Visit MPB Visit MPB Visit MPB

Focusing problem

sportyman531
Posted 22/04/2018 - 23:18 Link
I was out on Saturday photographing trains and I have two consecutive shots of different trains at the same location using the same focusing method: AF-S, Selective point, pre-focused on that point and waited for the train to come, then press the shutter release. The first image is not sharp anywhere across the frame, the second image is pin sharp. The exposures were similar, The first 400 ISO 1/800th @ f5.6, the second @ f5, focal length was 50mm, I was looking through the viewfinder and I was holding the camera (K3) steady, in both cases the image appeared to be in focus. Can anyone offer a reason for the discrepancy?
flickr ID: SteveW46243 or sw46246
DA AL 35mm f2.4, K3,18-135 WR, DA 55-300, Pentax KX, K5 x 2, 18-50 50-200 kit lenses, Tamron 17-50 f2.8, Fuji X10, 2x Mamiya 1000s 2x 80mm f2.8, 150mm f3.5, 210mm f4, Yashica 124G TLR, 2x Yashica FX3 Super 2000, Tamron 28mm f2.8, 90mm SP f2.5, 135mm f2.5, Olympus 35RC, 2x Yashica FX2, Kodak Pocket Instamatic, Kodak Box Brownie
pschlute
Posted 23/04/2018 - 07:36 Link
You say you "pre-focussed". But did you then turn off the AF ? Otherwise as soon as you press the shutter button when the train has arrived the camera will try and achieve focus again.
McGregNi
Posted 23/04/2018 - 09:06 Link
Yes, pre-focussing is a technique associated with manual focus mode usually.

Of course, if you used Back Button Focus for the pre-focussing, then did not press that again, it would stay fixed, so that is the way to use it in conjunction with autofocus. Of course the shutter button has to be set to not focus, or you have the problem Peter referred to.

I can't remember whether setting up BBF is a two step process, (ie the back button and shutter button are controlled independently), or if it's a one step process (where one button cuts out the other). Probably on our latest models there's a range of choices .....
My Guides to the Pentax Digital Camera Flash Lighting System : Download here from the PentaxForums Homepage Article .... link
Pentax K7 with BG-4 Grip / Samyang 14mm f2.8 ED AS IF UMC / DA18-55mm f3.5-5.6 AL WR / SMC A28mm f2.8 / D FA 28-105mm / SMC F35-70 f3.5-4.5 / SMC A50mm f1.7 / Tamron AF70-300mm f4-5.6 Di LD macro / SMC M75-150mm f4.0 / Tamron Adaptall (CT-135) 135mm f2.8 / Asahi Takumar-A 2X tele-converter / Pentax AF-540FGZ (I & II) Flashes / Cactus RF60/X Flashes & V6/V6II Transceiver
Edited by McGregNi: 23/04/2018 - 09:09
StephenHampshire
Posted 23/04/2018 - 09:10 Link
I photograph a lot of trains and use back button AF all the time. Was SR on? I have had some instances where shots in the middle of a sequence are soft (overall soft rather than just mis-focussed) which I blame on the SR still moving the sensor
pgweber
Posted 23/04/2018 - 13:15 Link
I sometimes pre-focus with my K5 using the Back (AF) Button

On the Button Customisation Menu:

For Back Button Focussing, I set:
AF Button - Enable AF
Shutter Button Half Press - Off

For Shutter Button Focussing, I set:
AF Button - Cancel AF
Shutter Button Half Press - Enable AF

...and then wonder again why I haven't bought a D750 or D500!
Peter

Pentax K5
Pentax DA 18-55 Mk1, 50-200 (Samsung), 16-45, 55-300 Mk1, 35 f/2.4
Pentax MZ6 + FA28-90, FA50 f/1.4, M 50 f/1.7
Tamron 80-210mm & 28mm
Edited by pgweber: 23/04/2018 - 13:18
JAK
Posted 23/04/2018 - 13:21 Link
Make sure the Button Customization setting in menu 4 is set to AF2 to disable focussing with the shutter button.
Otherwise all over blur is often simply camera shake.
John K
pgweber
Posted 23/04/2018 - 13:29 Link
JAK wrote:
Make sure the Button Customization setting in menu 4 is set to AF2 to disable focussing with the shutter button.
Otherwise all over blur is often simply camera shake.

On which body?
My K5 has "Enable AF/Off/Off (Live View only)" under the Shutter Button Half Press option.
Peter

Pentax K5
Pentax DA 18-55 Mk1, 50-200 (Samsung), 16-45, 55-300 Mk1, 35 f/2.4
Pentax MZ6 + FA28-90, FA50 f/1.4, M 50 f/1.7
Tamron 80-210mm & 28mm
JAK
Posted 23/04/2018 - 13:31 - Helpful Comment Link
pgweber wrote:
JAK wrote:
Make sure the Button Customization setting in menu 4 is set to AF2 to disable focussing with the shutter button.
Otherwise all over blur is often simply camera shake.

On which body?
My K5 has "Enable AF/Off/Off (Live View only)" under the Shutter Button Half Press option.

