wrote:
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One of the questions was "Does the focal length of a lens increase when the lens is attached to a DX format digital camera?". The answer was "No".
Lenses are marked by their actual focal lengths. It doesn't change when you mount the lens on a camera with a different sensor size.

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So that sounds like they are saying that if you put a DX 12-24 on a DX body you're going to get the full 12-24 focal range. Yes?
Sure, you're getting a 12-24mm lens. But, you'd have the same angle of view you'd get with an 18-36mm lens on a 35mm camera. ;-) IOW, the so called "crop factor" still applies.

If you use a smaller sensor or film size, the angle of view will be narrower (more apparent magnification) for any given focal length.

If you use a larger sensor or film size, the angle of view will be wider (less apparent magnification) for any given focal length.

The only reason to even have a so called crop factor or focal length multiplier is so that users familiar with using lenses on 35mm cameras have a better understanding of how angle of view compares using a DSLR with a sensor smaller than 35mm film.

If 35mm cameras were not so popular, there would be no need to use them at all.

A common misconception is that "made for digital" lenses (for example, Nikon's DX lenses) give a different angle of view compared to lenses made for a 35mm camera, when they're both used on a camera with an APS-C sensor. That's not the case. You'll have the same angle of view for a given focal length with either lens type on your D300.

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Old Mar 2, 2008, 5:45 PM   #4
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Thanks guys.

1 for "angle of view changes". 1 not.

And, if you put a FX lens on either a FX body or a 35mm SLR, neither the focal range or angle of view changes. Correct?

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Old Mar 2, 2008, 5:55 PM   #5
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DarkDTSHD wrote:
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Thanks guys.

But JimC. Can you clarify your position? First you say the field of view does change.
With a larger or smaller sensor or film size.

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Then in your last paragraph on misconception you end it by saying the angle of view does not change irrespective of the lens type used on my D300."
This is what I said:

"You'll have the same angle of view for a given focal length with either lens type on your D300."

I'm not sure where you're misunderstanding me. The angle of view for a given focal length lens will be the same on a camera with an APS-C size sensor, regardless if the lens is "made for digital" (i.e., a Nikkor DX lens), or not (for example, a lens designed for a 35mm camera). If the focal length is the same, the angle of view will be the same on your D300.




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Old Mar 2, 2008, 6:03 PM   #6
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JimC,

I misread. Thousand pardons!!
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Old Mar 2, 2008, 6:07 PM   #7
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DarkDTSHD wrote:
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And, if you put a FX lens on either a FX body or a 35mm SLR, neither the focal range or angle of view changes. Correct?
Changes from what?

If you really want to know the formula to compute angle of view, here it is:
  • Angle of View = 2 * ArcTan(Film Dimension / (2 * Focal Length * (1 + Magnification)))[/*]
If you want some simple formulas for how the larger format film compares to 35mm for a given focal length lens, here some are (you'll have a wider angle of view for a given focal length lens on a camera with film or sensor sizes larger than 35mm film).

645 focal length x 0.62 = equivalent 35mm focal length
6x6 focal length x 0.55 = equivalent 35mm focal length
6x7 focal length x 0.48 = equivalent 35mm focal length

IOW, if larger formats were more popular, we may be using "crop factors" and "focal length" multipliers to try and figure out how 35mm cameras compare for angle of view with a given focal length lens. ;-)

Here is a handy angle of view calculator. Just enter a focal length and the sensor or film dimensions to get angle of view:

http://www.mat.uc.pt/~rps/photos/angles.html

Note that you'll need the exact sensor dimensions in mm for weight and height.

If you look at lens specifications, you'll usually see angle of view listed for the camera type the lens was originally designed for. But, it will not be applicable if you use a lens on a body with a different film or sensor size.

Nikon started giving Angle of View for DX lenses in the specs assuming an APS-C size sensor would be used (since their DX series are similar to Canon's EF-S series lenses, or KM/Sony's DT lenses, or Tamron's Di II lenses, or Sigma's DT lenses) and they will only work on a camera with a sensor smaller than 35mm film without vignetting).

For example, the Nikon specs for the widest angle of view for the Nikkor 18-70mm f/3.5-4.5G ID-IF AF-S DX lens is shown as 76 degrees at a focal length of 18mm (as you would expect when a lens with a focal length of 18mm is used on a DSLR model with a smaller sensor).

