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Focal focus with crop sensor?


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If a lens shows everything in focus from 8ft to infinity at f5.6 (just making this up to ask the question) for a 24mm lens then is it the same on a crop sensor (CL) when this is now a 35mm lens and obviously at f5.6 the in-focus range is shorter .... not a trick question, anyone have the answer? 

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Let's also assume you shoot from the same distance when going from FF to cropped sensor frame - in this case, the field of view of your lens changes but not the focal length. To get a similar depth of field, you would only need to go to f/4 on the cropped sensor which is about f/5.6 on the full frame sensor. So at f/5.6 the in-focus range is longer and not shorter on a cropped sensor in comparison to FF. Or in other words: when you shoot at f/4 with a cropped sensor camera, you would need to stop down one more stop on a FF sensor to achieve a similar depth of field from the same distance (with a different field of view of course). 

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Let's also assume you shoot from the same distance when going from FF to cropped sensor frame - in this case, the field of view of your lens changes but not the focal length. To get a similar depth of field, you would only need to go to f/4 on the cropped sensor which is about f/5.6 on the full frame sensor. So at f/5.6 the in-focus range is longer and not shorter on a cropped sensor in comparison to FF. Or in other words: when you shoot at f/4 with a cropped sensor camera, you would need to stop down one more stop on a FF sensor to achieve a similar depth of field from the same distance (with a different field of view of course). 

 

So the range in focus lengthens or shortens? It would seem that it would lengthen so the 24mm lens' focus area is same for given f/stop regardless of whether it is on a FF or crop sensor?

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This topic generates enormously long threads without much light.  The source of confusion is that the lens hasn't changed, and the scene hasn't changed, but the image inside the camera is smaller, and if you are comparing, e.g.,  an SL with a CL, the pixels are smaller by a factor of 1.5 in linear dimension.  So if you want to print or display the resulting picture the same final size, the CL image will have to be enlarged by 1.5x as much.  Even though the portion of the image captured by the smaller chip is just as sharp as with the larger chip, you might want to have tighter standards and thus consider the depth of focus to be less with the smaller chip and its closer pixels.

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So the range in focus lengthens or shortens? It would seem that it would lengthen so the 24mm lens' focus area is same for given f/stop regardless of whether it is on a FF or crop sensor?

 

The range in focus lengthens on a cropped sensor - to get the equivalent one, you need to change one aperture stop. This can be seen as advantage for landscape shooting but as disadvantage where more shallow DoF might be required, for example in portraits. The extension factor is about 1.5x - rule of thumb is that this gives you a difference of about one stop in DoF terms between FF and cropped sensor - assuming you do not change the distance to the subject of course!

 

I have seen this issue when moving the other way from cropped to FF sensor in 2009. After I shot digital with the cropped sensor based camera for several years, I had to accustom myself again using a smaller aperture (larger aperture number) when shooting with my FF camera to get the same DoF. For me this is one reason not to go back to a cropped sensor based camera since I prefer the 35 mm/FF based DoF-aperture settings which I am used to. I can see that this must be a change for you now with the CL!

 

I recently started shotting film-based medium-format, and there the difference in DoF is even more extreme. A f/4 stop on a 6x7 cm frame gives the similar DoF of a f/1.4 stop in FF terms. 

Edited by Martin B
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If a lens shows everything in focus from 8ft to infinity at f5.6 (just making this up to ask the question) for a 24mm lens then is it the same on a crop sensor (CL) [...]

 

Not exactly. At 30ft for instance, everything should be in focus from 8ft to infinity on full frame and from 10ft to infinity on APS-C cameras. Same lens but different field of view and different circle of confusion. See http://www.dofmaster.com/dofjs.html.

 

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Not exactly. At 30ft for instance, everything should be in focus from 8ft to infinity on full frame and from 10ft to infinity on APS-C cameras. Same lens but different field of view and different circle of confusion. See http://www.dofmaster.com/dofjs.html.

 

ff_dof.png

aps-c_dof.png

 

Correct - shows that the hyperfocal distance with cropped sensor is longer than with FF - what I stated above. 

