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09-13-2008, 12:56 PM   #1
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Varifocal means?

Just got a new 28-105 f/3.2-4.5 and I'm wondering, where does it shift ? A constant-aperature zoom stays at, say, f/2.8 regardless of focal length, correct? Does varifocal lens like mine have fixed points where it goes from 3.2 to 3.5 to 4 to 4.5? For example, is the 28-50 range @ 3.2 and then the 51-105 @ 4.5? Or am I just misunderstanding the whole "zooming" process?
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09-13-2008, 01:27 PM   #2
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QuoteOriginally posted by FHPhotographer Quote
Just got a new 28-105 f/3.2-4.5 and I'm wondering, where does it shift ? A constant-aperature zoom stays at, say, f/2.8 regardless of focal length, correct? Does varifocal lens like mine have fixed points where it goes from 3.2 to 3.5 to 4 to 4.5? For example, is the 28-50 range @ 3.2 and then the 51-105 @ 4.5? Or am I just misunderstanding the whole "zooming" process?
The numbers you are talking about here are *maximum* apertures. The lens can be set to *any* aperture smaller than the maximum. So your 28-105 is happy to stay at, say, f/5.6 if that's where you set it in Av or M mode. But it's only *capable* of going to 3.2 at the wide end.

It's easy enough to see for yourself where the maximum aperture change on your lens - just zoom to 28mm, set the aperture to 3.2, and then watch the aperture change as you zoom. I'm guessing at point point - maybe around 35 - it will go to 3.5, then maybe around 60 it might go to 4.0, then maybe around 80 go to 4.5. Actually, I'd be curious for you to do this and report back... I've always been somewhat interested in this lens.
09-13-2008, 02:06 PM   #3
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"Varifocal" has nothing to do with the variable aperture! The variable aperture changes according to the focal length. Often cameras show an increase of the aperture number at a certain focal length, but that is more due to the camera electronics which will only show discrete steps in the aperture values ladder (like 1/3 f-stops).

Varifocal means, that the focus shifts with focal length. The very first zoom lenses (Zoomar) were of the varifocal design and later designers concentrated on fixing the focus on the same spot for any focal length of a zoom lens. This allowed to focus at the longest available focal length and then zoom out to a wider view, preserving critical focus.

Nowadays some zooms are again varifocal, because the lens designer simply assumes, that the AF function will compensate the focus changes during zooming. And quite like variable aperture a varifocus construction is simply cheaper to make and easier to compute...

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09-13-2008, 03:32 PM   #4
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Another advantage of the varifocal, as opposed to the true zoom, is the hit in your pocket book. If you want to keep f/4 all the way out to 200mm, it is going to cost money for the extra size of the lens elements.

09-13-2008, 10:39 PM   #5
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28-105 aperature points

QuoteOriginally posted by Marc Sabatella Quote
The numbers you are talking about here are *maximum* apertures. The lens can be set to *any* aperture smaller than the maximum. So your 28-105 is happy to stay at, say, f/5.6 if that's where you set it in Av or M mode. But it's only *capable* of going to 3.2 at the wide end.

It's easy enough to see for yourself where the maximum aperture change on your lens - just zoom to 28mm, set the aperture to 3.2, and then watch the aperture change as you zoom. I'm guessing at point point - maybe around 35 - it will go to 3.5, then maybe around 60 it might go to 4.0, then maybe around 80 go to 4.5. Actually, I'd be curious for you to do this and report back... I've always been somewhat interested in this lens.
It looks like it works out to
  • 28mm=3.2
  • 34mm = 3.5
  • 43mm = 4.0
  • 85mm=4.5
Could I assume, in some scenarios, that if I want to shoot at approximately 70mm in marginal light that it's time to boost the ISO to 400 or so and set the f-stop for maybe f/8 to keep the lens close to its optimium pont (I'm using the rule of thumb I've read that a lens hits its "sweet spot" roughtly two stops over the lowest f-stop... I know the 50 f/1.4 is sharper at f/4 or f/5.6 than it is at the widest aperature).
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09-14-2008, 03:52 AM   #6
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Going back to your question about apertures.

The basic principle is that the more light that you can get onto the sensor, the shorter the shutter speed can be. To get more light on the sensor you have to have as big a "hole" as possible - a large aperture. Boosting ISO does not change the amount of light on the sensor, it is just a case of turning up the "volume" (as an analogy think about listening to a radio station on shortwave - if the station is faint then you can turn up the volume on your radio which may be enough to hear the station, but you'll also hear the pops and whistles from radio noise too: the higher the ISO the more noise you'll see on your pictures).

