Quick questions about lens

[Eildosa]

Hi,

I was wondering, if I set a 15-55mm lens to it's 50mm focal lenght and take a shot,
then with the same camera use a 50mm prime lens and take the same shot.

Will I have the same thing in both shot? image cropped exactly the same way, same perspective, etc.

Also I saw that there is some 50mm lens that have a minimal focusing distance of 0.2m ans some that have a minimal focusing distance of 0.45m.

I always though that focal lenght was what was defining the minimal focusing distance, apparently not, what is defining the minimal focusing distance?

Thanks.

[tclune]

I was wondering, if I set a 15-55mm lens to it's 50mm focal lenght and take a shot,
then with the same camera use a 50mm prime lens and take the same shot.

Will I have the same thing in both shot? image cropped exactly the same way, same perspective, etc.

Focal length is only one aspect of an image. In addition, there is depth of field (typically a prime can have a larger aperture than a zoom can), distortion (typically, zooms will have some barrel distortion at the short focal end and some pin-cushion distortion at the long focal end), color and contrast (often, primes will be sharper in this sense) and bokeh (how smoothly the lens renders out-of-focus areas. This is caused by complex factors including how many blades are in the aperture and whether they are curved or straight and whether the lens suffers from astigmatism or not -- i.e., whether it loses sharpness equally in all directions. I'm sure there are other aspects to this of which I am unaware.) I tend to find inexpensive "nifty fifty" lenses to have pretty crappy bokeh, but that is a subjective judgment with each photographer. FWIW

[Manfred M]

Hi,

I was wondering, if I set a 15-55mm lens to it's 50mm focal lenght and take a shot,
then with the same camera use a 50mm prime lens and take the same shot.

Will I have the same thing in both shot? image cropped exactly the same way, same perspective, etc.

Not necessarily. Fixed focal length lenses (a.k.a. prime lens) tend to be sharper and have less distortion than zoom lenses. They will also have a larger maximum aperture. Zoom lenses are also more prone to focus breathing (size of the image changing during focus).

The 50mm equivalence is only correct for subjects that are "at infinity"; as you get closer to the subject, the focal length will be somewhat less. As an extreme example; the top of the line (expensive) Nikkor f/2.8 70 - 200mm lens has a focal length of 200mm when the subject is "at infinity", but is only 135mm at the closest focusing point (at the 200mm marking). It is my understanding that all variable focal length lenses do this to some extent.

The The question is always going to be, which option is more important to you; variable focal length versus higher image quality (that you may or may not notice).

Also I saw that there is some 50mm lens that have a minimal focusing distance of 0.2m ans some that have a minimal focusing distance of 0.45m.

That is totally dependent on the lens design. In theory the lens designer could increase the "throw" of the lens (the total distance the lens elements move from maximum to minimum during the focus operation, but larger throw = slower autofocus speed and more complex lens design (further rotation of the focus ring).

I always though that focal lenght was what was defining the minimal focusing distance, apparently not, what is defining the minimal focusing distance?

Thanks.

No; focal length is focal length, no more, no less. If you want closer focus distance, you'll need a different lens design (which is why there are macro lenses).

[chauncey]

To add to your confusion, let's add "field of view" into the equation...any lens, regardless of the
focal length, encompassing the same field of view (sans cropping), will exhibit the same DOF at
a given f/stop. In other words, were you to capture an image with a 50mm lens @ f/8, then
capture the same image using a 200mm lens @ f/8, distancing yourself enough to insure same
field of view...your depth of field will remain the same.

[William W]

I understand that it was a general comment about lenses, but, I'd advise care as to how exactly the Axiom of Depth of Field is expressed and interpreted:

will exhibit the same DOF at a given f/stop

I like to temper any comment about the Axiom with words to the effect of "for most commonly made shots" or similar.

The Axiom begins to fail for: close-up shots; extreme W/A Lenses: The Plane of Sharp Focus nearing the Hyperfocal Distance.

The Axiom does fail for Macro Photography.

I think that being precise about the Axiom in this particular thread is important. Not to make an 'Axiom' a 'Law' by using a definitive statement is relevant here, because the OP specifically mentioned "minimal focusing distance", which in turn implies, close-up work, may be a consideration and a predicate for the OP's question.

WW

[revi]

(...)

The 50mm equivalence is only correct for subjects that are "at infinity"; as you get closer to the subject, the focal length will be somewhat less. As an extreme example; the top of the line (expensive) Nikkor f/2.8 70 - 200mm lens has a focal length of 200mm when the subject is "at infinity", but is only 135mm at the closest focusing point (at the 200mm marking). It is my understanding that all variable focal length lenses do this to some extent.
(...)

