Background isolation...perception

Hi,

A friend and myself were discussing background isolation with large aperture lenses. We're both of the opinion that the blurring of the background (of the resultant image) is more extreme than we perceive it to be when looking through the viewfinder.....shooting with the lens at max aperture, of course.

Neither of us could arrive at a satisfactory explanation for this.....is it simply perception or is there another reason why it should appear this way.

Thanks in anticipation of any forthcoming sound reasoning.

Bob

Yes, you are right. "Modern" focussing screens have this effect (I think the first one was manufactured by Minolta under the name "acute-matte" on the X-700). Basically, instead of a matte screen (just "rough"), they are a collection of microscopic optical devices designed to better concentrate the light from the lens. The advantage is that they are much brighter with not so fast lenses (e.g. the typical f/5.6 zoom). The inconvenient is that each optical device only sees light from a relatively narrow angle, which means they only see part of the light from a fast lens. This effectively acts like a diaphragm, reducing the useful aperture of the lens.

For some cameras, alternative screens for fast lenses are available.

I understand the principal of the focus screen matte, Jerome, and I use an EC-S screen for my Zeiss manual focus lenses. The bit that I can't rationalise is that the screen is effectively sharpening that which the lens has already blurred.

Bob

The screen simply restricts the aperture of the lens. If you put a diaphragm behind a lens, it will also "sharpen what the lens has blurred".

Jerome Marot said:

The screen simply restricts the aperture of the lens. If you put a diaphragm behind a lens, it will also "sharpen what the lens has blurred".

Click to expand...

Of course....I feel stupid now. I'd never seen it that way! Thanks Jerome,

Bob

Bob Latham said:

I understand the principal of the focus screen matte, Jerome, and I use an EC-S screen for my Zeiss manual focus lenses. The bit that I can't rationalise is that the screen is effectively sharpening that which the lens has already blurred.

Click to expand...

Hi Bob,

It's more because the regular focus screens are not diffusing enough, and the laser pattern does redirect some of the light towards the viewer. If they were perfectly diffusing then (especially the corners of) the image would become much darker, and Fresnel lenses would be required to at least compensate for part of that.

I also use the EC-S screen in my Canon. It shows a much better preview of the DOF at wide apertures, but gets also darker with narrower than f/2.8 apertures faster. When using a regular focus screen and the aperture preview button, you will see that at anything wider than f/3.5 a regular screen will not show the DOF effect at all. The EC-S screen does allow to judge it better, but probably not good enough. Also the fact that it's a relatively small preview image, will hide some of the finer Bokeh effects.

Cheers,
Bart

Hi, Jerome,

Jerome Marot said:

The screen simply restricts the aperture of the lens. If you put a diaphragm behind a lens, it will also "sharpen what the lens has blurred".

Click to expand...

Thanks for your insights into this matter.

Those familiar with the principle of the "split prism" focusing aid will recall that, with respect to each side of the prism pair, the apparent aperture of the lens is at one side of the "whole real aperture" (in order to get the "rangefinder" effect of the split prism scheme), and is thus of necessity smaller than the "whole real aperture". Thus again we get a greater visible depth of field on each side of the prism pair than we do on the ground glass generally.

This is broadly parallel to what happens with the "microprism collar" that many traditional focusing screens had around the prism pair, and apparently the same holds for the overall focusing screen structure being discussed here.

Best regards,

Doug

Bob Latham said:

Hi,

A friend and myself were discussing background isolation with large aperture lenses. We're both of the opinion that the blurring of the background (of the resultant image) is more extreme than we perceive it to be when looking through the viewfinder.....shooting with the lens at max aperture, of course.

Neither of us could arrive at a satisfactory explanation for this.....is it simply perception or is there another reason why it should appear this way.

Thanks in anticipation of any forthcoming sound reasoning.

Bob

Click to expand...

A key factor could be hidden in the phrase "of the resultant image".

If the final image is bigger than what is seen through the viewfinder then magnification has happened. And blur is magnified and made more prominent just like everything else. A classic example is a contact sheet from 35mm film. Everything looks remarkably sharp on the contact sheet and it is not until an enlargement is attempted that the actual blurriness of the negative is discovered.

A counter-example I see often comes from composing a picture on the 8x10 ground-glass of a big view camera and finishing the photograph as an 8x10 contact. There is no apparent difference in blurriness between the original and final images. This is an hard-line example of "what you see is definitely what you get" and part-way contributes to the peculiar visual integrity of large format camera work.

So how does the modern LCD viewfinder perform with wide apertures to predict the final blur of the background?

Asher

Asher Kelman said:

So how does the modern LCD viewfinder perform with wide apertures to predict the final blur of the background?

Click to expand...

In theory, it should be a perfect image of the sensor. In practice, I find that the resolution, even with the best EVFs, is too low to judge the blur and sharpness.