Crop Factors

Started Sep 20, 2008 | Discussions
Sean Nelson
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Re: No such thing...
In reply to Guerito, Sep 21, 2008

Guerito wrote:

Again, you are confusing terms.

I'm afraid I understand exactly what the terms are being used for. I quote again from the Wikipedia article:
http://en.wikipedia.org/wiki/Crop_factor#Crop_factor_of_point-and-shoot_cameras

"Smaller, non-DSLR, consumer cameras, typically referred to as point-and-shoot cameras, can also be characterized as having a crop factor or FLM relative to 35 mm format, even though they do not use interchangeable lenses or lenses designed for a different format. For example, the so-called "1/1.8-inch" format with a 9 mm sensor diagonal has a crop factor of almost 5 relative to the 43.3 mm diagonal of 35 mm film. Therefore, these cameras are equipped with lenses that are about one-fifth of the focal lengths that would be typical on a 35 mm point-and-shoot film camera...."

My question to you would be this: If a P&S camera has a sensor 5X smaller than a DSLR, and if you're unwilling to describe this camera as having a "5X crop factor", then what term would you use to describe it?

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Guerito
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Re: No such thing...
In reply to Sean Nelson, Sep 21, 2008

Sean Nelson wrote:

My question to you would be this: If a P&S camera has a sensor 5X
smaller than a DSLR, and if you're unwilling to describe this camera
as having a "5X crop factor", then what term would you use to
describe it?

I would describe it as having a 35mm focal length equivalent of X. I wouldn't describe a Canon 5D as having a crop factor any more than I would a Pentax 645 or an 8x10 sheet camera if they are using the lenses designed for their native sensor/film size because then there would not be any magnification/crop factor. Since a compact digicam has a lens designed for its specific sensor size, there is no crop factor because nothing is being cropped.

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Sean Nelson
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Re: No such thing...
In reply to Guerito, Sep 21, 2008

Guerito wrote:

I would describe it as having a 35mm focal length equivalent of X.

...and in fact this is called the "Focal Length Multiplier". In other words, a 7.4mm lens on my Canon A650 with a sensor that is 1/5 the size of a 35mm film frame has a "Focal Length Multipler" of 5X, meaning it has a 35mm-equivalent field of view of 35mm.

And, in fact, there is NO DIFFERENCE between "Focal Length Multiplier" and "Crop Factor". They are both derived by dividing the 35mm film frame diagonal by the size of the smaller sensor. The result is the same number, and the number is used in the same way.

I wouldn't describe a Canon 5D as having a crop factor any more than I would
a Pentax 645 or an 8x10 sheet camera if they are using the lenses designed
for their native sensor/film size

And yet a Canon Digital Rebel with a kit lens designed specifically for that camera's sensor size is still described as having a 1.6X crop factor. And users refer to that crop factor when they say that the 18mm wide end of the lens is equivalent to a 29mm lens on a 35mm camera.

My Pentax DSLR has an APS-C sensor with a crop factor of 1.5X. The camera has the same crop factor whether I use a 100mm lens designed for an APS-C sensor or if I use 100mm lenses designed for 35mm or 6x7 cameras. If I glued the lens to the camera so that it couldn't be removed the crop factor would still be 1.5X. If I built a camera with a non-removeable lens with the same APS-C sensor then the same crop factor still applies. It has nothing to do with lens removeability or coverage and everything to do with the relative size of the sensor compared to a 35mm film frame.

(Crop factor can use other sensor sizes as the "reference size" as well - for example Pentax 645 users speak of the crop factor of that camera compared to a 6x6 or 6x7 film frame. But 35mm is the "de facto" standard and if people are using something else as a baseline then they're always careful to state that).

I've cited references and explained to the best of my ability. I'm apologize if you don't get it, but I've done my best.

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Guerito
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Re: No such thing...
In reply to Sean Nelson, Sep 21, 2008

I get it, focal length equivalent and focal length multiplier (crop factor) are not the same thing - and that's what I tried to explain. I give up.
--

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hbx2004
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Re: No such thing...
In reply to Sean Nelson, Sep 21, 2008

Let's take Canon G9 as example, which has focal length of 7.4-44.4mm. For average buyer, this number doesn't tell much about reach/coverage of the lens (except it's 6x-zoom).

People are comfortable with 35mm equivalent terminology, so manufacturers also give 35mm equiv. specification (35-210mm in this case). Now, from our experience (comparing with FF lens, actually), we can imagine what angle of view this lens covers at given focal lengths: about 63°(wide)-12°(tele).

