Thoughts on deliberately mis-adjusting auto focus point

[Cage]

OK, I've got two more sleeps till my new Sigma 150-600 Sport arrives and my brain has gone into overdrive, or maybe melt-down, about using it.

I'm particularly thinking about the narrow DOF. Picture a scenario where that seldom seen birdie lands on a branch about 10m away in fair light. After a couple of test shots you've already got your camera set at say ISO1000, 1/1000sec and f11 when about 20cm of Whipbird lands on the branch in a half face-on pose. These settings are of course flexible.

Focus on the eye, click, gotcha ! Examination of the shot reveals a sharp head, shoulder and upper back, the rest an increasing blur toward the tail.

Checking online DOF charts shows the following:

DOF Snip.JPG

At these settings you are only going to get half the bird in sharpish focus. That 12cm DOF is split evenly, half in front of your focus point (the eye) and half behind it, so almost half your area of sharp focus is wasted on vacant space.

So, what if you had an in-camera setting with a deliberate 5cm of back focus, which in theory should give you that amount of extra sharpness behind your focus point ?
This of course would only work with a co-operative subject, ie one that stays for longer than about 20 seconds to enable you to pull up that setting in your menu.

Am I going nuts? I think I need a cuppa and a lie down.

[ameerat42]

OK... and a lie down.

Forget the DOF calculator, and have a Bex with a good lie down...

Wait till you get it then try it out on a "mock" bird (not a mockingbird).
You might be surprised (or maybe at the Bex).

[Gazza]

OK, I've got two more ....sleeps.

Does that included afternoon naps?

[Cage]

Good onya, ya naysayers.

Of course pulling the lens back to 500mm increases the DOF to 22cm but that sort of defeats the purpose of the 600mm reach.

PS: RIP Glen Campbell. I'm enjoying his work on 2NUR FM (103.7) on my computer.

PPS: Is BEX still available?

[mudman]

google the lens and find out where the sweet spot is for focus
it will probably be less than 600mm
that will give extra dof and maximum sharpness

[ameerat42]

Good onya, ya naysayers.

YES, we are

Of course pulling the lens back to 500mm increases the DOF to 22cm but that sort of defeats the purpose of the 600mm reach.

Set f=500mm and then take a few paces forward

PS: RIP Glen Campbell. I'm enjoying his work on 2NUR FM (103.7) on my computer.

I concur.

PPS: Is BEX still available?

Apparently NOT!

[Brian500au]

it is hard enough to get a bird in focus. I would hate to try to deliberately put him out of focus and guess if I had him all in focus . Now can i share that pack of BEX with you?

Once you start shooting with your new lens - you will quickly find the sweet spot (aperture, shutter speed and ISO).

[Tannin]

Funny you should have this idea. Just last week I was thinking about adjusting a lens such that I could focus on the body of an average-size small bird (a honeyeater, say) and have te eye perfectly focused.

PS: Bex, cup of tea, good lie down.

[Hamster]

The DOF is only showing you the limits of acceptable focus. Nothing is truely in focus except the actual focus point, everything else is acceptable. I guess you'd have to ask yourself if having the eye 5 cm off the focus point is acceptable. Personally I think not, the eye needs to be the sharpest point and no other focus point is acceptable. If having as much of the bird as possible acceptably in focus is your goal then yes, your back focusing idea would work.

[mpb]

I'm particularly thinking about the narrow DOF. Picture a scenario where that seldom seen birdie lands on a branch about 10m away in fair light. After a couple of test shots you've already got your camera set at say ISO1000, 1/1000sec and f11 when about 20cm of Whipbird lands on the branch in a half face-on pose. These settings are of course flexible.

The real question is do you shoot the Whip Bird or the low backwards flying pink pig off to your left.

It is a good question and suggestion and worth considering.

I think I would prefer eyes in focus and tail OOF, therefore I would focus on the eyes to increase my chances of getting them in focus, however I am still in the circle of confusion over this issue.

[jim]

it is hard enough to get a bird in focus. I would hate to try to deliberately put him out of focus and guess if I had him all in focus . Now can i share that pack of BEX with you?

Once you start shooting with your new lens - you will quickly find the sweet spot (aperture, shutter speed and ISO).

This. Why on Earth would you want to do this to yourself?

[Cage]

Thank you all for your thoughts.

I guess the smart thing to do would be to wait till I get the lens and do some tests to see just how thick the slice of best focus is.

[J.davis]

Focus on the breast and you get it all in focus..............

[dacar]

Yes, JDavis. Surely it's much simpler to focus midway along the body. (Not that I havr any experience with birding.)

Sent from my SM-G900I using Tapatalk

[Cage]

Thank you all for your thoughts.

I guess the smart thing to do would be to wait till I get the lens and do some tests to see just how thick the slice of best focus is.

I wish it was that simple John.

Unfortunately there are variables, like available light, and it's effect on the 'f' stop you can use, and the distance from the subject, which along with your 'f' stop, determines your DOF.

- - - Updated - - -

Yes, JDavis. Surely it's much simpler to focus midway along the body. (Not that I havr any experience with birding.)

Sent from my SM-G900I using Tapatalk

Cheers dacar.

See above.

[J.davis]

I am generally not close enough to the bird to get an eye as the focus point.
At such a distance, F7.1 or F8 is my rule.

[Mark L]

Not to sure what you are asking other than the standard equation that the closer you are to a subject the less DOF you have.
It's even more important at 600m when the bird lands about 7 meters away. I took a couple of photos at f/8 and as my brain started the new f/# calculation the bird racked of.

These show what you need to practice focusing on though,

IMG_7827a.jpg

IMG_7830aa.jpg

Just enjoy your new lens and there's no need to over complicate things. You bought it for a reason and that's because it's a good lens.

[Cage]

Cheers Mark.

A couple of interesting shots you've posted, both at 600mm and ISO800. Were they both shot from the same spot and both with focus on the eye?

The first, at f6.3 and 1/100sec seems sharper with more DOF, the perch is sharper at the bottom, but the beak is a bit soft.

The next, at f8 and 1/60sec not so sharp, with the top of the branch a tad sharper and the beak is definitely sharper.

Do you have the OS on when shooting off your monopod?

[Tannin]

I'll answer for Mark. Yes, same spot. No, first shot has focus on the body - look at the outstanding feather detail and compare it to relatively undetailed head, which is behind the plane of sharp focus. In #1 we also see subject movement. Birds shots at 1/100th are strictly for thrillseekers! In the second, we have eye focus. Look at the much improved sharpness of head and bill, and softness of the body. Notice also that the perch is almost as sharp behind the bird as in front of it. (In #1, the perch beneath the bird is much sharper than the part beyond it.)

(Mark will correct me if I'm wrong. I'm just calling it from what I see in the picture.)

By the way, and back more closely on the main topic, the old rule always used to be that DOF is constant for any given framing. Stand back and use 600mm, or step closer and use 300mm, at any given aperture and sensor size, DOF is equal either way. The main gotcha with this rule (as I remember it) is that it is theoretically true only of lenses focused at a distance much greater than the focal length. With a macro subject with a 100mm lens at (say) 130mm from the film plane, it does not apply. But it should be pretty right with any birding subject.

[jim]

^ Yup.