"crop-factor" is still confusing me.

Started Dec 11, 2010 | Discussions
cg_25
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"crop-factor" is still confusing me.
Dec 11, 2010

I understand the premise behind full-frame (35mm) and cropped sensor (24mm) but I'm looking for a more technical description of the difference (perhaps a diagram).

24mm sensors are "cropped" but what happens optically when we use, say a 35mm lens, on a full-frame body such as the Canon 1D, versus the same 35mm lens on the Canon 60D?

My understanding is that there is a physical shift in the image, thus what is 35mm on the full-frame sensor is 56mm on the cropped sensor. Is this correct?

If so, as I understand it the difference between 35mm and 65mm is not only that we're capturing a smaller, "zoomed-in area", but that there is a "foreshortening affect" which is percieved as foreground and background objects being brought closer together. Is this true for cropped-sensor cameras? Is there a percieved foreshortening affect, or is it simply a "crop" of the full-frame area.

Anyways, the point I'm getting at is that the description I find on the internet is contradicting. We call it a "crop" factor yet describe what occurs in terms of focal length. A shift in focal length and and a physical crop do not achieve the same result. Am I making sense, can someone straighten me out?

markkuk
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Re: "crop-factor" is still confusing me.
In reply to cg_25, Dec 11, 2010

cg_25 wrote:

My understanding is that there is a physical shift in the image, thus what is 35mm on the full-frame sensor is 56mm on the cropped sensor. Is this correct?

No. Focal length is a physical property of the lens that doesn't depend on the body.

Is there a percieved foreshortening affect, or is it simply a "crop" of the full-frame area.

An APS-C body simply crops the image area of the lens.

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Guidenet
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Re: "crop-factor" is still confusing me.
In reply to cg_25, Dec 11, 2010

Nothing happens optically between using different sensors. A crop sensor camera doesn't magically change the focal length or give more magnification. The focal length stays the same as does the magnification of the lens. The crop sensor just captures a smaller portion of the same image.

Let me explain this simply. Take a 100mm lens not attached to a camera. Hold it up with a piece of carboard behind it. The image should form on that cardboard. Now, if you put a full frame sensor or a crop sensor on that cardboard, did anything change at all? No, see you just took a smaller crop of the same image. That's why it's called "crop factor" and the sensor is called a "crop" sensor. It's cropping the same exact image. No additional reach or magnification.

If you took a APS-C crop sensor camera with a 200mm lens on it and framed a bird on a fence pole at 30 meters, then did the exact same thing with a full frame camera, the only difference would be additional space around the bird with the full frame. The bird would project the same size bird on each sensor.

When people say that a 200mm lens is equivalent to a 300mm lens on a crop camera, they are giving a short cut answer or don't understand it. The only thing equivalent part is the angle of view. A 300mm lens on a full frame camera would have the same angle of view of a 200mm lens on a crop camera. That's because the crop camera crops a smaller view. This is another reason the pentaprism and viewfinder on a crop camera is smaller as well.
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cg_25
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Re: "crop-factor" is still confusing me.
In reply to markkuk, Dec 11, 2010

So would I be correct in saying:

The difference between a full-frame and cropped sensor (such as the Canon 60D) is that a cropped sensor captures (1/1.6) times the image area of the full-frame, all else being equal.

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cg_25
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Re: "crop-factor" is still confusing me.
In reply to Guidenet, Dec 11, 2010

Perfect explanation.

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Chas B
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Re: "crop-factor" is still confusing me.
In reply to cg_25, Dec 11, 2010

cg_25 wrote:

I understand the premise behind full-frame (35mm) and cropped sensor (24mm) but I'm looking for a more technical description of the difference (perhaps a diagram).

24mm sensors are "cropped" but what happens optically when we use, say a 35mm lens, on a full-frame body such as the Canon 1D, versus the same 35mm lens on the Canon 60D?

No, "crop" applies to the fact that a smaller sensor captures a smaller area of the image circle than a larger sensor.

My understanding is that there is a physical shift in the image, thus what is 35mm on the full-frame sensor is 56mm on the cropped sensor. Is this correct?

No, the image that the lens projects is not changed, merely the size of the sensor determines how much of that circle is captured.

If so, as I understand it the difference between 35mm and 65mm is not only that we're capturing a smaller, "zoomed-in area", but that there is a "foreshortening affect" which is percieved as foreground and background objects being brought closer together. Is this true for cropped-sensor cameras? Is there a percieved foreshortening affect, or is it simply a "crop" of the full-frame area.

Again, no. Perspective is determined by the camera to subject distance. If you stand in one spot and shoot two pictures with the same lens on two cameras, one with a smaller sensor, the camera with the smaller sensor will simply have part of the image captured by the larger sensor - hence, a crop. The perspective will be identical.

Anyways, the point I'm getting at is that the description I find on the internet is contradicting. We call it a "crop" factor yet describe what occurs in terms of focal length. A shift in focal length and and a physical crop do not achieve the same result. Am I making sense, can someone straighten me out?

Nothing happens to focal length; what changes is the field of view.

