Focus issues - a different analysis

[cmcbc]

I have discovered a focusing phenomenon with my 60d during a manual focus session with my 24-105L. Normally I use this lens on full auto but in manual focus a number of shots where out of focus despite being dead sharp in the view finder. I immediately thought of the infamous front or back focus problems associated with dslr's but after tests discovered a different kind of problem.

The following steps provide the basis of the test, best done with a medium telephoto lens.

1. Put camera on tripod and set Av for fastest aperture of lens. Set an object with fine detail or small text about 4 metres from the camera. Set lens to manual focus and move focus ring from a completely out of focus to the point of perfect focus on the object through view finder.

2. Take shot.

3. Do not touch the lens. Without moving anything, activate LiveView and activate the 10x screen zoom to see detail. In my tests this was slightly out of focus and the lens needed slightly re-focused to restore the sharp detail.

At face value this would suggest a front or back focus issue. However, when LiveView is then deactivated and the new focus checked through the viewfinder it was perfect and the resulting shot would be in perfect focus. It seemed as if the functioning of the mirror after LiveView somehow brought it back to a better seating. This is not the same as a consistent front or back focus problem. If the manual focus ring is then moved from one extreme to the other as in step 1 the problem would occur all over again for the next shot. This does not seem to happen for shots taken in auto focus.

I am convinced that extensive movement of the manual focus ring resulting in the movement of the internal structure of the lens causes tiny air pressure changes inside the camera which in turn cause microscopic movements of the mirror or screen out of position. I did tests with other lenses in manual focus and discovered the same effect. Has anyone else come across this with the 60d or other cameras in the range.

 

[Loswr]

Live View AF is the gold standard. If the VF manual focus gives a different result, it's either your eyes, or you didn't set the diopter properly.

 

[cmcbc]

Thanks neuroanatomist. Thankfully my eyes are good and Diopter is set correctly. I am a pro photographer with 26 years experience and I know when a subject is in sharp focus in the view finder, my business depends on that. Your right that LV is the real image via the imager and that is why I am using it to instantly see the difference between a sharply focused image in the VF and the resulting unsharp image that would be captured. There are strange things going on inside the camera in connection with either the mirror, focusing screen or possibly the imager itself during manual focus movements.

I have a 60D but most of my work is on larger format.

 

[CharlieB]

Live view is from the sensor... its always good.

The focusing screen is for crap in almost all modern SLR's.

To focus accurately on the screen, the screen must be EXACTLY perpendicular to the sensor, and the mirror must be EXACTLY at 45 degrees. I have set up Nikon F's and F2's to be critically in focus at any point on the screen. It is not an easy task. Those mirrors had adjustments, and the screen was 4 point shim-able. With a film camera its even worse really, because the film did not lay against the inner rails on any film camera (no matter what they hyped at you). It would be sort of flat the long way, but across the film from sprocket hole to sprocket hole, you got a wavy curve of a film plane... flat on the plate about 1/3 out from center in both direction, but bowing toward the lens in the center and edges.... but I digress.

Cheap cameras, back in the day, were made with the screen more or less at a 90 degree angle to the film, and the adjustment was made for CENTER focus by tilting the mirror in production (or repair).

Compare that today's autofocus sensor, which must be aligned with the sensor (and lens) and the mirror must be dead on in two ways... and oh yah... you want the screen sharp too? That is a very complex piece of alignment, and its no wonder why you see problems in top to bottom focusing. That is, top focusing points, not matching the bottom. Any wee little bit of misalignment at any point... its gonna get you a problem. Factor in stacked tolerances... its a problem.

I'd personally not trust the screen on any mid-tier or entry level digital SLR... it would be total luck to get one that was truly aligned.

 

[cmcbc]

Thanks CharlieB for your excellent explanation. You are right, I remember using cameras such as the Canon F1n long before digital and auto focus and experiencing some shots slightly unsharp because of film not being as flat as it could be and screen alignment needing a service.

