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Cropping a wide angle to increase focal length?

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kjkw said:
Chato said:
These two pictures were taken at the same time and place. The first three are:

1. The scene as it appears on a 28mm lens with a 1.7 cropped sensor...

2. This second is a 100 percent crop

3. The same crop interpolated up to 200 percent and then cropped again at 100 percent

Finally, the same scene at the same place and time with an 800mm lens, with only about ten percent of the image cropped away.
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I'd say that the final picture was focused on the bird and the first three weren't.
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The first "three" images are in fact the same image. And the shot was meant to show the bird in terms of the landscape. Which is why I carry that camera in the first place. In fact the bird is almost, if not perfectly centered. It's not completely centered because I wanted to also show the tidal "river."

The last shot is in fact the "second" shot, and was done with a different camera, but from the exact same spot, only a moment apart.

Dave
kjkw said:
Ken
http://mazurk.net/nature
Click to expand...
--
"Everyone who has ever lived, has lived in Modern Times"
 
kjkw said:
sherwoodpete said:
This greater degree of enlargement (8x instead of 4.2x) means a corresponding reduction in the size of the circle of confusion used in the DOF calculation. The end result is the depth of field is reduced when the image is cropped
Click to expand...
Does this mean that there's no fixed definition for "in focus", it's just a matter of degree of focus? If so, what would be a working definition for "in focus".

--
Ken
http://mazurk.net/nature
Click to expand...
A lens produces an image which is only precisely "in focus" at one specific distance.

When we consider depth of field, we are also interested in those parts of the resulting image, which when printed at a specified size and viewed from a specified distance, by a person with specified eyesight, appears acceptably sharp .

This concept, of an image appearing acceptably sharp underlies the whole concept of depth of field.

http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

Regards,
Peter
 
Chato said:
The first "three" images are in fact the same image. And the shot was meant to show the bird in terms of the landscape ... The last shot is in fact the "second" shot, and was done with a different camera, but from the exact same spot, only a moment apart.
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I know. I thought you were attempting to show the difference in DOF between a very cropped 28mm image and a slightly cropped 800mm image. My apologies if I misunderstood.

--
Ken
http://mazurk.net/nature
 
Sante Patate said:
Nelsonal's image has been "re-sized", not optically enlarged.
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I'm not following this at all.. Originally I was responding to the premise first articulated in Sante Patate's post which was:
Sante Patate said:
Sante Patate said:
In order to get a print the same size and the same angle of view with the shorter focal length lens you have to enlarge the cropped short focal length print, and that will make the DOF in that print exactly the same as in the longer focal length print.
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If Nelsonal's digital files for the 2 shots that he posted, are both sent to a printer for eg. a 5x7 print, the cropped 28mm shot would exhibit more DOF. The 2 posted shots can actually serve to show about how prints of the same size would look as they are posted online at the same size. Yes, they've both been resized (actually both are likely to have been downsized).. but who cares if we're talking about apparent DOF in a print?

--
'Everything in photography boils down to what's sharp and what's fuzzy.'
-Gaylord Herron
 
I think, but my dark room experience is limited to a single trip and I was way more focused on my cute lab partner than the job at hand, if you had say a negative taken with a 28mm lens and used an enlarging lens to print a cropped portion of the photo (similar to what I did, but optically rather than digitally) the depth of field would shrink.

That fits with my experience that if you move to keep the same framing and apeture, it doesn't really matter what length lens you're using you're depth of field is nearly the same.

Taking the digital output and using gimp's resize function however doesn't preserve the image properties. They were resized, because originally someone was asking how much image quality suffered, and I had no idea.
 
nelsonal said:
I think, but my dark room experience is limited to a single trip and I was way more focused on my cute lab partner than the job at hand, if you had say a negative taken with a 28mm lens and used an enlarging lens to print a cropped portion of the photo (similar to what I did, but optically rather than digitally) the depth of field would shrink.
Click to expand...
Yes, this would be the case.. but what I thought was being discussed was the effect of doing what you describe above, vs. taking another "negative" shot with eg. an 80mm and not cropping in your enlarger, to get the same subject framing. In which case the 28mm-shot negative would exhibit MORE DOF (per your example) than the 80mm shot.
--
'Everything in photography boils down to what's sharp and what's fuzzy.'
-Gaylord Herron
 
Chato said:
This is gibberish. When you use a longer focal lenght you are collect more light, you are not "enlarging" the image with your sensor. You're enlarging the image because you were able to collect eight, ten or whatever times the amount of light depending on your focal lenght.
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Maybe I’m not understanding what you’re getting at here, but I don’t see how this relates to the rest of the thread. Did you read the posts before and after the one you responded to?