Important is, 44mm lens on G6 covers the same area as 200mm lens on FF camera -there's nothing cropped (compared to anything) as both sensors will capture whole area projected by particular lens (at comparable focal length). That is: when printed at the same size, images from both camera will have the same content -nothing was cutted/cropped from what each lens projected.

This isn't the case when we talk about APS-C sensor cameras. Because APS-C sensor covers smaller (cropped) portion of projected image, angle of view is smaller. Thus, optically, 200mm lens on APS-C (1.6-crop) camera delivers the same result as 320mm lens on FF camera.

The point is:
AP-S cameras do crop (by factor), thus we can apply "focal length multiplier".

P&S cameras don't crop -we use focal length (35mm) equivalence for easier understanding only.

Greetings,
Bogdan
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My pictures are my memories
http://freeweb.siol.net/hrastni3/

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Sean Nelson
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Re: No such thing...
In reply to hbx2004, Sep 21, 2008

hbx2004 wrote:

The point is:
AP-S cameras do crop (by factor), thus we can apply "focal length
multiplier".
P&S cameras don't crop -we use focal length (35mm) equivalence for
easier understanding only.

What about a 4/3 camera? It has a sensor which is about half the diagonal of a full 35mm frame. It's interchangeable lenses are designed specifically to cover the smaller sensor size. Does it crop?

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PixelMinded
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Re: No such thing...
In reply to Sean Nelson, Sep 21, 2008

Sean Nelson wrote:

hbx2004 wrote:

The point is:
AP-S cameras do crop (by factor), thus we can apply "focal length
multiplier".
P&S cameras don't crop -we use focal length (35mm) equivalence for
easier understanding only.

What about a 4/3 camera? It has a sensor which is about half the
diagonal of a full 35mm frame. It's interchangeable lenses are
designed specifically to cover the smaller sensor size. Does it
crop?

This is a very controversial question on the Olympus SLR Forum, they argue that 4/3 lenses were designed with 4/3 sensor in mind from the very beginning. Not like full frame lenses used on APS-C sensor of other brands.

Anyway whether you call the subject of this thread, crop factors, or focal length conversion factor, I agree with you, that it applies to all formats regardless of whether the lens was originally intended or not intended for the sensor in question. Clearly substituting the expression "crop factor" with " focal length conversion factor" or "focal length multiplier", makes the calculations we are talking about applicable to all cameras/formats.

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hbx2004
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Re: No such thing...
In reply to Sean Nelson, Sep 21, 2008

Hi Sean,

Sean Nelson wrote:

hbx2004 wrote:

The point is:
AP-S cameras do crop (by factor), thus we can apply "focal length
multiplier".
P&S cameras don't crop -we use focal length (35mm) equivalence for
easier understanding only.

What about a 4/3 camera? It has a sensor which is about half the
diagonal of a full 35mm frame. It's interchangeable lenses are
designed specifically to cover the smaller sensor size. Does it
crop?

To avoid cropping (and size/weight/cost, of course), new 4/3 lenses were designed for 4/3 sensor -so why should they deliver cropped result? Cropped, compared to what?

But, we use to say, that 4/3 lens has 35mm equivalence factor 2.0 -so we know, for example, 14mm on 4/3 has the same view angle as 28mm lens on 35mm camera.

AFAIK, "crop factor" term can only be used for APS-C cameras -because they can use FF lenses (on smaller than FF sensor).

This leads me to another question... New Panasonic G1 is micro4/3, where normal 4/3 lenses can be used via adapter. What are the consequences? Nobody mentioned any...

I'll take a look in Panny forum..

Greetings,
Bogdan
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My pictures are my memories
http://freeweb.siol.net/hrastni3/

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Ron Parr
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Re: No such thing...
In reply to hbx2004, Sep 21, 2008

hbx2004 wrote:

This isn't the case when we talk about APS-C sensor cameras. Because
APS-C sensor covers smaller (cropped) portion of projected image,
angle of view is smaller. Thus, optically, 200mm lens on APS-C
(1.6-crop) camera delivers the same result as 320mm lens on FF
camera.

The point is:
AP-S cameras do crop (by factor), thus we can apply "focal length
multiplier".
P&S cameras don't crop -we use focal length (35mm) equivalence for
easier understanding only.

As a user of a lens we never need to know about or care about the size of the image circle a lens was designed to project. All we need care about is that the image is large enough to cover our sensor

Let's say we're viewing an images taken with several 50mm lenses, each designed to project different image circles. One is designed for a 1/1.8 P&S sensor, another is designed for a 1.6 crop sensor, another is designed for a standard 24x36mm sensor, and the last is designed for medium format. If all of these are captured with a 1/1.8 P&S sensor, then then they all will see exactly the same thing (excluding aberrations, of course).

