School me on Bokeh. Pics included.

edgar · Nov 12, 2009

Derrel said:
The easiest way to achieve a more shallow depth of field is to increase the size of the capture area. In other words, to step up to a full-frame sensor, which will allow you to use telephoto lenses with low f/numbers (like an 85mm f/1.2 or 85mm f/1.4 lens) from a distance of 20 feet, in order to frame a 6-foot tall man. Using an APS-C Canon, with a 1.6x FOV crop, to get the SAME field of view height, the camera must be 34 feet distant from the man. The depth of field differences are HUGE in this instance ,between the FF and the APS-C camera, using the same lens, but at widely different focus distances.

You are right concerning depth of field: the DoF depends both on the f-number and sensor size, and larger sensors give you shallower depths of field. However, background blur, as pointed out by Stosh, is a different issue. It turns out that background blur depends solely on the lens' real aperture, irrespective of both focal length and sensor size.

The only problem with APS-C sensors is that you have to step further from your subject, making long lenses a little bit inconvenient. Eventually, if you get too far, the background may not be the background (in Stosh's sense) anymore.

Consider as an example a 85/1.4 on both full-frame and APS-C. On FF it's a short tele lens with basically zero depth of field and huge background blur. On APS-C it's a medium tele with very narrow DoF (more than on FF, which is often a good thing) and huge background blur (same as on FF!). Still fantastic for Bokeh lovers, except you need more distance.

Edgar (new to this forum).

eric-holmes · Nov 12, 2009

So much information. I will have to process all of this. So am I understanding right? A 50mm lens would fare better for portraits as opposed to an 85mm or 105mm because it's not a full frame.

GeneralBenson · Nov 12, 2009

Derrel said:
_MG_2066_85AFD on 20D.jpg photo - Derrel photos at pbase.com
foreground bokeh test and then three seconds later

_MG_2067_85 AFD on 20D.jpg photo - Derrel photos at pbase.com
background bokeh

IMG_2153_EF 70-20028L-IS at f-2.8 bokeh.jpg photo - Derrel photos at pbase.com
Santa-Canon 200mm f/2.8 L-IS lens at f/2.8

DSC_0804_200VR at f2.8.jpg photo - Derrel photos at pbase.com
Jenni, Nikon 200mm f/2 VR-G at f/2.8

That last shot is gerat. The Nikon 200/2 is an awesome lens. Does the Nikon have rounded aperture blades, so that at f/2 it will be perfect circles? The Bokeh on the Canon lens looks like lemons, what's up with that?

Stosh · Nov 12, 2009

eric-holmes said:
So much information. I will have to process all of this. So am I understanding right? A 50mm lens would fare better for portraits as opposed to an 85mm or 105mm because it's not a full frame.

Forget about your sensor size. It's constant. The important thing right now is you have the capability of interchangeable lenses. Use this to your advantage. For every sensor size in the world, big or small, the following paragraph is true:

You can't say one lens is better for portraits. Every situation is different, unless you're in a studio. First thing is first - decide which blur applies to your situation - depth of field (background is close to subject), or background blur (background is distant to subject). Then knowing that you can best pick your lens based on the previous posts. Most blur for depth of field = lowest f-number (also called fastest lens). Most blur for background blur = largest aperture.

Edit: I just realized I didn't answer your post, but that's because it can't be answered. You only gave half of the information. Without giving the lens's f-numbers, we have no idea which lens is best for which situation. You would also need to know camera to subject distance as well as subject to background distance.

Derrel · Nov 12, 2009

Yes, the 70200 f/2.8 L-IS produces ellipsoidal out of focus highlights on point sources when shot at wide apertures...it's a characteristic of the lens. In this gallery minilights gallery 2 Photo Gallery by Derrel at pbase.com you can compare the 200/2 versus the Nikon 70-200 2.8 VR lens at different apertures. I tried the Canon zoom out, but it yielded ugly, ellipsoidal OOF point light sources. So ugly I decided I could not use it for this scene.

Almost any lens will produce round OOF hoghights if used wide-open--although many lenses have an internal baffling system, usually a square-ish shaped rectangular opening at the rear of the lens, an that can cause out-of-round highlights on point sources. The very rear part of the Canon 70-200 f/2.8 L-IS lens does NOT have a rounded opening--the opening is sort of square-is, which is what is creating the ellipsoidal or lemon-shaped bokeh.

The Sigma 30mm f/1.4 also has mechanical vignetting that causes a similar ellipsoidal rendering of point light sources. The Nikon 200/2 VR and the 70-200 2.8 have perfectly rounded rear sections that produce no mechanical vignetting of the light rays exiting the lens. I included the Santa and Jenni photos just to show that different lenses can produce a radically different bokeh "signature" on OOF highlights.

eric-holmes · Nov 12, 2009

Stosh said:
Edit: I just realized I didn't answer your post, but that's because it can't be answered. You only gave half of the information. Without giving the lens's f-numbers, we have no idea which lens is best for which situation. You would also need to know camera to subject distance as well as subject to background distance.

I'm sorry, I forgot to mention. I am looking at the 50mm 1.8D. I believe that most of the portraits I would do would have the background generally close to them.