Lower Apertures = Constant Failure.

SimplyShane

To keep this short, I will just say that I've been having major problems using lower (or faster) f/stops.

I recently picked up a 50mm f/1.8 lens and I struggle *constantly* with getting the correct area of my image in focus. My Auto-Focus, quite typically, will select regions that are close but not exactly what I had in mind for the shot, and the results have given me tons of headaches. (For instance, I had one shot where I wanted a page of a book in focus. The camera apparently chose a part of the bench directly behind the page. Things like that.)

If someone tells me how to post sample images, I will gladly supply some. I am using a Canon Rebel XS with the 50mm f/1.8 lens as I mentioned earlier... Auto-Focus was used.

Overall, the bottom line is that I need to learn how to use faster lenses. I had zero problems with my f/3.5 and slower kit lenses.. This prime is just throwing me for a loop..

time2smile

I'm a Nikon shooter, but it sounds as if you are letting the camera decide the AF area, you need to select your focus point manually.

Someone will come along shortly, and tell you how.

good luck

ziggy53

The Canon EF 50mm, f1.8 is Canon's least expensive lens and, unfortunately, contains the least accurate AF technology. I found it extremely difficult to AF successfully using that lens at large apertures.

About the only way to tame that lens, that I found worked consistently, was to use manual focus in conjunction with Live View mode. If you start live view and zoom in on the area requiring prime focus you can obtain very good results, even at f1.8. The lens does sharpen nicely by f2.2 however.

At f1.8 any lens will have extremely limited DOF so it's important to make several attempts of anything you really desire to capture sharply.

If you "must" use AF with that lens then try, as time2smile already suggested, using a single AF point directly on the area that requires prime focus. Remember what I said about the lens misfocusing and take several exposures re-focusing each time. About 1 in 3 should be good. Do not focus and recompose with any large aperture lens so make sure to choose an appropriate AF point.

If you can use a smaller aperture I found I got fairly consistently good AF by f4.

If you can afford it the EF 50mm, f1.4 USM is much, much better to AF consistently and correctly.

pathfinder

I never permit my Canon body to choose my AF point -I always manually choose a singular AF point for precise focus. If you let the camera choose autmatically, it will choose wrong most of the time.

You MUST manually choose a single AF point if you really want accurate focus. Smaller apertures, with their larger depth of field give you more slop, but as you are experiencing, large apertures are not nearly so forgiving.

chrisjohnson

Pages from books or printed papers are a bit tricky. I do this quite often in all kinds of light because it beats taking notes. When I started, the focus motor would be working overtime and it did not sound healthy.

My theory is that the camera sees the page as mostly white space - so no contrast. When you select a single focus point and are lucky enough to hit a letter square on then there is also no contrast - it is black. The better the camera the worse it gets. Otherwise you have a random mix of extremes - black and white. Ziggy certainly knows how this works technically but my feeling is that the sharpest auto focus comes when there is distinct contrast in the focal area.

At any rate, snapping book pages of text is tricky with all the lenses I have when I use autofocus and even when I think I have selected a single focus point. No need to buy a more expensive lens unless you like macro which is my lens of choice for books.

The method I use with the lens that happens to be on the camera is to set TV to 1/250 for sharpness and then FOCUS MANUALLY. Let the camera decide the aperture. When you bump up the ISO for f5.6 or higher then you won't have a problem with depth of field normally unless the page is very curved, but it is better to have a high f number because you are close to the subject and you don't want a bokeh effect. You can bump ISO up a long way before noise makes text difficult to read, depending on your camera. On my 40D text is readable on 1600 ISO even although this is rarely needed.

When you want to shoot color pictures then it is better (for me) to use a tripod and make sure lighting is even and the paper is flat. Focus manually. Live view is very handy if you have it. Even so it is possible to shoot sharply from the hand as long as shutter speed is right and the ambient light is even.

Maybe this long post does not answer your prime question - how to use faster lenses? I got distracted by your book example. Faster lenses are for bad light and no flash, bokeh on portraits, more flexibility - not for shooting books unless you stop the lens down.

pathfinder

As Chris pointed out, flat materials, like pages, are best shot at f8 or f11 or even
F 16, not at f1.8. Trying to shoot a page at f1.8 will be an exercise in frustration. The large aperture, and the short distance to the subject guarantee an extremely shallow DOF, and a page of text will confuse most AF systems. Shooting flat pages is usually best done with the camera mounted in a macro stand to align the sensor plane parallel with the plane of the text, due to the shallow DOF with short subject distances.

SimplyShane

pathfinder wrote: »

I never permit my Canon body to choose my AF point -I always manually choose a singular AF point for precise focus. If you let the camera choose autmatically, it will choose wrong most of the time.

