[debmalya]

This is a very interesting observation. I am completely new to photography. I have a Pentax DA 18-135WR on a K5 and just bought a Nikon AF-S 18-200VR on a D7000. Comparing the two I am observing, the Nikon has way more magnification at the same focal length than the Pentax - meaning, the Nikon produces a more close up image at 70mm than the Pentax does at 135mm. I was so surprised, I did a google search on it and landed this thread. Can anyone knowledgeable please care to explain this to me?

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Last edited by debmalya; 07-29-2011 at 12:47 PM. Reason: omitted little detail

[RioRico]

This is a very interesting observation. I am completely new to photography. I have a Pentax DA 18-135WR on a K5 and just bought a Nikon AF-S 18-200VR on a D7000. Comparing the two I am observing, the Nikon has way more magnification at the same focal length than the Pentax - meaning, the Nikon produces a more close up image at 70mm than the Pentax does at 135mm. I was so surprised, I did a google search on it and landed this thread. Can anyone knowledgeable please care to explain this to me?

At what focus distance? Different lenses, primes as well as zooms, may have different close-focus distances (CFDs). And with zooms, the CFD may change with changing the focal length. Being able to focus closer means the captured image will be of greater magnification. And lenses with IF (internal-focus -- only an internal element changes position) vs extension-focus (the front element changes position) means the CFD's will be different -- and at CFD, the real FL (focal length) of a lens may be different than its nominal FL, which is measured at infinity focus. So there are many factors involved there.

Bottom line: Lens focal lengths are measured at infinity focus. How they behave elsewhere varies with lens design.

[Lowell Goudge]

Bottom line: Lens focal lengths are measured at infinity focus. How they behave elsewhere varies with lens design.

It's just too bad that the focal length vs focus distance is neither documented by the manufacturer or in independent reviews

[RonHendriks1966]

Anyone compared it to DA*50-135mm? Just sold mine so can't look. I sometimes use my 60-250 for close-up:

[debmalya]

My (adhoc) comparison is with subject at approximately 10 feet distance or so, with both Pentax and Nikon lenses. But at that distance, should the CFD changes be significant? Also, is my DA 18-135 an IF lens? And what about my AF-S NIKKOR 18-200? Is it IF or not? And most important from a usage standpoint, I feel the Nikon is a more practically usable lens - but it's my personal uneducated opinion.

[dadipentak]

Yeah, it was an interesting thread--until Stone G had to go ruin it with mathematics.

[Laurentiu Cristofor]

I believe Nikon's $2400 70-200 VRII is only 135mm at MFD at 200mm.

I remember there were lots of Nikon users that were unpleasantly surprised to hear what IF means to focal length at close focusing distances. They felt cheated.

[debmalya]

Ok, so I figured out, my Nikon AF-S 18-200G VRII ED is not IF. My DA 18-135WR is an IF lens. I also read a little bit about IF lenses - and did not find any practical advantage of it. From what I understand, the IF lenses actually vary the focal length in the process of focusing. Thus a 70mm IF lens may not be a 70mm lens at closer than infinity focus. Now it seems clear why the focal length is specified at infinity focus. Anyways, I would rather prefer a conventional lens design over an IF design for most applications, exception may be if I need to get very close to the subject where an extension of the lens's length may interfere with the process - something very unlikely to happen in my case.

[Laurentiu Cristofor]

The only things that matter for close up photography are minimum focusing distance (working distance) and magnification factor - people should just look at these instead of wondering if the focal length changed or not when close focusing. Nobody should care what focal length their lens had if they got the magnification they wanted. Both the MFD and the magnification are provided by good sellers (adorama, B&H include this information for each lens they sell).

[newarts]

Yeah, it was an interesting thread--until Stone G had to go ruin it with mathematics.

Ok - no math.. .

There are two ways to get an image in focus

- use a particular focal length lens and vary the lens' distance from the sensor (old way)
- use a particular distance from the sensor and vary the lens' focal length (new IF way)

There is no guaranteed advantage to either of which I'm aware. Except maybe long IF lenses can be smaller and lighter than nonIF lenses.

Say we have two 100mm lenses: one purely IF and the other nonIF; when they are both focused at infinity the focal length of both is 100mm. When focused closer than infinity, the nonIF lens is moved away from sensor to bring the subject in focus, while the IF lens' internal elements are re-adjusted to a smaller focal length to bring the subject into focus.

