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08-11-2012, 09:55 AM   #1
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OK I'm reaching, but humor me...

Hey, I'm pretty positive that my next camera will be full-frame for multiple reasons, but when it comes to pentax crop sensor DSLR's, I have a question. One of the benefits of a larger sensor is more flexibility with shallow depth of field because of the field of view. So based on that, would a pentax k-x provide an EXTREMELY SLIGHT edge over say a K20D when it comes to shallow depth of field flexibility? the k-x's sensor is APS-C 23.6x15.8mm and the K20D's is slightly smaller at APS-C 23.4x15.6mm. Further more, since APS-C sensors for Pentax, Nikon and Sony are slightly bigger than APS-C Canon sensors (1.5 crop factor vs 1.6) would that give the former sensors a slight edge when it comes to shallow depth of field over the Canon ones? I know this is a bit of reach since we are talking very small margins of size difference, but I was curious about it nonetheless.

08-11-2012, 10:27 AM   #2
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Just an opinion but the differences you are talking about are so small I believe it would hardly be noticed, if noticed at all. The overall design of the sensor will make more of a difference than .2mm on the edges.

Those calculations about DOF are mathematical calculations based on optics...but when it comes to shooting real photos in real situations if you had a perfect world comparison with a same exact designed sensor but one of them is .2mm smaller on the edges... I don't think you would be able to tell the difference between those two.

On a FF--- that's a different animal---

I don't own the two cameras you mention... I have a K-5 and to me it seems as if the sensor design (overall design) is far more important at the end of the day.

Also in addition the lens will make more difference than the .2mm IMO when you are comparing the two Pentax models or any camera for that matter.

As far as comparing the Canon to the Pentax... I was ready to get a 7D until I saw K-5.

Canon has more glass of various designs and so forth but get ready to sell a kidney to get the good stuff.

With Canon once you get the glass you can move into FF simply by getting a new body... with Pentax...not as much the case.

Your question on DOF really I believe would matter as to the lens combo you use and the frame you are actually shooting.

A FF lens on a crop camera---don't think there will be a noticeable difference.... a lens designed for crop sensors... maybe. Some.

Last edited by alamo5000; 08-11-2012 at 10:40 AM.
08-11-2012, 10:28 AM   #3
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DOF is a lens characteristic. It doesn't change if you use the same lens with another crop, leaving everything else unmodified.

However, if you do achieve the same angle of view with another crop - in your example by using a 39mm or 41mm lens instead of a 40mm -, then the situation would of course change.
08-11-2012, 10:35 AM   #4
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QuoteOriginally posted by RKKS08 Quote
DOF is a lens characteristic.
Exactly.

But again noting, some lenses are manufactured to have a full frame circle, but these lenses can be used on crop cameras...

A lens designed for crop sensors would probably have a different DOF...so the issue isn't really the sensor I wouldn't think.An f1.8 on a full frame designed lens would probably have a different DOF characteristics than an f1.8 on a crop sensor designed lens even though both say f1.8 on them...and even that will vary lens to lens.

If my understanding is correct a crop sensor simply plucks out a section of the existing light circle coming into the camera. Period end of story.

If the lens projects a full frame circle of light then whatever that circle is, just pluck out your 1.5 or 1.6 crop. If it has good DOF so be it. If not that's just that. Its the lens that ultimately creates the light circle.

A crop designed lens... you get the idea.


Last edited by alamo5000; 08-11-2012 at 10:47 AM.
08-11-2012, 12:35 PM   #5
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QuoteOriginally posted by RKKS08 Quote
DOF is a lens characteristic.
No it isn't, you're forgetting about the circle of confusion which is sensor (and print) based.
08-11-2012, 01:23 PM   #6
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QuoteQuote:
No it isn't, you're forgetting about the circle of confusion which is sensor (and print) based.
I didn't forget. If you blow up the printed image to the same size as if the picture was shot with a bigger sensor, you will indeed see a difference. But this doesn't only affect the circle of confusion. EVERYTHING will be bigger. It is just not the same picture anymore.
Strictly sensor related, nothing changes. That's why I said DOF is a lens characteristic.

