« Tides of War | Main | Special Offer for TOP Readers »

Thursday, 21 April 2011

Comments

...wonder if this explains why I've never been able to manually focus any of the auto-focus cameras I've used since the 90's, but when I go back to my ancient Nikkormat, the image snaps in and out of focus all over the viewfinder, even with a 28mm? Everyone 'says' they can manually focus with a modern camera, but for the life of me, the difference is so subtle I don't understand how they're nailing it! Nikon seems to be better than Canon, but still difficult. Tried magnifiers and correction and...everything. But, when I go back to vintage equipment, easy as pie!

"...everyone who's saying that a good test of this is to take a fast lens and watch the viewfinder as you stop down has still designed a bad test."

But that's exactly what I wanted to find out - is f/1.4 visibly brighter in the viewfinder than f/2.8 with the same lens?

If f/2.8 is different on an f/2.8 lens than it is on a faster lens, I don't necessarily care. That wouldn't tell me whether or not the viewfinder continues to get brighter as I stop down the f/1.4 lens. I might care if f/2.8 on the faster lens was darker than f/2.8 on the slower lens, but that's not what I wanted to check.

To add another caveat: Not all AF systems honor the superfast lenses. Depending on the design of the AF sensors and the optical path reflecting the light on it, the effective f-stop for AF measurement can be several f-stops slower than the dialed-in aperture would indicate. Some DSLR bodies like my Sony A700 can compensate this partially with an extra sensor field for lenses of f2.8 or faster.
Add to this the fact that many old lens designs have a focus shift from wide open, where the AF measurement happens, to the f-stop effective during exposure, plus the one-shot system of most AF systems, i.e. calculating the amount of correction necessary and applying it, but *not* re-checking if it's really correct, and it becomes easily imaginable that the AF measurement at f1.4 can be wrong more often than not.
Together with the microlens design of the viewing screen mentioned by Oren, that also gives the optical impression of a slower f-stop, effectively using a f1.4 lens at its widest opening can be a real challenge (aka nightmare). And that's the reason my long awaited vintage Minolta 1.4/85mm doesn't get the exercise I had anticipated.

Here is Chuck Westfall's take:
http://digitaljournalist.org/issue0804/tech-tips.html

I think it should also be pointed out that the *depth of field* does not change, so you cannot use the view finder to evaluate areas of focus.

Good thing you just posted your favorite cartoon a few weeks back.

I was under the assumption this bottoms out at f/2.8. Meaning that with the default focus screen (not true of the special screens made for manual focusing, those benefit every extra stop at any aperture) you will see a brightness difference between an f/4 lens and an f/2.8, but you won't between an f/2 and an f/1.4.

I had a 40D and bought a focusing screen. It made it a bit dimmer, but I could see a difference between f/1.4 and f/2 when pressing the DOF preview button, whereas with the default screen I could not.

Fooled me, too. Canon's focusing screens for manual focus (such as the Ee-S for the 5D) are described with the warning that they are not recommended for lenses slower than f2.8 because the screens are "not very bright". That makes me presume that the faster lenses provide more light to the viewfinder screen. Perhaps Canon's viewfinders are not included in the category of "some" viewing screens.

I'd guess that those Canon focusing screens R. Edelman mentions are of the groundglass type instead the microlens type. This would explain why they are a) dimmer - microlenses were introduced because of the required higher brightness of the viewfinder image following the introduction of slow standard zooms and b) show a difference when stopping down the lens: On the ground glass a real image is created, showing the distribution of sharpness, whilst the microlenses set up a virtual image like a magnifying glass - it becomes visible only in the full optical system including the human eye.

You should think yourself lucky if you can still get your foot anywhere near your mouth at your age. I'm a couple of years behind you and I can't do it! : )

wait what?
an engineered light pipe with microlenses designed to accept light from a narrow field of view."

I thought I knew everything and this line threw me for a loop.

Please tell me you'll spend a post (or three) in the near future and explain what this means as opposed to what we thought it meant.

Oh yes, modern DSLRs tend to come with such a screen out of the box in order to provide a bright viewing image using slower AF lenses. It sucks big time for MF work, so I changed the screen in my DSLR for an accessory screen of the Nikon F3 and it works very well for MF now, especially with fast lenses.

Add to all the previous observations that the so-called manual focus screens (for Canon AF DLSR's at least) do not have a split image or other focus aid because they interfere with metering. I tried the 5DMkII manual focus screen with Leica R lenses and gave it up because you never are sure of correct focus(good eyesight, recently lasik'ed to 20/10). It was like focusing on a very mini view camera ground glass. Frustrated, I bought an M8.

Wow, I never even knew that there was a microlens solution to the focusing screen problem... but it explains a lot as to why when I mount a slow zoom on a ground glass focusing screen the image really gets significantly darker than on a microlens screen.

Here's a great article which explains this (with some examples):
http://www.jayandwanda.com/photography/dslr_man_focus/man_focus.html

Pak

Ctein can blame ink vapour from the 3880. Mike can blame chocolate shakes (the effects are very long lasting) and many months of automatic transmission frustration. Oren Grad for President!

Come on folks, it's easy; just whip out your trusty depth of field calculator. I have one on my iPhone which I use when shooting 5x4 (an ironic convergence?) And if that fails you, there's always hyperfocal. No?

It does happen, however, that wider aperture lenses snap in and out of focus with more alacrity than do slower ones--whether or not the viewfinder image is apparently brighter.

from Gordon Lewis
"The only thing of value I can add at this point is that Live View (for cameras that have it) is an excellent way to see exactly what depth-of-field you'll get at any given aperture."
That is not quite true either unless you are going to print or only view your image at teh same size as your Live View Screen. The depth of field on your much larger print may indeed look a lot less narrow.

Robert,

You are correct about depth-of-field (Mike's bete-noire) when using Live View. That's why I mentioned using 10X magnification. It may not be the exact magnification you use when enlarging but it's a better indication than 1:1 on a 3" display.

Solution to crow:
-Remove foot from mouth
-insert bar of wet soap
-massage soap in mouth
-remove soap
-insert crow, preferably deceased
-count quickly to numeral three
-removed deceased crow
-rinse mouth with fresh water
-insert bar of wet soap and massage
-rinse mouth with fresh water
-do not re-insert foot
Smile, you are good to go

Mike,
"everyone who's saying that a good test of this is to take a fast lens and watch the viewfinder as you stop down has still designed a bad test."
Can you explain why that is (or is not)? To my simple way of thinking, the fact that I can see no difference in the image on the screen between f1.4 and almost f2.8 on my 50mm Nikkor-S on my D700 shows I would have a bit of a problem trusting the focus if I were to shoot much at f1.4 (which I don't when critical focus is an issue).
Nor do I see any difference between the f1.4, f1.8, f2, and f2.8 (zoom) 50mm lenses I have. What am I missing?
I admit I have not shot much at all with all those other 50mm lenses in my "collection" so I am really asking if I should be taking a closer look at them.

The comments to this entry are closed.