Everybody seems to be taking this question far away from what the OP asked. Talking about sensor size, DOF, and resolution. I don't get any of that from the original question. Reply to my post above yours if you don’t agree.

I feel like sometimes people bring baggage from other threads and make assumptions about what’s being asked, and that takes it far off topic.

p.s. My enlarger analogy seems to be confusing people so I'll concede it must not be very clear, but if you think about using a traditional enlarger. You move the lens (increase the focal length) away from the sensor (paper) to enlarge the image and capture a magnified portion of the image. Sensor (paper) stays the same size and image gets cropped because it falls off the edges of the sensor (paper). How is this different than what happens inside a camera. An enlarger is a camera; you're just taking a picture of the negative.
 
Morris Sullivan said:
Chato said:
This is gibberish. When you use a longer focal lenght you are collect more light, you are not "enlarging" the image with your sensor. You're enlarging the image because you were able to collect eight, ten or whatever times the amount of light depending on your focal lenght.
Click to expand...
Maybe I’m not understanding what you’re getting at here, but I don’t see how this relates to the rest of the thread. Did you read the posts before and after the one you responded to?

Everybody seems to be taking this question far away from what the OP asked. Talking about sensor size, DOF, and resolution. I don't get any of that from the original question. Reply to my post above yours if you don’t agree.

I feel like sometimes people bring baggage from other threads and make assumptions about what’s being asked, and that takes it far off topic.

p.s. My enlarger analogy seems to be confusing people so I'll concede it must not be very clear, but if you think about using a traditional enlarger. You move the lens (increase the focal length) away from the sensor (paper) to enlarge the image and capture a magnified portion of the image. Sensor (paper) stays the same size and image gets cropped because it falls off the edges of the sensor (paper). How is this different than what happens inside a camera. An enlarger is a camera; you're just taking a picture of the negative.
Click to expand...
You are correct. My response was not truly responsive to your reply. I apologise, and at the same time agree that your enlarger analogy sucks... :)

It STILL sucks by the way. Personally I don't see the relevance at all. A small focal lenght is a small focal lenght no matter how enlarged you make it. When a lens has a larger focal lenght, it's not simply a question of "enlarging the image."

Dave

--
"Everyone who has ever lived, has lived in Modern Times"
 
Kenny123 said:
I have a question regarding cropping a wide angle. If for example a photo was taken with a 28mm lens (FF equivalent) and then the central section was cropped out, would the central section then have an increased focal length to make it the same as if it was taken with for example a 50mm lens (depending on how much was cropped)?

I know the resolution would be less but would the field of view and look of the photo be the same?
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Let's say your original was 6000 pixels by 4000 pixels. If you were to crop it "2x" to 3000 pixels by 2000 pixels, the resulting shot (ignoring resolution) would be the same as if you had shot it with 2x longer focal length and 2x larger (higher number - f/2.8 -> f/5.6) f-stop.

--
Lee Jay
(see profile for equipment)
 
Think about it like this:

If you shot a picture on a film camera, then cut the edges off the negative, you'd still have a picture taken with a 28mm lens. You'd have everything the same except just the middle of the piece of film.

If you shot with a 50mm and didn't crop, then to print from the cropped 28mm neg you'd have to enlarge it more to get a subject of the same size. But it would be an enlarged 28mm "image." So depending on the natuire of the subject, original distances, etc., there could be visible differences in the two images.
 
I think that talking in terms of enlargement (at least when it comes to digital cameras) cause more confusion than enlightenment. My digital image saves images to file. Those image files have no physical dimensions. I can print it at any size I choose to. For some image size/viewing distance combinations the image may be deemed low-quality. Figuring out that limit is a lot more complex than consulting "dpi/ppi" tables on the net.

-h
sherwoodpete said:
PicOne said:
FYI, a photo does not always need to be enlarged..
Click to expand...
That depends on how you define "enlarged". In the context of DOF discussions, it is the optical enlargement which matters.
Click to expand...
 
Kenny123 said:
I have a question regarding cropping a wide angle. If for example a photo was taken with a 28mm lens (FF equivalent) and then the central section was cropped out, would the central section then have an increased focal length to make it the same as if it was taken with for example a 50mm lens (depending on how much was cropped)?

