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Magnification by cropping
2 photos taken with 2 different cameras but of the same subject, at the same distance, same framing, with the same lens, say 100mm..

One camera is 10 megapixels, the other 8 megapixels..

If I crop (in Photoshop), the 10 megapixels photo down to 8 megapixels and print or view both on screen, the image of the cropped photo, will now appear to be magnified, compared to the uncropped one. To what extent? 1.25? (10 divided by 8)? If not what is the formula?..

Comments (15)

Jml9905 wrote:.

2 photos taken with 2 different cameras but of the same subject, atthe same distance, same framing, with the same lens, say 100mm..

One camera is 10 megapixels, the other 8 megapixels..

If I crop (in Photoshop), the 10 megapixels photo down to 8megapixels and print or view both on screen, the image of the croppedphoto, will now appear to be magnified, compared to the uncroppedone. To what extent? 1.25? (10 divided by 8)? If not what is theformula?.

You're asking based on the reduction in megapixels (a measurement of area)..

So 1 / sqrt(8/10) = 1.12x crop factor. I used the words "crop factor" in the same way that smaller sensors work, so the 8mp is like a 1.12 crop factor camera compared to a 10mp "full frame"..

[ e d @ h a l l e yc c ] http://www.halley.cc/pix/..

Comment #1

2 photos taken with 2 different cameras but of the same subject, atthe same distance, same framing, with the same lens, say 100mm..

One camera is 10 megapixels, the other 8 megapixels..

If I crop (in Photoshop), the 10 megapixels photo down to 8megapixels and print or view both on screen, the image of the croppedphoto, will now appear to be magnified, compared to the uncroppedone. To what extent? 1.25? (10 divided by 8)? If not what is theformula?.

It depends whether you mean length or area. The picture that has been cropped will cover four fifths of the area of the uncropped one, i.e. a magnification of 5/4 or 1.25 in terms of 'area of view' covered. If you do the calculation in terms of length you want the square root of that, i.e. the cropped image includes along it's edge a distance of 0.89 x that of the uncropped one, corresponding to a magnification of a length scale of 1.12..

Thus, a one-metre square in the cropped image would appear to be 12% longer along an edge, which corresponds to 25% bigger in it's area, compared to the uncropped image. It's a matter of taste how you express it..

Best wishesMike..

Comment #2

Jml9905 wrote:.

2 photos taken with 2 different cameras but of the same subject, atthe same distance, same framing, with the same lens, say 100mm..

One camera is 10 megapixels, the other 8 megapixels..

To be clear on my previous reply, it sounds more like you're comparing two cameras at the same effective focal length; what is in view and what is out of view is based on the SIZE of the sensor (given identical focal length), not how many pixels..

If you're cropping a 10mp image down to 8mp, it's AS IF the sensor were smaller but the sensor density were the same..

[ e d @ h a l l e yc c ] http://www.halley.cc/pix/..

Comment #3

This a strange question and I'm not quite sure why you are asking it, but here goes..

If you crop the 10Mg pic to 8Mg then you will losing 20% of the picture. The parts lost of course will be the edges if you crop symetrically..

Upi now are left with two images that are exactly the same size, on scree, if showing at the same resolution. If you print them (once again at the same dpi) yo will simmilarly have two picture the same physical size on the paper but the cropped (originally 10Mg) image will not show as much a sthe 8Mg image and hence everything in it'simage will be 20% larger.Hope that helps.Jules.

Jml9905 wrote:.

2 photos taken with 2 different cameras but of the same subject, atthe same distance, same framing, with the same lens, say 100mm..

One camera is 10 megapixels, the other 8 megapixels..

If I crop (in Photoshop), the 10 megapixels photo down to 8megapixels and print or view both on screen, the image of the croppedphoto, will now appear to be magnified, compared to the uncroppedone. To what extent? 1.25? (10 divided by 8)? If not what is theformula?.

Black moles do not destroy information...

Comment #4

I'm mostly interested in bird photography and of course when taking pictures of very distant subjects, one way "to bring them closer" is to crop the picture in Photoshop as much as is acceptable. I know that when you are the maximum focal length of your super zoom and that the subject is still too small your only choice is cropping in Photoshop (or buying a bigger len$). Of course it's always a compromise between getting a big enough picture of the subject versus image quality..

But lately I've been wondering what formula would let me know what apparent magnification is gained from cropping..

Thank you all.....

JulesJ wrote:.

This a strange question and I'm not quite sure why you are asking it,but here goes.If you crop the 10Mg pic to 8Mg then you will losing 20% of thepicture. The parts lost of course will be the edges if you cropsymetrically.Upi now are left with two images that are exactly the same size, onscree, if showing at the same resolution. If you print them (onceagain at the same dpi) yo will simmilarly have two picture the samephysical size on the paper but the cropped (originally 10Mg) imagewill not show as much a sthe 8Mg image and hence everything init'simage will be 20% larger.Hope that helps.Jules.

