autofocus vs manual focus

[bigdazzler]

Small Format Digital Single Lens Reflex I presume ??

[TOM]

[Kym]

Small Format Digital Single Lens Reflex I presume ??

4/3? APS-C? APS-H? 35mm?

[bigdazzler]

4/3? APS-C? APS-H? 35mm?

That wasnt the question .. hehe

[TOM]

4/3? APS-C? APS-H? 35mm?

correct

[kiwi]

My d3 has trap focus, I believe, not that I've ever tried it

[reaction]

Do any of the DSLRs here have 'catch in focus' feature?
That lets you pre-focus and when the DSLR catches the subject in focus the shutter fires.

The K7 has it, but never tried it. Sounds like it's for catching animals in garden at night type work...

[reaction]

The OP's point is how a lens can be perfect with MF, and off with AF. Usually when this occurs there is a fault and it's not that uncommon, have seen it reported quite often, Can be a calibration issue, or problem with the AF mechanism in general

I have AF issue in 1 lens that uses non SWM/USM motor. The MF on that lens is fine, as is AF/MF on my other lenses, which all use SWM. So I think it’s the lens issue, but not sure.
I’ve heard of Focusing Screens, but don’t get what they do. Why don’t DSLRs come with them installed? I can already see focus in VF, why do I need a Focusing Screen? Sites just show what they sell, but don’t explain how it works…

[arthurking83]

if you look into the mirror box of your camera, and up into the VF prism(towards the flash) you will see an opaque looking screen in there.
This is the focusing matte screen. The opaqueness of it is due to very fine etched lines on it. It obviously decreases the amount of light because it's not totally clear(like a window) but opaque.
This is to create a virtual DOF of the scene that you see through the vf, so that as the part of the scene that you want focused becomes clear in through the vf. If the focusing screen were not there, I suspect that the entire image would be crystal clear, and trying to determine a point of focus would be impossible as the DOF would be unlimited(just as it is looking through a window).

Two things will happen if you don't have a focusing matte screen:
you lose the ability to see the focus as the camera does. The focusing screen is a part of the calibration of the entire optical system, so if you remove it, what you see in focus is not what's actually in focus(dependant on aperture/DOF of course).
The removal of the matte screen creates a tunnel vision effect through the vf. so as you move your eye around there is massive(mechanical?) vignetting to various degrees dependent on how your eye is orientated. I'd say the matte screen acts as a diffuser too.

Remove the matte screen and the image through the vf is instantly brighter and more clear. But there is no perception of DOF at all. Everything appears to be more in focus. You can obviously focus/defocus with the lens, but once the lens is focused to a point, the DOF is much deeper through the vf.

I'm @ home with the kids today, and in between playing Jeeves to them, I'll try to get my focus screen info thread up and running today too.

As for the why you need a focusing screen.. well.. you don't. Well, you don't really need another, because you already got one in your camera(D90 I think?).
These aftermarket focusing screens allow better manual focusing only!
If you opt for the Plus screen from KatZeye, there can or will be exposure issues compared to how each lens's exposure performance was prior to the screen being fitted(that info is available on the Katzeye site too). But once that screen is fitted any exposure compensation required will the same across all lenses.

Which lens is giving you trouble?

[farmer_rob]

Just a brief addition to AK's detailed post - the aftermarket focus screens use a number of optical tricks to make focus "easier" to see. They replace the existing screen in your camera. As AK said, you only need them if you want more accurate manual focusing.

[reaction]

OK yeh, it's for my D90, still don't get why there's magic 'better' screens aftermarket - why not put them in the 1st place?
hmm...

The problem lens is my Tamsy 17-50/2.8 which has a regular motor. It gets close, but I can't MF override

[arthurking83]

cost!

I just placed myself in the financially abyss of ordering the Katzeye!

bloody cost over $200 in total.

have a read of my focusing screen tutorial to see some of the why's inherent in standard manufacturer matte screens.

Main reason is cost. But in the uber upper end of the DSLR(and SLR) camera range, the likes of D3's and 1Ds's, the manufacturers have interchangeable matte screens available for those level of cameras.
User changeable and very expensive to boot(even compared to the Katzeye) which is one of the most expensive.

Once my KatzEye is installed, I'll add more info to my Focus screen tute.

EDIT: Oh! the Tammy 17-50!! This is my only lens that has never caused me any issue whatsoever, at all!
All other lenses have some annoyance or bothersome failing or whatever, except my T17-50. Is it misfocusing with every single shot, or is it misfocusing at a certain focal length?

To determine focusing accuracy you need to start from the start... tripod and shots. Start with test targets of something against a wall.

Could be a lens-camera miscalibration of some kind. Not uncommon. If it's still under warranty take it to get it serviced/checked/calibrated.
if it's the screw driven type, one thing you can do is as I mentioned with my Tammy 28-75mm lens. The lens allows you to twist the focusing collar enough to play with the slack in the system.
Using Liveview lightly twist the focusing collar to see if the focus issue is due to play in the screw drive system. There should be about a millimeter of play in the focus collar from stop to stop.

