Crop Factors

Started Sep 20, 2008 | Discussions
2clueless New Member • Posts: 21
Crop Factors

Can anyone tell me how the crop factor is determined on a point and shoot camera? (Or any digital for that matter!) In other words, I don't know from reviewing the literature about any given camera how the crop factor is determined. Unfortunately, manufacturers are not required to reveal it. (A bit of false and misleading advertising here, but what can one do?) I appreciate your comments.

Steve

WT21 Veteran Member • Posts: 5,103
Re: Crop Factors

Crop factor has nothing to do with what is set by manufacturers. It is a descriptive term that is driven by the size of the sensor, and the manufacturers DO tell you the size of the sensors.

It's like saying a van is twice the size of a car and a school bus four times the size of a van. It's just descriptive.

Wikipedia is your friend: http://en.wikipedia.org/wiki/Crop_factor

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Sean Nelson
Sean Nelson Forum Pro • Posts: 12,846
Re: Crop Factors

2clueless wrote:

Can anyone tell me how the crop factor is determined on a point and
shoot camera? (Or any digital for that matter!) In other words, I
don't know from reviewing the literature about any given camera how
the crop factor is determined. Unfortunately, manufacturers are not
required to reveal it. (A bit of false and misleading advertising
here, but what can one do?) I appreciate your comments.

Bearing in mind that "crop factor" is just a way to convert a focal length to a field of view, you can deduce what it is if you know the focal length of the camera and the claimed "35mm equivalent field of view".

For example, my Canon A650 has a 7.4-44.4mm lens which Canon claims is "35mm film equivalent 35-210mm" Dividing 35 by 7.4 gives you a crop factor of about 4.7

lacix
lacix Contributing Member • Posts: 594
Re: Crop Factors

I’m maybe wrong, but all smaller than full-size sensor cameras employing reduction lenses. The, so called “tele lenses” used on this cameras are really cropped wide fields. The truth tele is a magnifying lens, and has a unique compressing effect (the tele effect), which reduction lens does not have.
The crop factor could be a very important info IMO.

UBL

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Sean Nelson
Sean Nelson Forum Pro • Posts: 12,846
Re: Crop Factors

LaciX wrote:

I’m maybe wrong, but all smaller than full-size sensor cameras
employing reduction lenses. The, so called “tele lenses” used on this
cameras are really cropped wide fields.

I'm a little perplexed by your post, but I think what you're trying to say is that a telephoto lens on a small-sensor camera has a relatively short focal length (35mm for example) than a lens with an equivalent field of view on a camera with a larger sensor (250mm as an example).

This is of course true. But you should understand that a telephoto lens on a small-sensor camera produces exactly the same perspective (ie, "compressing effect", as you call it) as a telephoto lens with the same field of view on a large-sensor camera. This is true even though the lenses may have radically different focal lengths.

For example, a 150mm lens on a 35mm camera, a 100mm lens on an APS-C camera, and a 20mm lens on a small-sensor camera (with a crop factor of 7.5) will produce images that all look the same (except for depth of field and potential IQ differences due to the lens design and sensor noise levels, etc).

lacix
lacix Contributing Member • Posts: 594
Crop Factors

Sean Nelson wrote

For example, a 150mm lens on a 35mm camera, a 100mm lens on an APS-C camera, and a 20mm lens on a small-sensor camera (with a crop factor of 7.5) will produce images that all look the same (except for depth of field and potential IQ differences due to the lens design and sensor noise levels, etc).

LaciX

You can’t get more DOF while have the same perspective! More DOF=wider angle of view.

My G9 has crop factor 4.55, which means that at 210mm has more DOF than full-size tele 210, but because, actually this is only a cropped image field of normal 45mm (210/4.55) lens.
And back again, 45mm lens has no tele effect or compression.

Of course, would not see the difference in close distances, but try-it on landscape. The difference is huge!

UBL

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Sean Nelson
Sean Nelson Forum Pro • Posts: 12,846
Re: Crop Factors

You can’t get more DOF while have the same perspective! More
DOF=wider angle of view.

That's true FOR THE SAME CAMERA.

But if we're comparing cameras with different sensor sizes, and using lenses which give the same field of view (and therefore the same perspective), then at the same aperture the depth of field is shallower on the camera with the larger sensor.

This is because depth of field is determined by the absolute aperture of the lens (ie, a 100mm f/4 lens has an absolute aperture of 100/4 = 25mm). On a camera with a larger sensor, you have to scale the optics of the lens up in order to retain the same field of view, and therefore the absolute aperture is larger.