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Not exactly. At 30ft for instance, everything should be in focus from 8ft to infinity on full frame and from 10ft to infinity on APS-C cameras. Same lens but different field of view and different circle of confusion.

 

Different circle of confusion, AKA degree of enlargement is what makes the difference here, but if both images are enlarged to the same degree, not the same sized print, there is no difference in DOF.

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All other things remaining equal what makes the difference is the sensor size which determines the circle of confusion.

 

More accurately the degree of enlargement makes the difference (given same viewing distance). No?

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All other things remaining equal yes but the OP was comparing different formats so for the same enlargement CoC depends on one thing only if i'm not mistaken, the size of the sensor. Are we in agreement?

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Correct - shows that the hyperfocal distance with cropped sensor is longer than with FF - what I stated above. 

 

You did not state that, you stated that the focus range is longer, which is incorrect (also according to the dofmaster calcs above), as is the statement that you made above that you need to open the aperture one stop more for a cropped sensor from the same distance... 

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See what I mean about threads on this subject?

 

The problem is that "in focus" depends on what you will do with the image.  It's probably least confusing to specify that the output is held fixed -- display on a particular size monitor, or printed to a fixed size.  Tools that tell you what to do without asking what you want to have as a result are part of the problem.  Remember that only one distance is precisely "in focus." And this thread postulates that we are comparing the same lens, say a nice new SL Summicron, used first on the SL and then on the CL.  This leads to different pictures.

Edited by scott kirkpatrick
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[...] you need to open the aperture one stop more for a cropped sensor from the same distance... 

 

+1 when using equiv. FoV lenses like 35 on FF and 24 on APS-C.

 

 

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If a lens shows everything in focus from 8ft to infinity at f5.6 (just making this up to ask the question) for a 24mm lens then is it the same on a crop sensor (CL) when this is now a 35mm lens and obviously at f5.6 the in-focus range is shorter .... not a trick question, anyone have the answer? 

 

Hmm.  Let me make sure I understand the scenario you are asking about.  You are asking if:

- You attach a 24mm lens to a full frame camera, set it to, say, f/4, and set the focus point to the hyperfocal distance such that everything from 8ft to infinity is in focus

- You now attach that same 24mm lens to an APS-C camera (where it will show the field of view of a 35mm lens on a full frame camera) and set it to the same f/4 and don't physically move the lens and camera...

 

Then, would you still have the same hyperfocal distance and the same 8ft to infinity depth of field?  Even though you now have a very different field of view?

 

If that's your question, then the answer is no.  You'll have less depth of field with the smaller format sensor.  Where you used to get everything form 8ft to infinity in your depth of field when set to the hyperforcal distance, you now will only get from 12ft to infinity. To understand what's going on, you need to distinguish between blur circle and circle of confusion.

 

The blur circle--the size of the blur at the image plane--is determined by the lens aperture (physical diameter, not f/stop) and the distances involved and nothing else.  Focal length of the lens doesn't matter and sensor size doesn't matter.  Since you didn't change your lens aperture and you didn't move the camera/lens, the blur circle is the same in both cases.  But, when you make a print or display your image on a monitor, the APS-C sensor will need 1.5x more magnification for a given size print or display.  So that blur circle that didn't change physical size on the sensor is going to look bigger in the image from the smaller sensor just because you enlarged it more to see it on your screen.  So your depth of field is going to be significantly narrower with the APS-C sized sensor.

 

So why do people think that larger formats generate narrower depths of field for a given f/stop?  It's because the scenario is usually a little different.  Most people would set up the comparison to use lenses of different actual focal lengths to get the same field of view out of each sensor.  So let's try that scenario instead.  Let's try a 24mm lens on the APS-C size camera and a 36mm lens on the full frame camera.  I know those aren't quite standard sizes for lenses, but it makes the math easy. The same field of view that way.  The blur circle is, again, dependent on the diameter of the aperture and the subject distance/focus distance and nothing else.  Here's our first complication, though... Since the focal length of the two lenses is different, the diameter of the aperture isn't the same unless we choose different focal ratios!  So let's shoot the 24mm on the APS-C camera at f/4.  That gives a lens opening of 6mm diameter.  To get the same diameter opening out of the 36mm lens (which happens to be connected to the full frame camera) we'd need a focal ratio of f/6.  If we did that, then we'd be back to the first scenario where the larger format sensor was actually generating more depth of field due to the lower magnification required for viewing.  If we slowly increase the size of the aperture on the 36mm lens until we have the same depth of field as the 24mm lens on the APS-C camera we would find equivalence right around f/5.6.  So, for a given field of view, you generally need about one stop larger an aperture when using an APS-C size chip to get the same depth of field as you would on a full frame chip.

