Auto Focus, and Why? (1 Viewer)

[Kevin Conville]

I've been using an old Canon S70 to digiscope through a homemade adapter on a Nikon ED82 and 30x DS EP and it works pretty good, in good light. In an attempt to improve low light performance I recently bought and subsequently returned a Canon S90. It wasn't much of an improvement for me over the 6 year old S70, surprisingly. That's the back story.

Now I'm very interested in the micro 4/3 cameras but am perplexed by something. Why/how do these auto focus through an eyepiece when I cannot get a Canon DSLR to do it with an optically faster lens? I never quite understood why P&Ss auto focus but the 4/3 stuff is configured much like SLR lenses, with near aps-sized sensors, and yet they auto focus. Excuse me if i seem a little dense here. Can someone please explain?

Cheers,
Kevin

 

[RJM]

FWIW Kevin, I also could not get AF to work when I tried handholding my Nikon D90 w/50mm F1.4 up to the 30x DS on the ED82. Memory is now fuzzy on the exact issue but at least the shutter would not fire. Figured I just needed to mount the camera or use a 35mm lens but never pursued it as the total possible focal length of ~1500mm was not worth the weight and hassle. But it could just as well be some quirk of the Nikon ED82 + 30x DS eyepiece.

In the end, the limited focal length range when using a dSLR/fieldscope is the primary reason I went back to a Panny FX150 compact digicam for digiscoping.

However I do think GH1 with a 200mm lens would be a great companion to have along with a digisco kit for close-in and flight shot opportunities.

cheers,
Rick

 

[Josef]

I think I'll probably trip over myself here and last time I used a SLR was 35mm but; 1. I used a canon s90/scope and using a magnifying glass focused my scope with the monitor and took pic with time delay and got satisfactory result. 2 Use the same process all the time with my P&S and get satisfactory results, usually only disappointed when in the panic of things camera goes to a branch I didn't see. 3. The 4/3 I believe uses a similar contrast defined focus technique as 1 and 2 above instead of optically like a typical DSLR.

I've sort of concluded that with the contrast defined P&S cameras the quality of focus (and speed) is in the way the camera is programmed to say yes there is or no there is not focus. Like i said I don't know how DSLRs can be so fast on moving birds and how they do it optically is amazing.

 

[iporali]

Now I'm very interested in the micro 4/3 cameras but am perplexed by something. Why/how do these auto focus through an eyepiece when I cannot get a Canon DSLR to do it with an optically faster lens? I never quite understood why P&Ss auto focus but the 4/3 stuff is configured much like SLR lenses, with near aps-sized sensors, and yet they auto focus.

Kevin,

It is because phase detection AF of the DSLRs require apertures of at least f/5.6, which can't be reached with exit pupils of 2-4mm. It does not help to have a fast lens behind the scope if it is stopped down to f/10 by the scope. Like Josef said the m4/3 use contrast-detection-AF, which is slow but can function at "any" apertures. The problem with phase-AF is not low light per se, but the narrow angle at which light rays enter the AF-cells. Film-SLR users may remember "blacking-out" of the split-prisms at small apertures - if the same occurs with phase-AF sensors... well, the AF just does not work .

BTW, the reason why phase-AF is so fast, is that the sensor "knows" immediately the direction and amount of necessary correction, whereas contrast-AF requires many samples before it finds where the maximal contrast is achieved.

HTH,

Ilkka :t:

 

[Paul Corfield]

My Canon 450D will auto focus with the kit lens through my 35X eyepiece up to around 30mm or maybe 35mm as long as the contrast is good. Certainly has no problem at say 28mm.

Paul.

 

[Neil]

Ilkka explaination sounds good to me. I have got my D90 Nikon to AF very slowly in good light but it's not worth the trouble at f11 ( Swaro 80mm scope).
Suprisingly my Olympus 4/3 rds 50 mm Macro won't AF with the G1 either so I have to use it in MF.
Neil.

 

[gergrd]

Neil said:

"Suprisingly my Olympus 4/3 rds 50 mm Macro won't AF with the G1 either so I have to use it in MF."

Per Panasonic's website: http://panasonic.jp/support/global/cs/dsc/connect/g1.html

As shown, a long list of auto-focus 4/3rds lensese (including the Oly 50mm macro) will not autofocus with the Panasonic micro 4/3rds cameras (regardless of whether or not they are being used for digiscoping). I read a vague explanation of why that is, but I don't have a source. I've also read that all four-thirds autofocus lenses will auto focus with the Oly micro 4/3rds cameras (albeit slowly).