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Cropping Images to Gain Extra Reach: Pros and Cons

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Post Options Post Options   Quote robsphotography Quote  Post ReplyReply Direct Link To This Post Topic: Cropping Images to Gain Extra Reach: Pros and Cons
    Posted: 20 January 2010 at 05:49
I have recently added a new page on my hobby web site titled:

Advantages and Disadvantages of Cropping Images to Gain Extra Reach

I am interested to receive feedback about whether you agree with the mathematical formulas that I have used. In addition, please give your opinions on the advantages and disadvantages of cropping images to gain extra reach.

There have been several threads on various forums about whether it's best to purchase, for example, the Sony 70-300G lens or the Sony 70-400G lens. To help with such an evaluation, I have included on the above page three comparative photographs, one taken with a 300mm lens, a cropped image with a field of view of 400mm, and another crop with a field of view of 500mm.

Regards
Rob

Edited by robsphotography - 20 January 2010 at 11:00
http://www.robsphotography.co.nz/Sony-A900.html
(Examples of the outstanding resolution of images from the Sony A900)
 



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Post Options Post Options   Quote Hobgoblin Quote  Post ReplyReply Direct Link To This Post Posted: 20 January 2010 at 10:07
Rob

I'm not going to comment on the maths but I believe that you missed mentioning one of the advantages of cropping that can be obtained.

If you go out shooting with the express intention of cropping in mind you can likely obtain a better image by forgetting some of the 'rules' of composition.
On a full frame camera just about every lens will lose definition at the edges so put the main subject as close to the centre as possible and you get the benefit of using the sharpest part of the image.

When you crop you can then put the perfectly focussed subject where you like in the final image.

Edited by Hobgoblin - 20 January 2010 at 10:07
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Post Options Post Options   Quote robsphotography Quote  Post ReplyReply Direct Link To This Post Posted: 20 January 2010 at 10:51
Originally posted by Hobgoblin Hobgoblin wrote:

Rob

I'm not going to comment on the maths but I believe that you missed mentioning one of the advantages of cropping that can be obtained.

If you go out shooting with the express intention of cropping in mind you can likely obtain a better image by forgetting some of the 'rules' of composition.
On a full frame camera just about every lens will lose definition at the edges so put the main subject as close to the centre as possible and you get the benefit of using the sharpest part of the image.

When you crop you can then put the perfectly focussed subject where you like in the final image.


Thanks very much for this contribution, if you don't mind, I have now added this advantage to my web page here:

http://www.robsphotography.co.nz/focal-length.html

I have noted that this advantage was contributed by "Hobgoblin" from this forum.

Best Wishes
Rob
http://www.robsphotography.co.nz/Sony-A900.html
(Examples of the outstanding resolution of images from the Sony A900)
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Post Options Post Options   Quote CKsam2 Quote  Post ReplyReply Direct Link To This Post Posted: 20 January 2010 at 15:34
Another factor to bear in mind when cropping is the increase in noise this can bring.
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Post Options Post Options   Quote robsphotography Quote  Post ReplyReply Direct Link To This Post Posted: 20 January 2010 at 21:31
Originally posted by CKsam2 CKsam2 wrote:

Another factor to bear in mind when cropping is the increase in noise this can bring.


Yes, good thought. I guess it is tempting to print a cropped image at the same size as you would print the full-sized image. But, if you do this, then any noise / imperfections in the image will be magnified. However, if you print the cropped image at the same pixels per inch as the original image then, in comparison with the original image, the print size will be correspondingly reduced, as shown in the article here:

Advantages and disadvantages of cropping images to gain extra reach

Therefore, if you reduce the print size of a cropped image in line with the calculations in the above article, there shouldn’t be any noticeable increase in noise and other imperfections.

Regards
Rob
http://www.robsphotography.co.nz/Sony-A900.html
(Examples of the outstanding resolution of images from the Sony A900)
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Post Options Post Options   Quote cputeq Quote  Post ReplyReply Direct Link To This Post Posted: 23 January 2010 at 13:33
Here's one maybe you didn't think of - Cropping will allow you to get a greater Depth of field than if you had made the shot "native"

For instance, we take a picture of an animal with the Sony A850/900 and a 300mm lens, at f/8, from 50 feet away. We have a total DOF of 3.99 feet (http://www.dofmaster.com/dofjs.html)

Now, when we crop to a 400mm view, we still have this 3.99 DOF to work with.

