Which is correct?

[litefoot]

You've only emphasized on point that you believe is a focal length issue, but care to elaborate what do you mean by that?

I believe the question is on the differences on the focal length althought both lenses are set at 70mm.

Assuming that both pix taken from the same distant and the same 70mm setting, they should show the same FOV. I am just surprised that he could be misled to conduct test based on the distance from the first element of the lens.

Now what other tests would you suggest to run on magnification ratio?

 

[litefoot]

could you elaborate on this point? how did the different FOV result in a bigger pringles can?

Simply narrower FOV.

 

[natnivek]

Hi guys... So sorry that my test is such a unprofessional one. :embrass:

yanyewkay, due to this being an unprofessional test, I didn't measure any distances while taking the pictures. What I took note was that both lenses was at focal length 70mm and both had no problem focussing. With that done, I guess the possibility of the minimum focus distance problem should be excluded.

litefoot, are you trying to say that this is working something like an extension tube? Where there is no magnification per se but just because the lens is further away from the CCD, the image is effectively enlarged?

Please correct me if my understanding of this matter is erred... Thanks!

 

[mpenza]

Just like micro glasses, the replicate the subject at 1:1.

For the 18-70 - ratio is 1:6.2

For 70-300 - 1:3.9

Therefore the image on the 70-300 is maginifed near 1.5x bigger.

1/6.2 = 0.16 maginfication
1/3.9 = 0.25 magnification

Thus at the same focal length, due to the reproduction ratio differences, your images size also looks different. Hope I explained it correctly

erm... it doesn't work this way lah!!! The ratio is the maximum magnification ratio at the closest focussing distance. when focussing distance change, the magnification changes.

if the focussing distance is the same at the same focal length (for non-macro lenses at least), the subject should by right be the same in the picture.

That said, wanglk's explanation is correct. Focal length is usually measured at infinity focus.

 

[natnivek]

erm... it doesn't work this way lah!!! The ratio is the maximum magnification ratio at the closest focussing distance. when focussing distance change, the magnification changes.

if the focussing distance is the same at the same focal length (for non-macro lenses at least), the subject should by right be the same in the picture.

That said, wanglk's explanation is correct. Focal length is usually measured at infinity focus.

Hi mpenza, so espn's explanation is not correct. So what is the reason why two images taken at the same focal length is different?

Am I missing something here? :dunno:

 

[espn]

I believe the question is on the differences on the focal length althought both lenses are set at 70mm.

Assuming that both pix taken from the same distant and the same 70mm setting, they should show the same FOV. I am just surprised that he could be misled to conduct test based on the distance from the first element of the lens.

Now what other tests would you suggest to run on magnification ratio?

Ok, so coming back to the question, what's the answer to the threadstarter's question? That both at 70mm and yet produces difference in image sizes? :dunno:

erm... it doesn't work this way lah!!! The ratio is the maximum magnification ratio at the closest focussing distance. when focussing distance change, the magnification changes.

if the focussing distance is the same at the same focal length (for non-macro lenses at least), the subject should by right be the same in the picture.

That said, wanglk's explanation is correct. Focal length is usually measured at infinity focus.

:bsmilie: I thought so, was just luan-pom one. I didn't think it would be the answer too :bsmilie:

So since litefoot has an answer, I'm still waiting for him to elaborate and explain.

 

[yanyewkay]

erm... it doesn't work this way lah!!! The ratio is the maximum magnification ratio at the closest focussing distance. when focussing distance change, the magnification changes.

if the focussing distance is the same at the same focal length (for non-macro lenses at least), the subject should by right be the same in the picture.

That said, wanglk's explanation is correct. Focal length is usually measured at infinity focus.

yups.. stated repro ratio on the lens is at minimum focus distance.. that is why I wanted to know the distance of subjects to camera in my previous post.

usually zoom lens have their repro ratio in a range. like 1:4-6.3 (labelled on the 70-300G) IIRC ( i sold my 70-300G quite some time ago).