Shorter focal length lenses can gather more light than a telephoto - they have a wider field of view and hence "see" more light. So to be able to have an "aperture number" that can give the amount of light on the sensor you have to take into account the focal length. The f-number you see is the ratio of the focal length to the effective aperture ("effective" to take into account that light is bent by the lens which again will be different for different focal lengths and lens designs).

So for example, I have an M50/1.4, the front element has a diameter of 36mm and so 50/36 = 1.38 for this lens the effective aperture is more or less the same as the diameter of the front element. For my M135/3.5 the front element is 38mm and 135/38 = 3.55, again, effective aperture is the same as the front element diameter.

But for my DA18-55 3.5-5.6 the front element is 36mm, and you can see that for a zoom the diameter of the effective aperture is nowhere near the front element diameter (otherwise the DA18-55 would be f/0.5 - f/1.5!). The reason, of course, is that the design of a zoom is complicated by the fact that it can be zoomed, and the bending of the light by the zoom design reduces the effective aperture, but not by the same amount over the zoom range (if you compare the actual f-number range of 3.5 - 5.6 with 0.5 - 1.5 you can see that they are not proportional).

But the principle still stands that the f-number depends on the focal length. The effective aperture is determined by the complicated lens formula of the zoom and also depends on the focal length. So as you change the focal length of the zoom the f-number will change continuously and not in the discrete steps you suggested earlier.

Richard
09-14-2008, 05:25 AM   #7
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Hello !
" Varifocal " is originaly the trade mark of first zoom produced in 1953 by Pierre Angenieux famous French manufacturer of hight quality optics.

Patent abstract :

Title:Varifocal objective and process for controlling it. Document Type and Number:European Patent EP0289392 Kind Code:A1

"Varifocal" is now generic word for zoom optics

Last edited by Edgar Lefuneste; 09-14-2008 at 05:39 AM.
09-14-2008, 05:48 AM   #8
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While history may suggest that all zooms are "varifocal" for photographers, the use of zoom and varifocal have specific meanings.

True zooms are lenses that hold focus when focal length is changes, varifocal lenses do not.

Additionally there has been a lot of concern and commant raised about apature in this discussion. Please note:

in addition to true zooms vs varifocal lenses, there are two different types of variable focus length lenses those which hold their apature constant as focal length is changed, and those that do not.

Constant apature zooms are usually much more expensive than variable apature zooms.

09-14-2008, 11:24 AM   #9
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QuoteOriginally posted by Lowell Goudge Quote
While history may suggest that all zooms are "varifocal" for photographers, the use of zoom and varifocal have specific meanings.

True zooms are lenses that hold focus when focal length is changes, varifocal lenses do not.
...
Lowell, forget it. Nobody tends to read serious and knowledgable answers here. I have written everything you rightly explained a couple of posts earlier, but nobody takes notice and adds his own suggestions, instead of simply looking up the facts...

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09-14-2008, 11:41 AM   #10
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QuoteOriginally posted by richard64 Quote
Going back to your question about apertures.

The basic principle is that the more light that you can get onto the sensor, the shorter the shutter speed can be. To get more light on the sensor you have to have as big a "hole" as possible - a large aperture. Boosting ISO does not change the amount of light on the sensor, it is just a case of turning up the "volume" (as an analogy think about listening to a radio station on shortwave - if the station is faint then you can turn up the volume on your radio which may be enough to hear the station, but you'll also hear the pops and whistles from radio noise too: the higher the ISO the more noise you'll see on your pictures).

Shorter focal length lenses can gather more light than a telephoto - they have a wider field of view and hence "see" more light. So to be able to have an "aperture number" that can give the amount of light on the sensor you have to take into account the focal length. The f-number you see is the ratio of the focal length to the effective aperture ("effective" to take into account that light is bent by the lens which again will be different for different focal lengths and lens designs).

So for example, I have an M50/1.4, the front element has a diameter of 36mm and so 50/36 = 1.38 for this lens the effective aperture is more or less the same as the diameter of the front element. For my M135/3.5 the front element is 38mm and 135/38 = 3.55, again, effective aperture is the same as the front element diameter.

But for my DA18-55 3.5-5.6 the front element is 36mm, and you can see that for a zoom the diameter of the effective aperture is nowhere near the front element diameter (otherwise the DA18-55 would be f/0.5 - f/1.5!). The reason, of course, is that the design of a zoom is complicated by the fact that it can be zoomed, and the bending of the light by the zoom design reduces the effective aperture, but not by the same amount over the zoom range (if you compare the actual f-number range of 3.5 - 5.6 with 0.5 - 1.5 you can see that they are not proportional).