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Leonard Migliore
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You would not be correct
In reply to cg_25, Dec 11, 2010

cg_25 wrote:

So would I be correct in saying:

The difference between a full-frame and cropped sensor (such as the Canon 60D) is that a cropped sensor captures (1/1.6) times the image area of the full-frame, all else being equal.

A Canon crop sensor is 1/1.6 the length and 1/1.6 the height of a full-frame sensor. Its area, therefore, is 1/2.56 (39%) that of a full-frame sensor. But that's not relevant to anything. You generally don't take area into account when you shoot pictures.

All you need to know is that crop sensor is smaller than a full-frame sensor. So when you use the same lens on a crop sensor as on a full-frame sensor, you capture a smaller area. Don't try using numbers.

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joey_B
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Re: "crop-factor" is still confusing me.
In reply to cg_25, Dec 11, 2010
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Robsphoto
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Re: "crop-factor" is still confusing me.
In reply to cg_25, Dec 12, 2010

cg_25 wrote:

I understand the premise behind full-frame (35mm) and cropped sensor (24mm) but I'm looking for a more technical description of the difference (perhaps a diagram).

24mm sensors are "cropped" but what happens optically when we use, say a 35mm lens, on a full-frame body such as the Canon 1D, versus the same 35mm lens on the Canon 60D?

My understanding is that there is a physical shift in the image, thus what is 35mm on the full-frame sensor is 56mm on the cropped sensor. Is this correct?

If so, as I understand it the difference between 35mm and 65mm is not only that we're capturing a smaller, "zoomed-in area", but that there is a "foreshortening affect" which is percieved as foreground and background objects being brought closer together. Is this true for cropped-sensor cameras? Is there a percieved foreshortening affect, or is it simply a "crop" of the full-frame area.

Anyways, the point I'm getting at is that the description I find on the internet is contradicting. We call it a "crop" factor yet describe what occurs in terms of focal length. A shift in focal length and and a physical crop do not achieve the same result. Am I making sense, can someone straighten me out?

You may find this article to be of help, it has quite a few pictures that illustrate the main principles:

http://www.robsphotography.co.nz/crop-factor-advantage.html

Regards
Rob

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chuxter
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Re: "crop-factor" is still confusing me.
In reply to cg_25, Dec 12, 2010

Just for your edification, all this angst is a result of camera manufacturers using the "effective focal length" as a substitute for "angular field of view"...

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Doug Haag
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Re: "crop-factor" is still confusing me.
In reply to chuxter, Dec 12, 2010

I always believed that what was occurring was precisely as described by most of the respondents to this question. But that can't be true, because when people pose Depth of Field questions about lenses affixed to full frame and cropped sensor cameras they are referred to calculators that say there is a DOF difference.

It always seemed to me that if you put a full frame lens of a particular focal length (and set to a specific f-stop) on each of a full frame and a cropped sensor camera and take shots from identical camera positions, a certain central portion of the full frame image would line up (or match) perfectly with the entirety of the cropped sensor photo. If the only differences were the side-to-side and top-to-bottom areas of the scene that is captured by each sensor, then the depth of field (i.e. foreground to background sharpness) of the common (i.e. matching) areas of the two images should be the same.

But apparently this is not the case, because all the depth of field calculators indicate that the DOF will be different depending on which sensor captures the image. As such, the "foreshortening" mentioned by the OP should also be affected.

It never seemed logical to me that a cropped portion of an image that is transmitted by a particular lens should have different front-to-back characteristics than the front-to-back characteristics of the larger, uncropped image produced by the same lens. But it must be so, unless all those DOF calculators are just WRONG.

And, by the way, it can't be that the DOF is influenced by the fact that the number of pixels in the matching central portion of the full frame image is significantly less than those in the entire image. That's because when applied to sensors of identical size (e.g. APS-C only) the DOF calculators do not show any DOF differences between, say, 6MPX and 12MPX cameras. DOF is seemingly not influenced by pixel count.

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joey_B
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Re: "crop-factor" is still confusing me.
In reply to Doug Haag, Dec 12, 2010

Read up on what 'depth of field' actually is. And what sensor size and sensel pitch to to sharpness of an image.

http://www.cambridgeincolour.com

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Chas B
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The DOF confusion ...
In reply to Doug Haag, Dec 12, 2010

The DOF confusion is largely from posing the wrong scenario.

If you take a picture from the same spot using the same lens on a DX and on a FX camera, the DX picture will be identical to a crop of the FX picture.

It's when you take the two pictures so that the framing is identical - and therefore the distance is different - that you will get differences in DOF. IT has to do with camera/subject distance, not sensor size, not pixel count, not pixel density.

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Johan Borg
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Re: "crop-factor" is still confusing me.
In reply to Guidenet, Dec 12, 2010

Guidenet wrote:

When people say that a 200mm lens is equivalent to a 300mm lens on a crop camera, they are giving a short cut answer or don't understand it. The only thing equivalent part is the angle of view.

Then again, how often do you choose a focal length for another reason than angle of view?

The entire concept of "crop" only makes sense when using a lens designed for 35mm on an APS body. I suspect most lenses used with an APS camera these days are made for APS in the first place, which makes them no more crop systems than FF systems are crop of MF.

