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06-24-2012, 09:49 PM   #1
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Lens Geometry Question

I have seen it said many times that 'if you have a 1.5 crop factor camera your lens will in effect have more zoom'... specifically a 100mm lens would be '150mm equivalent'...

Well here is my question...I assume the above type of statement was made using lenses built for full frame 35mm cameras. You in essence have the geometry of the lens ie the focal length, the diameter, and so forth where the lens allows a circle of light back to your sensor...

What a crop sensor effectively does is crop your photos at the time of taking the photo. A 100mm lens is still 100mm and the zoom is the zoom, but you are merely taking a recorded crop of that image.

So here is the crux of the question... if you have a lens built specifically FOR a 1.5 crop camera, ie a smaller barrel, a different angle of view than a full size counterpart, in short geometry and physics built around the smaller sensor...wouldn't a 100mm lens be considered '100mm' even in 1.5 crop sensorland?

Just to be clear with a full size normal lens for 35mm cameras they project a certain size light circle back to your sensor where it takes a crop of it...and thus leading to the 'appearance' that it has 'more zoom'... (whereas in reality if you had a full size sensor camera, took the same picture and then cropped it with software to the same dimensions it would be almost effectively the same thing)

With lenses 'build for the 1.5 crop sensor' do these dynamics change at all? Or does it even matter?


Last edited by alamo5000; 06-24-2012 at 10:01 PM.
06-24-2012, 09:59 PM   #2
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QuoteOriginally posted by alamo5000 Quote
...wouldn't a 100mm lens be considered '100mm' even in 1.5 crop sensorland?
That is correct. The actual focal length does not change regardless of the sensor size. The "equivalent" statement is a method to try to give a frame of reference using 135 format film focal lengths. Problem is some people are not familiar with that format, so they now have 2 meaningless numbers :-)
06-24-2012, 10:02 PM   #3
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The best thing you can do is to forget all these questions immediately. A 100mm is a 100mm is a 100mm. That is what it is considered always because that is what it is, no matter if the lens was made for a smaller or bigger sensor. You already get it -- it's cropped compared to the full-frame 35mm format. That's all you need to know, and really you don't even need to know that if you can forget this "equivalence" nonsense.
06-24-2012, 10:04 PM   #4
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QuoteOriginally posted by vonBaloney Quote
The best thing you can do is to forget all these questions immediately. A 100mm is a 100mm is a 100mm. That is what it is considered always because that is what it is, no matter if the lens was made for a smaller or bigger sensor. You already get it -- it's cropped compared to the full-frame 35mm format. That's all you need to know, and really you don't even need to know that if you can forget this "equivalence" nonsense.
Next time please try to be more honest with me and tell me how you REALLY feel... LOL!!!

06-24-2012, 10:07 PM   #5
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First of all, the phrase "more zoom" is pretty meaningless - a zoom lens is one whose focal length may be varied by the user (as distinct from a prime lens, which has a single fixed focal length).

Apart from that, you're quite right about the cropping effect - that's all that's happening here. But a 100mm lens is a 100mm lens regardless of the sensor it's expected to be used with. A lens designed with a crop sensor in mind, such as the Pentax DA series, will still give the same field of view as a lens of the same focal length which was designed for a "full-frame" (e.g. a 135 film) camera. However, you would probably find that the DA lens wouldn't be able to illuminate the entire frame if you mounted it on a full-frame camera, and you'd end up with a vignetted image.

Obviously, it works better the other way round: a lens intended for full-frame, mounted on a crop-sensor camera, would give results identical to the "native" lens of the same focal length.
06-24-2012, 10:11 PM   #6
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QuoteOriginally posted by SpecialK Quote
That is correct. The actual focal length does not change regardless of the sensor size. The "equivalent" statement is a method to try to give a frame of reference using 135 format film focal lengths. Problem is some people are not familiar with that format, so they now have 2 meaningless numbers :-)
If you somehow could put a 1.5 crop designed lens on a full frame camera you would/would not have serious vingetteing (SP?) issues due to the smaller circle of light?

I take it lenses made for 1.5 crop sensors are physically smaller than their counterparts? Kind of like the micro 4/3 thing?

Just a smaller lighter overall package...

The only difference is with lenses built around a 1.5 crop sensor they in effect 'waste' less light (as if you can waste light) so to speak...simply because they project a smaller diameter of light back to the sensor in the first place....
06-24-2012, 10:16 PM   #7
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QuoteOriginally posted by m42man Quote
However, you would probably find that the DA lens wouldn't be able to illuminate the entire frame if you mounted it on a full-frame camera, and you'd end up with a vignetted image.