He says K3 so I checked on that to see which settings might be affected. Also he says he used the viewfinder, not Live View. My K3 is set up permanently to focus that way, I hate leaving the focussing completely to the camera (of whatever brand.) The focus point is down to me, not the camera (which is a dumb tool and has no idea what I'm taking.) I'm not surprised those who rely totally on shutter press AF have issues, it's too late to alter anything if it doesn't get what you want.
I forget what the settings are on the K-5 but I used to do it so know it can be done without being in Live View (as I didn't use that at all.)
John K
Edited by JAK: 23/04/2018 - 13:44
JAK
Posted 23/04/2018 - 14:08 Link
Just a thought, if the failed image could be uploaded here it might help give others a better idea as to the issue with it. It's hard to diagnose a problem without seeing it!
John K
Edited by JAK: 23/04/2018 - 14:17
pgweber
Posted 23/04/2018 - 14:59 Link
Going back to what the OP said in the beginning, I am not convinced that once the focus has been set (either by shutter or AF button) that the camera will maintain that focus position. i.e. on my K5, I think the focus position will move.
Stephen, can you confirm if you used the AF button to pre-focus?
Peter

Pentax K5
Pentax DA 18-55 Mk1, 50-200 (Samsung), 16-45, 55-300 Mk1, 35 f/2.4
Pentax MZ6 + FA28-90, FA50 f/1.4, M 50 f/1.7
Tamron 80-210mm & 28mm
JAK
Posted 23/04/2018 - 15:04 Link
pgweber wrote:
Going back to what the OP said in the beginning, I am not convinced that once the focus has been set (either by shutter or AF button) that the camera will maintain that focus position. i.e. on my K5, I think the focus position will move.
Stephen, can you confirm if you used the AF button to pre-focus?

What would cause the focus to change if the camera is set to not refocus on shutter press?
FWIW, for the K-5 it's the setting: AF button > Cancel AF which disables the focus when the shutter button is pressed. Guess that's a strange use of logic there!
The AF button has several settings, one where the camera will focus only when the shutter is pressed while the AF button is pressed.
See camera manual page 124.
However, the OP is referring to using a K-3.
John K
Edited by JAK: 23/04/2018 - 15:16
pgweber
Posted 23/04/2018 - 15:31 Link
JAK wrote:
pgweber wrote:
Going back to what the OP said in the beginning, I am not convinced that once the focus has been set (either by shutter or AF button) that the camera will maintain that focus position. i.e. on my K5, I think the focus position will move.
Stephen, can you confirm if you used the AF button to pre-focus?

What would cause the focus to change if the camera is set to not refocus on shutter press?
FWIW, for the K-5 it's the setting: AF button > Cancel AF which disables the focus when the shutter button is pressed. Guess that's a strange use of logic there!
The AF button has several settings, one where the camera will focus only when the shutter is pressed while the AF button is pressed.
See camera manual page 124.
However, the OP is referring to using a K-3.

I think that there is some looseness or slop in my 16-45 which is where I have noticed this effect.
Peter

Pentax K5
Pentax DA 18-55 Mk1, 50-200 (Samsung), 16-45, 55-300 Mk1, 35 f/2.4
Pentax MZ6 + FA28-90, FA50 f/1.4, M 50 f/1.7
Tamron 80-210mm & 28mm
sportyman531
Posted 23/04/2018 - 22:08 Link
pschlute wrote:
You say you "pre-focussed". But did you then turn off the AF ? Otherwise as soon as you press the shutter button when the train has arrived the camera will try and achieve focus again.

I didn't turn the AF off but once I had focused I kept the shutter release half down.
flickr ID: SteveW46243 or sw46246
DA AL 35mm f2.4, K3,18-135 WR, DA 55-300, Pentax KX, K5 x 2, 18-50 50-200 kit lenses, Tamron 17-50 f2.8, Fuji X10, 2x Mamiya 1000s 2x 80mm f2.8, 150mm f3.5, 210mm f4, Yashica 124G TLR, 2x Yashica FX3 Super 2000, Tamron 28mm f2.8, 90mm SP f2.5, 135mm f2.5, Olympus 35RC, 2x Yashica FX2, Kodak Pocket Instamatic, Kodak Box Brownie
sportyman531
Posted 23/04/2018 - 22:10 Link
McGregNi wrote:
Yes, pre-focussing is a technique associated with manual focus mode usually.

Of course, if you used Back Button Focus for the pre-focussing, then did not press that again, it would stay fixed, so that is the way to use it in conjunction with autofocus. Of course the shutter button has to be set to not focus, or you have the problem Peter referred to.

I can't remember whether setting up BBF is a two step process, (ie the back button and shutter button are controlled independently), or if it's a one step process (where one button cuts out the other). Probably on our latest models there's a range of choices .....

Thanks Ni, I will have to try BBF and I must look up the process too.
flickr ID: SteveW46243 or sw46246
DA AL 35mm f2.4, K3,18-135 WR, DA 55-300, Pentax KX, K5 x 2, 18-50 50-200 kit lenses, Tamron 17-50 f2.8, Fuji X10, 2x Mamiya 1000s 2x 80mm f2.8, 150mm f3.5, 210mm f4, Yashica 124G TLR, 2x Yashica FX3 Super 2000, Tamron 28mm f2.8, 90mm SP f2.5, 135mm f2.5, Olympus 35RC, 2x Yashica FX2, Kodak Pocket Instamatic, Kodak Box Brownie
sportyman531
Posted 23/04/2018 - 22:11 Link
StephenHampshire wrote:
I photograph a lot of trains and use back button AF all the time. Was SR on? I have had some instances where shots in the middle of a sequence are soft (overall soft rather than just mis-focussed) which I blame on the SR still moving the sensor

Yes, SR was on, when do you use SR and when do you not use it?
flickr ID: SteveW46243 or sw46246
DA AL 35mm f2.4, K3,18-135 WR, DA 55-300, Pentax KX, K5 x 2, 18-50 50-200 kit lenses, Tamron 17-50 f2.8, Fuji X10, 2x Mamiya 1000s 2x 80mm f2.8, 150mm f3.5, 210mm f4, Yashica 124G TLR, 2x Yashica FX3 Super 2000, Tamron 28mm f2.8, 90mm SP f2.5, 135mm f2.5, Olympus 35RC, 2x Yashica FX2, Kodak Pocket Instamatic, Kodak Box Brownie

Add Comment

To leave a comment - Log in to Pentax User or create a new account.