M8.2 Back/Front Focus: should I lose any sleep?

[dfoo]

If you are focusing with a sufficiently small aperture, yes, its possible. However, for a large aperture with small DOF I don't think its possible to be accurate. For one thing on Canon cameras at least the matte screens don't show anything above 2.8 aperture.

With respect to using the focus indicator, thats cheating, unless you are using a MF lens (as I've done). However, that is a crutch which shows that you cannot really focus very accurately with a focus aid.

 

[rxmd]

If you are focusing with a sufficiently small aperture, yes, its possible. However, for a large aperture with small DOF I don't think its possible to be accurate. For one thing on Canon cameras at least the matte screens don't show anything above 2.8 aperture.

I don't think I understand what you're talking about when you say they don't show anything at f2.8 or higher, but in general there is a tradeoff between the brightness of a matte screen and its focusing accuracy.

A matte screen must not be 100% transparent. The idea is that an image is projected onto the matte screen, and that your eye focuses on this image. If the matte screen is too transparent, it will appear brighter, but your eye will focus on what is coming from the lens instead, and since your eye has built-in focusing, things will look sharper than they are and you can't focus accurately. That is not a problem with an autofocus camera, because you don't use the matte screen for focusing anyway, it's just for composition. In effect modern DSLR viewfinders are closer to what you have in your average rangefinder camera, except that the rangefinding bit is done by electronics and that the image is produced through the lens. That's why camera manufacturers have been putting more transparent, brighter screens in their cameras nowadays, which, consequently, make manual focusing more difficult and eventually impossible. You couldn't focus through an M3 viewfinder either even if there were coupled optics inside. Focusing aids such as a split prism or microprism ring are nice, but they're not the issue here; you can have one matte screen that is fine for manual focusing, and another that, while delivering a brighter image.

This tradeoff has been known for ages. Since 99% of customers don't do manual focusing, it is not a problem for most of the customer base, who enjoys a brighter viewfinder image instead. People who need manual focusing have been replacing screens in their DSLRs for something like a decade now, in analog SLRs even longer, professional DSLRs offer a broad range of focusing screens, for others there's plenty of aftermarket focusing screen options optimised for manual focusing, with focusing aids if you want them. It's neither news nor rocket science.

Philipp

 

[Gabriel M.A.]

If you are focusing with a sufficiently small aperture, yes, its possible. However, for a large aperture with small DOF I don't think its possible to be accurate. For one thing on Canon cameras at least the matte screens don't show anything above 2.8 aperture.

With respect to using the focus indicator, thats cheating, unless you are using a MF lens (as I've done). However, that is a crutch which shows that you cannot really focus very accurately with a focus aid.

The photos I posted above are with a 50mm f/1.8 Schneider-Kreuznach, wide-open.

Here's one with an auto-focus lens (Canon EF 50mm f/1.4) focused manually:

 

[Gabriel M.A.]

Another one with the Canon EF 50mm f/1.4 focused manually:




No split screen (nor "focus indicator") used in my Canon 5D with any of the shots above.

 

[dfoo]

What focusing screen are you using? I personally use the Ee-d. I've never tried the Ee-S, perhaps it is better. With the Ee-d screen I certainly cannot focus with any degree of accuracy with large apertures. The focus points in those images above are not the precise... try something critical, like the eye in a portrait and record your accuracy. I speak only for myself; it is very low.

 

[Gabriel M.A.]

I've used all three of them. I like the grid focusing screen, but it's the worst one for manual focusing. The standard one works fine for manual focusing, except for the Canon EF 50mm f/1.4 wide open and the 50mm f/2 Summicron-R, the other manual lenses are fine.

I never had a single issue with the 20D or 300D (aka Digital Rebel), though.

 

[markgay]

Be careful to avoid absolutes. Some people can focus a matte screen perfectly well. It depends on your eyes. I would argue that a split screen is a hell of a lot easier to focus (and easier = more keepers) but that is a relativity. It doesn't mean someone else (with sharp eyes) won't cope with other screens.

Erwin Puts did some calculations for rangefinder baseline accuracy which clearly showed the importance of the acuity of the user's eye. However as someone with dodgy eyesight, rangefinder focusing still beats other screens for me, with less eye strain over a full day and consequently fewer headaches.

Automatic focus is a totally different issue, not least because there are different types of AF and the fact that AF is not you - it doesn't know, precisely what you want to focus on. You can move AF targets around the screen but that's hardly faster than manual.

And then, there is a point where the rangefinder-eye combo just falls over - somewhere around 135mm.

It's nothing new. Often chewed over, rarely rowed over.

 

[jaapv]

Be careful to avoid absolutes. Some people can focus a matte screen perfectly well. It depends on your eyes. I would argue that a split screen is a hell of a lot easier to focus (and easier = more keepers) but that is a relativity. It doesn't mean someone else (with sharp eyes) won't cope with other screens.

Erwin Puts did some calculations for rangefinder baseline accuracy which clearly showed the importance of the acuity of the user's eye. However as someone with dodgy eyesight, rangefinder focusing still beats other screens for me, with less eye strain over a full day and consequently fewer headaches.

Automatic focus is a totally different issue, not least because there are different types of AF and the fact that AF is not you - it doesn't know, precisely what you want to focus on. You can move AF targets around the screen but that's hardly faster than manual.

And then, there is a point where the rangefinder-eye combo just falls over - somewhere around 135mm.

It's nothing new. Often chewed over, rarely rowed over.

It works both ways. Astigmatism makes it difficult to use a rangefinder - but is no problem on an SLR.