I know the resolution would be less but would the field of view and look of the photo be the same?
Click to expand...
Assuming ideal lenses (infinite resolution, no diffraction) and sensors (infinite resolution and noiseless):

I believe that if you took an image using a 50mm@f/2.8 on a FF camera and cropped until you had a central portion of 1/2 the width and 1/2 the height (1/4 the area), this would appear similar to a 100mm@f/5.6 on a FF camera centered at the same scene.

In practice, one does not expect these two images to be similar, and I think that is primarily because the available number of pixels will be lower when cropping, and a wider angle lense will typically not have the necessary sharpness to compete with the tele lense in this way. Further, no two lenses are equal.

-h
 
hjulenissen said:
I think that talking in terms of enlargement (at least when it comes to digital cameras) cause more confusion than enlightenment. My digital image saves images to file. Those image files have no physical dimensions.
Click to expand...
If we are discussing depth of field, the size of the circle of confusion is required. This is directly related to the physical dimensions of:
  • the final print or displayed image
  • the physical sensor size
hjulenissen said:
I can print it at any size I choose to. For some image size/viewing distance combinations the image may be deemed low-quality. Figuring out that limit is a lot more complex than consulting "dpi/ppi" tables on the net.
Click to expand...
Low-quality or high-quality when printing is a separate topic and is not directly related to depth of field.

Of course this topic causes confusion. How many times on these forums has this same topic been discussed, and explained. If you have a better suggestion for explaining how and why depth of field changes when the image is cropped I'd be glad to hear it.

Regards,
Peter
 
Chato said:
Morris Sullivan said:
p.s. My enlarger analogy seems to be confusing people so I'll concede it must not be very clear, but if you think about using a traditional enlarger. You move the lens (increase the focal length) away from the sensor (paper) to enlarge the image and capture a magnified portion of the image. Sensor (paper) stays the same size and image gets cropped because it falls off the edges of the sensor (paper). How is this different than what happens inside a camera. An enlarger is a camera; you're just taking a picture of the negative.
Click to expand...
You are correct. My response was not truly responsive to your reply. I apologise, and at the same time agree that your enlarger analogy sucks... :)

It STILL sucks by the way. Personally I don't see the relevance at all. A small focal lenght is a small focal lenght no matter how enlarged you make it. When a lens has a larger focal lenght, it's not simply a question of "enlarging the image."
Click to expand...
I'm not following. How would you describe the effect of a longer focal length? The way I see it, the image projects out from the focal point, just like a projector would onto a wall. As you get further from the wall (same as increasing focal length) the image gets bigger. So in essence you are exactly "enlarging the image". If I'm missing something here, please explain it to me.

I know, I know, I just created a new analogy. Please forgive me.

Here's another way to look at it. When you look through a zoom lens and turn the ring, what happens? The image gets bigger and the stuff at the edge of the frame falls outside the viewfinder image(I won't say it gets cropped off, some people are sensitive to that). Nothing else. It doesn't collect any more light, or anything like that.
 
sherwoodpete said:
If we are discussing depth of field,
Click to expand...
But I fail to see the relevance to the topic. My paraphrasing of the original question:

Ignoring loss of resolution, will cropping the image taken by a wide-angle lense (28mm) produce an image similar to a non-cropped image from a narrower lense (50mm)? Would this effectively alter the focal length?

In my mind, the answer to that question should be either:
"yes", "no", or "yes/no but only if you change the aperture by a factor of X".

I dont see how DOF, printsize, degree of enlargement, CoC should be included in any reply to that question? Ok, perhaps as an addendum for the tecnically inclined, but I think it can be answered better without.
sherwoodpete said:
If you have a better suggestion for explaining how and why depth of field changes when the image is cropped I'd be glad to hear it.
Click to expand...
My suggestion would be:

If you crop the image taken with a wide-angle, you can in principle get the same image as you would have using a tele lense if you compensate the aperture, everything else equal. In practice, you probably will not, as limited lense resolution and sensor resolution gives the non-cropped image an advantage.
 