I thought that effect occurred (only?) in lenses with "internal focussing" (the lens barrel doesn't move when focussing, but internal elements move). In such lenses, focusing is done by changing the focal length at inifinity of the lens. This has nothing to do with variable focal length as in zoom lenses (which change the angle of view by changing the focal lenght at infinity of the lens).

As for the marked focal length on a lens: as always, this is the focal lenght for subjects 'at infinity'. In the simplest case, getting closer to your subject means moving the lens away from the sensor (see e.g. http://en.wikipedia.org/wiki/Lens_%28optics%29), and the closer you get to the subject, the farther away from the sensor. So the minimal focussing distance depends on how much the maker wants to invest in longer barrels, and longer gear paths or more precise steering.

For lenses with internal focussing it's a bit different (the lens barrel seems not to move), but there also, closer focussing distance means more expensive construction, and might hit optical limits (distortion and aberrations getting out of hand)

[Eildosa]

Nice, thanks for all that.

[Inkanyezi]

I think it is a bit too much theorising on this matter. There are essentially two ways to focus an optical system, and they can of course be combined into a hybrid version.

The first method is using a fixed focal length optical system and moving it farther from the image plane. When it is twice the focal length away, it is focused at a distance four times the focal length and its image is life size. The angle of view of course is equivalent to a lens of twice the focal length, when that lens is focused at infinity. When a lens focuses in that manner, its effective "speed" is decreased while focusing, and at life size reproduction, the f-number may be considered twice (or half if you consider that it is a fraction) the one when focused at infinity. The main disadvantage for autofocusing is that when getting close to life size, not only the distance from the image plane changes, but also the distance from the lens to the subject will change considerably, which makes AF unreliable at close distances. When moving the lens farther away, the distance, as measured from the image plane, becomes larger, while reproduction scale increases. Hence four times the focal length is the closest distance from the image plane any lens can focus.

The second method is altering the focal length of the optical system, while it remains more or less in the same position, the same distance from the image plane. When doing this, the focal length is decreased to focus closer, and its field of view remains more or less the same at all focusing distances. When reaching reproduction of life size, the focal length has shrunk to half, effectively getting a lens of half the focal length and twice the f-number. Unlike the method that extends the lens, an inner focusing lens may be reliable for AF at all distances down to the closest possible.

In the past, the extension method used to be prevalent, except for a few cheaper cameras that had a positive front lens system which did the focusing. You may compare this method to adding positive close-up lenses in front of the lens, as it does almost the same, effectively decreasing the focal length of the system by adding positive dioptre values without further extending the system from the image plane.

Nowadays, most varifocal lenses use front lens focusing, although there are exceptions. Front lens focusing, where the front component moves away, effectively decreases focal length. However, it does not change angle of view to any large degree, so the notion that a zoom at 200 mm focal length becomes 135 mm is largely a moot point. Its angle of view remains about the same when focusing closer. Effectively this decreases its focal length, but for the photographer it is of no importance. A fixed focal length lens would have narrowed its field of view instead, making it appear as a longer focal length, although its focal length would remain fixed.

For shooting movie, narrowing the field of view by focusing is mostly undesired.

[tao2]

Whilst some are in danger of disappearing up their own focal length...ah'll see what ah can do...

Hi,

I was wondering, if I set a 15-55mm lens to it's 50mm focal lenght and take a shot,
then with the same camera use a 50mm prime lens and take the same shot.

Will I have the same thing in both shot? image cropped exactly the same way, same perspective, etc.

Also I saw that there is some 50mm lens that have a minimal focusing distance of 0.2m ans some that have a minimal focusing distance of 0.45m.

I always though that focal lenght was what was defining the minimal focusing distance, apparently not, what is defining the minimal focusing distance?

Thanks.

I was wondering, if I set a 15-55mm lens to it's 50mm focal length and take a shot,
then with the same camera use a 50mm prime lens and take the same shot.

Will I have the same thing in both shot? image cropped exactly the same way, same perspective, etc.

Let's say, e.g. ye set a camera with a prime 50mm lens at f8 and take a photo with a particular group of settings - saturation, sharpness, IQ etc.

Ye then put the 18-55mm zoom on the same camera and setting it at exactly 50mm at f8 and using the exact same camera settings as above and assuming that ye have the same distance, angle, lighting etc...ye'll get the exact same shot.

Also I saw that there is some 50mm lens that have a minimal focusing distance of 0.2m ans some that have a minimal focusing distance of 0.45m.

The minimal focusing distance is a property of the lens and the distance tae the film/sensor and is a result of lens design. It changes depending on the number, type and placement of elements in the lens construction even in lenses with the same focal length.