 

[litefoot]

Ok, so coming back to the question, what's the answer to the threadstarter's question? That both at 70mm and yet produces difference in image sizes? :dunno:



:bsmilie: I thought so, was just luan-pom one. I didn't think it would be the answer too :bsmilie:

So since litefoot has an answer, I'm still waiting for him to elaborate and explain.

To simply put. The 70mm at 28-70, 70-200, 35-70, 18-70 or 70 prime (if there is one) should technically produce to same FOV.

The magnification rating on the zoom lens is for comparing the images between the wide and tele end (eg 35-70 is a 2x zoom). In Micro/macro 1:1 will mean exact image size on the film plane at the tele end (if its zoom) with the closest focused distance.

I think I mentioned about the reason for the slight difference in FOV. Assuming same focused distance. Now it seems to me that he shot the pix as long as they are in focused at 70mm for both lens disregarding the distant which of course will also affect the FOV.

 

[yanyewkay]

To simply put. The 70mm at 28-70, 70-200, 35-70, 18-70 or 70 prime (if there is one) should technically produce to same FOV.

The magnification rating on the zoom lens is for comparing the images between the wide and tele end (eg 35-70 is a 2x zoom). In Micro/macro 1:1 will mean exact image size on the film plane at the tele end (if its zoom) with the closest focused distance.

I think I mentioned about the reason for the slight difference in FOV. Assuming same focused distance. Now it seems to me that he shot the pix as long as they are in focused at 70mm for both lens disregarding the distant which of course will also affect the FOV.

i will do some test with my rig tonight and post my findings. Hopefully it'll support my claims of repro ratio.

if the subjects are sufficiently far from the lens (~3m) at 70mm should produce the same pic.

This is an interesting debate. It's been long since I've seen such activities in Nikon forum. :thumbsup: I think it doesn't apply to Nikon systems but across all systems

 

[litefoot]

Hi guys... So sorry that my test is such a unprofessional one. :embrass:

yanyewkay, due to this being an unprofessional test, I didn't measure any distances while taking the pictures. What I took note was that both lenses was at focal length 70mm and both had no problem focussing. With that done, I guess the possibility of the minimum focus distance problem should be excluded.

litefoot, are you trying to say that this is working something like an extension tube? Where there is no magnification per se but just because the lens is further away from the CCD, the image is effectively enlarged?

Please correct me if my understanding of this matter is erred... Thanks!

No, nothing to do with extension tube. The macro magnification ratio rated on the lens is achieved with the shortest focusing distance of the lens. Using extension tube will allow you to focus even closer beyond the lens specification.

You asked: "Does this mean that different lenses have different "standards" of focal lengths? (at 70mm)". So I am assuming that you are comparing the FOV of both lenses at 70mm.

In fact I am assuming that the 18-70 has a closer shortest focusing distance (typical with wider lens as compared to tele) therefore you are able to achieve a higher magnification than the 70-300 at 70mm.

Confused?

 

[litefoot]

Hi guys... So sorry that my test is such a unprofessional one. :embrass:

Kevin, do this.

Mount the the cam on the tripod, if you dun have a tripod try sitting on a chair with a back rest. Fixed the distance between the cam and the subject. Shoot the pix with both lenses set at 70mm focused and lets compare the results.

 

[natnivek]

Now it seems to me that he shot the pix as long as they are in focused at 70mm for both lens disregarding the distant which of course will also affect the FOV.

Nope... Both were at the same distance from the subject. If it wasn't, this whole experiment would be a joke.

The only time the distance was changed was when it was suggested that the distance should be measured from the front element. That's when the camera moved about 3 inches back when using the 70-300. But the result was still the same.

 

[roygoh]

Just like micro glasses, the replicate the subject at 1:1.

For the 18-70 - ratio is 1:6.2

For 70-300 - 1:3.9

Therefore the image on the 70-300 is maginifed near 1.5x bigger.