Obviously, it works better the other way round: a lens intended for full-frame, mounted on a crop-sensor camera, would give results identical to the "native" lens of the same focal length.
You answered while I was typing the other post...so in essence since it can work one way but not the other all you are really losing is field of view....which might matter a lot for big old landscape photos possibly? But if you are shooting a baseball game or something where the subject is a fixed size (ie human) it just takes out a lot of the surrounding 'context' data when the image is cropped...which you probably would do anyway...unless that was your intended purpose...
06-24-2012, 10:27 PM   #8
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QuoteOriginally posted by alamo5000 Quote
If you somehow could put a 1.5 crop designed lens on a full frame camera you would/would not have serious vingetteing (SP?) issues due to the smaller circle of light?

I take it lenses made for 1.5 crop sensors are physically smaller than their counterparts? Kind of like the micro 4/3 thing?

Just a smaller lighter overall package...

The only difference is with lenses built around a 1.5 crop sensor they in effect 'waste' less light (as if you can waste light) so to speak...simply because they project a smaller diameter of light back to the sensor in the first place....
Lenses designed for APS-C, aren't necessarily any smaller than lenses designed for 35mm Full Frame.

The mount and registration distance are the same for Pentax, so that gives you characteristics that don't really lend themselves to making smaller lenses. Ever seen a Sigma lens? They make APS-C lenses, and they are massive compared to other lens brands

06-24-2012, 10:33 PM   #9
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Here is another follow up question since you have all caught on quickly to my original posting... with a 1.5 crop sensor... say that sensor is 16 megapixels...

Now if you had a full frame sensor that was say 24 megapixels...

If you were truly and literally cropping a full frame photo down, wouldn't you lose image quality vs the 16MP ?

Unless your full size sensor is a 32 megapixel camera the original light capture on a 16 MP crop sensor in theory could give you a sharper image than cropping down a 24MP full frame to the same size...

Right?

This is assuming a similiar type sensor...
06-24-2012, 10:47 PM   #10
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QuoteOriginally posted by alamo5000 Quote
Here is another follow up question since you have all caught on quickly to my original posting... with a 1.5 crop sensor... say that sensor is 16 megapixels...

Now if you had a full frame sensor that was say 24 megapixels...

If you were truly and literally cropping a full frame photo down, wouldn't you lose image quality vs the 16MP ?

Unless your full size sensor is a 32 megapixel camera the original light capture on a 16 MP crop sensor in theory could give you a sharper image than cropping down a 24MP full frame to the same size...

Right?

This is assuming a similiar type sensor...
Yes, the 16MP crop sensor has more pixels for a given area than a 24MP full-frame sensor, so in theory you may end up with sharper images. You'd need a 36MP full-frame sensor for equivalent resolution (for any given detail within the cropped area).
06-25-2012, 03:50 AM - 2 Likes   #11
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QuoteOriginally posted by vonBaloney Quote
That's all you need to know, and really you don't even need to know that if you can forget this "equivalence" nonsense.
The notion of "equivalence" is not nonsense. On the contrary, it is helpful to understand which lenses perform the same on different cameras with different sensor sizes.

For instance, a 50-135/2.8 lens on an APS-C camera produces the same images as a 75-200/4 lens on a full-frame camera (figures are approximate). Note the "f/4"; people often just convert focal length and then state that a 70-200/2.8 lens is much larger and heavier than a 50-135/2.8 on APS-C, neglecting the fact that a 70-200/2.8's on FF equivalent would be a 47-133/1.8 on APS-C. It is no wonder that a 50-135/2.8, i.e., f/2.8 instead of f/1.8 as the widest f-ratio, can be smaller and lighter. Very little to do with the image circle, but a lot with the maximum aperture size (which also dictates the front element size).

If you are interested in this stuff, I can recommend this article.

QuoteOriginally posted by alamo5000 Quote
I take it lenses made for 1.5 crop sensors are physically smaller than their counterparts?
As alohadave already wrote, it is difficult to turn a smaller image circle into a size/weight advantage. You gain a little on the size of the rear elements, but the overall size of most lenses is dictated by their light gathering abilities.

Lenses for smaller formats only often appear to be smaller because they often are "slower" (i.e., have less light gathering ability). This reduction in "speed' is not readily apparent because f-ratios of small sensor formats look "fast" before you convert them to their full-frame equivalent values. For instance, the full-frame equivalent of the 8.5/1.9 lens for the Q, is a 47/10. Doesn't look that fast anymore, does it?

QuoteOriginally posted by alamo5000 Quote
Unless your full size sensor is a 32 megapixel camera the original light capture on a 16 MP crop sensor in theory could give you a sharper image than cropping down a 24MP full frame to the same size...
To convert MP between different sensor sizes, you need to multiply/divide with the square of the crop factor. So, in this case, 36MP on FF has the same pixel pitch -- and thus allows cropping without loss of resolution -- as 16MP on APS-C.