What if we take the same shot, but with a 400mm lens?
Assuming we're still at f/8 and same distance, our DOF is cut to 2.23 feet.

So, although we have more megapixels to work with on the 400mm shot as opposed to the cropped 300mm shot, we also have a little over half the DOF compared to the crop shot - this might make a difference if your animal is lying down at an angle and you wish to get the entire body into focus, etc.


 



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Post Options Post Options   Quote pegelli Quote  Post ReplyReply Direct Link To This Post Posted: 23 January 2010 at 13:54
cputeq: qualitatively you're right, but your first step is a bit off. As you crop and enlarge the dof still decreases (due to the enlargement), however not as much as by taking the same shot with a longer lens.

If I use MHohner's dof calculator I get:

50 mm, FF, 10 meter, f4 ==> DOF = 12.5 meter
50 mm, APS-C, 10 meter, f4 ==> DOF = 7.1 meter
75 mm, FF, 10 meter, f4 ==> DOF is 4.5 meter
Mind the bandwidth of others, don't link pictures larger then 1024 wide or 960 pix high, see here
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Post Options Post Options   Quote cputeq Quote  Post ReplyReply Direct Link To This Post Posted: 23 January 2010 at 16:32
Pegelli - I may be misunderstanding you, but I will have to disagree with your assessment.

Let's say I take a photo of a tree from 10 meters away with a full-frame at 50mm with a Sony A850 or A900. I have a DOF of "X" amount.

Cropping this image to a 75mm field of view (or any FOV), on a computer, will not alter the image's DOF. The image's DOF was set at the time of image capture and would not be altered simply by "zooming" into the image. An object that was in focus slightly behind the tree at full-frame would still be exactly as in-focus if one cropped to a smaller FOV.

Cropping into the photo (for instance, to show more detail on the tree) will not somehow alter the captured "sharp pixels" any less; one is simply expanding the pixels allocated to the tree over a broader viewing surface, as to (hopefully) see the detail that wouldn't normally be seen in a full-frame view (unless printed very large!)

I have included examples below. I took a photo of a magazine at an angle at 70mm, f/2.8 As you can clearly see in the images, the text does not somehow lose or gain focus as we crop in - only details that were previously unseen (on a monitor or smaller print) are now seen, since they are dispersed over a larger viewing area.

Forgive me if I have misunderstood your point :)
I think the confusion may be that you're throwing APS-C measurements into the mix, whereas I'm only speaking of cropping images from the A850/900 sensor using different lenses, but keeping sensor the same.




Magazine, full frame but resized for web. We can see the text getting fuzzy below the picture of the lens on the left page.


At approximately 50% crop we see more detail, but the clear and blurry letters have not changed.


Finally at 100% we now see more detail than we could (reading small text on the right of image), but fuzzy / clear letters have not changed.
DOF is still the same.


Edited by cputeq - 23 January 2010 at 16:35
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Post Options Post Options   Quote robsphotography Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2010 at 03:03
Originally posted by cputeq cputeq wrote:

Here's one maybe you didn't think of - Cropping will allow you to get a greater Depth of field than if you had made the shot "native"

For instance, we take a picture of an animal with the Sony A850/900 and a 300mm lens, at f/8, from 50 feet away. We have a total DOF of 3.99 feet (http://www.dofmaster.com/dofjs.html)

Now, when we crop to a 400mm view, we still have this 3.99 DOF to work with.

What if we take the same shot, but with a 400mm lens?
Assuming we're still at f/8 and same distance, our DOF is cut to 2.23 feet.

So, although we have more megapixels to work with on the 400mm shot as opposed to the cropped 300mm shot, we also have a little over half the DOF compared to the crop shot - this might make a difference if your animal is lying down at an angle and you wish to get the entire body into focus, etc.




Thanks very much “cputeq” and "pegelli" for your posts. With regard to cropping to a 400mm field of view, a full sized image taken with the Sony A900 with a 300mm lens at F/8 from 50 feet away, I would agree with "cputeq" that the depth of field in a cropped image appears to look the same as that in the original image. But complex DOF theory may not agree with this viewpoint.