You MUST manually choose a single AF point if you really want accurate focus. Smaller apertures, with their larger depth of field give you more slop, but as you are experiencing, large apertures are not nearly so forgiving.

Hold up sir.

I ALWAYS use the center focus point. I hate having the camera choose its own point. As you said, it always gets it wrong..

However, my Rebel STILL gets it wrong even with the center focus point chosen.

Could my problems lie with all the "focusing and recomposing" I've been doing? I've always used that strategy and it works beautifully on everything but this lens...

If I had to change that about my shooting style, it would be very difficult. Not impossible, but a big pain in the arse...(Manually selecting AF points is just clumsy. Leaving it on the center point is so much better... *Sigh*)

SimplyShane

ziggy53 wrote: »

The Canon EF 50mm, f1.8 is Canon's least expensive lens and, unfortunately, contains the least accurate AF technology. I found it extremely difficult to AF successfully using that lens at large apertures.

About the only way to tame that lens, that I found worked consistently, was to use manual focus in conjunction with Live View mode. If you start live view and zoom in on the area requiring prime focus you can obtain very good results, even at f1.8. The lens does sharpen nicely by f2.2 however.

At f1.8 any lens will have extremely limited DOF so it's important to make several attempts of anything you really desire to capture sharply.

If you "must" use AF with that lens then try, as time2smile already suggested, using a single AF point directly on the area that requires prime focus. Remember what I said about the lens misfocusing and take several exposures re-focusing each time. About 1 in 3 should be good. Do not focus and recompose with any large aperture lens so make sure to choose an appropriate AF point.

If you can use a smaller aperture I found I got fairly consistently good AF by f4.

If you can afford it the EF 50mm, f1.4 USM is much, much better to AF consistently and correctly.

---

Thank you so much for the great reply.

You seem to confirm that my issue really lies with the "focus and recompose" line of thought I've always lived by.

(No, I never let the camera choose its own point. That's kind of silly unless you just have to blindly point and shoot..
However, you also mention that the AF technology used in the lens just happens to be quite bad by default. That is very sad to hear.)


Getting around that issue seems to involve either::

1. Taking multiple shots and praying for luck. or 2. Using manual focus with Live View.

Sadly, both of those options are terrible for me because I've been trying to photograph children for a local school district without anything being posed. (So the manual focus option is kind of thrown out the window.)

Apparently, the only other option I have is to spend more money, but I simply can't afford to do that and it is not an option. Bottom line. I'm very poor and am thankful that I can afford what little I have.


Your thoughts??? (Thanks for all the help thus far..)

chrisjohnson

With a higher aperture number (smaller opening) your depth of field increases. So if you shoot at f16, for example, your picture tends to stay in focus. Try it.

Ziggy did not say that AF is bad by default, he said it is difficult on your lens with big apertures (lower numbers). A simple approach would be to reduce the opening by moving to a higher aperture number, or moving further away from the subject, or both. Somewhere on the net there must be a depth of field table for your lens - I am too lazy to find it - but it is certainly in your manual. When you take pictures with a 50mm lens close up and aperture wide open your depth of field will be very small - at best a few millimeters only I guess. Things outside the focus point will be unsharp.

Welcome to the wonderful world of optics. The 50mm lens works great when your subject is several metres away. It barely works at all on close subjects. You might enjoy a macro lens which is designed for close range and also works decently at longer distances. Otherwise you need to reduce the aperture (higher f number), as said before.

pathfinder

You have two options.
Shoot at a smaller aperture -f4,f5.6,f8. Rather than f1.8 or f2.0
Replace your Rebel with a 7D or a used 1series camera.
Backing away from your subject-increasing the image plane to subject distance - will increase dof modestly as well.

An explanation of why focus recompose does not work - http://visual-vacations.com/Photography/focus-recompose_sucks.htm

zoomer

I did not read all the other responses.
You can not focus and recompose with large f-stop short depth of field lenses, especially the closer you are to the subject.
Your perspective and in focus plane changes as you move.

With my 85 1.4 I have done a lot of testing.
First I fine tuned the lens with my camera to make sure it was focusing where I wanted it to.
Then I went to Continuous focus, I use the same settings that I would use for shooting birds in flight.
When I did that my keeper rate went to almost 100%. Try that.

Don't be shy about using that large f-stop, that is why you bought the lens in the first place I image.
Switching to continuous focus and using the center focus point should solve your problem.....or at least improve your keeper rate greatly.
Of course first do some testing to make sure the lens is focusing where you think it is. You may have a front or back focus issue.
At 1.4 or 1.8 if your focus area is just a smidge off you will have problems.
My 85 1.4 took 14 points of adjustment to get it to focus where the camera said it was focusing.