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sherwoodpete
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Standard print size and viewing conditions
In reply to Doug Haag, Dec 12, 2010

DOF is calculated using basic assumptions about using a standard print size viewed from a standard distance and so on.

Because of the standard print size, the crop-sensor image must undergo additional enlargement compared with the full-frame image, when the print is made. It is this additional enlargement which leads directly to the difference in Depth of Field.

Regards,
Peter

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Chas B
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Re: Standard print size and viewing conditions
In reply to sherwoodpete, Dec 12, 2010

sherwoodpete wrote:

DOF is calculated using basic assumptions about using a standard print size viewed from a standard distance and so on.

Because of the standard print size, the crop-sensor image must undergo additional enlargement compared with the full-frame image, when the print is made. It is this additional enlargement which leads directly to the difference in Depth of Field.

No, if I capture the same field of view with a 12Mp DX D300 and a 12Mp FX D700, there is no difference in enlargement for a print. The only difference was the distance between the focal plane and the focus point.

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Leonard Migliore
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Please look at what you said
In reply to Chas B, Dec 12, 2010

Chas B wrote:

sherwoodpete wrote:

DOF is calculated using basic assumptions about using a standard print size viewed from a standard distance and so on.

Because of the standard print size, the crop-sensor image must undergo additional enlargement compared with the full-frame image, when the print is made. It is this additional enlargement which leads directly to the difference in Depth of Field.

No, if I capture the same field of view with a 12Mp DX D300 and a 12Mp FX D700, there is no difference in enlargement for a print. The only difference was the distance between the focal plane and the focus point.

"Enlargement" is the ratio between the original image size (used to be the negative size) and the size of the final print. Let's make 8" X 12" prints from DX (23.6mm X 15.8mm) and FX (36mm X 24mm). Since 12" is 305mm, we have enlarged the DX image 13X and the FX image 8.5X to get the same print dimensions. So there is indeed a difference in enlargement between DX an FX for the same print size.

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sherwoodpete
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Re: Standard print size and viewing conditions
In reply to Chas B, Dec 13, 2010

Chas B wrote:

sherwoodpete wrote:

DOF is calculated using basic assumptions about using a standard print size viewed from a standard distance and so on.

Because of the standard print size, the crop-sensor image must undergo additional enlargement compared with the full-frame image, when the print is made. It is this additional enlargement which leads directly to the difference in Depth of Field.

No, if I capture the same field of view with a 12Mp DX D300 and a 12Mp FX D700, there is no difference in enlargement for a print. The only difference was the distance between the focal plane and the focus point.

I was responding to the post by Doug Haag where it was said:

It always seemed to me that if you put a full frame lens of a particular focal length (and set to a specific f-stop) on each of a full frame and a cropped sensor camera and take shots from identical camera positions

Note in particular:
1. a full frame lens of a particular focal length
2. identical camera positions

Under these conditions the idea of capturing the same field of view does not apply.

Regards,
Peter

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Chas B
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I am clear on what I said...
In reply to Leonard Migliore, Dec 13, 2010

"Enlargement" is the ratio between the original image size (used to be the negative size) and the size of the final print. Let's make 8" X 12" prints from DX (23.6mm X 15.8mm) and FX (36mm X 24mm). Since 12" is 305mm, we have enlarged the DX image 13X and the FX image 8.5X to get the same print dimensions. So there is indeed a difference in enlargement between DX an FX for the same print size.

That is true for film, and would be true for digital if the number of pixels differed in proportion to the sensor size. My example posits the same number of pixels in the two images, hence no difference in enlargement. The information density is the same.

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Leonard Migliore
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Oh, I see what you're getting at
In reply to Chas B, Dec 13, 2010

Chas B wrote:

"Enlargement" is the ratio between the original image size (used to be the negative size) and the size of the final print. Let's make 8" X 12" prints from DX (23.6mm X 15.8mm) and FX (36mm X 24mm). Since 12" is 305mm, we have enlarged the DX image 13X and the FX image 8.5X to get the same print dimensions. So there is indeed a difference in enlargement between DX an FX for the same print size.

That is true for film, and would be true for digital if the number of pixels differed in proportion to the sensor size. My example posits the same number of pixels in the two images, hence no difference in enlargement. The information density is the same.

I agree the PPI would be the same in both cases. The subject of this sub-thread, however, is depth of field. Clearly, the FX pixels are bigger than the DX pixels (8.5 microns for a D700, 5.5 microns for a D300). So one has to be further out of focus to blur the same number of pixels in FX as you would in DX. The DOFmaster calculator uses a 30 micron circle of confusion for FX (about 3.5 pixels and a 20 micron circle of confusion for DX (about 3.6 pixels) in recognition of this condition.

If you had an FX camera with 5.5 micron pixels (that's 28 MP and I want one), the DOF calculation would still use a 30 micron COC because it's based on blur at a given final image size. If you used the DX DOF calculation, you wouldn't be able to see the out-of-focus region until you stuck your head nearer the picture because it's not enlarged as much.

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