Nowadays with a flat imager I would have expected better engineering and manufacturing tolerances. I use auto focus all the time as does almost everyone on the planet, but it is interesting to see that something is happening during manual focus that is highlighting the inaccuracy of the viewfinder. I suspect this is more prevalent to the prosumer models. It could be argued that the modern view finder is not fit for purpose and LiveView must now be used at all times. This would seriously effect how almost every pro user (particularly press and sports) interfaces with their cameras. Canon has been plagued in recent years with the short or long focus issue.

 

[marekjoz]

If you haven't done it yet, check this wonderful blog posts about AF, MF and LV in three parts by Roger Ciala from lensrentals.com. This guy knows what he's writing about:

http://www.lensrentals.com/blog/2012/07/autofocus-reality-part-1-center-point-single-shot-accuracy
http://www.lensrentals.com/blog/2012/07/autofocus-reality-part-ii-1-vs-2-and-old-vs-new
http://www.lensrentals.com/blog/2012/08/autofocus-reality-part-3b-canon-cameras

Maybe there you will not find an answer better, than given by neuro and CharlieB, but it gives a real good coverage on potential problems with focus of very different kind and for really unexpected reasons.

 

[cmcbc]

Great articles. These explaine a lot. They do however deal with auto focus issues using manual focus via LV as one of the performance benchmarks.

In my tests with manual focus via the view finder, results are even worse when checked against the LiveView image. Thankfully most people don't use manual focus via the view finder, but they should be aware of the possibility of really poor results if they ever do use it.

Many pro users sometimes shoot with fast lenses (f1.2, f1.4 etc) wide open, the subsequent depth of field at the imager/film plane is less forgiving of inadequate design or manufacturing tolerances. All these issues seem to be less apparent to users working in f stops such as 3.5 through 16 etc.

 

[cmcbc]

Judging by all the responses above, it would seem that on mid range dslr's (and possibly higher models), relying on the view finder for manual focus sharpness can produce worse focus problems compared to using auto focus despite front/back focus issues relating to auto focus.

 

[awinphoto]

For the most part, I would say that these cameras are relying so much on the AF in expense of MF, the non-changeable focus screens with the transmissive LCD would a big hint to where the camera industry is going... I've been shooting professionally also so I know how annoying OOF focus images are, especially when we need that money shot...

That being said, dont flame but in the 90's i used to work at a large pro lab in san diego that was one of the larger labs in the country, and we had a professional photographer client so shot medium format hassy's and without fail he would get these huge 20x30's or 30x40's canvas prints and towards the last year I was there his prints were just so slightly soft... We at the lab would look at the print and notice his focus was off... We would kinda gently hint to him that maybe he should get his eyes checked and he would go into tirades and even go as far as saying he's using a soft focus technique for his portraits, but on the 30x40's, it was unmistakable that his focus was not right... I dont know what ever happened to him, but when your eyes start going, it may be hard for some people to notice and or accept... Both my parents have coke bottle glasses and I so far have dodged the bullet with good vision, but I know my day is coming...

 

[Rocky]

Judging by all the responses above, it would seem that on mid range dslr's (and possibly higher models), relying on the view finder for manual focus sharpness can produce worse focus problems compared to using auto focus despite front/back focus issues relating to auto focus.

On top of all the explanation from this site, there is another 'inconvenience". ALL the AF lenses have very "coarse" movement compare to the real MF lenses. That make MF a chore on the AF camera body.

 

[Mt Spokane Photography]

The focus screen can be inaccurate. Canon will sell you shims to adjust the position to correct it, or you can ask them to adjust it. sometimes peoplle replace a focus screen but lose those tiny shims.
Live view uses the actual sensor, so you see the actual image being taken.

 

[marekjoz]

The focus screen can be inaccurate. Canon will sell you shims to adjust the position to correct it, or you can ask them to adjust it. sometimes peoplle replace a focus screen but lose those tiny shims.
Live view uses the actual sensor, so you see the actual image being taken.

And at the same time with fast lenses wide open, after having focused properly with liveview at 10x magnification it is educationaly useful to move a camera or focus ring just a little tiny distance to see how shallow the DOF was. It is really shocking when you realize after such an excercise, how low the probability was, that you would hit the target with the viewfinder and naked eye only.