But the principle still stands that the f-number depends on the focal length. The effective aperture is determined by the complicated lens formula of the zoom and also depends on the focal length. So as you change the focal length of the zoom the f-number will change continuously and not in the discrete steps you suggested earlier.

Richard
Richard, there is much truth in your post, but I think, the facts are much simpler:

aperture = focal length / open diameter

The open diameter is an engineering expression and should be substituted by "entrance pupil" in optics speak. In simple lens constructions, this is more or less the exact dimension of the front lens. In more complicated lens constructions, the front lens is usualy much bigger, than the max. aperture suggests. The entrance pupil is the diameter of the iris as imaged (projected) by the lens elements in front of the iris.

With zoom lenses, you have often lens elements that shift their position inside the optical train and thus vary the image ratio with which the iris is projected. Depending on the complexity of the construction, you can conmpensate for that change or not. If you do not compensate, the aperture varies with focal length. If you compensate, you can for example simply open the iris diameter according to the focal length. This is more expensive, not only because of the more complex iris control mechanism, but also, because you need to make your lenses simply bigger, than in a variable aperture lens.

Ben
09-14-2008, 12:51 PM   #11
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OP respopnse, I only know what the data tell me...

QUOTE=richard64;342287]Going back to your question about apertures.

The basic principle is that the more light that you can get onto the sensor, the shorter the shutter speed can be. To get more light on the sensor you have to have as big a "hole" as possible - a large aperture. Boosting ISO does not change the amount of light on the sensor, it is just a case of turning up the "volume" (as an analogy think about listening to a radio station on shortwave - if the station is faint then you can turn up the volume on your radio which may be enough to hear the station, but you'll also hear the pops and whistles from radio noise too: the higher the ISO the more noise you'll see on your pictures).

Shorter focal length lenses can gather more light than a telephoto - they have a wider field of view and hence "see" more light. So to be able to have an "aperture number" that can give the amount of light on the sensor you have to take into account the focal length. The f-number you see is the ratio of the focal length to the effective aperture ("effective" to take into account that light is bent by the lens which again will be different for different focal lengths and lens designs).

So for example, I have an M50/1.4, the front element has a diameter of 36mm and so 50/36 = 1.38 for this lens the effective aperture is more or less the same as the diameter of the front element. For my M135/3.5 the front element is 38mm and 135/38 = 3.55, again, effective aperture is the same as the front element diameter.

But for my DA18-55 3.5-5.6 the front element is 36mm, and you can see that for a zoom the diameter of the effective aperture is nowhere near the front element diameter (otherwise the DA18-55 would be f/0.5 - f/1.5!). The reason, of course, is that the design of a zoom is complicated by the fact that it can be zoomed, and the bending of the light by the zoom design reduces the effective aperture, but not by the same amount over the zoom range (if you compare the actual f-number range of 3.5 - 5.6 with 0.5 - 1.5 you can see that they are not proportional).

But the principle still stands that the f-number depends on the focal length. The effective aperture is determined by the complicated lens formula of the zoom and also depends on the focal length. So as you change the focal length of the zoom the f-number will change continuously and not in the discrete steps you suggested earlier.

Richard[/QUOTE]
Well, I won't pretend I understood most of your post, however, I do know that this lens does change in "discrete steps". That's what the exif data say.

Now I'm not coming onto your job site and trying to knock the shovel out of the hands of you, Ben and the other forum folk who work in this way, so don't take this as a criticism, but all I see is what the camera sees and all I know is what the camera data tell me. And the camera sees that at 33mm it's f/3.2 and at 34mm it's f/3.5.

What that means I will leave to others to ponder. I have the answer I sought,
FHPhotographer

Last edited by FHPhotographer; 09-14-2008 at 12:58 PM.
09-14-2008, 12:57 PM   #12
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OP response, fact is arguable, even here...

QuoteOriginally posted by Ben_Edict Quote
Lowell, forget it. Nobody tends to read serious and knowledgable answers here. I have written everything you rightly explained a couple of posts earlier, but nobody takes notice and adds his own suggestions, instead of simply looking up the facts...

Ben
This is a bit snarky isn't it? I suspect a lot of us read the "serious and knowledgable answers," but we may not understand the answers, or we may not care, or we may not agree. But I wouldn't assume the posts go unread or unconsidered.