For example, the DOF calculator at http://www.dofmaster.com/dofjs.html shows the following for a picture taken with a field of view equivalent to a 35mm lens on a 35mm film camera at f/4 with a subject distance of 3 feet:

Canon A650 at 7.4mm, DOF = 3 ft to 14.9ft
Pentax K100Ds at 23.2mm, DOF = 4.09 to 6.44 ft

OP 2clueless New Member • Posts: 21
Re: Crop Factors

Thank you for answering my question (as clumsy as it was!). What I was trying to determine is if a crop factor of, say, 5 on a small sensor point and shoot should be used to determine the "equivalent" or "apparent" focal length FOV relative to a 35mm film camera just as it does on dSLR's. In other words, if a point and shoot camera lens is rated 28mm-210mm 35mm Film Camera Equivalent, would we still use the crop factor (as an example: '5') times the focal length to determine the "equivalent" FOV. i.e. 28mm x 5 = 140mm. Thus letting us know that if we were using a 35mm film camera in this situation we would have to be using a 140mm lens. Or, the inverse, to obtain the FOV of a 140mm lens on a full frame camera we would need to use a 28mm lens selection on the point and shoot.

Sean Nelson
Sean Nelson Forum Pro • Posts: 12,846
Re: Crop Factors

2clueless wrote:

What I was trying to determine is if a crop factor of, say, 5 on a small
sensor point and shoot should be used to determine the "equivalent"
or "apparent" focal length FOV relative to a 35mm film camera just as
it does on dSLR's.

It can, but in most cases compact camera manufacturers already publish the "35mm equivalent" focal lengths anyway, and they DON'T publish the crop factors.

But knowing the crop factor does come in handy if you're looking at EXIF information for a picture and the 35mm equivalent isn't included.

hbx2004 Veteran Member • Posts: 3,162
No such thing...

2clueless wrote:

Can anyone tell me how the crop factor is determined on a point and
shoot camera? (Or any digital for that matter!) ...

There isn't such thing as "crop factor" for p&s cameras -because sensor covers full image projected by the lens at it's actual focal length.

Greetings,
Bogdan
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OP 2clueless New Member • Posts: 21
Mfr Literature and Sales Pitch Do Matter!

Of course, you are correct. However, that was not my point when I said that the camera manufacturer's standard literature (and sales personnel's "pitch") were not entirely legit. If I am looking at a small sensor point and shoot camera at the local retailer, the sales rep usually points out that the 28-210mm zoom on the camera has a great wide angle FOV. However, that is not true. If the "crop" factor for that particular lens happens to be "5", the effective FOV for that camera on the "wide" end is really a full frame equivalent of 140mm. A 140mm lens is hardly a "wide angle" lens. I can't believe how many people I've met who have bought the point and shoot with the purported wide angle lens and had no idea whatsoever that the "effective" or more appropriately "apparent" fov is 140mm. To me....that's misleading and it falls squarely on the shoulders of the manufacturers and their retail representatives to point this out. User fairly new to digital photo would not normally be expected to have researched this issue on their own. They wouldn't even suspect that it is an issue.
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Sean Nelson
Sean Nelson Forum Pro • Posts: 12,846
Re: No such thing...

hbx2004 wrote:

There isn't such thing as "crop factor" for p&s cameras -because
sensor covers full image projected by the lens at it's actual focal length.

From: http://en.wikipedia.org/wiki/Crop_factor :

"a crop factor is the ratio of the dimensions of a camera's imaging area compared to a reference format; most often, this term is applied to digital cameras, relative to 35 mm film format as a standard."

The only cameras that don't have a "crop factor" are those which have a sensor (or film) which is the same size as a standard 35mm film frame. On all other cameras, "crop factor" is used to translate a len's focal length into a field of view, expressed in terms of the focal length that would give the same field of view on 35mm film.

So my Canon A650IS camera with it's lens set to 7.4mm gives the same field of view as a 35mm lens would on a 35mm film camera. Therefore it's "crop factor" is a little less than 5.

Another example: my Pentax K100D Super camera uses an APS-C sensor with a crop factor of 1.5. The crop factor is the same whether I use one of my old film lenses which covers an entire 35mm film frame, or one of my new designed-for-digital zoom lenses which only covers the APS-C sensor area and which would vignette if I used it on a film camera.

OP 2clueless New Member • Posts: 21
Re: No such thing...

Really? This what I was trying to determine with my original question: if a point and shoot camera has a lens of say 28-110mm focal lengths, are you saying that the fov for this lens is exactly the same as the same lens on a full frame digital camera? I don't think so, but I am really to re-visit the subject. It has been my understanding that, on the Point & Shoot, the 28mm end of this example lens would yield the same fov as a 140mm lens on a full frame camera. (Assuming a crop factor of '5', which most Point & Shoots yield.) Help me out here.

OP 2clueless New Member • Posts: 21
Re: No such thing...

Thank you. I just responded to the above reply asking for him to re-educate me. I admit that my original question wasn't all that clear, but I got the answers I need thanks to people like you.
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OP 2clueless New Member • Posts: 21
Publish but no explanatin

Sean: thanks for jumping in on this. You statement is precisely my point. I am completely open to correction on this issue but I don't think publishing the information is enough without a thorough explanation. Even if it is published, as a new or relatively new person to digi photo I probably wouldn't even realize that there was an issue. Further thoughts?
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Sean Nelson
Sean Nelson Forum Pro • Posts: 12,846
Re: Mfr Literature and Sales Pitch Do Matter!

