Convergence vs Divergence

[ricktas]

Interesting that people seem to think these buttons and dials turned up in reasonable recent years, commenting as though they are a recent and sudden change. Yet here is the top of a Canon T70 which was introduced in 1984 - almost 30 years ago!!! I have one of these. It was the first SLR I bought with my own money..once I started working. Prior to that I was given cameras as presents and gifts (box brownie, kodak 110 instamatic)



and before that, the Canon T50 (in 1983)


This change has been gradual, not something that has happened overnight, and given us all plenty of time to adjust to it. Sometimes getting out of your comfort zone is good! Just as some do not like Convergence, some appear to not like change as well Sometime we spend to much time talking about the gear we use, than using it...anyone want to meet me on DPReview

[Tommo1965]

LOL..yeah I get that about "talking about the gear rather than using it"..Im very guilty of that

my X700 was introduced in 1981...but i never bought it then ..I started with a EOS 750..then a week later after buyers remorse ( yes we had that in the eightes too) a EOS 650 which was released in 1987 ..i then traded that in for the x700 as I wanted to even then, De Tech myself ...seem silly now as even the EOS650 was a basic camera by the D800 standard...but i want manual focus lenses etc..( also they were cheaper to Buy)




I still have the x700...its a beauty...Ill never shoot with it again.. so I do embrace the new Tech...just wish the interface was a little old-old Tech sometimes

[ameerat42]

(Last time I looked there was close convergence of opinion on the degree of divergence from whatever opinion the article's author was espousing.)

Am(generally in parallel).

[swifty]

I don't know much about STM lens(es) specifically, but if they are what I think they are, then their design and benefits are yet to be full seen on DSLRs.

My understanding of the old type DSLR lenses, is that they're optimised to focus speedily when using CDAF as was the case for DSLRs for an eternity.
Their inherent design is such that they need to be quick to 'hunt' to acquire focus. So for CDAF cameras, this makes sense as in reality the lens is actually hunting to achieve focus so that the camera/lens combo focuses quickly.
PDAF is different tho. The design is such that the AF sensor actually knows which way to turn the lens, so that AF is not done by hunting(the lens), it's achieve with a decisive and direct control communicating to the lens which way to turn and by exactly how much.

non PDAF optimised lenses(ie. any standard af lens type since day one in AF lens history) don't work well in PDAF mode compared to PDAF optimised lenses.

My (admittedly very small) understanding of the STM lens in Canon's line up is that it's PDAF optimised .. hence why it's fast to focus on the 70D.

With all the complexity and annoyances of inaccuracy in CDAF(did someone say D800! ) ... long before EVF replace OVF, PDAF will replace CDAF in DSLRs for many reasons.

I have no idea on how easy it would be for manufacturers to convert current lens types into PDAF optimised versions ..... if at all possible to do.

if this is indeed the case, I'm hoping that Nikon offer some sort of conversion process(like they did for non Ai to Ai) ... don't want to be stuck with a slow focusing lens that won't work well on some future camera that I'll end up with.

I have to admit tho, my only knowledge of the STM lens from Canon was the comparison that DPR did between a 70D+STM lens vs a Sony camera.

The point is: that this videocentric feature(or requirement) will eventually see benefits in that af modules and lenses will not 'hunt for focus' any more and will know exactly which way to focus and by exactly how much.

Sorry but I'm completely lost in this middle part of your post.
Firstly, aren't DSLR lenses (other than Canon STMs) designed for PDAF, not CDAF?

As you've said in PDAF, the AF system knows in what direction and by how much to focus the lens. This happens in both traditional PDAF and on-sensor PDAF. So all traditional DSLR lenses are designed to make quick jumps and in the right direction according to what the PDAF is telling it to do.

But before the introduction of on-sensor PDAF, the traditional PDAF designed DSLR lenses worked poorly in liveview mode of DSLRs which uses CDAF. Cos in CDAF the AF doesn't know which direction and by how much so it achieves focus by hunting/scanning back and forth and analysing the amount of contrast. The corresponding CDAF lens then needs to have smooth incremental AF motor steps as the CDAF system analyses the image for differences in contrast, which PDAF DSLR lenses weren't very good at doing because their motors are better for quick big jumps.

In traditional DSLRs, there's an alternative optical path for the PDAF sensor (the submirror just behind the main mirror directs light to the PDAF module). So when the mirror was removed in the mirrorless camera designs, the cameras had to rely completely on CDAF which was sensor based. So began a whole generation of new lenses for mirrorless cameras that are more opitmised for CDAF but they faired poorly in C-AF cos every time the subject moved, it didn't know which direction to adjust focus unlike with PDAF. But now virtually every manufacturer has figured out how to add PDAF on the sensor itself so now even traditional DSLR lenses worked better on mirrorless cameras that have on-sensor PDAF cos that's how traditional DSLR lenses were designed to work. Many of the manufacturers have moved to a hybrid design where the PDAF does the large directional AF adjustments and the CDAF takes over for the fine AF adjustments cos its more accurate. The OMD EM1 is an example of this.

CDAF and now on-sensor PDAF aren't inaccurate at all. They are very accurate because they are located on the sensor itself. Rather than a different location with its own optical path as with traditional DSLRs that also meant traditionally the alignment had to be very good. The D800 issue was a misalignment of the PDAF module with the sensor. This is also why micro-adjustments are very useful and necessary for the traditional PDAFs.
With on-sensor PDAFs, the micro-adjustments are necessary only for tolerances in the lens but the AF sensors are located on-sensor and shouldn't have any alignment issues.

STM lenses are designed with different motors for smooth incremental steps in focus change. I haven't read anywhere that its got any PDAF optimisation, rather its designed really to work well with CDAF. The first STM lenses appeared mid last year and the first on-sensor PDAF Canon camera (70D) only appeared this year. It is very much a video-centric lens design with silent focusing and smooth incremental AF changes as well as smooth aperture iris control (as opposed to stepped).

[arthurking83]

LOL yeah sorry Swifty .. late night post, arduous working hours .. stressed eyeballs, and good dose of goofed up brainfade by the dummy banging on the keyboard!! ..

I mixed up CDAF and PDAF in my reply there.

[swifty]

LOL yeah sorry Swifty .. late night post, arduous working hours .. stressed eyeballs, and good dose of goofed up brainfade by the dummy banging on the keyboard!! ..

I mixed up CDAF and PDAF in my reply there.

No worries, no biggie.