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03-24-2011, 09:03 PM   #31
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so the crop factor would have an effect on the f/stops number over all if you are using a FF lens on an aps-c camera that is all i want to know i know all about the dof alteration like a 50mm is a 75mm and so on but when using the f/stops do we multiply or divide the stop number thank you

03-24-2011, 09:38 PM   #32
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QuoteOriginally posted by lguckert79 Quote
so the crop factor would have an effect on the f/stops number over all if you are using a FF lens on an aps-c camera that is all i want to know i know all about the dof alteration like a 50mm is a 75mm and so on but when using the f/stops do we multiply or divide the stop number thank you
You keep it the same for the purpose of exposure.

However, to increase depth of field to match the equivalent full frame DoF, you divide. So, to get the "same picture" on a Pentax dSLR as you would get on a film K1000, you'd use a 50mm lens at f/1.8 on the dSLR and 75mm at f/2.8 (closest to f/2.7) on the film camera.

See lens - Can a smaller sensor's "crop factor" be used to calculate the exact increase in depth of field? - Photography - Stack Exchange for details on that.
03-24-2011, 09:42 PM   #33
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awsome thank you
03-25-2011, 04:04 AM   #34
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My statement that your response was misleading apPlies directly to the first line of your post specifically that you apply crop FA for to both focal length and aperture

This is simply untrue

03-25-2011, 04:17 AM   #35
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QuoteOriginally posted by lguckert79 Quote
so the crop factor would have an effect on the f/stops number over all if you are using a FF lens on an aps-c camera that is all i want to know i know all about the dof alteration like a 50mm is a 75mm and so on but when using the f/stops do we multiply or divide the stop number thank you
No, NO, HELL NO

F stops define the ratio of aperture diameter to focal length. This determines the amount of light hitting the sensor in terms of intensity per square mm

You need to go bac to basics. There is no change in an image or it's exposure or depth of field or the impact from camera shake when you cut the middle 60% out of a frame

That is all the croP factor is.

The confusion comes from taking that image and further enlarging it.

Further confusion about differences in depth of field come about when you compare shots done with the same lens on different formats to get the same image size. To do this you have to move and the result of focusing to a different distance is to change the Depth of field.

The whole confusion therefore comes from the fact that you end up changing shooting distances, focal lengths and perspective when changing formats and therefore depth of field also changes, not because of the sensor format but because you are taking different shots and enlarging them differently

It is much simpler to consider each format on it's own and not attempt any conversion between formats even when shooting both
03-25-2011, 05:51 AM   #36
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QuoteOriginally posted by Lowell Goudge Quote
You need to go bac to basics. There is no change in an image or it's exposure or depth of field or the impact from camera shake when you cut the middle 60% out of a frame

There's no change to exposure, but (assuming same print size and the same criteria for sharpness) the depth of field certainly does change.

QuoteOriginally posted by Lowell Goudge Quote
The confusion comes from taking that image and further enlarging it.
Which is what one normally does, right? Full-frame gives more room to print even larger, but assuming that you're printing both at 8×10. (Or 9×12 or whatever.)

QuoteOriginally posted by Lowell Goudge Quote
Further confusion about differences in depth of field come about when you compare shots done with the same lens on different formats to get the same image size. To do this you have to move and the result of focusing to a different distance is to change the Depth of field.
But if you change focal length *and* relative aperture (f-number) by the same amount (which is the same as changing focal length but keeping the absolute aperture same), and stand in the exact same place, and print at the same size with the same sharpness requirements, you will get very similar DoF. (Again, see my post on Photo-SE.)
03-25-2011, 09:59 AM   #37
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This is why I hate crap.factor thinking. Much dither and blather about calculations, rather than LEARNING WHAT A SPECIFIC FOCAL LENGTH DOES ON A SPECIFIC FRAME AT ANY DISTANCE. Don't f*cking WORRY about replicating the look of one camera+lens combo (that you don't have) on another camera+lens setup (that you DO have). Just put lens on camera; approach subject; look through finder; when desired image is framed, SHOOT.

The only real need for non-visual thought is in adjusting DOF with a FF lens on a dSLR. Nudge the DOF spread by a little over 1 f-stop. Otherwise, don't fret the numbers. I'll put an old M42 Tokina 21/3.8 on my K20D, set the aperture to f/11, and prefocus for DOF, just inside the f/8 marks, say around f/7. Which means, prefocus to 2m for DOF from 1m to infinity.