2clueless wrote:

If I am looking at a
small sensor point and shoot camera at the local retailer, the sales
rep usually points out that the 28-210mm zoom on the camera has a
great wide angle FOV. However, that is not true. If the "crop" factor
for that particular lens happens to be "5", the effective FOV for
that camera on the "wide" end is really a full frame equivalent of
140mm.

I think you or the salesman is mixing the actual focal length and the "35mm equivalent focal length". For example, if you look at the lens information on Canon web page for the A650:

http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&fcategoryid=183&modelid=15658

it states: 7.4-44.4mm f/2.8-4.8 (35mm film equivalent: 35-210mm)

If the camera saleman tells you that this is a 35-210mm lens, you don't apply the crop factor a second time to come up with the idea that the "wide" end of this lens is equivalent to 175mm on a 35mm camera. He's already quoting you the converted figure.

On the Canon A cameras, at least, the actual focal lengths are engraved on the lens barrel. If there's any doubt as to which focal length someone is talking about, then that's where to look.

PixelMinded Senior Member • Posts: 1,032
Re: Crop Factors

Sean Nelson wrote:

2clueless wrote:

What I was trying to determine is if a crop factor of, say, 5 on a small
sensor point and shoot should be used to determine the "equivalent"
or "apparent" focal length FOV relative to a 35mm film camera just as
it does on dSLR's.

It can, but in most cases compact camera manufacturers already
publish the "35mm equivalent" focal lengths anyway, and they DON'T
publish the crop factors.

But knowing the crop factor does come in handy if you're looking at
EXIF information for a picture and the 35mm equivalent isn't included.

They publish the sensor size and from that you know the diagonal of the sensor, for example G10 has 1/1.7" sensor, which means the diagonal is 9.5mm. Devide the diagonal of 35mmFF ( 43.3mm) by the diagonal of G10 --> 43.3mm/9.5mm = 4.6x (this the crop factor of the G10).
To verivy:

At the wide end of the zoom, G10 is 6.1mm --> 6.1mm x 4.6 = 28mm equivalent. The tele end is 30.5mm x 4.6 = ~ 140mm equiv.

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Guerito Veteran Member • Posts: 4,874
Re: No such thing...

Sean Nelson wrote:

hbx2004 wrote:

There isn't such thing as "crop factor" for p&s cameras -because
sensor covers full image projected by the lens at it's actual focal length.

So my Canon A650IS camera with it's lens set to 7.4mm gives the same
field of view as a 35mm lens would on a 35mm film camera. Therefore
it's "crop factor" is a little less than 5.

Actually, Bogdan has it right, there's nothing to crop from the fixed lens. You're confusing crop factor with equivalent FOV.

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Sean Nelson
Sean Nelson Forum Pro • Posts: 12,846
Re: No such thing...

Sean Nelson wrote:

So my Canon A650IS camera with it's lens set to 7.4mm gives the same
field of view as a 35mm lens would on a 35mm film camera. Therefore
it's "crop factor" is a little less than 5.

Guerito wrote:

Actually, Bogdan has it right, there's nothing to crop from the fixed
lens. You're confusing crop factor with equivalent FOV.

I refer again to the widely accepted definition of the term "crop factor":
From: http://en.wikipedia.org/wiki/Crop_factor :

"a crop factor is the ratio of the dimensions of a camera's imaging area compared to a reference format; most often, this term is applied to digital cameras, relative to 35 mm film format as a standard."

Since my Canon A650IS's sensor has a diagonal measurement that is about 1/5 the size of a full 35mm film frame, it's crop factor is 5.

I believe you're referring of the idea of cropping the image from a full-frame sensor because you're using a lens designed for APS-C coverage. It's true that in that case the lens doesn't cover the full frame of view, and therefore you're effectively using a smaller sensor (actually the central portion of the full-frame sensor). The term "crop factor" applies here too. But the term is used because you're using a smaller effective sensor size, not because the lens doesn't cover the full sensor.

To illustrate the difference, I can attach medium-format Pentax 6x7 lenses to my APS-C Pentax K100D DSLR. These lenses actually cover about twice the image area of even a full-frame sensor, so if your notion of "crop factor" were true then these lenses would have a crop factor of less than 1 (about 0.5, in fact). But that's not the case, all lenses mounted on my K100D have the same crop factor - 1.5 - that's determined by the ratio of the K100D's sensor to the size of a full 35mm frame. Therefore, if I mount a 100mm 6x7 lens on my K100D, the 1.5 crop factor tells me that I'm getting the same field of view as a 150mm lens would give on 35mm film.

Guerito Veteran Member • Posts: 4,874
Re: No such thing...

Again, you are confusing terms. The term "crop factor" refers to the relationship of the native focal length of the lens being used to that of the sensor/film size. If I were able to remove the lens from say a G9 and replace it with a 100mm lens designed for a larger sensor like the 5D, then you would have a crop factor because only a portion of the image circle is being used (cropped). In other words, the focal length of the lens would still be 100mm, but the image would appear larger because of the focal length magnification (crop factor).

Since the G9 uses a lens designed for its particular sensor size, there is nothing being cropped (magnified), the focal length of the lens is a true 7.4-44.4mm, however one can still compute a 35mm focal length equivalent as being 35-210mm.
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