But I don't want to start us argueing about semantics, as it happens in this forum with every thread about perspectivic distortion.
08-11-2012, 01:48 PM   #7
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from: Depth of field - Wikipedia, the free encyclopedia
QuoteQuote:
Affecting DOF are camera-to-subject distance, lens focal length, selected lens f-number, format size, and circle of confusion criterion.
08-11-2012, 04:11 PM   #8
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QuoteOriginally posted by brandonbpm Quote
Hey, I'm pretty positive that my next camera will be full-frame for multiple reasons, but when it comes to pentax crop sensor DSLR's, I have a question. One of the benefits of a larger sensor is more flexibility with shallow depth of field because of the field of view. So based on that, would a pentax k-x provide an EXTREMELY SLIGHT edge over say a K20D when it comes to shallow depth of field flexibility? .
Theres no such thing as shallow DOF flexibility. FF has the DOF range moved one stop towards shallower DOF than APS (it looses in the other end). This is an advantage if want as shallow DOF you can get with a certain lens but a disadvantage if you don't (most of the time if you don't shoot wide open all the time). For the same DOF you have to use one stop higher ISO value on FF than on APS in order to achieve the same shutterspeed at a certain DOF.

08-11-2012, 04:15 PM   #9
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QuoteOriginally posted by RKKS08 Quote
DOF is a lens characteristic. It doesn't change if you use the same lens with another crop, leaving everything else unmodified..
DOF is a lens characteristic but the lens you are using for a certain format to achieve a certain magnification is dictated by the format. So although DOF is a lens characteristic the DOF characteristics you end up with is dictated by the fomat. Different foirmat will display different DOF characteristics cause yoiu'll use different lenses to achieve the same image...
08-11-2012, 10:36 PM   #10
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Can you explain a little better the relationship between ff and dof if shooting wide-open? It was a little confusing to me.
08-11-2012, 11:46 PM   #11
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QuoteOriginally posted by brandonbpm Quote
Can you explain a little better the relationship between ff and dof if shooting wide-open? It was a little confusing to me.
I am not an expert but I will try. Others are much more knowlegeable than I am.

QuoteOriginally posted by Pål Jensen Quote
Theres no such thing as shallow DOF flexibility. FF has the DOF range moved one stop towards shallower DOF than APS (it looses in the other end). This is an advantage if want as shallow DOF you can get with a certain lens but a disadvantage if you don't (most of the time if you don't shoot wide open all the time). For the same DOF you have to use one stop higher ISO value on FF than on APS in order to achieve the same shutterspeed at a certain DOF.
Lets assume you had an f1.8 lens on a full frame and a f1.8 lens (same focal length and all things being equal) on a 1.5 crop camera... you will have different DOF for each at the exact same distance to the focal plane.

A Flexible Depth of Field Calculator

Use the calculator above to see...

According to it using a 50mm f1.8 at a 5 meter distance to the focal plane with a FF will give you 1.16meters of DOF...

Same exact calculation using a 1.5 crop camera...gives you a .76 meters DOF. According to that its actually thinner with the crop sensor using the exact same lens.

In a full frame format according to that calculator you need f1.2 to get a .77meter DOF at the same exact focusing distance mentioned above.

He's saying this same phenomenon continues on throughout the range regardless if you are focusing at 5 meters away, 10 meters, 20 meters or so forth...its the relationship of fstop # to actual DOF relationship...according to my calculation on that calculator a FF camera inherently has a 'fatter' DOF.

The calculator is just a rough guide though.

I am not sure if the calculator takes into account different lens designs... some lenses are actually built for crop sensor cameras. Everything is simply sized down to match the sensor so it alters the optical formula. With Pentax (and Canon I assume) you can use a lens designed for a FF camera on a crop sensor camera and this too would have an effect on 'actual effective DOF' and it might not be exactly what that calculator says it will be...it just depends on the combo.

In the second part of his post (if I read and understand it myself) was that--as I said above to get an equivalent DOF on a FF vs a crop you will have to use a different fstop #. He is in effect saying (again my words not his--so maybe I am wrong)...that if you start your calculation by saying 'I want this much DOF' and then work backwards you will have to use different fstops to obtain the same thing on the different formats and because its literally a different fstop you then have to adjust the ISO if the shot requires an equivalent shutter speed.

If you have leeway to go with slightly faster or slower shutter speeds you might have to either use a higher ISO on the camera or a slower shutter speed to get the same shot....

Again, I am just an innocent bystander here

I am also confessing up front that I am relatively new to photography (but I have read a lot)...again the others can feel free to correct me if I am wrong...

