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08-04-2011, 08:55 PM   #31
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QuoteOriginally posted by Laurentiu Cristofor Quote
Is this at the same distance from the subject and what is that distance?
Yes, this comparison is at the same distance from the subject with both lenses, which I would estimate to be approximately 10' or so.

08-04-2011, 09:13 PM   #32
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QuoteOriginally posted by Lowell Goudge Quote
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
Your arguments in favor of the IF lens design is valid. The reduced magnification, which I believe is what made the OP term this as a "rip off" in casual terms does not seem to be related to IF or non-IF.
I don't see any reason to dispute the benefits of an IF design, but the observed phenomena of reduced magnification of this lens does not seem to be IF related.
I understand that focal length is measured from the center of the lens to the point where the light rays converge behind the lens caused by parallel incident rays; because rays coming from infinity are considered parallel.

By the way, in case you may not have known, the Pentax DA 18-135WR lens thats in discussion here, even though an IF lens, it does change in terms of length wise dimensions being a a zoom lens. I am not sure if it also pumps air in and out of the camera body when it is zoomed out and in respectively; my hunch is: it does!
08-04-2011, 09:28 PM   #33
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FWIW, there was severe gnashing-of-teeth, pulling-of-hair and gouging-of-eyes in the Nikon forums last year when folks unfamiliar with this phenomenon discovered that their new $2300 70-200 2.8 VR II was really only a 135mm lens at the long end for closer-in stuff.

In other words, it's nothing really new, it is what it is, and you're not alone.
08-05-2011, 03:52 AM   #34
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QuoteQuote:
I understand that focal length is measured from the center of the lens to the point where the light rays converge behind the lens caused by parallel incident rays; because rays coming from infinity are considered parallel.

By the way, in case you may not have known, the Pentax DA 18-135WR lens thats in discussion here, even though an IF lens, it does change in terms of length wise dimensions being a a zoom lens. I am not sure if it also pumps air in and out of the camera body when it is zoomed out and in respectively; my hunch is: it does!

All normal lenses obey the thin lens formula; that means focal length, distance, and magnification are related as:

Distance = FocalLength(1+ 1/magnification)

So at a fixed distance magnification a change in focal length gives a change in magnification.

The 18-55wr's length changes in a complex way as it is zoomed; a few rearrangements are taking place inside - not only is the focal length being changed, the distance from the lens to the sensor is also being changed.

IF lenses may change their overall length as the distance between optical elements is adjusted.

If the overall length of a lens changes, air is pumped in or out.

08-05-2011, 04:15 AM   #35
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QuoteOriginally posted by debmalya Quote
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.
In a sense all zoom lenses are IF because the relative positions of the elements are changed when zoomed.

When focal length is changed the distance of the lens from the sensor must also be changed. An IF lens has a mix of lens motions, some changing focal length & some changing distance to the sensor. The details of these motions will differ with particular lenses.

These distance changes are sufficient to explain your observations. It is not exactly a "rip-off"; it simply is what it is. The bottom line is that for an IF lens the correlation between focal length, distance to the subject, and magnification is more complex than in the non-IF case.

In all cases it is possible to take a photo with any magnification from zero to the specified maximum - in that sense there is nothing to complain about.
08-05-2011, 05:10 AM   #36
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So it sounds like lenses should advertise their "Close Focus Yield" in addition to their min/max focal lengths at infinity. Just like banks advertise APY in addition to APR. That way those that haven't read this thread won't be caught unaware.

As a newbie, I'm really glad I ran across this. I'm not lucky enough to afford the lenses being discussed (yet) but, thanks to this forum, I now am aware of and have a decent understanding of a difference I was previously unaware of.

Since it has been noted above that there are other threads on the topic, it may actually be worth a sticky in the beginner's corner.
08-05-2011, 05:41 AM   #37
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QuoteOriginally posted by debmalya Quote
Your arguments in favor of the IF lens design is valid. The reduced magnification, which I believe is what made the OP term this as a "rip off" in casual terms does not seem to be related to IF or non-IF.
I have to disagree with this.

the zoom and focusing of the lens involves movement and changes in the relationship of all the internal elements relitive to each other.

this is the cause of the problem, whether it is a 100% IF issue or a combination of IF and zoom lens design I can't answer but it is definitely a function of both

If the zoom function changed magnification only and the focus moved the entire group only, then the discussion would not be taking place, because focus would not alter the zoom ratio.

BUT.....

although I have not gone through the designs in detail, I can gaurantee that to hold a fixed focal length, while focusing from infinity to minimum distance, with a zoom would result in a much larger and more complicated lens.

as I said all along, this is a design trade off,, not a rip off. It is just too bad that it is not specified more clearly, at least in the detail of the specifications for the lens
08-05-2011, 05:46 AM   #38
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QuoteOriginally posted by HockeyDad Quote
So it sounds like lenses should advertise their "Close Focus Yield" in addition to their min/max focal lengths at infinity. Just like banks advertise APY in addition to APR.
except that no one "advertises" weaknesses

we could, for example insist that they add a second set of focal lengths to the lens.

let me see now, we have a 18-135 F4-5.6, so lets make this a 18-135 (16-70) F4-5.6 so that the focal length at minimum distance is also noted?

now, in extending this further, what does the SR think about all this. If you set the lens to 135mm, and focus at min distance, does the SR still think it is 135mm

08-05-2011, 06:01 AM - 1 Like   #39
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QuoteOriginally posted by Lowell Goudge Quote
let me see now, we have a 18-135 F4-5.6, so lets make this a 18-135 (16-70) F4-5.6 so that the focal length at minimum distance is also noted?

now, in extending this further, what does the SR think about all this. If you set the lens to 135mm, and focus at min distance, does the SR still think it is 135mm
Well of course, we simply need more advanced lens chips that report the lens' *actual* focal length and aperture at any given focus distance -- yeah, they need to report the focus distance too. And since f-stop is the ratio of iris diameter to focal length, the aperture changes with focus also, and should be reported. More chips! Better chips! Tell the SR'bot the truth!

