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Archive 2012 · Crop Factor
  
 
PaulCal
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p.1 #1 · p.1 #1 · Crop Factor


Re-hashing this important term again and reading some threads specifically this one which seems to explain it:
http://www.fredmiranda.com/forum/topic/1162859/0?keyword=crop,factor#11088631

Heres my question/observation:

Does crop factor always relate to sensor size ie. the smaller the sensor the greater the crop factor or magnification.
So if you were to take the 7D with a sensor size of 22.3x14.9 and change that to say 36x24 mm you would remove the 1.6 crop factor?
Or conversely put all those 18mp in an even smaller sensor like 18x12mm to increase the crop factor/ magnifier to sat 1.8 approx.

So the reference then for crop factor is always 36x24 which is full frame or 1.0

Does this make sense FMers?



Dec 15, 2012 at 02:56 PM
dhphoto
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p.1 #2 · p.1 #2 · Crop Factor


PaulCal wrote:
So the reference then for crop factor is always 36x24 which is full frame or 1.0



Yes.

It was the standard for so many years it has become synonymous with 'full frame'

A crop sensor is basically a 'full frame' sensor with the edges cut off, nothing more nothing less. It means long lenses appear longer and wide lenses appear less wide. The lenses themselves don't change at all, just their angle of view is reduced because the top, bottom and sides of the full frame sensor are 'missing' (or cropped).

The reason the apparent depth of field changes is because you have to position the camera in a different place to obtain the same image size - you would need to move back for a portrait for example, which would give you more depth of field because you are further away.

This is a simplification but essentially correct.


Edited on Dec 15, 2012 at 03:23 PM · View previous versions



Dec 15, 2012 at 03:21 PM
jcolwell
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p.1 #3 · p.1 #3 · Crop Factor


There are two important magnifications to consider. One, the magnification of the real life subject to its image size on the sensor, and two, the magnification from the captured image size on the sensor to its viewed size on a monitor or as a print. The "crop factor" effect is related only to the second magnification.

The reference for full frame "small format" is 36mm x 24mm.



Dec 15, 2012 at 03:22 PM
waldr_p
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p.1 #4 · p.1 #4 · Crop Factor


Yes, crop factor does relate to sensor size.

Personally I find it more useful to consider the pixel density of the camea rather than the crop factor.

For example, I have two APS-C cameras (the 40D @ 10 Megapixels and the 7D @ 18 Megapixels). These are both 1.6x crop sensors however the 7D has a higher pixel density than the 40D which means that in focal length limited shooting the 7D lets me put more pixels on the target and gives me a better image. The difference between these cameras is quite significant (the 40D has approx 3M pixels per sq cm of its sensor vs approx 5.4M on the 7D).

Assuming your main concern (regarding crop vs full frame) is about reach I feel that looking at pixel density is far more informative than considering the 1.6x crop factor.

Of course there are other issues at play here, like the pixel level quality of the cameras being compared, which pixel density does not take into account.

Paul




Dec 15, 2012 at 03:27 PM
dhphoto
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p.1 #5 · p.1 #5 · Crop Factor


waldr_p wrote:
Yes, crop factor does relate to sensor size.

Personally I find it more useful to consider the pixel density of the camea rather than the crop factor.

For example, I have two APS-C cameras (the 40D @ 10 Megapixels and the 7D @ 18 Megapixels). These are both 1.6x crop sensors however the 7D has a higher pixel density than the 40D which means that in focal length limited shooting the 7D lets me put more pixels on the target and gives me a better image. The difference between these cameras is quite significant (the 40D has approx 3M pixels
...Show more

I don't agree with this at all.

The physical SIZE of the sensor, or film type or camera type or whatever you want determines the angle of view of the lenses placed on that camera.

The angle of view of a lens on a 40D and a 7D is identical, the only thing that is different is the *size* of the resulting image file, the 7D file is bigger, it contains more pixels, the lenses perform the same function in the same way on both, the 'crop factor' is the same.

It is only when you change the physical dimensions of the recording film or sensor that the lenses produce a different angle of view.



Dec 15, 2012 at 03:38 PM
ggreene
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p.1 #6 · p.1 #6 · Crop Factor


Crop factor affects FOV/DOF. Pixel density is solely responsible for the perceived extra reach you
get (ex. FF D800 (36MP) has more reach then a 1.3x crop 1D4 (~27MP)).



Dec 15, 2012 at 03:57 PM
dhphoto
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p.1 #7 · p.1 #7 · Crop Factor


ggreene wrote:


Crop factor affects FOV

Yes, sensor size affects angle of view

Crop factor affects DOF

No, the lens focal length, focusing distance and aperture determine depth of field, also circle of confusion and viewing distance

Pixel density is solely responsible for the perceived extra reach you
get (ex. FF D800 (36MP) has more reach then a 1.3x crop 1D4 (~27MP)).


