I currently have a 50mm 1.8 and 85mm 1.8 and would want to know if the bokeh or sharpness would be any different at all. Many seem to say that the 85mm woud work best, but wouldn’t I have to bump up my Aperture to get a sharp image? I’m shooting on a full frame camera not a crop sensor.
If you want more depth of field in your subject, stand further away, &/or close the aperture.
The added distance will mean the 'percentage' of distances from front to back of your subject will be less, increasing apparent DoF, at the expense of your subject taking up less of the frame.
If you want more bokeh with that setting, separate the background from the subject by a greater distance.
You can't really have both at once.
Longer focal length will give a shorter DoF for a given distance. Shorter focal length will give you a wider background.
I don't have the math to calculate what the actual difference would be. I'd try both & see which you prefer.
To get the same framing of your subject using the same camera, you'll have to shoot from further back with the 85mm lens than with the 50mm lens. The decrease in DoF from using an 85mm lens compared to a 50mm lens when both are used from the same distance is offset by the increase in DoF when using the same lens from a greater distance. In the end at most distances they usually offset each other almost exactly.
Assuming an 8x10 display size viewed from a distance of 12 inches, shooting from 20 feet with the 50mm lens at f/1.8 on a FF camera will give you a DoF of about 1.86 feet or 22.32 inches. Shooting from 34 feet with the 85mm lens at f/1.8 on a FF camera will give you a DoF of about 1.86 feet or 22.32 inches.
Assuming an 8x10 display size viewed from a distance of 12 inches, shooting from 10 feet with the 50mm lens at f/1.8 on a FF camera will give you a DoF of about 0.46 feet or 5.52 inches. Shooting from 17 feet with the 85mm lens at f/1.8 on a FF camera will give you a DoF of about 0.46 feet or 5.52 inches.
In both cases the total depth of field is the same when the 85mm lens is used from 1.7X the distance of the 50mm lens to get the same framing of the same subject as the 50mm lens. What does change is how much of that DoF is on front of and behind the point of focus.
At 20 feet, the 50mm lens distributes the DoF 0.89/0.97 feet in front of/behind the point of focus.
At 34 feet, the 85mm lens distributes the DoF 0.9/0.95 feet in front of/behind the point of focus.
At 10 feet the 50mm lens distributes the DoF 0.22/0.24 feet in front of/behind
the point of focus.
At 17 feet the 85mm lens distributes the DoF 0.23/0.23 feet in front of/behind the point of focus.
Please note: All distances are rounded to the nearest two significant digits to the right of the decimal, which can introduce rounding errors between the sum of the distances on each side of the point of focus and the total distance.
Having said that, which lens is sharper, a 50mm or an 85mm, depends entirely upon the two lenses in question.
A budget 50mm lens will probably not be as sharp as a premium 85mm lens. But a budget 85mm lens will probably not be as sharp as a premium 50mm lens, either.
Then, depending on how you define "sharp" one lens might be better than the other or vice versa.
Do you define "sharp" as absolute performance at the center of the lens? Average performance over the lens' entire field of view? Do you measure "sharpness" over the entire field of view using a flat test chart? Or do you base it on the shape of the space occupied by a three dimensional subject?
Lenses that are well corrected for field curvature and thus do better on flat test charts tend to have "rougher", "harsher", or "busier" bokeh than lenses that don't correct as much for field curvature but tend to have "smoother" bokeh.
What aperture you need to use to get all of your subject within the depth of field depends upon:
- The focal length of the lens.
- The distance from the camera to the point on the subject you want to be most in focus. There's always only one distance that is truly in focus. The rest of what we call "depth of field (DoF)" is not actually in sharpest focus, but is close enough that our eyes can't tell the difference when the image is displayed at a specific size from a specific viewing distance. In short, depth of field is an illusion, although a very convincing one.
- Which way your subject's body is oriented in relation to the camera. If they are standing perpendicular to the optical axis of the lens you'll need much less DoF than if they are lying prone along the same direction as the lens' optical axis.
- The size you intend to display the image.
- The distance from which you intend the image to be viewed.
For full body portraits, though, neither a 50mm or 85mm is probably your best choice unless you plan on shooting from halfway across a small parking lot, even with a full frame sensor.