Warning on Flange Focus Adjust

Alastair Traill · September 2nd, 2009, 05:15 AM

[QUOTE=Vincent Oliver;1086317]Daniel, you are right in sating the backfocus should be set up correctly. However, at the wide setting of the lens there is a lot of depth of field which makes manual focus very difficult.

Vincent,
It is true that at the there is a lot of depth of field at the wide angle end of the focal length range however the depth of focus is much less. That is to say that the lens/film plane or lens/sensor plane distance is much more critical at short focal lengths than longer focal lengths. The long focal length lens is bringing the light rays to a focus over a much greater distance than the wide lens that has much less distance to do so. i.e With the wide lens the light rays are refracted more sharply as they pass through the focus making the distance setting more critical. Draw yourself some ray diagrams one with the gently converging rays of a long lens and one with steeply converging rays. You will find that you have to be more precise placing the film plane with the wide lens if you want exact focussing.

Vincent Oliver · September 2nd, 2009, 10:24 PM

[QUOTE=Alastair Traill;1301565]

Quote:

Originally Posted by Vincent Oliver

Daniel, you are right in sating the backfocus should be set up correctly. However, at the wide setting of the lens there is a lot of depth of field which makes manual focus very difficult.

Quote:

Vincent,
It is true that at the there is a lot of depth of field at the wide angle end of the focal length range however the depth of focus is much less.

I wondered if anyone would pick up on this. You are absolutely correct in your reply, the depth of rear focus (nodal point) does need to be more precise with wide angle lenses. The depth of field (whats in front of the lens) will be greater, hence my statement that focusing doesn't need to be as critical.

Like most other things, to get the best results your kit does need to be in balance, i.e. setting the back focus is just as important as setting the focus on your subject.

Simon Wyndham · September 3rd, 2009, 03:38 AM

It would be nice if the stock fuji lens had a manual back focus. However I believe that the reason why is because even in full manual mode the focus ring is still operating a servo with no mechanical connection. So any back focus adjustment has to be servo operated too.

Alastair Traill · September 3rd, 2009, 05:57 AM

In my post above I said that the depth of focus of a wide-angle lens is much less than for a longer lens. Here is an example that shows what a slight change in lens/sensor distance would be expected to make. The calculation is based on a simple formula that relates the focal length of a lens, the focusing distance and the extension of the lens from its infinity setting. It is a very useful formula to use when making a lens adaptor and you do not know how long the mount should be.

If for some reason such as a build up of dust etc or some damage causes the EX3 lens to be 0.1 mm further from the sensor than the correct adjustment, and the lens is set to infinity one could expect the following: -

At a focal length of 5.8 mm the lens would focus on a subject about 3.3 m away.
At a focal length of 81.2 mm the lens would focus on a subject at about 660 m away.

I.e. in this case the change at the long end would not be noticeable whereas at the wide end there may be a problem

Glynn Morgan · September 4th, 2009, 08:20 AM

can someone explan depth of focus?

Also, I believe the Z7 has manual back focus adjustment, I didn't think the Zeiss lens on that camera was fully manual even on full manual? (i.e. was using a servo). You just pressed a button when it was in focus at Tele and at wide and it would set it. Sounded pretty sweet to me.

Vincent Oliver · September 4th, 2009, 09:07 AM

Depth of focus is the point that the lens projects the image on the CMOS,CCD or film plane. This a very small but critical distance. Think of using a magnifying glass on a sunny day to project the suns rays onto a piece of paper, if it is in focus then it will burn, if not then it won't. Then think of the small distance you have to move the magnifying glass in order to make it work, and then compare that to the distance you are away from the sun. Extreme I know, but I hope it makes the point clear.

Depth of field is the area in front of your lens, i.e. your subject matter, this varies according to which aperture is in use.

Th

Glynn Morgan · September 4th, 2009, 10:14 AM

learn something new everyday, cheers.

September 4th, 2009, 09:07 AM	#21
Vincent Oliver Trustee Join Date: Sep 2004 Location: Chislehurst, London Posts: 1,724	Depth of focus is the point that the lens projects the image on the CMOS,CCD or film plane. This a very small but critical distance. Think of using a magnifying glass on a sunny day to project the suns rays onto a piece of paper, if it is in focus then it will burn, if not then it won't. Then think of the small distance you have to move the magnifying glass in order to make it work, and then compare that to the distance you are away from the sun. Extreme I know, but I hope it makes the point clear. Depth of field is the area in front of your lens, i.e. your subject matter, this varies according to which aperture is in use. Th __________________ Eyes are a deaf man’s ears. Ears are a blind man’s eyes

September 2nd, 2009, 05:15 AM	#16
Alastair Traill Major Player Join Date: Nov 2008 Location: Melbourne Australia Posts: 626	[QUOTE=Vincent Oliver;1086317]Daniel, you are right in sating the backfocus should be set up correctly. However, at the wide setting of the lens there is a lot of depth of field which makes manual focus very difficult. Vincent, It is true that at the there is a lot of depth of field at the wide angle end of the focal length range however the depth of focus is much less. That is to say that the lens/film plane or lens/sensor plane distance is much more critical at short focal lengths than longer focal lengths. The long focal length lens is bringing the light rays to a focus over a much greater distance than the wide lens that has much less distance to do so. i.e With the wide lens the light rays are refracted more sharply as they pass through the focus making the distance setting more critical. Draw yourself some ray diagrams one with the gently converging rays of a long lens and one with steeply converging rays. You will find that you have to be more precise placing the film plane with the wide lens if you want exact focussing.

September 3rd, 2009, 03:38 AM	#18
Simon Wyndham Trustee Join Date: Dec 2003 Location: Malvern UK Posts: 1,931	It would be nice if the stock fuji lens had a manual back focus. However I believe that the reason why is because even in full manual mode the focus ring is still operating a servo with no mechanical connection. So any back focus adjustment has to be servo operated too.

September 3rd, 2009, 05:57 AM	#19
Alastair Traill Major Player Join Date: Nov 2008 Location: Melbourne Australia Posts: 626	In my post above I said that the depth of focus of a wide-angle lens is much less than for a longer lens. Here is an example that shows what a slight change in lens/sensor distance would be expected to make. The calculation is based on a simple formula that relates the focal length of a lens, the focusing distance and the extension of the lens from its infinity setting. It is a very useful formula to use when making a lens adaptor and you do not know how long the mount should be. If for some reason such as a build up of dust etc or some damage causes the EX3 lens to be 0.1 mm further from the sensor than the correct adjustment, and the lens is set to infinity one could expect the following: - At a focal length of 5.8 mm the lens would focus on a subject about 3.3 m away. At a focal length of 81.2 mm the lens would focus on a subject at about 660 m away. I.e. in this case the change at the long end would not be noticeable whereas at the wide end there may be a problem

September 4th, 2009, 08:20 AM	#20
Glynn Morgan Regular Crew Join Date: Feb 2009 Location: Australia Posts: 119	can someone explan depth of focus? Also, I believe the Z7 has manual back focus adjustment, I didn't think the Zeiss lens on that camera was fully manual even on full manual? (i.e. was using a servo). You just pressed a button when it was in focus at Tele and at wide and it would set it. Sounded pretty sweet to me.

September 4th, 2009, 10:14 AM	#22
Glynn Morgan Regular Crew Join Date: Feb 2009 Location: Australia Posts: 119	learn something new everyday, cheers.