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09-02-2012, 12:06 PM   #1
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not sure how to word this question..

well, here goes

when you use a shorter focal length with the lens close to wide open, you need to be relatively close to the subject to get a good bokeh.
if you get a long lens and stand back you can stop down and get just as good bokeh, usually better most times for me.
so what focal length (prime) does this transition start to really take off? (focal length) when doing people shots?
I realise most think 70-120 or so make the best for portraits, but which length (even outside those focal points) give the best balance

like I said not sure how to word this..

randy

09-02-2012, 12:25 PM   #2
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The effect you are describing is the depth of field. You observe correctly - short lens wide DoF, long lens short DoF. The bokeh character depends on the lens design, and the details of the iris are particualrly important. The DoF effect is on a continuous scale, no special length at which it kicks in. It depends on what you want to achieve when it is 'right' for your purpose. All I can suggest is that you experiment to get a feel for what it is like - doing things like bracketing when you have the time with different apertures and focal lengths taking essentially the same subject. That way you can develop a sense of what will get you the efect you want.
09-02-2012, 12:36 PM   #3
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Spatial compression? Subject isolation?
For portrait lenses people tend to go with 85mm and 135mm lenses, but I don't think there is any 'optimal' focal length. They have different pros and cons. The more tele it is, the more it will blur backgrounds and the more wide angle it is, the bigger its DoF will be at farther distances. This gives the illusion that things are closer or farther apart.
09-02-2012, 01:19 PM   #4
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For me, it's 85mm. And the Rokinon 85mm 1.4 is an excellent solution.

09-02-2012, 02:36 PM   #5
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QuoteOriginally posted by tim60 Quote
You observe correctly - short lens wide DoF, long lens short DoF. The bokeh character depends on the lens design, and the details of the iris are particualrly important.
All wrong unfortunately.

Sorry i'm lazy so going to link you through.
Bokeh
09-02-2012, 02:40 PM   #6
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For big blur spots in the background it's the easiest to use a what longer lens 50mm is enough and a faster aperture, move close to the subject to great as much distance differnce between the background.
Either you get good or bad bokeh is a different question but you ain't asking about bokeh i believe

Last edited by Anvh; 09-02-2012 at 02:46 PM.
09-02-2012, 02:43 PM   #7
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I did some calculations a while back to answer a question of depth of field using diffence lenses and standing back futher with longer focal lengths.

It turns out that if you keep image size the same, the DOF is the same if you keep aperture the same. I.e. F2.8 for any two lenses gives the same depth of field for the same magnification ratio. ( not exact but to a first order approximation of within 10%)
09-02-2012, 02:57 PM   #8
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QuoteQuote:
Originally posted by tim60
You observe correctly - short lens wide DoF, long lens short DoF. The bokeh character depends on the lens design, and the details of the iris are particualrly important.
QuoteOriginally posted by Anvh Quote
All wrong unfortunately.

Sorry i'm lazy so going to link you through.
Bokeh
Bokeh actually does depend greatly on lens design and aperture. If the aperture is curved or straight, whether is has five blades or nine, it will differ. The lens will also affect the quality of the bokeh, wehther the OOF circle is consistent, has a bright edge, or even donut shaped.

DOF on the other hand defies logic to some extent when you start to realize, as mentioned, that for the same object when viewed at the same size DOF does not change regardless of focal length. Probably for that reason, since longer focal lengths appear to compress long distances into a smaller portion of view, DOF will appear to change more radically.

Probably the best way to see how DOF is affected is to go to DOFMaster.com and play around with their calculator.

