My friend and I were talking as we were shooting. I will spare you the whole conversation. Which would have more bokeh (granted you had the same framing)? I have no idea how to do the calculation while adjusting for the different subject distances due to different focal lengths.
Well, you can use DOFMaster's online calculator to find out how much you'll have in focus by plugging in the different parameters. That might give you an idea, at least...
Like Sean said, check out www.dofmaster.com:
100mm @ f/2 with a subjectdistance of 10 feet the total DoF is 0.22ft
200mm @ f/2.8 with a subjectdistance of 10 feet the total DoF is 0.08ft
These figures are assuming you are using your Canon Rebel 1000D/XS
Originally Posted by Mark Elberson
You know Mark, I started to post example figures just like that. However, then I realized that the subject distance would have to change in order to achieve the same framing. I'm assuming you'd need to double the subject distance in order to get the same framing. Wouldn't that be right? If so, the total DOF using a 200mm lens (assuming the same framing) would be .32 feet.
All of that was going through my head this morning as I was getting ready for work. So I deleted the example I had typed out and just posted the site recommendation until I could think it through without having to do fifteen other things at once. :-)
Great point Sean and valuable lesson! Thanks for bringing that to my (and everyone else's) attention.
You know Mark, I started to post example figures just like that. However, then I realized that the subject distance would have to change in order to achieve the same framing. I'm assuming you'd need to double the subject distance in order to get the same framing. Wouldn't that be right? If so, the total DOF using a 200mm lens (assuming the same framing) would be .32 feet.
All of that was going through my head this morning as I was getting ready for work. So I deleted the example I had typed out and just posted the site recommendation until I could think it through without having to do fifteen other things at once. :-)
The "same framing" (original post specification) would put the 200 @ f2.8 with a subject distance of 20ft, the total DoF is .32ft
The 100 @f/2 wins the shorter DoF calculation but does it produce better bokeh?
The EF 100 f/2 USMhas8 aperture blades, but I can't find any review that tests it's bokeh. Bryan has some nice sample shots and you'll notice in this colorful grass photothat even though the background is blurred there is still a bit of edginess/nervousness to the bokeh. This lens probably has straight rather than curved aperture blades. It would also be interesting to see how the bokeh changes stopped down and subject-distance get's shorter. Bokeh can also change across the frame as well as have oinion like rings in speculars rather than flat circular spots. There is also the dreaded bokeh CA. Uuuoooohhhh!!
The 200 f2.8 has 8 aperture blades as well but I believe that twice the reach will yield better bokeh even though the DoF calculations favor the 100 f2. Looking at Bryans Coneflower Photoit's obvious that whatever is in the background has melted away in to ooF land in a very dreamy manner. Even the stem on the flower is slowly melting away so "buttery smooth".
If all-of-a-sudden Ifell into some major extra coin I would not hesitate to purchase a used 200 f1.8L or the new 200 f2.0L. Even the 300 f2.8L would be very tempting. The creamy bokeh produced by these lenses is the reason they're so highly coveted.
Bokeh seems to be a factor of many things not just DoF and not necessarily the lens design. (Though the more aperture blades and the more curved they are the better.) Also, the FOV vs. focal length playsa major role. For any given aperture, as the fov angle decreases the smoothness of the bokeh increases because less of the background is being rendered by the lens. I have some killer closeups that I took years ago with the el-cheapo Tamron 70-300 f3.5-5.6 Di LD at focal lengths of 180-260 at f4-f8 that have very beautiful creamy bokeh.Often, it's the time of day and the harshness of specular reflections and/or contrast areas like limbs against the skywhich either serve to compliment or detract from a len's bokeh ability. I posted 2 photos of my cat in Postyour Petsthe second one is th EF 50 f1.4 @ f1.4. I find the bokeh extremely pleasing in the ooF areas. I do however, have shots of my son playingin the shadeof a crepe myrtle in bright daylight where the speculars from the surrounding leaves creates an extremely nerve racking bokeh. Same lens in two different situations can yield two different results. The EF 100 f2.8 USM Macro... ? I can't say I've ever been displeased with the bokeh performance of that lens. I'm rambling......
So, my vote is for the longer reaching 200 f2.8....
The 200mm f/2.8 will have more out of focus blur, but the 100mm f/2 will have thinner DOF. The amount of the difference for the background blur will depend on how close you are to the hyperfocal distance. The reason why the 200mm f/2.8 has more blur even though it has a slower f-number is because it has higher magnification.
For example, using 35mm and h/CoC=1440 (CoC=0.025mm):
100mm f/2 @ 3 meters: 84mm DOF
200mm f/2.8 @ 6 meters: 118mm DOF
You can see the 200mm f/2.8 DOF is 40% deeper when framed the same way. Even so, it has more OOF blur.
(By the way, bokeh is not the "proper" term for this, because it only relates to the quality of the out of focus blur, not the quantity.)
Thanks guys. I was excited to see so many replies by only 930am! So just to make sure. To get the same framing for different focal lengths the subject distance is proportional to the focal length? (50mm-5ft, 100mm-10ft, 200mm-20ft, 300-30ft, and on and on (even in the reverse direction?)) And just to throw another wrench in the machine. I have the 100mm f/2.0 and the 300mm f/4.0. This is even another question, but I guess I can calculate it now for sure granted the subject distance assertion is correct.
Yes. There is a six-fold difference between 50mm and 300mm, so to get the same field of view (not angle of view), you need a six-fold difference in subject distance.
The 300mm will have slightly more diffuse OOF blur and deeper DOF. (When framed the same for distances far from the hyperfocal distance.)
Calculating OOF blur is much less common than calculating DOF. The only calculator I know of is Bob Atkin's:
http://www.bobatkins.com/photography/technical/bokeh_background_blur.html
I plugged in your numbers and used a 2 meter focus distance and typical 35mm 8x10 CoC values:
Interesting, but... does he say *what* he is calculating? How does he define OOF blur? Do all these numbers come together to make a single measure of bokeh?
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