Jml9905 wrote:.

2 photos taken with 2 different cameras but of the same subject, atthe same distance, same framing, with the same lens, say 100mm..

One camera is 10 megapixels, the other 8 megapixels..

If I crop (in Photoshop), the 10 megapixels photo down to 8megapixels and print or view both on screen, the image of the croppedphoto, will now appear to be magnified, compared to the uncroppedone. To what extent? 1.25? (10 divided by 8)? If not what is theformula?.

Black moles do not destroy information...

Comment #5

Jml9905 wrote:.

But lately I've been wondering what formula would let me know whatapparent magnification is gained from cropping..

Yup, same thing, SORTA. The 1.12x number is also how you'd compute the effective focal length difference. So your 100mm lens is now acting like a 112mm lens on the same size sensor, albeit with "larger" pixels..

How that equates to "magnification power" like telescopes is a bit different, but I think you get the gist..

[ e d @ h a l l e yc c ] http://www.halley.cc/pix/..

Comment #6

Yes, thanks Ed,.

From your formula I was able to calculate that to get the same "magnification" as a 1.4x teleconverter, I would need to crop down down from 10mp to 5mp..

1 / sqrt(5/10) = 1.414.

Ed Halley wrote:.

Jml9905 wrote:.

But lately I've been wondering what formula would let me know whatapparent magnification is gained from cropping..

Yup, same thing, SORTA. The 1.12x number is also how you'd computethe effective focal length difference. So your 100mm lens is nowacting like a 112mm lens on the same size sensor, albeit with"larger" pixels..

How that equates to "magnification power" like telescopes is a bitdifferent, but I think you get the gist..

[ e d @ h a l l e yc c ] http://www.halley.cc/pix/..

Comment #7

There are a whole bunch of "yeah but"s in your project, like what's the quality of the lens when you crop down. Among other things, remember that all pixels are not created equal. And pixel quality can vary depending on exposure conditions and ISO settings..

One experiment that's quick and easy is to have a 5x7 print made from, more or less, the whole frame, and an 8 x 12 print made from the whiold frame, and then chop off the edges of the 8x10 intil you get a 5x7 area..

And, as they say in public school; compare and contrast..

BAK..

Comment #8

Two pieces to this puzzle. First, you need 4 times the pixels to allow doubling the equivalent focal length. I.e. your 10 megapixel camera becomes 2.5 when you crop to the center half horizontally and vertically. Second, cameras with more pixels in the same size sensor usually have more noise (but by no means always when it comes to DSLR's)..

Try a teleconverter or a telescope...

Comment #9

Jml9905 wrote:.

That when you are the maximum focal length of your super zoom andthat the subject is still too small your only choice is cropping inPhotoshop.

The cold hard fact is that if you're at your maximum focal length and the subject is too small, it's pretty much a waste of time. Cropping is not a substitute for filling the frame with your subject, especially when the subject is far away as it usually is with birding. Even at 500mm for a small bird (sparrow size) you pretty much have to be less than 20ft away (10ft is more ideal). Also, using a prime lens will be a little more forgiving than a super zoom..

Don't think of cropping as magnification, think of it as dillution. ie, an 8x12 printed with 10MP will have considerably more information than a 10MP image cropped to 5MP and enlarged to 8x12. Thus you're spreading less information over a larger area...

Comment #10

I've long felt that "digital zoom" is the work of the devil - it's effectively doing the same thing "Magnifying by cropping". The difference is that digital zoom (depending on the camera) stretches the cropped area afterwards... evil!.

That said, it really depends on what you want to do with the final image. If you're starting with a 10Mp image and it's only going to end up as a 1Mp on a web page there's nothing wrong with cropping back to the 1Mp as you'd be losing all those pixels in scaling anyway. If doing large prints it's different..

Of course, the quality of the original image will determine what level of detail you can pull out of it. If you're going down to full resolution it quite possible it'll look godawful.Fuji S6500fd newbie..

Comment #11

Doesn't increase the size of an image. Cropping simply discards portions of the image other than what is selected leaving you with only the portion selected..

You get enlargement when you interpolate the crop back to the original image size thus adding pixels so the increase in size is done via software..

This is different than a "sensor crop". A "crop-factor" or reduced field of view sensor actually vests the full resolution of sampling sites on an area smaller than a 35mm frame in the relevant sense. So the image captured by a crop-factor sensor with a given lens from a specific distance at a specific focal length has more pixels on the cropped subject area than a capture made from the same distance with the same lens with a full 35mm frame camera of identical optical resolution and pixel count..

This then gives the "crop" from the smaller sensor greater enlargement potential because it consists of the same frame of subject reference but containing more pixels within that subject area..