That makes no sense at all



Dec 15, 2012 at 04:03 PM
Monito
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p.1 #8 · p.1 #8 · Crop Factor


waldr_p wrote:
Yes, crop factor does relate to sensor size. [...] however the 7D has a higher pixel density than the 40D which means that in focal length limited shooting the 7D lets me put more pixels on the target


dhphoto wrote:
I don't agree with this at all

The physical SIZE of the sensor, or film type or camera type or whatever you want determines the angle of view of the lenses placed on that camera. The angle of view of a lens on a 40D and a 7D is identical, the only thing that is different is the *size* of the resulting image file, the 7D file is bigger, it contains more pixels, the lenses perform the same function in the same way on both, the 'crop factor' is the same. It is only when you change the physical dimensions
...Show more

That exactly what waldr_p was saying. What part of "more pixels on the target" do you not understand? PetKal would say something like "more pixels per duck".


ggreene wrote:
Pixel density is solely responsible for the perceived extra reach you
get (ex. FF D800 (36MP) has more reach then a 1.3x crop 1D4 (~27MP)).


dhphoto wrote:
That makes no sense at all


Actually it does. The key word is "perceived". The idea is that when you enlarge the pixels from both cameras to the same size on the print, the duck is bigger on the print from the D800.

Especially true when you correct ggreene's mistake: the 1D4 has 16 MegaPixels.

Photograph a duck at a certain distance so that a 300 mm lens makes it 10 mm long on the sensor (both sensors, of course, at the same distance and focal length).

The D800 has 7,360 sensels per 36 mm long side of the frame. That makes 4.9 micrometer sensels, so there are 1000 / 4.9 per millimeter = 204 per mm or 2044 per 10 millimeter duck image length.

The 1D4 has 4896 sensels per 28 mm long side. That makes 5.7 micrometer sensels and 175 per mm or 1750 per 10 millimeter duck image length.

Now if you make prints so that the pixels are all 1 mm square to equalize the effect of pixellation, then the D800 print is 7.4 x 4.9 meters or 291 x 193 inches or 24 feet 3 inches x 16 feet 1 inch. The duck image will be 2 meters long or 79 inches long.

The 1D4 print will be 4.9 x 3.3 meters or 193 x 130 inches or 16 feet 1 inch x 10 feet 10 inches. The duck image will be 1.75 meters long or 69 inches long.

That's what more reach means. 79 inches is more than 69 inches. More pixels per duck.



Dec 15, 2012 at 06:05 PM
Monito
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p.1 #9 · p.1 #9 · Crop Factor


ggreene wrote:
Crop factor affects DOF


dhphoto wrote:
No, the lens focal length, focusing distance and aperture determine depth of field, also circle of confusion and viewing distance


Wrong. ggreene is correct. Crop factor does affect Depth of Field. You've never done the experiment. Do it, or look at the many examples on the net. I hope I don't have to post links for you.

To make the proper comparison for this paradigm, you have to have the same subject distance (for same perspective), same aperture, same viewing distance, and same print size. Given the same observer, the viewing distance and print size will give the same circle of confusion, due to the visual acuity of the viewer.

You have to change the focal length to compensate for the crop factor, otherwise the pictures will not depict the same thing (one will have more things in the frame).

When you make the same picture by using the same distance, same aperture, adjusted focal length, same print size, and same viewing distance, then the print from the crop factor camera will have more depth of field. It's a fact.

The reason is that circles on the image sensor do not have the same effect as the circles of confusion on the print. The CoC ("blur disks") on the prints are derived from different sized circles on sensor due to the different degree of enlargement.



Dec 15, 2012 at 06:05 PM
gdanmitchell
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p.1 #10 · p.1 #10 · Crop Factor


This is pretty much correct. You can almost always count on the term "crop factor" being relative to the full-frame format. I have heard MF photographers use the term somewhat differently to refer to their own set of conversion calculations relating both to DSLR full frame and to some of the "mini-MF" formats that are larger than full frame but considerably smaller than 45 x 60. (For example, the 33x44 format...)

So, if you want to keep life simple - and why not!? - your understanding is correct.

By the way, despite the endless theorizing to the contrary, in the real world format does affect DOF in photographs reproduced at some given size, along with a number of other related and semi-related things including how large you might be willing to print and how far you might be willing to stop down in order to increase DOF. A hint: absolute size of "fuzziness" (e.g. - circle of confusion) is essentially irrelevant, but relative size (e.g. - percentage of frame width) is significant.

But that's not keeping life simple, is it? ;-)

Dan

PaulCal wrote:
Re-hashing this important term again and reading some threads specifically this one which seems to explain it:
http://www.fredmiranda.com/forum/topic/1162859/0?keyword=crop,factor#11088631

Heres my question/observation:

Does crop factor always relate to sensor size ie. the smaller the sensor the greater the crop factor or magnification.
So if you were to take the 7D with a sensor size of 22.3x14.9 and change that to say 36x24 mm you would remove the 1.6 crop factor?
Or conversely put all those 18mp in an even smaller sensor like 18x12mm to increase the crop factor/ magnifier to sat 1.8 approx.

