Which is correct?

[espn]

I believe the maximum reproduction ratio rating for a lens is a function of the closest focusing distance as well as the focal length (if we can loosely apply the lens equation for thin lens here). In most cases for zoom lenses, the maximum reproduction ratio occurs at the closet focusing distance of the largest focal length. As such, you cannot just compare the reproduction ratio of the 2 lenses in this case, as we are talking about 70mm focal length, with is the minimum focal length on one lens and the maximum focal length of the other (the max reproduction ratio of 1:3.9 for the 70-300mm lens most likely applies to the 300mm focal length setting).

Furthermore, the subject distances are held constant (more or less) when comparing the 2 lenses, and the distance is not the closest focal distance for both lenses.

Therefore, I don't think the different max reproduction ratio is the explanation.

Roger, thanks for the effort in explaining.

 

[Ektar25]

The initial assumption that a lens marked 70mm does in fact mean an exact true focal length of 70mm is not justifiable unless the lens has been calibrated

In addition to what was earlier said about lenses changing focal lengths at different focusing distances, all manufacturers have engineering, manufacturing and marketing tolerances – IIRC they allow a variance of +/- 5%

Therefore your 18-70mm could actually be a 18.9-66.5 zoom (18+5% and 70-5%) – and why not the other way around (-5/+5) and make it 17.1-73.5? – because then the Marketing Department would want to call it a 17-75mm – so much easier to sell

So in reality you may be conducting a FOV comparison of 2 lenses with true focal lengths of 66.5mm and 73.5mm - naturally the pix don't match

Why don’t manufacturers state the true focal lengths? Well who wants to buy a 72.5-193.4mm f/2.92 lens instead of a 70-200/2.8? – and yes the fudge factor applies to apertures as well

You can refer to photodo and popphoto websites for published tests which have the true focal lengths

cheers

 

[Pablo]

May I just ask a question ?

If the setting of the lenses are according to the marker saying "x" mm. is it possible to not be exact.
Would not the slightest margin off the mark give quite a bit of difference at this distance ?

Hope you get my question.

Cheers

 

[natnivek]

Hmmm...

Dear experts, so far we have a lot of theories in here. But no conclusion. Do we just live with this or can some expert help me conclude?

Much appreciated...

 

[jnet6]

Hmmm...

Dear experts, so far we have a lot of theories in here. But no conclusion. Do we just live with this or can some expert help me conclude?

Much appreciated...

Try not to go too much details for the start.

understand the basic from wide to tele first/crop factor.

 

[roygoh]

Hmmm...

Dear experts, so far we have a lot of theories in here. But no conclusion. Do we just live with this or can some expert help me conclude?

Much appreciated...

Actually I think wanglk's explanation in post#2 is the most accurate one.

To prove that, you can do the following:

1. Take the 18-70mm lens, zoom to 70mm, and then manually turn the focusing from infinity to the minimum focusing distance (0.38m) while looking through the view finder. Does the FOV change while the focus setting is changed?

2. Conduct the same experiment again like you did in your first post, but this time take outdoor landscape shots with the lens focused to infinity.

- Roy

 

[reflecx]

Quite a few people (wanglk, mpenza) have already stated the correct answer, which is that internal focusing lenses have reduced focal lengths when focused at close distances. Focal length is measured when focused to infinity.

At min focusing distance (0.38m), the focal length of the 18-70mm zoomed all the way out is about 45mm.

 

[natnivek]

Hi all,

Thanks for all your effort to explain this. I think now i've got a much better idea about this as compared to 2 days ago.

Hope that this thread helped others who are wondering about this question too.

Cheers!

 

[litefoot]

Actually I think wanglk's explanation in post#2 is the most accurate one.

To prove that, you can do the following:

1. Take the 18-70mm lens, zoom to 70mm, and then manually turn the focusing from infinity to the minimum focusing distance (0.38m) while looking through the view finder. Does the FOV change while the focus setting is changed?

2. Conduct the same experiment again like you did in your first post, but this time take outdoor landscape shots with the lens focused to infinity.

- Roy

Technically speaking, the FOV should be the same even if its focused near distance or infinity assuming the lens are calibrated accurately.

FOV are determined by focal length and size of the focal plane. Therefore, if Kevin had set the focal length of 70mm to have the image shot on the same focal plane (ie D70) with a fixed distance (not varying the position of the focal plane). In an ideal situation the result should get identical FOV.

I had mentioned that the discrepency could be due to the focusing design both lenses and Ektar's had also provided another logical explanation on the rounding of the actual focal length.

 

[wanglk]

Technically speaking, the FOV should be the same even if its focused near distance or infinity assuming the lens are calibrated accurately.

er..sorry to say but this is NOT true:nono: . Technically speaking, for an IF (Internal Focus), the FOV does change when it focuses from near distance to infinity even though the lens was calibrated accurately. Remember, the focal length is measured when the lens focuses to infinity. This is nothing to do with calibration or manufacturing tolerance. This is a "feature" of IF lenses (or you want to call it a "bad point").

Why like that one? hehe... here really need some knowledge of geometric optics. I will stop at this point before go too technical. Interested? Go and get a B. Sc degree in Optics :bsmilie:

 

[litefoot]

Maybe I should use the word theoretically instead.

 

[spraylord]

this is a really funny thread!:bsmilie:
DX and non DX, that is the question.:bsmilie:

 

[espn]

this is a really funny thread!:bsmilie:
DX and non DX, that is the question.:bsmilie:

Get DX, but get all the expensive ones. :bsmilie:

 

[spraylord]

that was not what i meant espn! hehehe.. cant buy buy buy always right?:nono:

the only diff bet the 2 lens mentioned is the DX AND non DX.
maybe someone can look into it?:dunno:

 

[rexlim]

the only diff bet the 2 lens mentioned is the DX AND non DX.
maybe someone can look into it?:dunno:

I think this is so too. Because DX Nikkor design actually reduces the size of the image circle projected on the sensor(in this case, the smaller DX sensor, compared to a full frame 1). As for the non-DX 70-300 lens, image circle which is comparable larger, projected on the smaller DX sensor will threoratically produce a more 'zoom-in' image. (hyperFocal had mentioned earlier on post #39)
My tots...
Experts care to correct if I'm wrong...:embrass:

 

[spraylord]

ahhh.. finally someone sharing the same view!

 

[litefoot]

ahhh.. finally someone sharing the same view!

Ah! I got what you mean now. Silly me that I did not notice the kit lens is a DX. What you are trying to say is that the image formed by this DX lens could be the center of the "fisheye" which is undistorted or with little distortion.

I am barking up the wrong tree. Thanks for the joke.

 

[d7t3]

i agree with ektar25's point. i've read lens reviews where they said that "actual measured focal length was xx.xx mm". this is quite normal. 70mm is prob. +/- a couple of mm.

why not use both lenses at same camera position, but focus at infinity instead? that would take out the focusing distance factor.