Last edited by alamo5000; 08-11-2012 at 11:55 PM.
08-12-2012, 03:30 PM   #12
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QuoteOriginally posted by brandonbpm Quote
Can you explain a little better the relationship between ff and dof if shooting wide-open? It was a little confusing to me.
Fundamentally all you really need to know is this:

- Larger format sensors need a longer focal length to achieve the same field of view from a given perspective (perspective = camera position).
- Longer focal lengths have less depth of field than shorter ones for the same aperture and perspective.
- Thus a larger format will have less DOF for the same field of view and aperture.

On APS-C a given lens will produce a FOV and DOF approximately equal to a lens on FF with 1.5x the focal length and 1.5x the aperture f-number when viewed at the same size (e.g. a 6x4 inch print). Thus, a 50mm f/1.4 lens on APS-C will look roughly the same as a 75mm f/2.1 on FF. In practice, you can just approximate the aperture equivalence to be 1 stop just to simplify things (e.g. f/1.4 -> f/2.0). Note that this is for DOF purposes only, shutter speed/ISO is linked only to the true aperture, in this case f/1.4.

There are lots of technical details, formulae and whatnot on this topic, but that usually only serves to confuse the average shooter.

Last edited by Cannikin; 08-12-2012 at 03:42 PM.
08-13-2012, 04:36 AM   #13
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WOW I was trying to read way too far into that! So tell me if I have this right then. That calculator business is very technical, and i dont know how accurate it is in real world usage, but lets try and talk real world use for a second (i know I posed an extremely technical question before but bear with me here). From what I understand, the DOF of a crop sensor and a FF sensor using the EXACT same lens at the EXACT same aperture and the EXACT same distance from the subject, with the same exposure for each will have pretty much the exact same depth of field (at least to any reasonable margin to the human eye), but the field of view will be different. But making the field of view exactly the same for a FF and a crop with same lens, aperture and exposure WILL affect DoF. So if you were to frame them the EXACT same way in camera instead of worrying about the distance from the subject, you would either have to get closer to the subject with the full frame camera, or use a longer focal length, or just do the opposite on the crop (get further away, use a wider lens) and THAT is what would give the FF a different depth of field, in this case, shallower.

Just to be sure I used google for a couple of these explanations:
Busting the FF vs Crop DoF Bokeh Myth: Canon SLR Lens Talk Forum: Digital Photography Review
Full Frame vs Crop Sensor Depth of field myth
(the end of the video he does a comparison)

This all has helped me so much. So this means that when some [misinformed or vague] people say "full frame gives you shallower depth of field", what they really mean is a full frame enables you to frame your subject easier when it comes to the depth of field you may want. because for example, if I wanted to use an 85mm f1.4 at f1.4to frame a subject 15 feet in front of me on a FF, in order to frame it the same way on a crop with that lens, I would have to have the space to move back further in order to frame it the same way, and that would increase the depth of field. Or if I wanted to use a different lens at the same distance, I would use a lens about 55mm f1.4 (I rounded down from 56.66), and that would give me a deeper depth of field as well, so if I wanted comparable depth of field for that focal length and distance, I would either have to get an even FASTER lens (55mm f1.0 or something ridiculous lol), or I could just scrap it all and deal with the different framing haha. When I look at it like this, the whole sensor format wars seems a tad bit silly, because I would be willing to bet that in most real world situations, you arent gonna be worrying about switching out cameras for composition purposes, you would more likely worried about how to get the shot you want with what you have in your hands.

With all that being said, my next camera will most likely be full frame. Not so much because of what was said before, but even moreso just because I like the feel better. There's just something about looking through the viewfinder of a 35mm that has always felt so special and badass to me. I also want that same great experience I get with some wide lenses now of getting closer to your subject in order to get the shot you want (getting "in the mix", as I call it, some say "in the zone") with some of my favorite primes and short tele's.

Last edited by brandonbpm; 08-13-2012 at 04:47 AM.
08-13-2012, 09:02 AM   #14
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QuoteOriginally posted by brandonbpm Quote
From what I understand, the DOF of a crop sensor and a FF sensor using the EXACT same lens at the EXACT same aperture and the EXACT same distance from the subject, with the same exposure for each will have pretty much the exact same depth of field (at least to any reasonable margin to the human eye), but the field of view will be different.
The lens will indeed throw out the same image on the film/sensor plane so the DOF of the lens is the same so to say but since APS-C capture less it means that CoC is lower because of the extra enlargement you need to make to make same size print as FF sensor. So with APS-C you are always at 1,5x difference.
Since APS-C is enlarge more you see the details closer so something will appear out of focus faster than with FF.
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