focal length, and maximum magnification

tkbslc wrote in post #14833640
The 100mm will make a larger picture because it can get MUCH closer. That's why lenses specify magnification figures. It reconciles the focal length + min focus distance into a comparable number.

The problem is that some zooms don't maintain their focal length (measured at infinity) when they focus close. Some lose nearly half their focal length. The 15-85 must be doing that to have a worse magnification at double the focal length of the 17-40.

But it will not make the subject larger if you shoot it from 50 feet away. The 200/300/400mm lens will.

The whole point of this thread is as the OP said "so i was looking at which can focus closer...the 17-40L or the 15-85mm".

Take Care,
Cheers, Patrick

But just focusing closer is irrelevant without magnification numbers. The 15mm fisheye can focus as close as the 100mm macro, but since it is a 15mm lens with poor magnification numbers, it will not produce a close-up photo.

I think he's trying to figure out why the 15-85 has more magnification then the 17-40, but they're listed the other way around. I think that the numbers are given at the lens' smallest focal length. As in, 17-40 @17 and 15-85 @15 will yield .25 and .21 respectively. Since both lens' mfd are only a couple inches off, it would make sense that at their shortest length, the 15 has a lower magnification number. You can physically get closer and 2mm longer with the 17-40 thus the higher magnification.

But, when zoomed all the way in, at mfd, the 15-85 will magnify the image more. So maybe if the they listed it at their max focal, the 15-85 will have a higher magnification number.

No, magnification numbers are given at the focal length that has the greatest magnification. I can't think of any examples where it isn't the longest focal length.

tkbslc wrote in post #14833758
No, magnification numbers are given at the focal length that has the greatest magnification. I can't think of any examples where it isn't the longest focal length.

Haha if that's the case then there goes my theory . So can 2 inches give that much of a difference with double the focal length? Maybe something with crop factor might be at play. That 85 on crop is equivalent to 50ish on ff. So comparing 40 and 50ish (53 to be exact) on full frame, and their mfd, maybe you'll get those magnification numbers? i.e. 11.02" @40mm vs 14.4" @53mm would maybe give .25 and .21

Or, you could even go as far as comparing 40mm on crop. So, 11.02" @ 64mm vs 14.4" @ 85mm.

Saint728 wrote in post #14833692
The whole point of this thread is as the OP said "so i was looking at which can focus closer...the 17-40L or the 15-85mm".

sorry for the confusion i changed the wording in my first post after you replied...i'm not looking for just which can focus closer...i mean i can read numbers and know which one will be farther away

it's more about magnification, and why the 15-85mm is lower, but yet in my eyes should give higher magnification

dscri001 wrote in post #14833749
I think he's trying to figure out why the 15-85 has more magnification then the 17-40, but they're listed the other way around. I think that the numbers are given at the lens' smallest focal length. As in, 17-40 @17 and 15-85 @15 will yield .25 and .21 respectively. Since both lens' mfd are only a couple inches off, it would make sense that at their shortest length, the 15 has a lower magnification number. You can physically get closer and 2mm longer with the 17-40 thus the higher magnification.

But, when zoomed all the way in, at mfd, the 15-85 will magnify the image more. So maybe if the they listed it at their max focal, the 15-85 will have a higher magnification number.

i like that theory...at least it makes sense to me...although i clearly have no clue how magnification numbers are calculated...although there's a link at the bottom to a formula of 1/(MFD/focal length), but if that's the case the 17-40L max mag is like .14 or something...

tkbslc wrote in post #14833541
It means that there is no way the 15-85 is still 85 at min focus distance. Magnification numbers are not relative to focal length.

it also has me wondering if the crop lens mag is relative to a crop sensor.

yeah it could be not 85mm...i don't have either lens in question so mine were done with 18-55IS, and 28-135IS w/extension tube

if magnification doesn't deal with focal length it must just be related to the sensor size i guess...

I think that you are overthinking this. Maximum magnification is related to both focal length and subject distance, but not to sensor size. Typically, maximum magnification will be at maximum focal length (if the lens is a zoom), and minimum focus distance.

Now, it may be, as suggested by others, that lenses may not actually possess their stated focal length at their minimum focus distance, so, as they say, "your mileage may vary".

From the numbers you quote, it's clear that the 17-40 gives the greater magnification. Some of what it lacks in focal length, it recovers in MFD; the difference is down to how FL changes with FD for each lens.

mwsilver wrote in post #14833690
The 15-85 is specified at 35cm, The 1.2 ft is only an approximation. I can achieve focus with my copy at around 32 -33 cm at both the wide end and the long end.