For example, assume that you print the full-sized 300mm image from the Sony A900 at 175 pixels per inch, then you will get print dimensions of about 35 inches x 23 inches (6048 / 175 = 34.56 inches).

If you crop this 300mm image to the same field of view that you would get from using a 400mm lens, and print it at 175 pixels per inch, you will obtain print dimensions of about 26 inches x 17 inches (4536 / 175 = 25.92 inches).

Now, if you crop a full-sized image, and then print the cropped image at the same number of pixels per inch as the full-sized image, wouldn’t this give the same result as physically cutting with scissors the actual 35 inches x 23 inches print, so that it has new print dimensions of 26 inches x 17 inches? In other words, although the “cut” print is only 75% of the width and height of the original image, the image quality and depth of field in the cropped area of the full sized print appear to have not been altered in any way. However, complex DOF theory may not agree with this viewpoint.

With regard to capturing the same image with a 400mm lens (with a Sony A900 at F/8 from a distance of 50 feet), then the depth of field (DOF) tables do confirm that the original DOF has been cut from 3.99 feet to 2.23 feet. If a 500mm lens is used to capture this same image, the DOF is cut further to 1.42 feet. (I guess it could also be a disadvantage if you particularly wanted lesser depth of field?)

So, would you think this depth of field factor would convince some people that they should buy a 300mm lens, instead of a 400mm or 500mm lens?

Regards
Rob

http://www.robsphotography.co.nz/focal-length.html

Article about advantages and disadvantages of cropping images to gain extra reach

Edited by robsphotography - 24 January 2010 at 21:52
http://www.robsphotography.co.nz/Sony-A900.html
(Examples of the outstanding resolution of images from the Sony A900)
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Post Options Post Options   Quote cputeq Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2010 at 03:38
Originally posted by robsphotography robsphotography wrote:


So, would you think this depth of field factor would convince some people that they should buy a 300mm lens, instead of a 400mm or 500mm lens?

Regards
Rob

http://www.robsphotography.co.nz/focal-length.html

Article about advantages and disadvantages of cropping images to gain extra reach


I would say it would not convince many people to use 300mm over 400mm or 500mm.


For the most part, there are good reasons people will choose a 70-300 over the 70-400 or other lens.

For instance, a 500mm lens buyer is probably a birder or wildlife shooter, and if you're a birder specifically, you're almost always wanting more focal length! Cropping will only get you so far.

Also, with cropping heavy, one must be very careful to get exacting sharpness.

For instance, say I am very lucky and almost fill a frame at 500mm with a bird - Even if I'm not exactly 100% sharp, I am able to produce a fine image because we are not looking at each exact pixel (unless printing huge or viewing 1:1 on computer monitor, which isn't very helpful)

Now imagine I crop a 300mm to a 500mm view - I must be more accurate in my focus, because now we are "seperating" the pixels more, so any movement will be noticeable.


I think mainly there are weight and cost concerns.
500mm lenses (primes) are not very small and very heavy and expensive.
The 70-400mm isn't exactly travel-friendly (and is very noticeable color), whereas the 300mm zoom is lighter, less expensive and not as noticeable. I think these are the primary concerns when using lenses.

I do agree with your article's point, however; the high-MP count of the A850/900 are a real benefit to anyone that wants to use a 300mm lens as opposed to something larger - they are still able to make fine prints with heavy crops while still retaining detail.

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Post Options Post Options   Quote dca1213 Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2010 at 03:41
Call me stupid but, the DOF of the original is the same as the DOF of the crop since the image has already been captured. The only differnce is perhaps the distances? Does that make sense or have I been too deep into the the Dewers tonight?...
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Post Options Post Options   Quote robsphotography Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2010 at 08:20
Originally posted by dca1213 dca1213 wrote:

Call me stupid but, the DOF of the original is the same as the DOF of the crop since the image has already been captured. The only difference is perhaps the distances? Does that make sense or have I been too deep into the the Dewers tonight?...


DOF theory is extremely complex, and I don't think the situation can be summed up in just one sentence!

