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-   -   Back button focus (http://www.railpictures.net/forums/showthread.php?t=17233)

lalam 09-10-2014 02:48 PM

Back button focus
 
Finally , I was able to step into world of DSLR(D3300,18-55kit) and practicing now.
After going through many threads on focusing techniques, I have assigned AF-L key to Focus-On. Now the focusing is through this key and not half pressed shutter button. The method that I understood from the threads is 1) Press the back button to hear the focus beep and then 2) Press the shutter immediately. Is this correct?
Would like to know the advantages. I understand that if I shoot in burst mode, I can have the sharp frame when train enters focus point (Single point).
But how advantageous this method is, if I am using single frame at a time?
It may be possible that I use single frame at a time only for slow moving trains here. Please guide if I am correct. Thanks in advance.

Mr. Pick 09-10-2014 03:58 PM

Jim Thias will be along shortly to give you some great guidance. Jim got me started on the back button focus a couple of years ago and I've used it ever since. Jim...all yours!

JimThias 09-10-2014 07:17 PM

Yes! Another convert. :-D

Lalam, if you prefocus on a subject, you don't have to push the shutter button right away. The focus will not change until you push the back button again, or accidentally bump the lens. However, when the train is coming toward you, you may want to refocus at different spots in front of the train just before pressing the shutter button as you are zooming out. With a little practice, you can become very quick at that technique. I don't shoot that way very often, so most of my shots are prefocused in a particular spot on the rails where I want to capture the nose of the train at a predetermined focal length (that comes from using my 10-22 and 17-40 a lot more recently when shooting trains).

However, for moving objects that I want to continue firing on, like a football or basketball player coming toward me, I'll return the focus control to the shutter button and then change the focusing to servo mode. I do that because you never know where exactly they are going to run to, so a quick prefocus in a particular spot can prove to be a failure. A train on a fixed path is a different matter.

lalam 09-10-2014 07:28 PM

Quote:

Originally Posted by JimThias (Post 181464)
Yes! Another convert. :-D

Lalam, if you prefocus on a subject, you don't have to push the shutter button right away. The focus will not change until you push the back button again, or accidentally bump the lens. .

Thank you. So if I decide to focus on rails where I want the loco nose to be, I need to compose the frame, push back button just once and wait for the train to reach that spot . correct?. The focus won't change even if I lift my thumb from back button. Or in other words this locks the focus point on a particular spot.

bigbassloyd 09-10-2014 07:41 PM

Once you're focused, it doesn't change unless you hit the back button again, or bump the lens.

I've used the back button focus since the beginning, and it's second nature for all my focusing needs. I even servo focus with it.

Loyd L.

JimThias 09-10-2014 07:44 PM

Quote:

Originally Posted by bigbassloyd (Post 181467)
I even servo focus with it.

I haven't tried to servo focus with it yet, as I figured servo only worked with the shutter button. I'll give it a go sometime though. :smile:

JimThias 09-10-2014 07:46 PM

Quote:

Originally Posted by lalam (Post 181466)
Thank you. So if I decide to focus on rails where I want the loco nose to be, I need to compose the frame, push back button just once and wait for the train to reach that spot . correct?. The focus won't change even if I lift my thumb from back button. Or in other words this locks the focus point on a particular spot.

Correct. :-)

lalam 09-10-2014 07:51 PM

Thank you. Will be experimenting on coming Saturday.

Mr. Pick 09-10-2014 08:32 PM

Loyd is correct. Back button focus works in the servo mode as well, which is what I do when shooting race cars and rapidly moving grandkids.

Mr. Pick 09-10-2014 08:33 PM

Quote:

Originally Posted by lalam (Post 181470)
Thank you. Will be experimenting on coming Saturday.

It may seem a little awkward at first, but once it becomes second nature you will rarely get a missed focus. These new focusing systems on today's cameras think way too much for you and end up making some poor decisions on where to focus. In my experience anyway...

kszok 09-10-2014 11:55 PM

I use back button focus in servo mode at air shows and my daughter's swim meets and have had good results.

JRMDC 09-11-2014 02:19 AM

Could someone please explain what is happening with back button focus in servo mode? What does it mean when you use the button to focus, what is happening over time as the servo kicks in, etc.?

Mr. Pick 09-11-2014 02:29 AM

Quote:

Originally Posted by JRMDC (Post 181478)
Could someone please explain what is happening with back button focus in servo mode? What does it mean when you use the button to focus, what is happening over time as the servo kicks in, etc.?

