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02-24-2013, 02:07 PM   #1
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AF in VERY low light

I was out last night doing some pictures of a ship that during a storm last month has landed on a beach some miles away from my home.

I was with a friend that has a sony alpha55 camera..

There was no light but the one from the moon, wich is ofcourse very very dim.

The problem was the sony was not having problems getting focus and on the K-30, i had to do it manually, and that was trial and error, with 30seconds exposure... The light was so dark, i couldn't see anything with the focus peaking on!!!
That was ok, because there was really no light, but WHY did the sony get focus so good and quickly??? Was that some setting on my camera?
I was using f8.0 to get everything better in focus even if the focus was not spot on, as a safe measure!

Wich is the best setting for a trying to get focus on a low light environmment??

Zorza

ps. here is one of the pictures

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02-24-2013, 02:26 PM   #2
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Perhaps the Sony was on multi-point AF and it locked onto the brightest part of the frame, and your K-30 was on centre point? Perhaps the Sony had a better lens? Or it's simply just better AF in very low light. Or it lies and an AF lock isn't actually - were his photos actually in focus?
02-24-2013, 02:33 PM   #3
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It's probably that the Sony was using a different type of AF system to begin with, not being a DSLR. (Or, I forget, does sony only call its DSLRs alphas? I'm not familiar with the rated specs on either of these: While I believe it's far less of a problem now than on, say my aging K20d, there's often particular issues about the *color* of the light that's present in lower lighting conditions: (because of the *wavelength* differences) for instance, something lit exclusively by *neon* may seem quite enough to the human mind or eye (or contrast-detect AF) but fool the optical AF on a DSLR.

Pentax's AF system has never been considered their strongest suit, anyway. It's been a matter of incremental improvements for a while now, given their corporate history and stuff. Having Ricoh behind them now will probably help.

Last edited by Ratmagiclady; 02-24-2013 at 02:40 PM.
02-24-2013, 02:54 PM   #4
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QuoteOriginally posted by zorza Quote
Wich is the best setting for a trying to get focus on a low light environmment??
In very low light, I prefer to wok with MF. I think that there are much more controls in this way. I set the focus using the focal distance on the focus ring (after it was properly checked). Then I set the aperture or shutter speed to get the best conditions I want.

Hope that the comment helps....

02-24-2013, 03:00 PM   #5
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You can assist the camera by one of both of these techniques to help with low light focus:

1. Flashlight:

1. ask someone to point it at your subject while you try to focus.
2. Change the focus switch from AF to manual focus mode once focus is acquired, then turn off the flashlight
3. Take a picture without you/camera or your subject moving.

A laser pointer can also work.


2. Fast Lens:

What is the maximum aperture of your lens? Does not matter if you are shooting at f8. The camera will try to AF with the lens wide open and then stop down to take the shot.

If you are using the kit lens at the long end then wide open it is still only f/5.6 which does not get enough light. If you have an f/2.8 or faster lens the AF may work better.
02-24-2013, 03:09 PM   #6
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By default, when you turn Off Sony alpha body, they put the lens on infinity.

Maybe the Sony detect it couldn't not focus and jut said "ok" by default.
02-24-2013, 03:31 PM   #7
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Hi.. I was using the kit lens at 18mm wich are f3.5.

I did not try to use my 55da* f.14 because I needed the 18mm wide.. One thing, neither of these lens have numbers on them to try to set the focus manually to a certain distance. And I coulnd not find if infinity was all the way to the right.. Some lens it is a little bit febore the end.

The Sony was getting focus. His pictures where focused and when pressing teh shutter, he got the green square giving focus.

I was using one centre point af, and the switched to 5point to see if the camera could give me some more...
Which do you think is the most accurate? how many of them are cross type?

Even in live view, wich the focus turns to Contrast AF, it wouldn't focus.. It was do dark it was no contrast so start with..

