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Assuming I framed a shot of a person for example, filling the frame in exactly the same way, with an 85mmf1.8 and then a 50mm f1.8 - would I get more blur/bokeh with the 85mm ? - I love my bokeh and my 80-200mm f2.8 is just too heavy and conspicuous for carrying around to use ad-hoc, so it doesn't get used as it should. Incidentally does anyone have an 85 f1.8 & 200f2.8 - how do they compare in the same situation previously mentioned. I'm considering selling the beast for something more compact is why I ask.
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From the same position, if you set 85mm on the 80-200mm, and f2.8, and f1.8 on the 85mm, the 85 should give more blur of the background. If the weight and size bother you, maybe take a look at m4/3. I did.
For the same size magnification in the viewfinder a longer focal length lens has more blur (narrower depth of field) than a shorter focal length lens, if both lenses are used at the same aperture.
Whether the bokeh is better depends on the lens. The 50mm f1.8 G has better bokeh than the 50mm D and generally 85mm "portrait" primes have better bokeh than 50 mm lenses.
m4/3 for the same viewfinder magnifications has two stops more sharpness than FX and one stop more sharpness than DX. This is the opposite of the effect you seem to want to achieve.
If I recall correctly, if you change position to get the same framing with a different focal length, then the depth of field is the same. However bokeh is dependent on lens construction among other things so this is unpredictable - for example using two lenses of same focal length but from different manufacturers can give quite different bokeh.
Quote: From the same position, if you....
That's not what he asked. He said:
Quote: filling the frame in exactly the same way,
..which means changing position and, thereby, altering the perspective.
I think the short answer is that there is no answer, other than what you might discover by trial and error. Changing shooting position to retain the framing will mean different depth of field which may, or may not affect bokeh. But, as LS states, bokeh is often a characteristic of a particular lens, determioned, in part by the lens construction and also by the iris diaphragm.
PS: I use an old Nikkor 85mm f/1.8D lens and it does have a nice bokeh at f/4 on head and shoulders shots.
Forgetting perspective - just wondered on depth of field and thereby blur of background - if I frame the shot the same will 85mm f1.8 give more background blur than 50mm f1.8 ? As I say 50mm f1.8 doesn't quite give me the highly blurred background I want and the 80-200mm f2.8 is a bit heavy for regular usage.
If the subject is the same size in the frame, the depth of field will be the same, even if you change lenses, though the perspective will change (as mentioned)
A subject at 10 feet on a 50mm lens at f/2 will have the same depth of field as a subject at 20 feet on a 100mm lens at f/2 and will be the same size in the viewfinder.
(Around 12" or so, dependent on your calculation constants).
Quote: Forgetting perspective - just wondered on depth of field and thereby blur of background - if I frame the shot the same will 85mm f1.8 give more background blur than 50mm f1.8 ? As I say 50mm f1.8 doesn't quite give me the highly blurred background I want and the 80-200mm f2.8 is a bit heavy for regular usage.
As above, it depends on lens construction. But it isn't just about 'how much blur', it is also the way the bokeh is rendered - for example with one lens straight edges can have an appearance like camera shake (with seeing parallel lines), but another lens will just blur it completely into a vague coloured stripe. A lot of lens reviews include samples of bokeh so you could look for those.
Apologies - for some reason the first few responses didn't show when I first looked - what your saying is basically the 85mm used for a head and shoulders shot for example will give more blur/bokeh than a 50mm used for the same shot.
Thanks - that's what I wanted to know..
...but don't confuse "blur" and "bokeh". See mikehit above.
Quote: what your saying is basically the 85mm used for a head and shoulders shot for example will give more blur/bokeh than a 50mm used for the same shot.
It will if you stay in the same place, but then the subject will be much bigger in the frame with the longer lens. If you move further away so the person fills the frame by the same amount as it did with the 50, the depth of field (and the amount of blur) will, to all intents and purposes be the same.
The amount of blur - ie the out of focus area will be the same for the combination of focal length, distance and aperture.
The bokeh - ie the 'quality' of the out of focus areas will vary with the construction of the different lenses. It's how the out of focus areas are rendered, and depends on the number and shape of the aperture blades, number and type of elements, type of glass etc etc. It is pretty subjective and hard to quantify, whereas depth of focus uses definite numerical values.
To sum up:
If you move twice as far away and use a lens twice the focal length, the subject will be the same size in frame and the depth of field will remain the same.
Right - so if I want a head and shoulders shot, using a large aperture will give more blur( I knew this bit), the larger the aperture the more blur. However an 85mmf1.8 is not going to give more blur than a 50mmf1.8 (both at the same ap.) when framing the person in the foreground the same. That makes sense - I was thinking regarding macro lenses , greater focal lengths don't reduce DoF at 1:1 do they. An 85mm would affect perspective and let me shoot further away. I'd be better off going for a 50mm f1.4 then for what I want. So why do people go for 85mm for portraits then?
Quote: An 85mm would affect perspective and let me shoot further away
To be pedantic it is the fact you are further away that changes perspective, not the fact you are using an 85mm lens.
which leads us onto:
So why do people go for 85mm for portraits then?
Perspective matters a lot in portrait photography - what you want is to get the apparent ratios between facial features correct. To take the two extremes: shoot from 3 feet with a wide angle lens and you get that huge bulbous nose, shoot from 20 feet with a 200mm lens and the features look flat (because of the effect of distance and perspective compression). Photographers found that using a lens in the focal range 90 to 135nm they were standing at a distance that the features 'looked right'.
So if you are using a APS-C camera you would want a lens between 60mm and 90mm and for micro 4/3 you would want a lens 45mm to 70mm because these lenses on those formats you are standing in the same position (for what its worth, it is one of the few times that quoting the crop factor is actually of any use because you are comparing formats).
As I understand it, depth of field reduces exponentially as the the distance to the subject decreases.
That makes Real world DOF calculations difficult.
There's lots of things that affect the depth of field, and endless pointless Internet arguements about them. In my experience distance to subject is the most important, particularly when shooting reasonably close subjects - shooting a face and shoulders portrait would put you about 0.5 - 1.5 meters away depending on focal length. Which makes the distance to subject a massive factor in the amount of 'blur' you get.
Shooting a 35mm or 50mm 1.8 would mean you would be very close to your subject for a head and shoulders portrait. Admittedly there would be distortion , which may or may not be a good thing. ( Lee Jeffrie's portraits on Flickr are a good example of portraits shot at 24mm. )
Shooting a head and shoulders portrait at 85mm would give you low distortion and a 'flattering' look (hence why thats the "classic" portrait focal length ). You'd be quite abit further away from the subject though, which has a massive impact on DOF.
I've got fast 28, 50 and 85mm primes and if I want a very blurred head and shoulders portrait I'd always go as wide as possible (depending on the amount of distortion that would be acceptable). A 20mm 1.8 head and shoulders portrait would give you ridiculous amounts of blur (and distortion).
I'd use the 85mm for my wife, the 50mm for my young son, the 28mm for my grandfather, with the 20mm reserved for my little brother when Gurning.
I guess the only way to answer the question is to borrow a couple of lenses and test them out .
Quote: and endless pointless Internet arguements about them
As far as the OP is concerned it is quite simple and explained above.
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