Last edited by Class A; 06-25-2012 at 04:07 AM.
06-25-2012, 05:25 AM - 1 Like   #12
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QuoteOriginally posted by Class A Quote
For instance, the full-frame equivalent of the 8.5/1.9 lens for the Q, is a 47/10. Doesn't look that fast anymore, does it?
It's still an f/1.9 lens for light gathering.
The f/10 equivalence is for depth of field/bokeh.
06-25-2012, 08:16 AM   #13
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QuoteOriginally posted by alamo5000 Quote
You answered while I was typing the other post...so in essence since it can work one way but not the other all you are really losing is field of view....which might matter a lot for big old landscape photos possibly? But if you are shooting a baseball game or something where the subject is a fixed size (ie human) it just takes out a lot of the surrounding 'context' data when the image is cropped...which you probably would do anyway...unless that was your intended purpose...
Everyone has been tap dancing around the 2 magic words so far. You have touched on them in this post. What a different sensor size changes on a given lens, for example the DA 40/2.8 Limited is the Field of View and Angle of View. The FOV and AOV will be different on full frame, aps-c or m4/3. It is still a 40mm f2.8 lens. On a separate but related note, not all manufacturers use the same size aps-c sensors. Canon's is slightly smaller for example.
06-25-2012, 09:03 AM   #14
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QuoteOriginally posted by Class A Quote
The notion of "equivalence" is not nonsense. On the contrary, it is helpful to understand which lenses perform the same on different cameras with different sensor sizes.
Helpful to who? Who's running around with a full stable of lenses of different focal lengths with multiple bodies of different formats trying to take "equivalent" images with each?

QuoteQuote:
For instance, a 50-135/2.8 lens on an APS-C camera produces the same images as a 75-200/4 lens on a full-frame camera (figures are approximate). Note the "f/4"; people often just convert focal length and then state that a 70-200/2.8 lens is much larger and heavier than a 50-135/2.8 on APS-C, neglecting the fact that a 70-200/2.8's on FF equivalent would be a 47-133/1.8 on APS-C. It is no wonder that a 50-135/2.8, i.e., f/2.8 instead of f/1.8 as the widest f-ratio, can be smaller and lighter. Very little to do with the image circle, but a lot with the maximum aperture size (which also dictates the front element size).
See, statements like these are fantastically confusing because they make it sound like you can't get "2.8 DOF" from a 2.8 lens on an APS-C camera, which of course you can. Just as 50mm is 50mm is 50mm, f/2.8 is f/2.8 is f/2.8.
06-25-2012, 10:31 AM   #15
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QuoteOriginally posted by lytrytyr Quote
It's still an f/1.9 lens for light gathering. The f/10 equivalence is for depth of field/bokeh.
Not true.

There is no difference between "light gathering" and "blurring ability" (bokeh). That 8.5/1.9 lens on a Q gathers as much light as a 46.75/10.45 on a full-frame camera. Both lenses have the same aperture diameter wide open. The latter determines "light gathering" ability.

QuoteOriginally posted by vonBaloney Quote
Helpful to who?
For instance for those who want to understand what kind of blur and/or noise they can expect from a 8.5/1.9 lens on the Q (and have some experience from full-frame or APS-C cameras). If you translate the specs of the Q lens to FF-equivalent values, you notice that this lens cannot gather a lot of light and hence will produce noisy images and very little background blur (unless focused very closely). Noise and large DOF are often attributed to small sensors, but as a matter of fact, the associated small lenses are the true culprits.

QuoteOriginally posted by vonBaloney Quote
See, statements like these are fantastically confusing because they make it sound like you can't get "2.8 DOF" from a 2.8 lens on an APS-C camera, which of course you can. Just as 50mm is 50mm is 50mm, f/2.8 is f/2.8 is f/2.8.
Yes, a 50/2.8 lens is a 50/2.8 lens on all cameras. The lens never changes its physical properties. But the very same physical properties have different effects depending on the sensor size.

So if you are claiming that you can FF-f/2.8 (i.e., the amount of background blur you can get from an f/2.8 lens on an FF-camera) from a f/2.8 lens on APS-C camera, you are wrong. You can only get APS-C-f/2.8 blur from an f/2.8 lens on an APS-C camera (which corresponds to ~FF-f/4.3). Note that I never said that any lens changes its physical properties or specifications. I only point out that one can translate specifications into equivalent specifications on a different format.

Surely you will accept that a 50mm lens produces a different AOV on a FF-camera compared to an APS-C camera. The same holds true for the maximum background blur that can be achieved (when keeping everything else equal). This change in effective properties can be expressed in terms of equivalent specifications.

Last edited by Class A; 06-25-2012 at 10:39 AM.
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