This is the amount of focus error I see in my Tammy 28-75mm at most portrait distances.

[reaction]

it's misfocusing with almost every single shot, at 2.8. I wouldn't notice it at other f/#
I did some test targets and it's always front focusing. It even looks 'off' in the VF. The VF matches the output focus.
I now try to AF many times before a shot, to hope it gets 'closer' each beep. At 1st I thought it was my fault since it's my 1st f2.8. Anyway, this causes me to lose shots where I move fast and the subject is moving too.
I got it on hols, I don't think Oz will take it under warranty

[para]

when I am photographing wakeboarding from the boat I pre focus the rider no matter what he is doing a 1080 or an ollie(tricks)
He is always at 80ft or whatever he rides at

[arthurking83]

it's misfocusing with almost every single shot, at 2.8. I wouldn't notice it at other f/#....
I got it on hols, I don't think Oz will take it under warranty

makes sense, and .. "bother!"(on getting it on hols)

My favourite test for measuring back/front focus is to do the 'battery test':

Battery test involves 5 batteries(but you can substitute batteries for other similar stuff).

line the batteries up so that the central one is furthest from the lens to the two nest to it, an din turn those two on either side are slightly further back then the two batteries next to them.

So from a birdseye view they're arranged like this:

      X
    X  X
  X      X
      _
   camera

(Code is required or I lose the formatting of the X's)

Line up the batteries to be sure that only one battery is seen by any one AF area box.

Take multiple exposures at different apertures up to f/5.6 will be enough.

Take a series of exposures using both vf AF normally, and via Live View. as well.
of course on a tripod. Lighting conditions are irrelevant too. but make sure you wont get any camera vibration like mirror slap, etc.

Begin by using the vf AF system and using the central focus area on the central battery. Determine if the any of the closest batteries on the sides are more in focus than the the central one you focused on.

Then do the same with Live View and zoom to approx 100% view and focus on each battery to see if there is any difference in focus performance(compared to vf AF).

Another thing to check, if you try this test, is to determine if the slack in the screw drive system is part of the issue.
using the vf AF system again, acquire AF, and then lightly twist the focus ring to either side of the slack points to see if there is any difference in focus.

Note tho, if the camera is still set to only shoot once focus is acquired then you may not get any response from the camera as you do these tests.
I use the AF-On button on my D300 to get focus and have separated it from the shutter release. You can set the AE-L button to act as the focus drive to separate it from the shutter release. it''s better to have the focus independent of the shutter release for more user control over the camera.

sorry, I'm rushed because now dinner awaits......

if you had too pay for a service, I think it''d be worth the effort and expense. This lens is wickedly sharp ... when it works properly of course.

[reaction]

I use a target on screen, cuz it's bright and easy. I take at 45deg left and right, it's easy to see the issue.

On my K-7 it's the same, but not as bad, plus K-7 has focus set, so it's now at -3 and seems fine.

Hoping for a D400 soon...

[reaction]

ok, there is almost NO slack in the motor at all. so it's not that issue.
is there slack in screw types?

[arthurking83]

ok, there is almost NO slack in the motor at all. so it's not that issue....

I would guess that there should be some very small amount of slack in the screw drive system(taking into account both lens and camera).
**BUT I'M NOT 100% SURE!!**(just offer my experience with what I have at my disposal).

Both of my Tammys, 28-75/2.8 and 17-50/2.8 have approximately 1mm or so of slack.
That is, when you rock the focus ring from one end to the other without dragging the camera's screw drive motor, you should feel about 1mm of free play in the focus ring.
At the two end points you need to use much more force to drag the camera's focus motor.
Note you can do this without causing harm.

I only have two screw type lenses to work with, all others are HSM, AF-S, built in lens motor type(Tammy 70-200/2.8) or manual manual focus.

note that some Nikon AF/AF-D lenses which rely on the screw drive may not have any slack in the lenses focus ring as the lenses focus ring locks up(80-200/2.8 AF-D was my one and only lens that did). So, with that lens you can't drag the camera's focus motor even if you wanted too because the lens locked up the focus ring.
With that 80-200mm lens it actually made sense that it operates in that manner too. Sometimes you would end up holding(cradling) the lens from up near the front of it.. right where the focus ring is. So when the lens was focusing the focus ring would not turn. In effect it doubled up as a hand rest, and you wouldn't want to inadvertently move the focus ring once the lens was focused.

[reaction]

well I mailed tammy and they said they'd "change your warranty card to international" by sending it to them. not bad but I still worry about the Sydney side.

[Analog6]

Back before the days of autofocus (yes, I am that old, children!) I did a lot of equestrian photography of moving subjects. Jumping was easy, as someone said, you simply pre-focus. But because the split image screens were more suited to manual focussing, although frustrating with their 'black spots' it was pretty easy to know when you were sharply focussed.

If you wanted to do any extensive manual focussing know you would need to change your screen, it is impossible to do on the modern DSLR ones (IMHO).