Warning on Flange Focus Adjust
 

I just got back from a shoot where the lens simply came off in my hand while shooting. I neglected to run the Auto FB ADJ right afterwards and a lot of my wides were soft. Sony's manual states that "repeated adjustment is not necessary as long as the same lens is used", but I think this should read " if you change or remove lens, run the Auto FB ADJ test".

I finally spotted the problem in the viewfinder and ran the test against a blank wall and it worked.

Not quite sure why the lens popped off--the barrel release is not a snap-lock, rather just a pressure on the flange, and maybe the release lever got bumped when removing the camera from the bag and just took a while to work its way free.

Hope this helps others avoid the problem.

My lens came off my EX3 lat last year. Cause? In and out of my camera bag which caused the lens to work free of its housing. Glad I was holding on to the lens and body at the time as it was sitting on my shoulder.

As my lens has manual back focus, I refitted the lens and carried out flange focus immediately. All is good.

Hi

I think Sony is mistaken in that instruction as well unless they have unbeknown to us modified the laws of physics with new alloys as it pertains to backfocus calibration.

Backfocus should be calibrated any time a lens is mounted, there is change in ambient temperature of 5 degrees or more for an hour or more and before every shoot if the camera has traveled in a vehicle. A backfocus chart should be used whenever possible of course.

Backfocus is useful if you intend zooming whilst shooting, for most shots you would set up the focus before shooting. In other words you will not be out of focus as long as the image in the viewfinder looks sharp.

Vincent, My experience is if the backfocus is off it makes it difficult to judge the overall sharpness on the wide part of the lens. Most viewfinders are still not sharp enough to have complete confidence in their ability to show small focus errors on wide shots. I would check the backfocus with a chart before I started using the camera or after a lens change if possible.

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.

When on location it is not always practical to perform the backfocus routine, especially if you have the client, talent and other crew members waiting about. On some days I may do two, three or more lens changes.

I would really caution against relying on the viewfinder--It crossed my mind that I might need to re-calibrate, but everything looked fine in the viewfinder. It wasn't until the subject was backlit against a blown-out window that I noticed the fuzz. I think one should re-calibrate every time the lens is changed or removed, even if it means using a blank wall (which I did and which yielded perfectly focused shots thereafter).

I would strike me as being odd that you can't rely on a viewfinder for accurate focus, surely this is one of its many functions. If you are using autofocus then perhaps viewfinder accuracy is not such a big issue, but then how many professionals actually have 100% confidence in the autofocus. (for any camera)

Yes, re-calibrate often, but also use your eyes to check the focus often.

I was zooming in on every shot, getting focus, and then pulling back--nothing looked out of focus until I got back to my 42", and virtually every wide was soft. It was not noticeable in the viewfinder.

Mikes experience points out why ignoring checking the backfocus at the beginning of a shoot or when changing the lens can be much more expensive than the time to do the check would cost. Circumstances often drive us into bad decisions as far as the results are concerned. Of course you usually can see a difference in the viewfinder once the backfocus is correct compared to it being off but your eye adjusts to the softness on a small screen pretty easily.

Howdy,

Assuming a professional lens with manual back focus adjustment, you can set back focus after every lens change in less than ten seconds by having a chart on the side and panning to it. True some clients don't have 10 seconds but as Daniel points out they may suddenly have it when the costs of missing critical focus is understood. My approach is brightness up, contrast down, peaking minimum. Others prefer a variation of the three. The only problem I see with repeating back focus is throwing off a calibrated viewfinder (VF focus and luminance). Iif you use a separate monitor that's solved.

Using 800 or less lines on a small surface area for critical focus on a 1920 or 1440 image is not the same as using full resolution on a large viewing plane eg. a 20" broadcast engineering HD monitor. Even if you have full res on some theoretical small built-in LCD, the less the physical dimensions of a hi-res display the sharper the image and the less detectable focus. Web clips of footage on video sites from new small format cameras look so good because they're so small. Same footage on a viewer's 46" flatscreen or movie theater is a different experience.

Considering DOF as a product of footcandles as well as focal length, a wide focal length can still have narrow DOF at full open iris and be quite plainly obvious to the eye in certain lighting situations and camera/lens combinations. I've seen this many times in broadcast jobs using jibs where after backfocus was calibrated things improved.

Regards

I take the point that a small LCD screen is not the optimum way for accurate focusing. However, short of taking a large monitor on location, we have no other choice. The whole point of the EX1/3 is that it is a portable unit.

Despite what messages I may be giving out, I do backfocus my gear often. I also use the magnify function and peaking to check for accurate focus. And yes, I admit that I also zoom in to my subject to focus and then zoom out again to shoot, in this case you do need to ensure the backfocus is set up correctly.

Agreed. For just about the same amount of time as going from play to review, you can save your shoot.

I totally agree Vincent. Portability is the number one biggest feature for me with the EX-1. For my use EX-3 starts to go against the main purpose of a small format camera. So plainly when the production calls for maximum portability the viewfinder/LCD is the option for backfocus.

The supplied EX3 lens doesn't have manual backfocus adjustment and my auto adjustment was so hard to calibrate I just took it to a service center and told them my footage was soft on the computer.

Seems to be ok now - I don't want to touch auto FB adjust again. The Fujinon lens should really have a manual FB adjust too.

[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.

[QUOTE=Alastair Traill;1301565]
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.

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.

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

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.

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

learn something new everyday, cheers.