In simple terms, I may get it, but maybe I just don't agree with it. Besides, fact is funny stuff, a bit slippery at the very best...and even at the very best, fact is not truth.

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09-14-2008, 03:22 PM   #13
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QuoteOriginally posted by FHPhotographer Quote
QUOTE=richard64;342287]
I do know that this lens does change in "discrete steps". That's what the exif data say.
That's only because it is photographic tradition to round off all aperture aperture calculations to a series of fairly standardized "stops": 2.8, 4.0, 5.6, 8, 11, 16, 22, plus additional half-stops or third stops depending on the camera and/or lens. But I would not doubt what you were being told - probably the maximum aperture *does* get smaller and smaller as you zoom but the camera just keeps reporting it rounded off to the nearest "stop" setting.

This would be reasonably easy to test: Just take a series of pictures at maximum aperture, using the same shutter speed for all shots at a given aperture (ie, use M mode). So if, for example, you say that it shows f/3.5 as the maximum focal length between 35mm and 43mm, then take a shot just bove 35mm and just below 43mm both at f/3.5 and using the same shutter speed. Then compare in the images in an editing program to see if the one at 43mm isn't perhaps measurably darker (compare the actual RGB values of a pixels at the same places in the image).
09-14-2008, 03:43 PM   #14
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QuoteOriginally posted by FHPhotographer Quote
It looks like it works out to
  • 28mm=3.2
  • 34mm = 3.5
  • 43mm = 4.0
  • 85mm=4.5
Thanks for the info! I had passed on that lens earlier because I was convinced 28mm was not wide enough for a general purpose zoom lens. And indeed, sometimes I'd miss the wider end. But then, I'm also often happy walking around with just my DA40, or maybe an M28/2.8 and something telephoto. I'd keep my kit lens for the wide angle - or perhaps someday get the DA21 or DA15 (when it comes out) for wider angle possibilities.

As it is, I suspect this lens probably *would* have made me quite happy the last couple of years, but it's time is probably past for me - I'm becoming more a prime snob.

QuoteQuote:
Could I assume, in some scenarios, that if I want to shoot at approximately 70mm in marginal light that it's time to boost the ISO to 400 or so and set the f-stop for maybe f/8 to keep the lens close to its optimium pont
I don't know what you call marginal light, but in, say, the typical living room lit only by artifical light fixtures (overhead and/or lamps), ISO 400 and f/8 will get only a shutter speed of say, half a second. Which is to say, totally useless for most of the kind of photography you'd normally be doing. You've got no choice but to open up to f/4 (and yes, give up some sharpness), and crank ISO up to 800 or 1600 (and yes, accept some noise) just to get a shutter speed that has *any* hope of stopping camera and subject motion. SR helps with camera shake, of course, but at 70mm it cannot work miracles, and it won't stop your subjects, either. And really, f/4 at ISO 1600 *still* won't give you fast enough shutter speeds in many situations. f/2.8 is usually considered a bare minimum, and faster is even better.

Which is why, even if a 50/1.4 or 50/1.7 is sharpest at f/5.6 or so, it will still get used a lot at f/2.8, because it's sharp *enough* there, and in many situations, that's the only way to get enough light even at ISO 1600. And in other situations, it's a way of getting a shot at ISO 400 or better, and the difference in image quality between ISO 400 and ISO 1600 is often more significant than the difference in in image quality between f/2.8 and f/5.6. Similarly, f2 might be the only way to get a shot even at ISO 1600 in some cases, and it other cases, it's a way of getting the shot at ISO 200.
09-14-2008, 10:06 PM   #15
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QuoteOriginally posted by FHPhotographer Quote
Well, I won't pretend I understood most of your post, however, I do know that this lens does change in "discrete steps". That's what the exif data say.

Now I'm not coming onto your job site and trying to knock the shovel out of the hands of you, Ben and the other forum folk who work in this way, so don't take this as a criticism, but all I see is what the camera sees and all I know is what the camera data tell me. And the camera sees that at 33mm it's f/3.2 and at 34mm it's f/3.5.

What that means I will leave to others to ponder. I have the answer I sought,
FHPhotographer
What is really happenng is that the aperture is changing continuously, but the data in EXIF only marks discrete steps. It's like the trip odometer on an electronic display in my Toyota Tundra. It only shows 100 m intervals, even though one could actually do it in 1 mm intervals. Your aperture does not jump, it changes values in a complex manner, but not in descrete steps.
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