Zorza
02-24-2013, 04:55 PM   #8
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Based on what you have stated in the comments above and on your subject, I am going to make a few suggestions:
  1. Ignore what your friend's camera is doing. If it locked focus in "available darkness", you can can be sure that it was not a lock on the subject. As noted above, his camera may have simply picked up the moon, which may indeed have resulted in an acceptable image, but was not the bow of the ship.
  2. Accept the fact that your camera cannot focus in the dark. This is true for all AF cameras with available light auto-focus systems. The same is true for all manual focus systems. The sole exception might be if the subject is silhouetted and there is sufficient light from the background to create a strong boundary.
  3. Learn the basics of manual focus in dim light. As noted above, this means estimation of distance or use of supplemental light for focus. In the case of your subject (the ship) at 18mm focal length, I would have just set the focus at 15 and stopped down to f/5.6. With those settings everything from about 5 feet to infinity should be in acceptable focus.


Steve

BTW...if you have a smart phone, there are some great DOF calculator apps that are helpful for this type of shooting. Carry a small flashlight (torch) so that you can see the lens barrel to do the setting. It is also helpful if your lens is a vintage model made for manual focus. These generally have longer focus throw and are easier to set with precision.


Last edited by stevebrot; 02-24-2013 at 05:03 PM.
02-24-2013, 05:03 PM   #9
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QuoteOriginally posted by stevebrot Quote
  1. Learn the basics of manual focus in dim light. As noted above, this means estimation of distance or use of supplemental light for focus. In the case of your subject (the ship) at 18mm focal length, I would have just set the focus at 15 and stopped down to f/5.6. With those settings everything from about 5 feet to infinity should be in acceptable focus.
You are right.. If only the lens had the numbers on it for me to see the 15 or the infinity sign That is the main problem..

Zorza
02-24-2013, 05:11 PM   #10
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I would say the Sony has a more sensitive AF sensor. The k-30 has an old AF sensor which is not very light sensitive. In the k5II Pentax have finally upgraded the AF sensor and it is very sensitive and able to focus exceptionally low light quite quickly. If you want that level of performance you will have to upgrade, however for static scenes like yours with ample time to set up, AF is not really required or recommended.
02-24-2013, 06:30 PM   #11
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What lens did your friend have on his Sony? That could make all the difference in the world.
02-25-2013, 02:00 AM   #12
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QuoteOriginally posted by IchabodCrane Quote
What lens did your friend have on his Sony? That could make all the difference in the world.

Well, he was using a 16-35mm with a fixed f2.8 A very good lens.. (and expensive as hell)!!

But at the start it is only f2.8 to f3.5 diference for my kit lens.. It that a that big diference?

I have to get back there and try the 55da* F1.4


Zorza
02-25-2013, 02:19 AM   #13
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Yes, thats a big difference in light available to the sensor, and likely the main reason. Additionally, did he actually get sharp shots? Was he using f5.6 or similar, therefore putting more of the scene into focus and making it "easier"
02-25-2013, 07:27 AM   #14
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Its hard to say.
A lot of possible variables.

1. Maybe you tried to focus on a part w/o contrast and your friend focus on something with contrast (eg. outline of ship vs you pointing on dark hull)
2. Maybe the A55 was on auto-AF points and focus on the distant Sun (as shown in your photo); while you on single point pointed at the dark ship hull.
3. Perhaps the f2.8 lens was the difference in the dark (almost 1 stop diff to you f3.5)

But as Stevebrot suggest on point #2 and 3, if it can't do what another camera can, it can't (of course eliminating possible user error 1st).
Finding a way round it with the knowledge of the camera and the basics will probably see you thru this and any future situations.
03-04-2013, 01:56 AM   #15
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I don't think the lens aperture makes any difference, because the K-30 does not have f2.8 sensitive focus points. It means that, even if you put a f1.4 lens on the camera, the phase detection sensor sees the field with a f5.6 aperture.
Did you actually see your friend pictures?
Anyway, low light AF has never been (until the release of the K-5II) a Pentax strong point!
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