SimplyShane

pathfinder wrote: »

You have two options.
Shoot at a smaller aperture -f4,f5.6,f8. Rather than f1.8 or f2.0
Replace your Rebel with a 7D or a used 1series camera.
Backing away from your subject-increasing the image plane to subject distance - will increase dof modestly as well.

An explanation of why focus recompose does not work - http://visual-vacations.com/Photography/focus-recompose_sucks.htm

---

If I have to use f4 on up... Why the heck did I get this lens? Granted, it is dirt cheap ($100 or lower), but still...

Makes no sense.

And what would a 7D or 1series camera do differently? Why would that matter?

Finally, backing up from the subject obviously changes composition. Granted, that may work in some cases, but in others... No.

Yikes, this isn't looking good for me. If someone tells me how to post images, I'd love to send you guys samples...

SimplyShane

zoomer wrote: »

I did not read all the other responses.
You can not focus and recompose with large f-stop short depth of field lenses, especially the closer you are to the subject.
Your perspective and in focus plane changes as you move.

With my 85 1.4 I have done a lot of testing.
First I fine tuned the lens with my camera to make sure it was focusing where I wanted it to.
Then I went to Continuous focus, I use the same settings that I would use for shooting birds in flight.
When I did that my keeper rate went to almost 100%. Try that.

Don't be shy about using that large f-stop, that is why you bought the lens in the first place I image.
Switching to continuous focus and using the center focus point should solve your problem.....or at least improve your keeper rate greatly.
Of course first do some testing to make sure the lens is focusing where you think it is. You may have a front or back focus issue.
At 1.4 or 1.8 if your focus area is just a smidge off you will have problems.
My 85 1.4 took 14 points of adjustment to get it to focus where the camera said it was focusing.

---

Thanks for the reply.

However, I'm VERY confused. How are you "fine tuning" the lens? What are these "14 points of adjustment" you speak of? Lastly, I assume continuous focusing is an Auto-Focus mode available through the menu?

Sorry for my newbishness.

zoomer

SimplyShane wrote: »

---

Thanks for the reply.

However, I'm VERY confused. How are you "fine tuning" the lens? What are these "14 points of adjustment" you speak of? Lastly, I assume continuous focusing is an Auto-Focus mode available through the menu?

Sorry for my newbishness.

My Nikon cameras allow me to fine tune the focus on my lenses to the camera, maybe the Canon's don't do that, if not disregard that part.

Yes you want to use the same focusing mode that you use for tracking moving objects. If you are not familiar with doing that you will want to read up on in your manual or research on the internet. It is very easy to set the camera up to track movement.
The idea is that since the depth of field is so razor thin that any movement at all puts things out of focus if you are not using the movement tracking abilities of your camera.

As I said I did a lot of testing and this works.
If it does not work for you then you may have a front focus or back focus issue with your lens.
To find out do some test shots to see if your lens, when it focuses properly, is focusing on the place you want it to focus at your largest aperture.

adbsgicom

50D, 7D, 5Dmk2 (don't know about 1-series) all allow microadjusting the focus point. You can do it per lens (camera has a lookup table in it). Look on the custom function section of the menu.

anonymouscuban

Haven't read all the posts (little lazy today) so this may have been said already. Are you focusing and then recomposing? I struggled with this same issue but with my f2.8 lens. I would focus on my subjects eyes, then recompose, shoot, check the image in the LCD, scream obsenities, then start the cycle over again. I later realized it was my practice of recomposing after grabing focus. At f2.8, even the slightest movement would change the focal plane, making for a soft shot on the eyes. What I do now is to move my focal point to the closest one to where I want my focus to be, compose, THEN focus and fire. This has resulted in sharp eyes and no more Spanish curse words ala Ricky Ricardo.

pathfinder

anonymouscuban wrote: »

Haven't read all the posts (little lazy today) so this may have been said already. Are you focusing and then recomposing? I struggled with this same issue but with my f2.8 lens. I would focus on my subjects eyes, then recompose, shoot, check the image in the LCD, scream obsenities, then start the cycle over again. I later realized it was my practice of recomposing after grabing focus. At f2.8, even the slightest movement would change the focal plane, making for a soft shot on the eyes. What I do now is to move my focal point to the closest one to where I want my focus to be, compose, THEN focus and fire. This has resulted in sharp eyes and no more Spanish curse words ala Ricky Ricardo.

No more Ricky Ricardo cursing clap

This post just make my day!

JohnBiggs

There is a shortcut to focus point selection using the joystick. Maybe that will reduce the clumsiness.