Morris Sullivan said:
Chato said:
Morris Sullivan said:
p.s. My enlarger analogy seems to be confusing people so I'll concede it must not be very clear, but if you think about using a traditional enlarger. You move the lens (increase the focal length) away from the sensor (paper) to enlarge the image and capture a magnified portion of the image. Sensor (paper) stays the same size and image gets cropped because it falls off the edges of the sensor (paper). How is this different than what happens inside a camera. An enlarger is a camera; you're just taking a picture of the negative.
Click to expand...
You are correct. My response was not truly responsive to your reply. I apologise, and at the same time agree that your enlarger analogy sucks... :)

It STILL sucks by the way. Personally I don't see the relevance at all. A small focal lenght is a small focal lenght no matter how enlarged you make it. When a lens has a larger focal lenght, it's not simply a question of "enlarging the image."
Click to expand...
I'm not following. How would you describe the effect of a longer focal length? The way I see it, the image projects out from the focal point, just like a projector would onto a wall. As you get further from the wall (same as increasing focal length) the image gets bigger. So in essence you are exactly "enlarging the image". If I'm missing something here, please explain it to me.

I know, I know, I just created a new analogy. Please forgive me.

Here's another way to look at it. When you look through a zoom lens and turn the ring, what happens? The image gets bigger and the stuff at the edge of the frame falls outside the viewfinder image(I won't say it gets cropped off, some people are sensitive to that). Nothing else. It doesn't collect any more light, or anything like that.
Click to expand...
Actually I have a lot of sympathy for your views.

Just this last part may be questionable: "It doesn't collect any more light, or anything like that."

If this is a constant-aperture zoom, as the focal length is changed, the aperture changes in proportion. That means the exposure remains the same. But it also could be said that it is now gathering the same amount of light from a smaller part of the subject.

Whether this is considered "collecting more light" may be a matter of interpretation.

Though of course, not all zoom lenses maintain a constant aperture. A great many do not.

Regards,
Peter
 
That depends how many sensels you have before, and therefore after, the crop and how big you want to print. An 8 x 10 print at 300dpi is 7.2 MP, so if you have a 16 MP sensor you can crop half of the image area and still print 8 x 10 at 300dpi just the same as with the full 16 MP image. But if you wanted to print 16 x 20 that is 28.8 MP at 300dpi, so printing a 16 MP image at that size requires either using fewer dpi (ie, making the pixels bigger) or interpolating new pixels, and it requires twice as much to print an 8 MP image that size.
 
Morris Sullivan said:
Chato said:
Morris Sullivan said:
p.s. My enlarger analogy seems to be confusing people so I'll concede it must not be very clear, but if you think about using a traditional enlarger. You move the lens (increase the focal length) away from the sensor (paper) to enlarge the image and capture a magnified portion of the image. Sensor (paper) stays the same size and image gets cropped because it falls off the edges of the sensor (paper). How is this different than what happens inside a camera. An enlarger is a camera; you're just taking a picture of the negative.
Click to expand...
You are correct. My response was not truly responsive to your reply. I apologise, and at the same time agree that your enlarger analogy sucks... :)

It STILL sucks by the way. Personally I don't see the relevance at all. A small focal lenght is a small focal lenght no matter how enlarged you make it. When a lens has a larger focal lenght, it's not simply a question of "enlarging the image."
Click to expand...
I'm not following. How would you describe the effect of a longer focal length? The way I see it, the image projects out from the focal point, just like a projector would onto a wall. As you get further from the wall (same as increasing focal length) the image gets bigger. So in essence you are exactly "enlarging the image". If I'm missing something here, please explain it to me.
Click to expand...
You are enlarging an imge that has already been captured. Whatever detail in that image is "fixed" and no longer subject to change. Whereas the amount of detail by using a longer lens actually increases. My extreme example, demonstrates the amount of detail captured by the longer lens. For that matter the depth of field doesn't change by enlarging the image. What is in focus remains in focus, what is not in focus doesn't change. You are not altering the DOF, you would merely be cropping it. Whereas the DOF of a different lens speaks for itself.
Morris Sullivan said:
I know, I know, I just created a new analogy. Please forgive me.

Here's another way to look at it. When you look through a zoom lens and turn the ring, what happens? The image gets bigger and the stuff at the edge of the frame falls outside the viewfinder image(I won't say it gets cropped off, some people are sensitive to that). Nothing else. It doesn't collect any more light, or anything like that.
Click to expand...
Longer focal lenghts collect more light than smaller ones. A zoom lens actually works by increasing the amount of light collected. You are misled by the fact that cheaper zooms have a variable aperture, and the implication is that less light is being collected. Such is not the case.



Dave
--
"Everyone who has ever lived, has lived in Modern Times"
 
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