Composing on rangefinders with shallow DOF

[panda81]

I'm a newbie to rangefinders, so I apologize for the newbie question. I just received my first rangefinder last week, and I'm having a blast using it as much as I can.

However, one of the problems I'm facing is composition on the rangefinder. I really love using shallow DOF, but of course, I don't always want the subject of my photo to be exactly in the middle, where the patch is. I'm afraid focusing and recomposing will be too inconsistent if the DOF is too shallow. I've seen people suggest focusing on another object in the same plane as the object, but that only works if there is another object on the same plane where the patch is.

Does anyone have any other suggestions on how to deal with this issue?

Thanks in advance!

 

[ferider]

If you do some basic geometry, you'll find that recomposing using 1/3rd or so is no problem, even with the fastest RF lenses available, since the focus point stays well inside the DOF of the lens.

Other issues like RF and human focusing accuracy, lens focus shift, non-planar focal plane, etc., are typically more dominant.

Roland.

 

[Benjamin]

If you're using an f/2 lens, for example, you should be able to get away with focusing and recomposing. A few degrees shouldn't change your relationship to the subject in terms of distance too much, if at all.

Practise makes perfect, you'll soon figure it out.

Also, you don't make it clear as to wether or not you've actually had any problems on paper so far?

 

[ferider]

Here is an example of the math:

say you use a 50/1.4 lens wide open and a 1/3rd composition at 1m focus distance. Your lens has a flat focal plane. 50mm has 40 degress FOV. With 1/3rd composition you deflect by 40/6 = 6.7 degrees. The error will be 1m - cos(6.7 degrees) * 1m = 0.68cm, well within the 50/1.4's DOF, even at f1.4 (DOF is a bit more than 3cm).

Best,

Roland.

 

[Benjamin]

Here is an example of the math:

say you use a 50/1.4 lens wide open and a 1/3rd composition at 1m focus distance. Your lens has a flat focal plane. 50mm has 40 degress FOV. With 1/3rd composition you deflect by 40/6 = 6.7 degrees. The error will be 1m - cos(6.7 degrees) * 1m = 0.68cm, well within the 50/1.4's DOF, even at f1.4 (DOF is a bit more than 3cm).

Best,

Roland.

Nicely done.

 

[mfogiel]

BTW Roland has sent you a cloud of smoke with his maths, but did not reveal his old trick: focus, and then shift your body instead of changing the lens angle...

 

[ferider]

Traitor 🙂

 

[panda81]

Thanks all for your replies!

Benjamin - I have one or two out of focus shots that I'm unsure if it's due to recomposing or motion due to slow shutter speed. I just want to make sure that my recomposition technique is correct since I've never shot with rangefinders before.

Roland - Thanks! I appreciate the math...I'm a scientist-in-training so I always appreciate seeing the math and science behind photography as well.

Mfogiel - Thanks for revealing the secret tip 🙂 It will be very helpful for me to make sure my technique is correct.

 

[uhligfd]

If you think geometrically, you will realize that with a planar lens and turning the camera away from the focus spot will result in this spot being a little bit further back. This was given numerical values above.

So my trick with faces is to focus a bit in front of the eyes, maybe on the forehead, cheeks, ... and then turn to compose and shoot.

Now the actual difference is not that much ... so do so only if you are deadly serious!

 

[Tim Gray]

Thanks all for your replies!

Benjamin - I have one or two out of focus shots that I'm unsure if it's due to recomposing or motion due to slow shutter speed. I just want to make sure that my recomposition technique is correct since I've never shot with rangefinders before.

Probably slow shutter speed. Is anything in focus or is it all blurry?

I've found that with RF lenses limited to .7m or 1m, focus and recomposing even with a 50/1.4 isn't really a problem. Maybe it is with a 50/1.0 or something. I would think a far bigger culprit for most people is the combined body sway of the photographer and subject during the time of focussing and the time of the shutter click.

 

[Papercut]

+1 to tim -- I don't do shallow DoF all that often, but when I've missed focus on them with my 50/1.4, it's almost always due to body sway: focus, recompose, and then waiting for the right moment to trip the shutter ... those times when the focus is off it's almost always because I've leaned slightly forward while concentrating on focusing and then settled back into a more comfortable "waiting" posture 🙁