1/6.2 = 0.16 maginfication
1/3.9 = 0.25 magnification

Thus at the same focal length, due to the reproduction ratio differences, your images size also looks different. Hope I explained it correctly

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.

 

[natnivek]

Kevin, do this.

Mount the the cam on the tripod, if you dun have a tripod try sitting on a chair with a back rest. Fixed the distance between the cam and the subject. Shoot the pix with both lenses set at 70mm focused and lets compare the results.

Opps... Just read your reply.

My first result was based on the setup you mentioned.

 

[natnivek]

No, nothing to do with extension tube. The macro magnification ratio rated on the lens is achieved with the shortest focusing distance of the lens. Using extension tube will allow you to focus even closer beyond the lens specification.

You asked: "Does this mean that different lenses have different "standards" of focal lengths? (at 70mm)". So I am assuming that you are comparing the FOV of both lenses at 70mm.

In fact I am assuming that the 18-70 has a closer shortest focusing distance (typical with wider lens as compared to tele) therefore you are able to achieve a higher magnification than the 70-300 at 70mm.

Confused?

Haha... I reading all you guys posts over & over again.

First part about the extension I understand.

2nd part leh... Wouldn't it contridict your previous statement?

To simply put. The 70mm at 28-70, 70-200, 35-70, 18-70 or 70 prime (if there is one) should technically produce to same FOV.

FOV should only change when there is a change in focal length right?

Oh yah, thanks for not trying to cheem me out. I still learning...

 

[litefoot]

Haha... I reading all you guys posts over & over again.

First part about the extension I understand.

2nd part leh... Wouldn't it contridict your previous statement?


FOV should only change when there is a change in focal length right?

Oh yah, thanks for not trying to cheem me out. I still learning...

Nope, in the second situation, you are varying the focusing distance, the 18-70 zoom allowing you to get closer at 70mm as compared to 70-300. You get closer, the subject gets bigger on your pix.

FOV will change according to size of focal/film plane and focal length.

If you want to dwell deeper.

http://www.photo.net/learn/fov/

 

[Watcher]

The 18-70 lens is an IF (Internal Focusing) lens. That means, the length of the lens wont change during focusing. The good thing is fast focusing, but on the other hand, the focal length will change. This 70mm focal length can only be achieved when it focuses at infinity. When you focus an object nearby, the focal length will reduce. This is true to all the IF zoom.

You are right (just to emphasize the correct info, but nobody bothers to understand).

FL is measured when focus is at infinity.

 

[natnivek]

You are right (just to emphasize the correct info, but nobody bothers to understand).

FL is measured when focus is at infinity.

Hi there... Most of us trying to understand also lah...

Based on this, then my question would be since I was focussing on a nearby object and the focal length reduces, why does the lens still say that the focal length is 70mm? Even the EXIFs would show 70mm.

 

[hyperFocal]

Actually, I thought it has to do with the image circles produced by the lens and the 1.5x crop factor DX sensor. The 18-70mm kit lens is DX designated, thus producing an image circle that fits the DX sensor; while the 70-200mm is usable on a 135 format, thus a larger image circle. Consequently, the image recorded on the DX sensor with the 70-200mm is just the center portion.

So, my guess is that if you shoot the 70-200mm on a 135mm with the same setup, you should get exactly the same image as the 18-70mm on DX.

Just my thoughts.

PS : Btw, Nice Axe!

 

[natnivek]

Nope, in the second situation, you are varying the focusing distance, the 18-70 zoom allowing you to get closer at 70mm as compared to 70-300. You get closer, the subject gets bigger on your pix.

FOV will change according to size of focal/film plane and focal length.

If you want to dwell deeper.

http://www.photo.net/learn/fov/

Thanks for the link. Will need more time to absorb everything in there.

I understand if you get closer, the subject will be bigger. That's easy to understand.

But what I didn't understand was why the subject became bigger when I did not get closer.

Thank you...