And then I won't think of equivalences AT ALL. I won't think, HAY I'M REALLY AT 32/16. I'll just look, move, frame, shoot. It ain't rocket science.
03-25-2011, 10:12 AM   #38
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QuoteOriginally posted by Lowell Goudge Quote
You need to go bac to basics. There is no change in an image or it's exposure or depth of field or the impact from camera shake when you cut the middle 60% out of a frame
The sensor size does affect DOF. Online Depth of Field Calculator
If it didn't, the first entry field in the calculator would be unnecessary.

03-25-2011, 02:58 PM   #39
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QuoteOriginally posted by Parallax Quote
The sensor size does affect DOF. Online Depth of Field Calculator
If it didn't, the first entry field in the calculator would be unnecessary.
No. Depth of field is only a product of the lens itself and the print enlargement ratio

People confuse higher enlargement ratios with sensor size. If you take a 50 mm lens and shoot a subject from 20 feet on both film and ASP-C and print both with the same enlargement ratio the depth of field for both is the same.

As I said, if you cut the middle 60% out of a print from film nothing in the remaining middle portion has changed, has it. The depth of field in the middle 60% only reduces if you further enlarge that further. At the end of the day the sensor format is irrelevant it is a middle number only once you print that number actually drops out of the calculation.
03-25-2011, 05:07 PM   #40
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QuoteOriginally posted by Lowell Goudge Quote
My statement that your response was misleading apPlies directly to the first line of your post specifically that you apply crop FA for to both focal length and aperture

This is simply untrue
Lowell, please. It is true given the assumptions I'm making. You are making different assumptions so you arrive at different conclusions.

The assumption I'm making (which is shared by others in this thread) is that when comparing sensor formats it is difficult to make statements when you are comparing apples with oranges, e.g., prints of different size, images taken from different positions, etc.

Hence the assumption is that a photographer wants to capture a specific scene and print it to a specific output size. It simply doesn't make sense to change the size of prints just because you changed the sensor format.

This assumption gives rise to the notion of an "equivalent image", i.e., same content, same size.

Let's say you take an image with an FF camera with a 75mm lens at f/2.1. You can produce the equivalent image with an APS-C camera with a 50mm lens at f/1.4. Period. There is no debating about this. Use a DOF calculator or run the experiment in real life, if you are not convinced.

QuoteOriginally posted by Lowell Goudge Quote
Depth of field is only a product of the lens itself and the print enlargement ratio
And since one shouldn't change output size when comparing sensor formats, the enlargement ratio changes. Also, one has to change the lens (and its settings) in order to achieve the same image. Comparing images with different content just isn't useful in a sensor format discussion. Of course you can look at all sorts of different images that can be created by various sensor formats, lens variations and changes in camera positions (Bob Atkins does it in his article) but I find these a bit academic. A photographer (rather than a gearist) is interested in a particular result and hence the notion of an equivalent image makes the most sense.

QuoteOriginally posted by Lowell Goudge Quote
As I said, if you cut the middle 60% out of a print from film nothing in the remaining middle portion has changed, has it.
No one disputes that, it is just not a very helpful assumption that your APS-C prints are always 40% smaller than your FF prints and that you don't try to recreate the same FOV when changing the sensor format.

Everything you say makes sense with the assumption that one doesn't change the lens but changes the output size.

Everything others and I said makes sense when you are talking about equivalent images. I would appreciate if you stopped stating that my statements are "misleading" or "untrue".

Last edited by Class A; 03-25-2011 at 05:14 PM.
03-25-2011, 05:48 PM   #41
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QuoteOriginally posted by Class A Quote
Lowell, please. It is true given the assumptions I'm making. You are making different assumptions so you arrive at different conclusions.

The assumption I'm making (which is shared by others in this thread) is that when comparing sensor formats it is difficult to make statements when you are comparing apples with oranges, e.g., prints of different size, images taken from different positions, etc.

Hence the assumption is that a photographer wants to capture a specific scene and print it to a specific output size. It simply doesn't make sense to change the size of prints just because you changed the sensor format.

This assumption gives rise to the notion of an "equivalent image", i.e., same content, same size.

Let's say you take an image with an FF camera with a 75mm lens at f/2.1. You can produce the equivalent image with an APS-C camera with a 50mm lens at f/1.4. Period. There is no debating about this. Use a DOF calculator or run the experiment in real life, if you are not convinced.


And since one shouldn't change output size when comparing sensor formats, the enlargement ratio changes. Also, one has to change the lens (and its settings) in order to achieve the same image. Comparing images with different content just isn't useful in a sensor format discussion. Of course you can look at all sorts of different images that can be created by various sensor formats, lens variations and changes in camera positions (Bob Atkins does it in his article) but I find these a bit academic. A photographer (rather than a gearist) is interested in a particular result and hence the notion of an equivalent image makes the most sense.