I don't think that it's an approximation. Rather, I believe that the MFD is the distance from the sensor, not the end of the lens (or the lens mount). I'm pretty sure I read that somewhere (probably this forum), although I can't remember where exactly. Someone, let me know if this isn't true.

ohata0 wrote in post #14834447
I don't think that it's an approximation. Rather, I believe that the MFD is the distance from the sensor, not the end of the lens (or the lens mount). I'm pretty sure I read that somewhere (probably this forum), although I can't remember where exactly. Someone, let me know if this isn't true.

Yes, this is true. There is minimum focus distance (from the sensor), and working distance (from the end of the lens).

Remember that lenses change their effective focal length not only when being zoomed, but also as soon as you set their focus distance closer than infinity. So the EF-S 15-85 mm f/3.5-5.6 IS USM isn't at all any 85 mm any longer, when zoomed max out and then focused at closest range.

This is true also for prime lenses. Especially when they can focus at short distances. The EF-S 60 mm macro f/2.8 USM is significantly shorter than 60 mm when focused at its closest range, which is 19 cm.

The technical reason for this is that the larger extension of the optical system from the camera you get, the more light do you loose. You have to extend the optical system from the camera to focus closer, either by simply moving the whole lens system physically or by doing some internal optical focusing trick to achieve the same result. So the 60 mm macro has to open up beyond the f/2.8 aperture size to effectively remain an f/2.8 lens when focused closer than infinity. Obviously it can't, since the aperture blades would have to range outside the lens barrel.
But by using an optical trick it can reduce its focal length when focusing close up. The aperture's physical size is 60 mm/2.8 = 21 mm. When focusing at 1:1 scale, you normally loose two f-stops, thus making the lens an f/5.6 one. But since the physical size of an f/5.6 aperture is 60 mm/5.6 = 10.5 mm, we can "make" that an f/2.8 aperture if we reduce the focal length to 30 mm. Then we get 30 mm/10.5 mm = 2.8, and thus we are back where we started.

The same applies to normal lenses, primes as well as zooms, but it's not so obvious with them, since they normally do not allow focusing at such close distances.

If you instead put a 60 mm extension ring (just an empty barrel, not any tele-photo focal length extender with optics inside) on a 60 mm lens, with the lens still focused at infinity, you get a reproduction scale of 1:1 again. But now the lens is still set at infinity, so it does not change its focal length. Thus your f/2.8 is now effectively an f/5.6 lens. The camera will still tell you it can open to f/2.8, but instead it looks like the light available has been reduced to one quarter of what it was before the extension ring was mounted.

ohata0 wrote in post #14834447
I don't think that it's an approximation. Rather, I believe that the MFD is the distance from the sensor, not the end of the lens (or the lens mount). I'm pretty sure I read that somewhere (probably this forum), although I can't remember where exactly. Someone, let me know if this isn't true.

i measured from the little mark next to the lens mount on the camera...i think it must just come down to lens actual focal length as others have suggested...although i'm thinking of asking one of the lensrental companies if they can waste 2 minutes to actually show me which can get closer

ohata0 wrote in post #14834447
I don't think that it's an approximation. Rather, I believe that the MFD is the distance from the sensor, not the end of the lens (or the lens mount). I'm pretty sure I read that somewhere (probably this forum), although I can't remember where exactly. Someone, let me know if this isn't true.

I probably should have been more clear. In the post I was responding to , he indicated that the close focusing length is 1..2 ft, which he indicated was equal to 35.68cm. The actual specified distance is 35cm which converts to approximately 1.2 feet. In any case, after careful measurement on a few occasions I was able to achieve focus at approximately 33 from the sensor marker to the subject on my 15-85. I've noted that the close focus distance of lenses is not always the same as the specification.

The image ratios 17-40L as .25X, and the 15-85mm as .21X show that the 17-40L will provide a 1:4 ratio while the 15-85 will provide slightly better than a 1:5 ratio.

Therefore the 17-40L will provide a larger image in closeup shooting... However, the amount of difference is not realy that great.

RPCrowe wrote in post #14837140
The image ratios 17-40L as .25X, and the 15-85mm as .21X show that the 17-40L will provide a 1:4 ratio while the 15-85 will provide slightly better than a 1:5 ratio.

Therefore the 17-40L will provide a larger image in closeup shooting... However, the amount of difference is not realy that great.

but also just using the numbers provided like i stated before the 17-40L at 40mm at it's minimum focus distance would provide an image that encompasses 6.2" horizontally...and the 15-85mm at 85mm at it's minimum focus distance would yield an image that's 3.4" horizontally....

so it leads me to think that some number is wrong...probably the case of the focal lengths not actually being 85mm at the MFD...

maybe we can get a 17-40L shot at MFD of a quarter, and one with a 15-85mm at MFD as well...i just have a hard time thinking that the 17-40L can actually focus closer when we're talking double the focal length...and only a 2" difference in MFD...