Easy way to avoid such complexities: Stick to prime lenses. Shoot in M(anual) mode with Green button. Eschew automation (and obfuscation). K.I.S.S.
08-05-2011, 06:20 AM   #40
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QuoteOriginally posted by RioRico Quote
Well of course, we simply need more advanced lens chips that report the lens' *actual* focal length and aperture at any given focus distance -- yeah, they need to report the focus distance too. And since f-stop is the ratio of iris diameter to focal length, the aperture changes with focus also, and should be reported. More chips! Better chips! Tell the SR'bot the truth!
I am not sure the aperture is changing. I suspect that part of the whole design has the diaphram placed in a spot where it does not get impacted with the focal length.
QuoteQuote:

Easy way to avoid such complexities: Stick to prime lenses. Shoot in M(anual) mode with Green button. Eschew automation (and obfuscation). K.I.S.S.
sticking to primes does not soplve the issue, if they are internally focused.
08-05-2011, 07:01 AM - 1 Like   #41
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QuoteOriginally posted by Lowell Goudge Quote
sticking to primes does not soplve the issue, if they are internally focused.
Then there is no cure. Of course, there is no real issue, except that users don't understand the lens design trade-offs. No issue, a victimless crime, nothing to see here, move along folks. Ah, maybe each lens needs a disclaimer sticker, or some small print in the user manual: Focal length and aperture are measured at infinity focus. At other distances, your mileage may vary.

Or, stick to lenses that we *know* have no internal focus: enlarger-projector-copy lenses on extension! When I slap the Industar 100-U 110/4 on my Bellowscope, I know pretty much where it's going and what it's doing. No nasty surprises. No vain expectations. To change focus, move the bellows in or out. To know magnification, measure the extension. Simple and clean (and edge-to-edge flatfield sharp). No hay problemo!
08-05-2011, 08:16 AM   #42
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QuoteOriginally posted by Lowell Goudge Quote
I have to disagree with this.

the zoom and focusing of the lens involves movement and changes in the relationship of all the internal elements relitive to each other.

this is the cause of the problem, whether it is a 100% IF issue or a combination of IF and zoom lens design I can't answer but it is definitely a function of both

If the zoom function changed magnification only and the focus moved the entire group only, then the discussion would not be taking place, because focus would not alter the zoom ratio.

BUT.....

although I have not gone through the designs in detail, I can gaurantee that to hold a fixed focal length, while focusing from infinity to minimum distance, with a zoom would result in a much larger and more complicated lens.

as I said all along, this is a design trade off,, not a rip off. It is just too bad that it is not specified more clearly, at least in the detail of the specifications for the lens
The ratio of focal length change during focusing, versus the ratio of focal length change during zooming, would be quite different is my understanding. So, will focusing have that much significant effect on magnification that a non-IF lens casts a much bigger image at 70mm than an IF lens does at 135(?) mm to a 10' distant subject? Let's call this "design trade off" and not "rip off", but my question is - what percent of this reduced magnification is being contributed to by IF and what percent by something else (unknown to me)? My expectation is, if I have two zoom lenses set at 100mm, one IF and another not, I can understand if the IF lens produces an image which is maybe 5-7-maybe 10% smaller than the non-IF lens, but not this much that I am seeing when comparing my two lenses. Again, I am not an expert in this field, so my expectations and understandings are more of a lay man's.
08-05-2011, 08:27 AM   #43
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For the sake of further verification, I will try to take pictures using my IF and non-IF lenses set at same focal lengths, of the same subject from inifnity distance, and compare the results. I will report my findings once I have the opportunity.
08-10-2011, 06:54 AM   #44
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IF image one sensor width wider

QuoteOriginally posted by debmalya Quote
For the sake of further verification, I will try to take pictures using my IF and non-IF lenses set at same focal lengths, of the same subject from inifnity distance, and compare the results. I will report my findings once I have the opportunity.
I look forward to seeing your results to check this prediction from simple lens theory: for an IF lens that focuses by changing its focal length (its lens-image distance doesn't change).

Simple thin lens theory predicts that for lenses of the same focal length when focused at infinity, the IF lens' field of view will be one sensor width wider than that of the non-IF lens at all distances.

field.width.IF.lens = field.width.non-IF lens + sensor width at all subject distances for lenses with same focal length at infinity

This implies that image size differences between lens types are significant only for close-ups.

The working distance (subject to lens) & magnifications of an IF lens of focal length F" and normal lens F' are simple:

Distance = F'(1+1/m') = F"/m" where m=sensor.width/field.width

The most dramatic effect I can see in this prediction is for IF macro lenses: for the same nominal focal length macro lens the working distance at 1:1 mag is only 1/2 that of a non-IF lens; at 2:1 only 1/3 that of the non-IF lens etc...

Disclaimer - this is only for the hypothetical case of a constant distance between the lens and the sensor... I'll try to estimate what happens if that changes too....if the effective position of the IF lens moves away from the image as the focal length is changed the magnification discrepancy will be greater.

Last edited by newarts; 08-10-2011 at 08:53 AM.
08-10-2011, 08:21 AM   #45
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I looked into this last weekend with my DA*60-250mm. Large contrast in the distance so it was easy to see how the composition changed. There is a mayor loss in FOV when going from infinity to closest distance. Never thought about that before. Wil try to test it on another occasion when I have a little meeting to look at other lenses.
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