[Manfred M]

I thought that effect occurred (only?) in lenses with "internal focussing" (the lens barrel doesn't move when focussing, but internal elements move). In such lenses, focusing is done by changing the focal length at inifinity of the lens. This has nothing to do with variable focal length as in zoom lenses (which change the angle of view by changing the focal lenght at infinity of the lens).

As for the marked focal length on a lens: as always, this is the focal lenght for subjects 'at infinity'. In the simplest case, getting closer to your subject means moving the lens away from the sensor (see e.g. http://en.wikipedia.org/wiki/Lens_%28optics%29), and the closer you get to the subject, the farther away from the sensor. So the minimal focussing distance depends on how much the maker wants to invest in longer barrels, and longer gear paths or more precise steering.

For lenses with internal focussing it's a bit different (the lens barrel seems not to move), but there also, closer focussing distance means more expensive construction, and might hit optical limits (distortion and aberrations getting out of hand)

Remco - I'm not totally sure that this is correct, but I admit that my knowledge of optics is still primarily based on university physics courses that I took over 40 years ago, and my recall is anything but perfect... How does shifting the front element of a lens a set amount differ than shifting the internal elements the same amount to focus a lens make any difference regarding the effective focal length? The offset will be the same in both cases.

Moving the front element is certainly less complex to do than moving around the internal elements, but the offset (which drives the focus) will be identical. There will also have to be some small (?) impact on focal length as these shifting the relative position of the elements is how a zoom lens works.

[Manfred M]

Ye then put the 18-55mm zoom on the same camera and setting it at exactly 50mm at f8 and using the exact same camera settings as above and assuming that ye have the same distance, angle, lighting etc...ye'll get the exact same shot.

I would hesitate to use the word "exact", because that can have different meanings. I would suggest that I'd agree if you had said something like "approximately the same" or the likes, but exact is impossible. Two identical lenses will never be exactly the same just because of manufacturing tolerances (although they should be darn close). Two different lenses that are different designs, even when stopped down will not give you exactly the same results.

If we look at the level of distortion in the two lenses he is thinking about (remember he is trying to maximize getting an accurate capture so that his software can create a quality map from the captured images.);


http://www.lenstip.com/424.6-Lens_re...istortion.html


http://www.lenstip.com/304.6-Lens_re...istortion.html


And just for fun, lets look at the 18-200mm lens, which has even more distortion. ( throw this in only to show the issues that we get with lenses that have a very long focal length range, and this lens would perform brilliantly when measured against the point & shoot cameras he was working with originally).

http://www.lenstip.com/306.6-Lens_re...istortion.html


The fixed f/1.8 50mm lens is pretty good (the older Nikkor f/1.8 50mm is better), but the 18-55mm shows exactly what one one expect; barrel distortion on the short side and pincushion distortion on the long side. Now what I can't answer is whether this is going to be an issue for the type of work he is trying to do. Only he can do this.

For run of the mill photography; I would agree with you, that the results will be effectively identical. But this is not what he is trying to accomplish.

[jcuknz]

Eildosa Nothing to do with your current question but a comment after seeing your 180 degree set-up photo and somebody else's comment about flare.
I think that it will be highly desirable for each lens to have custom-made lens-hood for it so that the lens only sees what the camera is recording and nothing else and lights slightly or quite out of view do not shine on the lens ... even then a 360 degree set-up would be hazardous to image quality I think and though it raises problems from subject movement if not recorded at same instance.

Some system of taking sections at slightly different times and with lighting not needed for each exposure to be turned off ... not sure how quickly your lights gain full power and loose their power which could be the detirming factor. I envisage each photo group being made after the fashion of a Gatling Gun of the 1800 century. Perhaps that is not the best of comparisons

[revi]

Remco - I'm not totally sure that this is correct, but I admit that my knowledge of optics is still primarily based on university physics courses that I took over 40 years ago, and my recall is anything but perfect... How does shifting the front element of a lens a set amount differ than shifting the internal elements the same amount to focus a lens make any difference regarding the effective focal length? The offset will be the same in both cases.

Moving the front element is certainly less complex to do than moving around the internal elements, but the offset (which drives the focus) will be identical. There will also have to be some small (?) impact on focal length as these shifting the relative position of the elements is how a zoom lens works.

Sorry, my mistake. Of course, focussing by moving the front element(s) of the lens comes down to the same thing as moving internal elements: you change the focal length of the system. I was thinking of (older?) lenses, where focussing moved the whole optical block. There the focal length stays the same independent of object distance.

And there's another catch: what we call focal length is in fact the focal length of a thin lens that would give the same field of view... (there is no way we could have a lens with a "real" focal length of 10mm on our SLR's... ). So the whole notion of focal length is iffy at best with modern camera objectives. But the model works most of the time, so we keep on using it. (a bit like Newton and Einstein)