In reality, we almost never know about or care about the exact size of the image circle projected by our lenses. We usually just know that it's large enough and it's purely a supposition based upon an assumption of economy that the image circle is no larger than required the typical use of the lens.

For these reasons, there is absolutely no reason to attach the notion of a crop factor to some guess about what the lens maker had in mind when he designed the lens. Even if we guess correctly, it doesn't change anything. Therefore, the notion of a crop factor cannot be fundamentally about about lenses; its about the size of the image captured relative to a 24x36mm frame.

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PixelMinded
Senior MemberPosts: 1,022
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Re: No such thing...
In reply to Ron Parr, Sep 21, 2008

Ron Parr wrote:

hbx2004 wrote:

This isn't the case when we talk about APS-C sensor cameras. Because
APS-C sensor covers smaller (cropped) portion of projected image,
angle of view is smaller. Thus, optically, 200mm lens on APS-C
(1.6-crop) camera delivers the same result as 320mm lens on FF
camera.

The point is:
AP-S cameras do crop (by factor), thus we can apply "focal length
multiplier".
P&S cameras don't crop -we use focal length (35mm) equivalence for
easier understanding only.

As a user of a lens we never need to know about or care about the
size of the image circle a lens was designed to project. All we need
care about is that the image is large enough to cover our sensor

Let's say we're viewing an images taken with several 50mm lenses,
each designed to project different image circles. One is designed
for a 1/1.8 P&S sensor, another is designed for a 1.6 crop sensor,
another is designed for a standard 24x36mm sensor, and the last is
designed for medium format. If all of these are captured with a
1/1.8 P&S sensor, then then they all will see exactly the same thing
(excluding aberrations, of course).

In reality, we almost never know about or care about the exact size
of the image circle projected by our lenses. We usually just know
that it's large enough and it's purely a supposition based upon an
assumption of economy that the image circle is no larger than
required the typical use of the lens.

For these reasons, there is absolutely no reason to attach the notion
of a crop factor to some guess about what the lens maker had in mind
when he designed the lens. Even if we guess correctly, it doesn't
change anything. Therefore, the notion of a crop factor cannot be
fundamentally about about lenses; its about the size of the image
captured relative to a 24x36mm frame.

The smaller the format, the sharper the lens needs to be. Because the smaller frame have to be magnified many more times when viewed or printed. So while a medium format lens can be RELATIVELY softer than a 1/1.8" P&S lens and still produce perfectly sharp images on a medium format sensor or film, it won't suit a tiny 1/1.8" sensor. Even using full frame lenses on APS-C DSLR's result in softer performance than they do on full frame, but not a big deal because the two formats are relatively close to each other. Ignoring this sharpness issue however, if multiple 50mm lenses intended for different formats are used on one particular format then as you said they will all give the same focal length equivalent on that particular format.

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Sean Nelson
Veteran MemberPosts: 9,979
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Re: No such thing...
In reply to PixelMinded, Sep 21, 2008

PixelMinded wrote:

This is a very controversial question on the Olympus SLR Forum, they
argue that 4/3 lenses were designed with 4/3 sensor in mind from the
very beginning. Not like full frame lenses used on APS-C sensor of
other brands.

Interesting that this debate is going on there as well.

Ron Parr wrote:

In reality, we almost never know about or care about the exact size of the
image circle projected by our lenses. We usually just know that it's large
enough and it's purely a supposition based upon an assumption of
economy that the image circle is no larger than required the typical use
of the lens.

You've stated my argument most eloquently!

I think the reason we have a disagreement here is because some folks have a fairly narrow, technical notion of what the term "crop factor" should mean, whereas I'm a pragmatist and accept that the term is actually in much wider use. Kind of like when generic ASA is refered to as "Aspirin". The Bayer company gets upset, but there's not too much they can do about it because the word has entered the vernacular.

It starts with the fact that sensors smaller than 35mm film are called "cropped sensors", and is reinforced by the fact that everyone describes field of view by referring to "35mm equivalent" focal lengths, thereby creating the need for a conversion factor. The conversion factor, which is technically called the "Focal Length Multiplier", is the same number calculated the same way as the "crop factor" on small-sensor DSLRs. And since "Crop Factor" rolls off the tongue more easily then "Focal Length Multiplier" and logically associates with the notion of a "cropped sensor", it's become the defacto term for the conversion factor.

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