The Three Great Lens Mysteries

Mystery 1:
How can different lenses have different FF/BF characteristics on one body?

Mystery 2:
Why is it not possible to use non-centre AF points with a manual lens?

Mystery 3:
Why is it not possible to use multi-zone metering with a manual lens?

Regarding M1:
My understanding of AF is that the camera uses a feedback loop to achieve maximum sharpness. The lens should then not matter at all. My hypothesis is that lenses differ in chromatic aberrations and that the latter can throw off the prism based phase comparison. Another hypothesis is that there is no 100% feedback but that the camera always applies a small offset step after having found the correct point. This would be necessary if the optical path lengths between mount and AF unit and mount and sensor respectively where slightly different in each camera and instead of doing an optimal physical alignment, some software correction takes place. If lenses differ in how they actuate this final offset step then they would yield different FF/BF phenomena.


Regarding M2:
Centre focusing and then recomposing can introduce intolerable focusing errors when the DOF is very thin. It'll be nice to be able to use all AF points for focus confirmation. My hypothesis is that the camera doesn't know the aperture setting and if the aperture is stopped down too much, outer AF points may not work anymore. Is there a way, using information about the mount contacts, to make the camera believe that the manual lens can support multiple AF points? Not sure why even "A" lenses don't support them.

Regarding M3: I know that the camera needs to know the maximum and minimum apertures of the lens and manual lenses don't communicate them. But why not ask the user to enter this data? Again, perhaps, using information about the mount contacts, one could make a manual lens appear more capable.

With respect to M2 & M3 I could imagine that there are actually no technical obstacles but that Pentax wants people to buy their new lenses rather than being snug as bug in a rug with their old Takumar glass.

Although it is great that you can use manual lenses on your Pentax DSLR with just an extra button press added, I wonder why the following support hasn't been implemented:

Allow the user to enter an aperture value in Av mode. If it doesn't match the aperture set on the lens, tough. But if it does then you wouldn't need to dial in exposure compensation yourself. This shooting mode allows quicker shooting than "manual" + button press + shutter release.

Why can't there be a mode where the camera doesn't meter wide open and then exposes, but 1. stops down. 2. meters. 3. exposes? In other words, why can I not make the camera press the AE-L/green button itself every time I press the shutter release? Would the shutter lag be too large?

P.S.: If there really are no "technical obstacles" and Pentax indeed wants to entice users to buy new lenses, that'll be totally legitimate to me. Right now, Pentax is just the right brand for me and I'm grateful that I can play with the old glass at all. Just wondering why some things are not supported and if there is a technical explanation for it.

This sort of procedure goes back over thirty years to when Nikon users had to set their lenses to f5.6 before mounting them and then rotate them from largest to smallest aperture to set the meter to the lens. The meter even had an external pin for this. A very unelegant design.

I think most of the points you raise are valid in the sense that these things could be done, but Pentax engineers have future development on their mind, not putting in features that 99% of users would find quite mystifying.

As it stands we can use very old lenses if we want to, which is a bonus indeed.

Regarding #2

You can't use auto focus with a manual focus lens.

Are you referring to the red dots on the image area? If so, these are not 'focus confirmation' indicators but an indication of the area AF is attempting to focus on and offer by themselves no indication of sharpness, focus confirmation is the green hexagon at the bottom of the viewfinder. This has to be enabled in the menu (depending on which camera you have) for use with MF lenses.

Regarding #2

Most of us use 'A' series manual lenses without any problems whatsoever, 'M', 'S' and 'K' series lenses are more limited in their use.

Of course not. I was indeed referring to "focus confirmation". The red indicators are not as unrelated to focus confirmation as you describe them. When manually focusing, they pop up exactly when the AF sensor determines something to be in focus. As you say, the green hexagon is the final authority, but I cannot recall a case, when focusing manually, when the two indicators disagreed.


The point of some of my questions was to find out whether there are technical explanations for some of these limitations or whether Pentax just built in some incentive to use more modern lenses.

Thanks for your comment. I'm not sure about the 99%. There seems to be a large number of Pentax users which use old glass with new bodies. Being able to use your legacy glass is one of the reasons to go for Pentax rather than Canikon.

I find it quite puzzling that no one seems to know the reason for why different lenses show different FF/BF behaviour on one body. This is an often discussed topic, yet the underlying technical issues seem to be unknown.

No I'm not sure about 99% either, but whatever the figure the majority of buyers will not be enthusuasts like we are. Otherwise sales of Pentax DSLRs would be very low indeed!

I agree & fervently disagree why in heavens sake keep on about old lenses when the world has gone digital I have just sold all my 6x7 gear & my 645 stuff to go completely digital. as John said no quote but how will Pentax (as jethro tull) advance if we keep on living in the past. I an

THANKS FOR YOU WHO ARE OUT THERE THANKS FOR LISTENING.

I agree with you about the red indicators - I do and always have used these to confirm focus on my K10D without any problems. The red indication 'flash' occurs when the green hexagon lights up - I have checked, as I was put in doubt after following another thread and discussion on this forum some time ago.

I find that this technique improves my images under some circumstances, as I don't have to divert my eyes away from the focus point.

The Magic Lantern Guide for the K10D specifically refers to using the red indicators to confirm focus as well.

A quality lens will always be a quality lens irrespective of whether it is used on digital or film. When it comes to manual focus I have yet find any modern lens that is a patch on the "K" and M series

There are some issues... lens coatings are more relevant when using digital cameras compared to film, and for this reason some of the old very wide angle and certainly fisheye lenses are not as practical as a modern "made for digital" lens.

I agree. I did some very informal comparisons between my DA* 50-135mm (at 135mm) and my K series 135mm f/2.5. Apart from issues with the K20D AWB giving the two lenses different WB values they were very comparable.

I won't be giving up my old glass, in fact quite the opposite.

K.

what has old lenses got to do with 'going digital' ?
a lens is a lens is a lense even

as Jethro Tull was want to say
' Really don't mind if I sit this one out '

Pete

when they are a zoom ?

I agree, I used to enjoy my old SMC 50mmf1.2 for instance

Pete

my Jethro Tull reference was apparently eroneous as here I am again
I bought a K85/2 a while ago.
Certainly no multi zone and off centre point
A deal breaker on the K-7 so thinking of tossing it.
Might swap it for another K50/1.2 if I could find one.

Pete



'...the iron tower smiles down apon the silver sea
and along the golden mile they'll swigging mugs of tea...'

seems more apropriate