However, if the original image and the cropped image are both printed at the same pixels per inch, then the cropped area will print at the same size in both the print of the full-sized original image and in the print of the cropped image. Enjoy the Dewar's!

Regards
Rob
http://www.robsphotography.co.nz/focal-length.html
Advantages and disadvantages of cropping images to gain extra reach

Edited by robsphotography - 24 January 2010 at 21:43
http://www.robsphotography.co.nz/Sony-A900.html
(Examples of the outstanding resolution of images from the Sony A900)
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Post Options Post Options   Quote pegelli Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2010 at 08:37
Originally posted by cputeq cputeq wrote:


Cropping this image to a 75mm field of view (or any FOV), on a computer, will not alter the image's DOF. The image's DOF was set at the time of image capture and would not be altered simply by "zooming" into the image. An object that was in focus slightly behind the tree at full-frame would still be exactly as in-focus if one cropped to a smaller FOV.



I think this is the basic problem with your logic. Optically only one distance is optically perfectly sharp. However some distance before and hehind this distance the amount of "unsharpness" or also called the circle of confusion, is so small our eye still perceives this as sharp.

The dof is set by the size of the circle of confusion at a "normal" viewing distance.

Since a cropped picture with the same lens needs to be enlarged more to achieve the same size/viewing distance vs. the Full Frame version all circles of confusion will be enlarged as well, hence the smaller depth of field when viewing the same size picture from the same distance. However it is still larger than shooting the same scene on FF with a 1.5 x longer lens. So the advantage is there, but not as large as you say in your original post.

It's not an opinion or assessment, it's the laws of optical physics (and governed by formulas). If you read the article on the website I linked to, you will see that.

Edited by pegelli - 24 January 2010 at 09:39
Mind the bandwidth of others, don't link pictures larger then 1024 wide or 960 pix high, see here
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Post Options Post Options   Quote robsphotography Quote  Post ReplyReply Direct Link To This Post Posted: 24 January 2010 at 11:04
Originally posted by pegelli pegelli wrote:

Originally posted by cputeq cputeq wrote:


Cropping this image to a 75mm field of view (or any FOV), on a computer, will not alter the image's DOF. The image's DOF was set at the time of image capture and would not be altered simply by "zooming" into the image. An object that was in focus slightly behind the tree at full-frame would still be exactly as in-focus if one cropped to a smaller FOV.



I think this is the basic problem with your logic. Optically only one distance is optically perfectly sharp. However some distance before and hehind this distance the amount of "unsharpness" or also called the circle of confusion, is so small our eye still perceives this as sharp.

The dof is set by the size of the circle of confusion at a "normal" viewing distance.

Since a cropped picture with the same lens needs to be enlarged more to achieve the same size/viewing distance vs. the Full Frame version all circles of confusion will be enlarged as well, hence the smaller depth of field when viewing the same size picture from the same distance. However it is still larger than shooting the same scene on FF with a 1.5 x longer lens. So the advantage is there, but not as large as you say in your original post.

It's not an opinion or assessment, it's the laws of optical physics (and governed by formulas). If you read the article on the website I linked to, you will see that.


It’s turning out to be an interesting discussion, would you agree with the following summary of what has been discussed so far:

* An image taken with a 300mm lens will provide greater depth of field (DOF) than an equivalent image taken with a 400mm or 500mm lens. For example, if a 300mm image is taken with a full frame camera (sensor size 36mm x 24mm) at F/8 from a distance of 50 feet, then a depth of field table shows that the "near limit" is 48.1 feet, the "far limit" is 52.1 feet, and the total depth of field is 4 feet.

However, if a 400mm image is taken with a full frame camera at F/8 from a distance of 50 feet, then the DOF "near limit" is 48.9 feet, the "far limit" is 51.1 feet, and the total depth of field is 2.2 feet. If a 500mm lens is used to capture this same image, the DOF is further reduced to just 1.4 feet.

Note: If you crop a full-sized image that was taken with a 300mm lens, to the same field of view that is provided by a 500mm lens, then the depth of field of the cropped image is ??????

Is it possible to calculate a numerical value to replace the ?????? above.

Thanks for your help.

Regards
Rob
http://www.robsphotography.co.nz/Sony-A900.html
(Examples of the outstanding resolution of images from the Sony A900)
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