I believe that most IF lenses actually change both the focal length and distance from the sensor simultaneously. A Zoom lens is a type of IF lens in that the focal length and distance to the sensor are changed simultaneously.

There is no magic going on; when two lenses are stacked a new, compound lens is formed. The new lens has a focal length that depends on the original focal lengths and on the distance between them. So if you move one lens with respect to the other the focal length changes.

There is no image or magnification created by an IF lens that can't be created by an equivalent nonIF lens although for the IF lens you won't usually know the actual focal length for a subject not at infinity (you could maybe calculate it if you want.) See my next post regarding a difference in magnification/perspective)

[newarts]

There is no image or magnification created by an IF lens that can't be created by an equivalent nonIF lens although for the IF lens you won't usually know the actual focal length for a subject not at infinity (you could maybe calculate it if you want.)

But there is a subtle difference between IF and nonIF lenses.

The images formed are identical at infinity focus only.

That's because when focused at the same subject distance the focal lengths are different; since the focal lengths are different the magnification (what you see in the viewfinder) is different - smaller for the IF lens because it has a smaller focal length.)

Therefore IF lenses and nonIF lenses cannot create identical images with the same perspective. To create identical images with IF and nonIF lenses one must shoot at the same subject distance (perspective the same) then crop and enlarge the IF image to make magnification the same.

OP noticed that IF lens magnification at a constant distance was smaller than with a nonIF lens....that's because the IF's focal length decreases as subject distance decreases. It usually makes no difference - just move closer.

==========(math alert) DO NOT READ FURTHER (math alert)===================

Lenses with focal lengths f' and f" positioned a distance d apart create a compound lens with focal length f:

f = f' f"/(f '+ f" -d )

As you change the distance d the focal length f changes.

There was a class of lenses preceding modern zoom and IF lenses called "Varifocal" lenses in which the user could independently change the distance from the sensor and the lens' focal length.

Sorry about the math - I couldn't control myself

[Laurentiu Cristofor]

That's because when focused at the same subject distance the focal lengths are different; since the focal lengths are different the magnification (what you see in the viewfinder) is different - smaller for the IF lens because it has a smaller focal length.)

There is probably little practical impact from this.

I was also looking at the longer macro lenses - the Sigma 150 and the Tamron 180 - they are both IF lenses, which means that at 1:1 magnification they would have somewhat shorter focal lengths. But whatever focal lengths they have at maximum magnification, they both provide a longer working distance than the Sigma 105, which is non IF:

Sigma 105: 31.24cm
Sigma 150: 38.10cm
Tamron 180: 47cm

Just thought this was interesting.

[debmalya]

Ok, as far as my observation goes, during focusing the length of a non IF zoom lens changes very slightly as compared to the length change when zooming in or out. But, my non IF Nikon 18-200 lens produces a much more zoomed in image at 70mm position than my Pentax 18-135 does at 135mm position. It is not slight difference in magnification, it is significant difference in magnification. So, it does not seem to be due to IF or non IF factor - it is got be something else. based on all the explanations above I refuse to believe that this is "ok" because the Pentax lens is IF. I am now more inclined towards the OP's "rip off" remark.

[Lowell Goudge]

It is not a rip off. It is a technical trade off. Nothing more nothing less. You want fast focusing which requires the reduced mass of an IF lens. You want WR lenses that require the reduced seals of IF lenses and you don't want the overall dimensions of a WR lens to change and pump air in and out of your body

If you don't want any of these things fine then you can live without IF.

And note the change in focal length when focusing is not new, it was first reported on the K28/2.0 which has the fixed rear element , an essential forerunner to IF. That was in the 1970's.

As for the definition of focal length, the definition at infinity has been the only definition of focal length since the origins of optical design 500 years ago

As I said above the only shame is that no one tests to characterize the phenomena or specify this aspect of performance,, not the lens reviewers and not the manufacturers

[Laurentiu Cristofor]

But, my non IF Nikon 18-200 lens produces a much more zoomed in image at 70mm position than my Pentax 18-135 does at 135mm position.

Is this at the same distance from the subject and what is that distance?