So the reference then for crop factor is always 36x24 which
...Show more



Dec 15, 2012 at 06:16 PM
 

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Monito
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p.1 #11 · p.1 #11 · Crop Factor


When I referred to "aperture", I should have instead referred to "f/number" since the later is the needed ratio and the former is a physical size not generally accessible.



Dec 15, 2012 at 06:28 PM
gdanmitchell
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p.1 #12 · p.1 #12 · Crop Factor


Monito wrote:
When I referred to "aperture", I should have instead referred to "f/number" since the later is the needed ratio and the former is a physical size not generally accessible.


However, and in your defense, when people use the term "aperture" in photography, it is taken to be synonymous with f-stop.

Dan



Dec 15, 2012 at 06:38 PM
ggreene
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p.1 #13 · p.1 #13 · Crop Factor


Monito wrote:
Especially true when you correct ggreene's mistake: the 1D4 has 16 MegaPixels.


27MP at FF and since I was comparing it to a FF camera I thought that would be the clearer metric.



Dec 15, 2012 at 07:32 PM
CW100
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p.1 #14 · p.1 #14 · Crop Factor


pixel density, pixels on target, 'perceived' reach, DOF, angle of view, etc.
the OP is now even more confused



Dec 15, 2012 at 09:20 PM
speedmaster20d
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p.1 #15 · p.1 #15 · Crop Factor


yes the reference for crop factor is 35mm film/ FF sensor.

As pointed out it has nothing to do with FL/reach it just changes FOV.

What is referred to as telephoto "reach" is solely a function of effective pixel size and not the sensor size.



Dec 15, 2012 at 09:23 PM
rprouty
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p.1 #16 · p.1 #16 · Crop Factor


Given the same size sensor does more pixels (say 10MP vs. 18MP) "always" mean better IQ? I'm old and don't understand all this technical stuff.

Rod



Dec 16, 2012 at 03:20 AM
Paul Mo
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p.1 #17 · p.1 #17 · Crop Factor


rprouty wrote:
Given the same size sensor does more pixels (say 10MP vs. 18MP) "always" mean better IQ?


No, not always, but sometimes it can. There are many variables. Generally, the higher the MP the higher the resolving power. That means that finer details should be rendered more accurately by a higher MP sensor; think eyes and hair, and other dense textures.

There are so many factors when it comes to any given image.

A lower MP sensor can have beautiful characteristics. IMO see the 5D classic vs the D800. I'd rather view 5D images all day long because they are more film-like.



Dec 16, 2012 at 03:48 AM
gdanmitchell
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p.1 #18 · p.1 #18 · Crop Factor


CW100 wrote:
pixel density, pixels on target, 'perceived' reach, DOF, angle of view, etc.
the OP is now even more confused


It is amazing how the forum can turn a simple question worthy of a simple answer into a giant, multi-page post-fest. (When I wrote my first response, I basically wanted to simply reply, "Yes, you have the right idea." Because the OP did. But I knew that if I didn't at least give a nod to the other peripheral stuff that I knew would soon raise its head, the other people reading my reply would look for things to jump on.)

Phunny things, photography phorums.

Dan



Dec 16, 2012 at 03:50 AM
DavidP
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p.1 #19 · p.1 #19 · Crop Factor


dhphoto wrote:
No, the lens focal length, focusing distance and aperture determine depth of field, also circle of confusion and viewing distance


Well, technically "yes", but practically "no.

Two lenses of identical focal length at the same subject distance and f-stop will have the same DOF (assuming same assumptions for circle of confusion and the viewing distance), REGARDLESS of the "crop factor" (sensor size) of the camera.

However, in a practical sense, this is almost meaningless. Why? Because the two pictures won't show the same angle-of-view. The larger "crop factor" (smaller sensor) camera will result in a cropped image compared to what you get out of the other camera.

The assumption is that the photographer wants the same "framing" of the two images, so he'll move further away with the camera that has the larger "crop factor" (smaller sensor) to compensate. And THIS is what changes the DOF.



Dec 16, 2012 at 04:51 AM
AJSJones
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p.1 #20 · p.1 #20 · Crop Factor


DavidP wrote:
However, in a practical sense, this is almost meaningless. Why? Because the two pictures won't show the same angle-of-view. The larger "crop factor" (smaller sensor) camera will result in a cropped image compared to what you get out of the other camera.

The assumption is that the photographer wants the same "framing" of the two images, so he'll move further away with the camera that has the larger "crop factor" (smaller sensor) to compensate. And THIS is what changes the DOF.


Or he wants the same image (including perspective) - so he can't move any further/closer and must therefore use a different focal length for the different sensor - that also changes the DoF (in the standardized print/view situation)



Dec 16, 2012 at 06:33 AM
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