Back/Front Focus Questions

[dandaluzphotography]

Hi Guys,

I have a couple of questions about this. From everything I've read, this is a problem that happens when using auto-focus. Is it possible to have this problem when using Manual focus?

Also, If you're shooting something flat and somewhat far away, would a back/front focus issue be evident (using auto-focus)? Would it make your image appear of our focus and soft? I'm pretty sure if you had items in your photograph in front of behind the main subject of the shot, the back/front focus issue would be seen, but with a flat subject like a cityscape for example, would you see it?

Thanks,
Danny

 

[Big Mike]

The best way to check for this, is probably with a focus test. In simple terms, you can shoot something flat, but from 45 degrees. A newspaper or magazine page for example. Focus on something specific...like a line of text. Then closely examine the image to see if the line you focused on is sharp, or whether lines above or below are more sharp.

There are all sorts of 'focus test sheets' that you could print off and use. There is even a device that you put on a tripod/light stand. It has a flat face/scale (flat to the camera) and an angled scale. It looked like a good idea, but it was stupidly expensive.

 

[dandaluzphotography]

The best way to check for this, is probably with a focus test. In simple terms, you can shoot something flat, but from 45 degrees. A newspaper or magazine page for example. Focus on something specific...like a line of text. Then closely examine the image to see if the line you focused on is sharp, or whether lines above or below are more sharp.

There are all sorts of 'focus test sheets' that you could print off and use. There is even a device that you put on a tripod/light stand. It has a flat face/scale (flat to the camera) and an angled scale. It looked like a good idea, but it was stupidly expensive.

Thanks Mike. But would this be a problem using manual focus?

 

[Big Mike]

I would think that yes, front or back focusing could still be an issue with manual focus. Because when focusing, you are looking at the focusing screen (clear screen inside the viewfinder assembly) so if that is out of whack, it might look good to you, but when the mirror flips up, the focus at the sensor may not be the same as you saw on the focus screen.

But I'd guess that the AF accuracy is a separate issue. Because as I understand it, the AF system on these cameras is below the mirror (not above, like the viewfinder). There are two mirrors, a smaller one to send some light down to the AF system.

I'm a bit out of my reckoning here...hopefully someone else will chime in.

 

[kasperjd4]

You shouldn't have front/back focus issues when manual focusing. What you see in your view finder should be what you get on file. Front/back focus issue's are caused by the focus motor. Most camera's are a +/- 5 points in quality control for AF. If you want to tune your AF do like Big Mike said. But there shouldn't be any issue when manually focusing as it does not involve the focus motor.

 

[RacePhoto]

When focusing manually it shouldn't be an issue. Having the mirror that far out of whack would be almost impossible. Either that or the sensor would have to be loose and moved. Highly unlikely, which brings me to answer, NO you shouldn't have any focus issues, other than depth of field or vision problems, using manual focus.

I suppose someone could have the diopter on the viewfinder set so far out of normal that their eyes would compensate and create out of focus images that looked perfect when shot.

Generally you should be able to say, what you see is what you get. That's the advantage of a DSLR.

The best way to check for this, is probably with a focus test. In simple terms, you can shoot something flat, but from 45 degrees. A newspaper or magazine page for example. Focus on something specific...like a line of text. Then closely examine the image to see if the line you focused on is sharp, or whether lines above or below are more sharp.

There are all sorts of 'focus test sheets' that you could print off and use. There is even a device that you put on a tripod/light stand. It has a flat face/scale (flat to the camera) and an angled scale. It looked like a good idea, but it was stupidly expensive.

 

[dandaluzphotography]

Thanks guys.

Danny

 

[analog.universe]

When I changed the focusing screen in my 60D, I initially did not seat it 100% correctly, and it was front focusing during manual focus by about 1cm at 2m. Pulling the screen out and reseating it solved the issue. So, yes, you most certainly can have discrepancies between the apparent focus in the viewfinder and the actual focus to the sensor when in manual mode. But this represents a different issue, and different internal components, than any inaccuracies in the autofocus system (unless of course somehow the sensor plane is out of whack).

 

[tonifiddle]

I've got a second hand lens which has significant back focusing.By saying significat I mean that I could not correct the issue by recalibrateing the focusing point from the camera menu (available on Nikon D300 and higher end D-SLRs as well as EOS 50D and EOS 7D)
If I take a shot in life view mode (contrast detect focusing) the focus is spot on.But in phase detect mode (regular D-SLR focusing system) the focus is way off.
What I see in the view finder is not so easy to judge where exactly is the sweet spot so in other words it is almost impossible human eye to replace electronic focusing system.So I use the camera's focusing system to guide me where exactly I am in focus while manual focusing.The result is exactly the same focal point with a back focus.This leads me to the conclusion that the front or back focusing issues are irrelevant to the lens motor drive .

 

[1holegrouper]

DSLR's are generally optimized for autofocus IMHO. So, I find it is harder to precisely manual focus than compared to a film camera. What I typically do on tight criteria focus shots is increase my DOF more and/or just take a test shot and review it on the viewer under magnification via a loope. This also avoids the issue of the mirrors possibly being out of alignment. You may also want to check how clean your sensor is.