Confusion sometimes arises because with film, people sometimes crop to get a close-up of a particular part of a capture. But the close-up can't be enlarged any more than it could have had that portion remained with the entire negative enlarged rather than selecting only that "cropped" portion to enlarge..

So the bottom line is that cropping isn't a magical way of enlargement or "magnification" - it's simply a way of selecting a part of the capture to frame. The only time there is a true magnification potential is when a sensor smaller that some reference size captures at full resolution less than that full reference size image and then only because sampling is done at full resolution on less than the full reference size (such as 35mm)..

Best regards,.

Lin.

Jml9905 wrote:.

2 photos taken with 2 different cameras but of the same subject, atthe same distance, same framing, with the same lens, say 100mm..

One camera is 10 megapixels, the other 8 megapixels..

If I crop (in Photoshop), the 10 megapixels photo down to 8megapixels and print or view both on screen, the image of the croppedphoto, will now appear to be magnified, compared to the uncroppedone. To what extent? 1.25? (10 divided by 8)? If not what is theformula?..

Comment #12

I hope you are not a colege lecturer Lin. I can hear the students yawning! Lol.Jules.

Lin Evans wrote:.

Doesn't increase the size of an image. Cropping simply discardsportions of the image other than what is selected leaving you withonly the portion selected..

You get enlargement when you interpolate the crop back to theoriginal image size thus adding pixels so the increase in size isdone via software..

This is different than a "sensor crop". A "crop-factor" or reducedfield of view sensor actually vests the full resolution of samplingsites on an area smaller than a 35mm frame in the relevant sense. Sothe image captured by a crop-factor sensor with a given lens from aspecific distance at a specific focal length has more pixels on thecropped subject area than a capture made from the same distance withthe same lens with a full 35mm frame camera of identical opticalresolution and pixel count..

This then gives the "crop" from the smaller sensor greaterenlargement potential because it consists of the same frame ofsubject reference but containing more pixels within that subject area..

Confusion sometimes arises because with film, people sometimes cropto get a close-up of a particular part of a capture. But the close-upcan't be enlarged any more than it could have had that portionremained with the entire negative enlarged rather than selectingonly that "cropped" portion to enlarge..

So the bottom line is that cropping isn't a magical way ofenlargement or "magnification" - it's simply a way of selecting apart of the capture to frame. The only time there is a truemagnification potential is when a sensor smaller that some referencesize captures at full resolution less than that full reference sizeimage and then only because sampling is done at full resolution onless than the full reference size (such as 35mm)..

Best regards,.

Lin.

Jml9905 wrote:.

2 photos taken with 2 different cameras but of the same subject, atthe same distance, same framing, with the same lens, say 100mm..

One camera is 10 megapixels, the other 8 megapixels..

If I crop (in Photoshop), the 10 megapixels photo down to 8megapixels and print or view both on screen, the image of the croppedphoto, will now appear to be magnified, compared to the uncroppedone. To what extent? 1.25? (10 divided by 8)? If not what is theformula?.

Black moles do not destroy information...

Comment #13

Do you mean "college" lecturer? Maybe staying awake in class would help?.

Lin.

Lin.

JulesJ wrote:.

I hope you are not a colege lecturer Lin. I can hear the studentsyawning! Lol.Jules..

Comment #14

Jml9905 wrote:.

I'm mostly interested in bird photography and of course when takingpictures of very distant subjects, one way "to bring them closer" isto crop the picture in Photoshop as much as is acceptable..

The thing is ... a higher res camera *only* gives you extra "room to crop" if your lens is good enough to give you extra detail. Consider:.

You shoot a bird with a 400mm lens using a 6MP camera and a 10MP camera. Looking at both shots on your computer screen at a particular size (not 100%) or printing them both to the same size, the bird doesn't fill the frame enough. So you crop. Crop them both. Let's say you crop them both down to half size (area) so you've got a 5MP and a 3MP image. OK ...

If printing to 5x7, maybe 7x10, you might not see much detail between them, depending on how you print. But let's say the 3MP image doesn't have enough detail. The image from the higher res camera has more pixels ... but that doesn't mean it has more detail. If you shot with a 70-300/4.5-5.6 zoom and a 1.4X teleconverter wide open ...

If you shot with a 300/2.8 and a matched 1.4X APO, then it's probably noticably sharper..

And as the MP increases, so likelihood of capturing that much more detail decreases. With a decent lens, you might see more detail on 10MP than 6MP; with a really good lens, you'll see a bit more on 12 than 10. But if you think that going from 10MP to 20MP (using same lenses and same sensor size) gives you a corresponding increase in enlargability, you're probably not going to get it..

In summary, 10MP versus 8MP lets you crop away 20% of the area, or 11% in each dimension and end up with the same file size, but the result isn't necessarily as detailed as if you had the longer lens you're trying to simulate..

- DennisGallery at http://kingofthebeasts.smugmug.com..

Comment #15

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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