09-02-2012, 03:02 PM   #9
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I generally prefer something in the 55-100mm range for standing or sitting portraits but there are exceptions. Taking pictures of people is fairly easy with that length of lens but doing a portrait of an animal can sometimes be easier with a longer lens. There are actually some photographers out there who will use a wide angle lens for portraits, (being careful of lens distortion) but I'm not too fond of portraits taken with a wide angle myself but again there are exceptions. 75-100 is ideal for my purposes but I've been known to use a 55 to good effect too. I did a shoot not too long ago with someone who had a very narrow face and a rather prominent nose. Using a 28 and a 55 actually worked out a bit better for him. It all depends upon what type of portrait subject, their proportions, and how close you can get.
09-02-2012, 03:48 PM   #10
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QuoteOriginally posted by magkelly Quote
I generally prefer something in the 55-100mm range for standing or sitting portraits but there are exceptions. Taking pictures of people is fairly easy with that length of lens but doing a portrait of an animal can sometimes be easier with a longer lens. There are actually some photographers out there who will use a wide angle lens for portraits, (being careful of lens distortion) but I'm not too fond of portraits taken with a wide angle myself but again there are exceptions. 75-100 is ideal for my purposes but I've been known to use a 55 to good effect too. I did a shoot not too long ago with someone who had a very narrow face and a rather prominent nose. Using a 28 and a 55 actually worked out a bit better for him. It all depends upon what type of portrait subject, their proportions, and how close you can get.
I am trying to re-word this question...
at say, 40mm it would take F2.8 to blur the background. If I choose 200mm and took the same photo (size in frame) you can get the same (or better) at F5.6 or more.... that way the lens is stopped down so that it will sharper. is there a focal range where you can have a balance of both worlds??

hope this explains it better

randy
09-02-2012, 04:02 PM   #11
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Its all physics, you cannot change one without changing something else to compensate.

But a full frame or larger camera will make DOF appear smaller, so if you stop down your lens a stop (from f2.8 to f4), you can obtain a similar looking DOF with the sharper fstop... assuming all else being equal. I believe because you'd have to move in closer to retain the image size on the full frame sensor, DOF will actually be equal with the stopped down aperture as it was on the cropped sensor. To stop down even further, you'd increase sensor size and move in again.

I believe this is the only reason a full frame sensor makes sense over cropped (IMOHO, you can agree or disagree on that point :-) ) -- you can get shallower DOF with a less expensive (smaller aperture) lens.
e.g. If for some reason you like the f1.4 look on a full frame sensor and you wanted the same on your crop sensor, you'd need a f1.0 lens. Very expensive or impossible to get.
09-02-2012, 04:15 PM   #12
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QuoteOriginally posted by slip Quote
I am trying to re-word this question...
at say, 40mm it would take F2.8 to blur the background. If I choose 200mm and took the same photo (size in frame) you can get the same (or better) at F5.6 or more.... that way the lens is stopped down so that it will sharper. is there a focal range where you can have a balance of both worlds??

hope this explains it better

randy
No actually. As I said, check with a DOF calculator, you will find that for the same image size, the depth of field is the same at identical apertures .

I did this calculation once for someone who was looking to do portraits with longer lenses and was wondering about background distance to get similar effects. I used the DOF calculator and found that the setup remains the same I.e. subject to background if you step back proportionally to the change in focal length so that the image size remains the same
09-02-2012, 04:39 PM   #13
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QuoteOriginally posted by slip Quote
I am trying to re-word this question...
at say, 40mm it would take F2.8 to blur the background. If I choose 200mm and took the same photo (size in frame) you can get the same (or better) at F5.6 or more.... that way the lens is stopped down so that it will sharper. is there a focal range where you can have a balance of both worlds??

hope this explains it better

randy
I'd say that sort of thing is a matter of experience and personal taste. Best approach is to systematically test, evaluate results, and decide what works for you.
09-02-2012, 07:53 PM   #14
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Also, stopping down will cause differing effects depending upon the lens. More often then not, you'll wind up with hexagonal/octagonal/etc circles as opposed to the usually more preferable circle effects.

Some lenses are built to counteract this, but usually you'll wind up with oddly shaped bokeh as a result of stopping down which may or may not be to everyone's taste.
09-02-2012, 08:52 PM   #15
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QuoteOriginally posted by Lowell Goudge Quote
I did some calculations a while back to answer a question of depth of field using diffence lenses and standing back futher with longer focal lengths.

It turns out that if you keep image size the same, the DOF is the same if you keep aperture the same. I.e. F2.8 for any two lenses gives the same depth of field for the same magnification ratio. ( not exact but to a first order approximation of within 10%)
It is very close to exact if you are far from hyperfocal conditions (ie. the far background is noticeably out-of focus).
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