No one disputes that, it is just not a very helpful assumption that your APS-C prints are always 40% smaller than your FF prints and that you don't try to recreate the same FOV when changing the sensor format.

Everything you say makes sense with the assumption that one doesn't change the lens but changes the output size.

Everything others and I said makes sense when you are talking about equivalent images. I would appreciate if you stopped stating that my statements are "misleading" or "untrue".
The point is, that depth of field is a purely a function of focal length, aperture and ultimate magnification ratio from subject size to final print size. By stating the sensor size determines depth of field is simply not correct, because until the final image is printed, the entire discussion is hypothetical.

In fact, depth of field is almost irrelevant in today's world because images are viewed on different varying sized monitors, therefore every one viewing an image could have a different measure of depth of field simply because they are looking at a different projected size of the image.

if you impose one image size then perhaps you can make some statement about depth of field but we don't have one image size any more than one magnification ratio.

It is better to make it clear that depth of field is a function of final magnification, which then removes all assumptions, than to make a position based upon an assumption.

The reason is, all assumptions have finite limits, the statement that DOF is inversly proportional to enlargement size does not
03-26-2011, 05:36 AM   #42
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QuoteOriginally posted by Lowell Goudge Quote
By stating the sensor size determines depth of field is simply not correct,...
I didn't make such a statement. I would regard it as misleading.

Fact is, sensor size determines FOV. If the FOV changes because of a change in sensor size and one wants to take the same image then one has to change the focal length. Because of the change in focal length one has to also change the f-ratio to keep the same DOF, as the DOF is determined by the aperture diameter (focal length / f-ratio).

QuoteOriginally posted by Lowell Goudge Quote
In fact, depth of field is almost irrelevant in today's world because images are viewed on different varying sized monitors, therefore every one viewing an image could have a different measure of depth of field simply because they are looking at a different projected size of the image.
  1. There are still standard print sizes, like 8x5.
  2. Any given individual has a particular screen size that doesn't change for them if they compare images from different sensor sizes.

QuoteOriginally posted by Lowell Goudge Quote
if you impose one image size then perhaps you can make some statement about depth of field but we don't have one image size any more than one magnification ratio.
Not "perhaps". And yes, it still makes sense to talk about one image size (see the list above).

QuoteOriginally posted by Lowell Goudge Quote
It is better to make it clear that depth of field is a function of final magnification, which then removes all assumptions, than to make a position based upon an assumption.
Everyone's mileage will vary here. I think the "equivalent" image assumption is not just "an assumption" but a perfectly natural one. But I'm happy for anyone else to disagree.

QuoteOriginally posted by Lowell Goudge Quote
The reason is, all assumptions have finite limits, the statement that DOF is inversly proportional to enlargement size does not
Fine, but if I want to redo one of my earlier shots on a different format with a new format then a trivial crop factor multiplication comes in a lot handier than an invariant that is true but has little direct practical value.

When I do the shot with the calculated parameters, I don't even have to worry about the "final magnification" as long as I keep the output size the same for old and new shot.
03-28-2011, 11:16 AM   #43
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Let's put it this way: 'Crop factor' is just comparing fields of view to the 24x36mm standard. It's become a yardstick. (Particularly in the compact camera markets where lenses are down to pretty short numbers and the sensors vary a fair bit. )


It's actually not that big a deal, except as a referent on FOV and in certain technical debates. Compact cameras confuse people coming in.


Film people who've used different formats just kind of tended, I think, to develop different scales in our minds and image circles, maybe, and not worry about the comparisons unless we had like a zoom flash head or something.


Since digital, there's a lot of comparison shopping and even some differences between like a 1.5 or 1.6x crop when it comes to being picky about FOV in wide and moderate ranges.
03-28-2011, 03:13 PM   #44
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I am amazed why it is so hard to comprehend that a smaller sensor only captures a smaller part of the projected image.

Take your really expensive DSLR and with a black felt-tip draw on the sensor a rectangle half its size and blank out the outside. Now put on your really expensive 35mm f2.8 lens. Does it magically become a 70mm lens ? Does it magically change from f2.8 to f2 ?
03-28-2011, 03:19 PM   #45
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QuoteOriginally posted by kh1234567890 Quote
I am amazed why it is so hard to comprehend that a smaller sensor only captures a smaller part of the projected image.

Take your really expensive DSLR and with a black felt-tip draw on the sensor a rectangle half its size and blank out the outside. Now put on your really expensive 35mm f2.8 lens. Does it magically become a 70mm lens ? Does it magically change from f2.8 to f2 ?
You have just described what nikon and canon do when you put an ASP-C intended lens on a full frame body
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