But, if you look at the specs for a non-DX lens, the angle of view shown for given focal length assumes it will be used on a 35mm model.For example, the Nikkor 18-35mm f/3.5-4.5DED-IF AF lens (non DX lens) shows an angle of view in it's specifications of 100 degrees at a focal lenth of 18mm (as you would expect when an 18mm lens is used on a 35mm camera, since you'll have a wider angle of view compared to a model with a smaller sensor using the same focal length lens).

The focal length at the widest setting for both lenses is identical. What changes is the angle of view, depending on the size of the sensor/film the lens is being used with.

If you used the non-DX lens (designed for 35mm cameras) on a DSLR with an APS-C size sensor, the angle of view would be identical to the DX (digital only lens design) for the same focal length setting on that camera.

Since we have lenses that can be used on cameras with more than one sensor or film size, it's tougher to give angle of view for these (although they could give multiple angle of views in the specs, showing it for multiple sensor/film sizes).


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Old Mar 2, 2008, 6:35 PM   #8
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IOW, the primary purpose of the so called "crop factors" and "focal length multipliers" is to give buyers a better idea of how angle of view compares when the sensor size is different (as you have between models using 35mm size sensors, and APS-C size sensors).

That way, someone accustomed to using a 35mm film camera will have an easy way to determine what focal length lens they need to get comparable coverage, compared to lenses on 35mm models.

On an Olympus DSLR, you have to multiply the actual focal length of the lens by 2x to see how the angle of view compares to a lens used on a 35mm camera (because it's sensor is even smaller than the APS-C size sensors used by Nikon and Canon).

Yet, there's no "crop" involved (as the lens was designed with a smaller image circle for this camera type). But, you still have a narrower angle of view for a given focal length lens, compared to the same focal length lens on a 35mm camera.

The opposite is true for formats larger than 35mm film (you'll have a wider angle of view for any given focal lengh, compared to the same focal length on a 35mm camera).

Again, if 645 format was more popular than 35mm, we may have be seeing "focal length multipliers" to help medium format users make the transition, so that users could understand that a lens will have more apparent magnification (narrower angle of view) for any given focal length when used on a 35mm camera versus a medium format model. ;-)

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Old Mar 2, 2008, 7:27 PM   #9
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When I said "And, if you put a FX lens on either a FX body or a 35mm SLR, neither the focal range or angle of view changes. Correct?" I just meant that the focal length wouldn't increase as it would if put on a DX body...nor would the angle of view (FX lens on FF DSLR or SLR).

Thanks for the information!
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Old Mar 2, 2008, 8:02 PM   #10
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DarkDTSHD wrote:
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When I said "And, if you put a FX lens on either a FX body or a 35mm SLR, neither the focal range or angle of view changes. Correct?
The angle of view is dependent on the sensor or film size for a given focal length lens. It's not going to change if the sensor or film size is the same between the camera bodies you're using it on.

Quote:
I just meant that the focal length wouldn't increase as it would if put on a DX body...
The focal length does not change if you put it on a DX body. The focal length is still the same. ;-)

When people refer to multiplying the focal length of a lens by 1.5x, that's so you can tell what focal length lens you'd use on a 35mm camera to get the same angle of view. The focal length of the lens does not change with film or sensor size. A lens with a 50mm focal length still has a 50mm focal length, regardless if you're using it on a body with an 35mm size sensor, or a body with an APS-C size sensor.

That's just used so you can tell how lenses compare when sensor or film sizes change, and if 35mm film wasn't so popular, there wouldn't be a need for these types of multipliers.

Now, what you typically see in a non-DSLR models specifications, is a 35mm equivalent focal range to help users understand how cameras compare.

These models typically have very tiny sensors, so you'll have a much narrower angle of view for a given focal length lens. Take the Canon Powershot A470 just reviewed here as an example. It's got a "35mm equivalent" focal range of 38-132mm. But, that's just so users know how it compares. If you look at the front of the lens, you'll see where Canon has the actual focal range printed on it.

The actual focal range of the lens on this new Canon model is only 6.3-21.6mm. But, because of the small sensor, it gives you the same angle of view that a 38-132mm lens would on a 35mm camera. It's focal length is not changing. It's really only 6.3mm on it's wide end, and 21.6mm when zoomed into it's long end. The range given is just to help users understand what lens range on a 35mm camera it compares to (because 35mm cameras are very popular).

With an interchangeable lens camera, if you want to see what focal length lens is needed to give you the same angle of view on a camera with a different film or sensor size, you'll need to do the math yourself. They make it easier for point and shoot models by including this kind of comparative information in the specs and features.


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