Back button activates the servo which will continuously focus on whatever you have the focus box on for as long as you hold the button down. Works the same as holding the shutter button down halfway in servo mode.

Chris Z 09-11-2014 02:35 AM

Now I'm confused. It's like the same only different? Must be a Canon thing.

Chris Z

Mr. Pick 09-11-2014 03:54 AM

Quote:

Originally Posted by Chris Z (Post 181480)
Now I'm confused. It's like the same only different? Must be a Canon thing.

Chris Z

I probably didn't explain it very well. Hopefully someone else can explain it better than my attempt..... I could show you real easy!

Ron Flanary 09-11-2014 04:11 AM

I'm really confused. I thought everyone knew to prefocus and lock it it. I use a Nikon, but I just press the button and it should be right. I don't count on the servo for continuous focus to get it right. It can go nuts if it catches a ditch light, for instance.

The first camera I got with autofocus was a Nikon film camera (an N90?---can't recall the model right off hand). My concern was my eyesight, which wasn't as good as it once was (plus, I wear glasses, which is a pain in the rear for framing up a shot and getting it in focus). I would get slides back that were manually focused, and they weren't as tack-sharp as I wanted them to be. So---the autofocus replaced my eyeballing the focus manually. I do like it, and I don't think I could go back to manual focus with any hope of success.

JimThias 09-11-2014 12:54 PM

Quote:

Originally Posted by Ron Flanary (Post 181483)
I'm really confused. I thought everyone knew to prefocus and lock it it. I use a Nikon, but I just press the button and it should be right. I don't count on the servo for continuous focus to get it right. It can go nuts if it catches a ditch light, for instance.

I don't use servo for shooting trains and I'm not sure anyone else does either. It's really not necessary, IMO.

bigbassloyd 09-11-2014 01:43 PM

I never use it for trains. I do use it for sporting events.

Loyd L.

magicman_841 09-12-2014 02:02 AM

Aren't you guys are overthinking this? It seems that in 99.9% of train photos, you're at infinity anyway, especially with wider lenses. :confused:

miningcamper1 09-12-2014 04:33 AM

Quote:

Originally Posted by magicman_841 (Post 181495)
Aren't you guys are overthinking this? It seems that in 99.9% of train photos, you're at infinity anyway, especially with wider lenses. :confused:

I was never a fan of blurry foregrounds, so I usually set at the hyperfocal distance rather than infinity.

bigbassloyd 09-12-2014 04:41 AM

Proper focus is the most important thing you have when you're shooting. I learned that the hard way years ago with inferior equipment that required rather precise focusing methods to achieve a decent result.

And infinity focus is a bit of a misnomer in regards to actual focal distance. The majority of today's AF lenses will focus past the infinity point to accommodate temperature variances, and camera auto focus systems.

So back button focus + single shot focusing + center focal point + complete disregard of the focal distance meter on the lens is a winning combination for me when it comes to choo choos.

Loyd L.

lalam 09-12-2014 05:10 AM

Quote:

Originally Posted by bigbassloyd (Post 181497)

So back button focus + single shot focusing + center focal point + complete disregard of the focal distance meter on the lens is a winning combination for me when it comes to choo choos.

Loyd L.

Agreed. Instead of center point we may use lower or upper(Left or right) depending upon the requirement of the proper frame.

miningcamper1 09-12-2014 05:12 AM

Quote:

Originally Posted by bigbassloyd (Post 181497)
...complete disregard of the focal distance meter on the lens.

Disregard a critically useful tool in situations where important elements of the photo are both near and far? I don't think so!

bigbassloyd 09-12-2014 12:54 PM

Quote:

Originally Posted by miningcamper1 (Post 181501)
Disregard a critically useful tool in situations where important elements of the photo are both near and far? I don't think so!

There are very few instances in which the scale has any value today. Macro does comes to mind, as AF systems struggle with close distance focusing. In my opinon, it's just another previously useful but now obsolete item from the film days. AF systems, and live screen preview for manual focusing are the ways of the world now. I've made a decent living without having a scale on my lenses in the past. To be honest, I didn't realize my new lenses had them for several weeks.. :D

Loyd L.

JimThias 09-12-2014 02:01 PM

How can you even see this scale on TOP of the lens when you're looking through the viewer and focusing on the subject? :lol:

(cue the replies like, "Sounds like Jim has never had to focus manually before blah blah blah." :twisted: )


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