 

If you plug the numbers into a DoF calculator, you will find that a 24mm lens on an APS-C sized camera shot at f/4 has a hyperforcal distance of 23.7ft and will cover everything from 11.9ft to infinity.  Likewise, a 35mm lens on a full frame camera shot at f/5.6 has a hyperfocal distance of 23.8 ft and will likewise cover everything from 11.9 ft to infinity.  So, for a given field of view and a given depth of field, the full frame camera will need to be stopped down one stop.  

 

Hope that helps.

Edited by Jared
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  So, for a given field of view, you generally need about one stop larger an aperture when using an APS-C size chip to get the same depth of field as you would on a full frame chip.

 

 

+1. That's exactly what I wrote in my earlier comments, too. 

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Jared above made all the right points, but here is an illustration that clarifies the points, because many people here seem to confuse fixed distance and fixed field of view, which is not the same.

(I used dofmaster for all the calculation)

 

Fixed distance at 3m, with 50mm with two different sensors:

 

Leica M9 at f5.6  total depth of field   1.25m

Leica X1 at f5.6   total depth of field   0.94m

Leica X1 at f7.1   total depth of field   1.21m

 

So for a fixed distance with an APS-C sensor you need to stop down by about 2/3 to 1 stop to get the same depth of field

 

Fixed field of view (i.e. get the same image)  

 

To get the same image you need to step back with the APS-C sensor, so the distance to the subject will increase from 3m to 4.5m.

 

M9  at 3.0m at f5.6  total depth of field   1.25m

X1  at 4.5m  at f5.6  total depth of field  2.2m

X1  at 4.5m  at f3.4  total depth of field  1.26m

 

So for the same field of view (i.e. making the same image) for the APS-C sensor you need to open the aperture by 1 to 1+1/3 stops to get the same depth of field.

 

In conclusion, from the same distance if you use APS-C sensor, stop down about 1 extra stop, but if you compare two sensors in order to get the same image, then you need to open the aperture by about 1 stop with an APS-C sensor, because you will be further away from the subject.

 

This of course means that when you use a manual lens on your CL, and you want to do zone focusing using the dof lines of your lens, you need to use 1 more stop, i.e. in order to you use the f5.6 dof lines of your lens, stop your lens down to f8.

Edited by Daedalus2000
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I think there is one more comparison that people have in mind when asking how to compare FF and APS-C, and that is two different lenses, with focal lengths chosen to produce the same angle of view.  For example, 35 mm on the CL and 50 mm on the SL  That's the scenario in the later half of Jared's last epistle.  Where, before corrections for different degrees of enlargement, you need to stop down the full frame camera more.

 

And, of course, the folks who want everything in focus in their landscape ask different questions from the ones who are seeking the shallowest possible depth of field.

Edited by scott kirkpatrick
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So many words and still a lack of simple clarity.

Math does not make images.

 

We are the evaluators. How is the image presented? At what magnification? What size and from what viewing distance? Then work backwards. It is really that simple.

.

Edited by pico
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So many words and still a lack of simple clarity.

Math does not make images.

 

We are the evaluators. How is the image presented? What size and from what viewing distance? Then work backwards. It is really that simple.

.

 

OK, here is a simple, clear answer to the specific question originally asked.  No, the depth of field will be different.  The image taken with the larger format sensor will have more depth of field.  This is true regardless of size and viewing distance.

 

The longer answers explain, "why" and "by how much" and "what about if you use lenses with equivalent fields of view."  

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